C-009 Book 3 Electrical Fundamentals

AIR SERVICE TRAINING (ENGINEERING) LIMITED A Subsidiary of Perth College

Electrical Fundamentals EASA Part 66 – C/009 Book 3 Module 3

BRAHAN BUILDING CRIEFF ROAD PERTH PH1 2NX TEL: 01738 877105 FAX: 01738 553369

© Air Service Training (Engineering) Ltd

Aeronautical Engineering Training Notes These training notes have been issued to you on the understanding that they are intended for your guidance, to enable you to assimilate classroom and workshop lessons and for self-study. Although every care has been taken to ensure that the training notes are current at the time of issue, no amendments will be forwarded to you once your training course is completed. It must be emphasised that these training notes do not in any way constitute an authorised document for use in aircraft maintenance.

All Rights Reserved The copyright in these technical training notes remain the physical and intellectual property of Air Service Training (Engineering) Ltd, (AST). Copying, storing in hard copy or electronic format, transmission to third parties and use for teaching by establishments other than AST is forbidden, except with the written permission of the AST Chief Executive Officer.

J Dobney Theory Training and Exam Manager

September 2006

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Contents Page Number 3.17 AC Generators 3.17.1 AC Generator Principle ..................................................................... 1 3.17.2 Generator Types ................................................................................ 3 3.17.3 Single, Two and Three Phase Alternators .......................................... 7 3.17.4 Three Phase Star and Delta Connections ...................................... .11 3.17.5 The Brushless (Permanent Magnet) AC Generator ........................ .13 3.18 3.18.1 3.18.2 3.18.3

AC Motors ........................................................................................... . Construction, Principles and Operation ........................................ …15 Motor speed control ..........................................................................27 Types of Single Phase AC Motor .................................................... .29

3.12. Direct Current Generators/Motors………………………………………... 3.12.1 Basic Motor and Generator Theory .................................................. 37 3.12.2 Construction and Purpose of Components in DC Generator ........... 43 3.12.3 Operation and Factors Affecting Generator Output and Current Flow…53 3.12.4 DC Motor Principle of Operation ..................................................... 61 3.12.5 Operation and Factors Affecting Motor Output Power and Torque .. 65 3.12.6 Series Wound, Shunt Wound and Compound Motors...................... 75 3.12.7 Starter / Generator Construction ...................................................... 81

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3.17: AC Generators 3.17.1: AC Generator Principle

Simple AC Generator The principle of electromagnetic induction, relates to both dc and ac generators. When a conductor is cut by magnetic lines of force, a voltage will be induced in the conductor. The direction of the induced voltage will depend on the direction of the magnetic flux and the direction of movement across the flux. As shown, a bar magnet is mounted to rotate between the faces of a soft-iron yoke on which is wound a coil of insulated wire. As the magnet rotates, a field will build up first in one direction and then in the other. As this occurs, an alternating voltage will appear across the terminals of the coil, and the shape of this ac voltage will roughly approximate a sine wave. This is the principle of a simple ac generator. Rotor and Stator During this module, the terms ROTOR and STATOR will be used frequently. These terms apply to ac machines where the rotor is the assembly that is rotated and the stator is the stationary section. AS shown, the rotor is the bar magnet and the stator is the soft-iron yoke. Fleming’s Right Hand Rule

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Fleming’s Right Hand Rule Fleming’s Right Hand Rule for generators is used to determine the direction of the induced emf. The thumb, first finger and second finger are used as shown. 

The first finger points in the direction of the field (north to south external of the magnets).



The second finger points in the direction of the current flow.



The thumb points in the direction of motion.

When two of these three factors are known, the third can be determined by the use of this rule as can be clearly seen from the following figure.

Finding the Direction of Magnetic Flux, Current and Motion

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3.17.2: Generator Types There are two basic types of generator, the rotating armature type and the rotating field type. The rotating armature type is similar in construction to a DC generator where the armature rotates through a steady magnetic field. The rotating field type has stationary armature windings and a rotating field. Rotating Armature Generator The attached figure shows a schematic diagram of a rotating armature generator. The rotating armature cuts the magnetic field and produces an alternating emf in the armature windings. The main load current is carried by the slip rings.

Rotating Armature Generator

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Rotating Field Generator The rotating field or fields do not change their flux direction with respect to the rotor. The figure shown is a schematic diagram of a rotating field generator. The rotating magnetic field can be: 

A permanent magnet, this would only allow a very small voltage output as in a Tachogenerator.



DC wound coils, as in some aircraft ac generators.

The basic ac generator therefore cannot be a true self-excited generator. It requires a separate dc power source. The field is rotated and cuts the stationary windings. An alternating emf is produced in the stator windings. The slip rings only carry the field supply which is the smaller dc voltage and current.

Rotating Field Generator Some advantages of the rotating field type generator are:

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

The rotating field is an electromagnet fed with dc. This excitation current is much smaller than the output so the slip rings are smaller than would be necessary for the output current.



More efficient cooling is achieved on the stationary output windings allowing higher loads to be carried.



When the current is greater, the output windings must also be larger. Consequently, these heavier windings are not subject to centrifugal forces.

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Frequency There are two ways to change the output frequency of a generator. 

The first, as with voltage, is to increase the rotational speed of the rotor, which will increase the output frequency or decrease the rotational speed which will decrease the output frequency.



The second method is by changing the number of pairs of poles. In a generator, the voltage and current pass through a complete cycle of values each time a conductor passes under a north and south pole of a magnet as shown.

Frequency Output with One Pair of Poles The number of cycles for each revolution of the conductor is equal to the number of pairs of poles. The frequency, therefore, is equal to the number of cycles in one revolution multiplied by the number of revolutions per second. Expressed in equation form: N FREQUENCY (F) = P  Hz 60 Where N = Revolutions per minute P = Pairs of Poles. Because this frequency is measured in Hertz and we know that 1 Hertz is equal to 1 cycle per second, the ‘P × N’, must be divided by 60 to achieve the correct figure for the frequency. The first formula values show a frequency of 1 Hz over a set period of time with one pair of poles rotating. The second formula values show that the

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number of pairs of poles has been increased to two, and, because the North/South of the second pair do the same, the output frequency has been doubled to 2 Hz over the same period of time. Substituting these two examples into the equation where the RPM = 60. N 60  F P  1  1Hz 60 60

N 

F



60

60  P  60  2  2 Hz

Frequency Output with Two Pairs of Poles

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3.17.3: Single, Two and Three Phase Alternators Single Phase Alternator A generator that produces a single, continuously alternating voltage is known as a single phase alternator[generator]. The stator windings are connected in series. The individual voltages, therefore, add to produce a single-phase ac voltage as shown.

The definition of phase as you learned in studying ac circuits may not help too much, however, remember ‘out of phase’ meant ‘out of time’. Perhaps it is easier to think of the word phase as meaning voltage as in single voltage. The need for a modified definition of phase in this usage will be easier to see as we proceed. Single phase alternators are found in many applications. They are most often used when the loads being driven are relatively light. For this reason it is unlikely that you will come across any with an aircraft application unless they are used as ac tacho-generators for speed indication. Two Phase Alternator Two phase implies two voltages if we apply a new definition of phase, and, it’s that simple. A two phase alternator is designed to produce two completely separate voltages. Each voltage, by itself, may be considered as a single phase voltage. Each is generated completely independent of the other. Certain advantages are gained, which will be covered later. The figure shows a simplified two-pole, two-phase alternator. Note that the windings of the two phases are physically at right angles [90º] to each other. You would expect the outputs of each phase to be 90º apart, which they are.

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The graph shows the two phases to be 90º apart, with ‘A’ leading ‘B’. They are out of phase with each other.

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Three Phase Alternators The three phase alternator, as the name implies, has three single-phase windings spaced such that the voltage induced in any one phase is displaced by 120º from the other two. The simplified schematic shows all the windings of each phase joined together as one winding. The rotor is omitted for simplicity. The voltage waveforms generated across each phase are drawn on a graph, phase displaced 120º from each other. The three-phase alternator as shown is made up of three single-phase alternators whose generated voltages are out of phase by 120º. The three phases are independent of each other.

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Notes:

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3.17.4: Three Phase Star and Delta Connections Rather than having six leads coming out of the three-phase alternator, the same leads from each phase may be connected together to form a star [wye] connection. The neutral connection is brought out to a terminal when a singlephase load must be supplied. Single phase is available from neutral to ‘A’, neutral to ‘B’ and neutral to ‘C’. In a three phase, star connected alternator, the total voltage, or line voltage, across any two of the three line leads is the vector sum of the individual voltages. Each line voltage is 1.73 times one of these voltages. Because the windings form only one path for current flow between phases, the line and phase currents are the same [equal]. A three-phase stator can also be connected so that the phases are connected end-to-end; it is now delta connected. In the delta connection, line voltages are equal to phase voltages but each line current is equal to 1.73 times the phase current. Both star and delta connections are used in alternators and are more efficient than either two-phase or single-phase alternators.

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Notes:

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3.17.5: The Brushless (permanent magnet) AC Generator

Brushless AC Generator Schematic The permanent magnet, which can be up to 18 pole pieces, is fitted to the drive shaft and is rotated by the gearbox. The developed high frequency (up to 1200 Hz) is sent to the Generator Control Unit where it is processed to provide a dc input to the exciter generator (a set of windings mounted on the drive shaft). The resultant induced emf is rectified by a full wave bridge unit and fed to the main generator field as a dc. This dc cuts through the main generator field windings, wound on the stator. The resultant three phase output is supplied to the ac busbar.

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Notes:

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3.18: AC Motors Construction, Principlesand Characteristics 3.18.1: Introduction The basic principles of magnetism and electromagnetic induction are the same for both ac and dc motors, but the application of the principles is different because of the rapid reversals of direction and changes in magnitude characteristic of alternating current. Certain characteristics of ac motors make most types more efficient than dc motors; therefore such motors are used commercially whenever possible. During recent years, ac power systems have been developed for large aircraft with the result that a much larger amount of electrical power is available on aircraft than would be available with dc systems of the same weight. Thus one of the main advantages of the ac power system is that it provides more power for less weight.

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Production of a Multi-Phase Rotating Field A rotating field may be produced by applying a three-phase supply to a threephase stator. The field produced is of unvarying strength and its speed of rotation depends upon the frequency of the supply.

Typical Three-Phase Stator The figure shows a typical three-phase stator. The two windings in each phase (for example A and A 1) are connected in series and are so wound that current flowing through the two windings produces a North pole at one of them and a South pole at the other. So, if a current is flowing in the A phase in the direction from the A to the A 1 terminals, pole piece A becomes a North Pole and A1 a South Pole. The three-phase stator is connected in delta, so that only three terminals, each common to two of the windings, are provided for the three-phase ac input.

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At any instant, the magnetic field generated by one particular phase is proportional to the current in that phase. Therefore, as the current alternates, so does the magnetic field. As the currents in all three phases are 120 out of phase with each other, then so must the magnetic fields be and the resultant magnetic field will be the vector sum of these three.

Output from Three-Phase Stator From earlier studies, it will be remembered that the flux path follows the line of least resistance and this can be clearly seen in the figure. At position 1, phase A of the input supply is at zero with both B and C phases providing an output. The two flux paths B-C and B1-C1 are the lines of least reluctance and magnetically form a single resultant axis with which any permeable material located within its sphere of influence would tend to align. Staying with position 1, the current in the A phase is zero, the current in the C phase is positive and flows in the direction C to C 1 and the current in the B phase is negative and flows in the direction B to B 1. Equal currents therefore flow in opposite directions through the B and C windings and magnetic poles are established as shown in Fig 2. The shortest path for the magnetic lines of flux is such that the lines leave B 1 (North Pole) and go to C 1 (South Pole) with a similar result for C to B. Because the magnetic fields of the B and C phases are equal in amplitude (due to equal currents) the resultant field lies in the direction of the arrow.

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Moving on to position 2, where the supply cycle has advanced by 60 , the current in C is now zero, A is positive and B is negative. The resultant magnetic field is produced in the same way as described for position 1, and the other positions show the conditions at intervals of 60 . Thus, the magnetic field rotates one complete revolution (in a clockwise direction in this case) during one complete cycle of three-phase supply, so it is in time with, or synchronous with, the ac input. Types of AC Motor There are two principal types of ac motors. They are the 

Induction.



Synchronous.

The Synchronous Motor The ac generator, like the dc generator, is a reversible machine; if supplied with electrical energy, it runs as a motor. Thus synchronous motors have the same construction as rotating-field ac generators. The input alternating current is applied to the stator and the rotor carries the magnetic field windings which are supplied with dc from a separate source. NOTE:

The rotor may in theory (and practice) be either a permanent magnet or a wound rotor separately excited from a dc source.

If the rotor is energised with dc it acts like a bar magnet and will therefore try to line itself up with the magnetic field produced by the stator. In the synchronous motor the three-phase ac produces a rotating magnetic field, which causes the rotor to follow the field, (assuming that the motor is already running). The synchronous motor will not start of its own accord, because the rotating magnetic field moves too quickly to provide a starting force.

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

The inertia of the rotor does not allow it to respond to the rapidly rotating field.



It has to be started and run up to speed by another motor, usually a small induction motor.



When the speed of the driven rotor approaches that of the rotating magnetic field, the rotor and the field ‘lock together’ and the rotor then rotates synchronously with the field of its own accord.

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Synchronous Motor Characteristic The synchronous motor is a ‘single-speed’ machine, its speed of rotation being determined by the speed of the rotating magnetic field which, in turn, is decided by the frequency of the three-phase ac input to the stator windings. 

The synchronous motor is therefore most useful for applications requiring constant speed, eg ventilation fans and gyroscopes.

Equally, it is clear that the synchronous motor is most appropriate to light mechanical loads, because if the load became excessive, the ‘synchronous lock’ would be broken and the motor would stop. It is unusual to find them on large passenger carrying aircraft. Induction Motors The ac motor most commonly used on aircraft is the induction type and, dependant upon application, may be designed for operation from a threephase, two-phase or single phase power supply. It is robust, simple and cheaper than other types. The basic three-phase induction motor has no slip rings or commuter and has little to go wrong. The following figure shows the stator of the induction motor, which is almost the same as that of the synchronous motor, i.e. it has three-phase windings and associated pole pieces, which as usual produce a rotating magnetic field when supplied with three-phase ac. The rotor consists of a series of heavy copper bars connected at each end by a copper or brass ring. No insulation is required between the bars and the core on which they are mounted because of the very low voltages induced Issued September 2006

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in the rotor bars. This type of rotor is a squirrel-cage and no external electrical connections are made to it.

Three-Phase Induction Motor The basic principle of operation of the induction motor may be explained below, where a conductor is set at right angles to a magnetic field. If the conductor is stationary and the field moves from right to left, the change of flux through the conductor induces a voltage in it. If the conductor is part of a closed circuit, current flows in the conductor in the direction shown (the right hand rule for generators). This current-carrying conductor in the magnetic field then experiences a force tending to move it in the same direction as the field’s motion (the left-hand rule for motors). The conductor therefore tends to follow the movement of the field.

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Movement of a Conductor in a Field Applying this principle to the squirrel-cage induction motor we see that the rotating magnetic field produced by the stator induces a voltage in the bars of the rotor. 

Because the bars are thick and have a low resistance, a large current flows in them which set up a magnetic field.



The rotor field interacts with the stator field and, as usual when a currentcarrying conductor is placed in magnetic field, causes the rotor to turn so as to line up the two magnetic fields.



However, since the stator field is rotating, the rotor never quite catches up but follows a little behind.



As the rotor follows the field, the relative motion between the two is reduced, so also is the voltage induced in the rotor bars.



This reduces the rotor current and the turning force acting on the rotor. The rotor speed is automatically adjusted to something less than that of the rotating field; otherwise there would be no relative motion, no current and no movement of the rotor.



Thus in practice the rotor runs slightly slower than the rotating magnetic field, the amount depending upon the load.



The difference in the two speeds is the slip speed and the ratio of slip speed to the speed of the rotating field, is the slip.

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For example, if the magnetic field is rotating at 1000 rpm, the rotor may be running to 960 rpm. The slip speed is: 1000  960  rpm, and the slip is; 40  100  4% 1000 This is a typical value of slip. As noted earlier, the slip depends upon the load; the larger the load, the greater is the slip. But in practice very little speed change occurs between a light and a heavy load and the main use of an induction motor is as a constant speed drive to a load. This motor is only started under ‘no load’ conditions. The speed varies little between ‘no load’ and ‘full load’ when running.

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

This makes the motor suitable for driving such machines as lathes, bench drills and small generators.





The starting current of all squirrel-cage motors is heavy (4-6 times the running current). This is because, if the stator windings are energised from the three-phase supply whilst the rotor is stationary, the slip is maximum and so also is



the emf induced in the rotor.



The low resistance of the rotor gives rise to a large rotor current which produces a magnetic field opposing and weakening the stator flux (Lenz’s Law).



The back emf induced in the stator windings by the changing flux is therefore reduced so that a heavy current is taken by the stator on starting.

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Two-Phase Induction Motor A rotating magnetic field is also produced if two phases, 90° out of phase with each other are used instead of a three-phase supply. A two-phase induction motor is illustrated below.

TWO-Phase Induction Motor

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The production of a rotating magnetic field from a two-phase supply, 90° out of phase, is shown. It is a similar idea to the one previously drawn and described for a three-phase supply, and its action may be found in a similar manner. Two-phase induction motors are less efficient than three-phase types and the latter are used, where possible, in preference to two-phase motors.

Rotating Magnetic Field From a Two-Phase Supply Typically, two-phase induction motors find their greatest applications in systems requiring a servo control of synchronous devices.

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

for example as servomotors in power follow up synchro systems. In this instance the windings are also at 90° to each other but, unlike the motors thus far described, they are connected to different voltage sources.



One source is the main supply for the system and, being of constant magnitude, it serves as a reference voltage.



The other source serves as a control voltage and comes from a signal amplifier in such a way that it is variable in magnitude and its phase can either lead or lag the reference voltage, thereby controlling the speed and direction of rotation of the field and rotor. 3.18.1

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Single-Phase Induction Motors Single-phase induction motors are used extensively in low-power applications such as: 

blower fans and switch motors used in communication equipment.



A single-phase induction motor has only one stator winding so it is not capable of producing a rotating magnetic field of the type described earlier. The field produced by the single-phase winding alternates according to the frequency of the supply, and can be said to alternate along the axis of the single winding, rather than to rotate.

As the field changes polarity every half cycle, it induces currents in the rotor which tries to turn it through 180°, but as the force is exerted through the axis shown, there is no turning force on the rotor. This type of motor cannot, therefore be self-starting. If the rotor is given a start however, it will be given a push every half cycle that will keep it rotating. Since the field is pulsating, rather than rotating, single-phase induction motors produce a pulsating torque and are not as smooth running as two or threephase motors.

Single Phase Induction Motor

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Motor Speed Control

Single phase motors are not normally speed or direction controlled. Two-phase motor control uses reference and a control phase winding. The control winding input can be varied in amplitude and could either lag or lead the reference input so that speed and direction of rotation can be changed. Three phase motors rely on changing over any two windings, clockwise or anti-clockwise, to reverse the direction of rotation. Speed can be adjusted by the physical / electrical removal or addition, usually through relay control, of any pair of windings. Reducing the pairs of poles to increase the speed and increasing the pairs of poles to reduce the speed

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Notes:

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3.18.3: Types of Single Phase ac Motor It is impracticable to start a motor by turning it over by hand, so an electric device must be incorporated into the stator circuit such that it will cause a rotating field to be generated on starting. Once the motor has started, this device can be switched out of the stator, since the rotor and stator together will generate their own rotating field to keep the motor turning. 

The starting device takes the form of an auxiliary stator winding spaced 90° from the main winding, and connected to series ‘impedance’.



This ‘impedance’ is chosen to produce as great a phase displacement as possible between the currents in the main and auxiliary windings so that the machine starts up virtually as a two-phase motor.



A switch, usually operated by centrifugal action, cuts out the auxiliary winding when approximately 75% of synchronous speed has been attained and the machine continues to run on the main stator winding.



Alternatively, contacts in the auxiliary winding circuit may be closed by the high stator current which flows through a relay coil when the supply is switched on; the contacts opening as the motor current falls during acceleration from rest.

Electric Starter Using an Auxiliary Winding

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The impedance device used can be inductive or capacitive, or a combination of both. Consider the figure below, which shows a simplified schematic of a typical capacitor start motor. The stator consists of the main winding, and a starting winding which is connected in parallel with the main winding and spaced at right angles to it. The 90° electrical phase difference between the two windings is obtained by connecting the auxiliary winding in series with a capacitor and starting switch.

Schematic of a Capacitor Starter Motor 

On starting, the switch is closed, placing the capacitor in series with the auxiliary winding.



The capacitor is of such a value that the auxiliary winding is effectively a resistive-capacitive circuit in which the current leads the line voltage by approximately 45°.



The main winding has enough45°. inductance to cause the current to lag the line voltage by approximately



The two currents are therefore 90° out of phase, and so are the magnetic fields which they generate.



The effect is that the two windings act like a two-phase stator and produce the revolving field required to start the motor.

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When nearly full speed has been attained, a device cuts out the starting winding and the motor runs as a plain single-phase induction motor. Since the special starting winding is only a light winding, the motor does not develop sufficient torque to start heavy loads. Because a two-phase induction motor is more efficient than a single-phase motor, it is often desirable to keep the auxiliary winding permanently in the circuit so that the motor will run as a two-phase induction motor. The starting capacitor is usually made quite large, in order to allow a large current to flow through the auxiliary winding. The motor can thus build up a large starting torque. When the motor comes up to speed, it is not necessary that the auxiliary winding shall continue to draw the full starting current, and the capacitor can be reduced, therefore two capacitors are used in parallel for starting and one is cut out when the motor comes up to speed. Such a motor is called ‘capacitor-start, capacitor-run induction motor’ A disadvantage of this type of split-phase motor is the high starting current (nearly four times the full load current). The direction of rotation can be change by reversing the connections to either of the stator windings.

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Shaded-Pole Induction Motor The single-phase motors considered in the preceding sections all employed stators having uniform air gaps with respect to their rotor and stator windings, which are uniformly distributed around the periphery of the stator. The starting methods employed thus far were generally based on a split-phase principle of producing a rotating magnetic field to initiate rotor rotation. The great virtue of the shaded-pole motor lies in its utter simplicity; 

It has a single-phase rotor winding, a cast squirrel-cage rotor, and special pole pieces.



It has no centrifugal switches, capacitors, special starting windings, or commutator.



It has a single-phase winding and it is inherently self-starting.



There must be some auxiliary means of producing the effect of a rotating magnetic field, therefore, with a single-phase supply and only one stator winding.

The illustration shows the general construction of a two-pole shaded-pole motor.

The Shaded Pole Motor The special pole pieces are made up of laminations, and a short-circuited shading coil (or a single-turn solid copper ring) is wound around the smaller segment of the pole piece. The shading coil, separated from the main ac field

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winding serves to provide a phase-splitting of the main field flux by delaying the change of flux in the smaller segment. As shown below, when the flux in the field poles tend to increase, a shortcircuit current is induced in the shading coil, which by Lenz’s law opposes the force and the flux producing it. 





Thus, at point ‘B’, as the flux increases in each field pole, there is a concentration of the flux in the main segment of each pole, while the shaded segment opposes the main field flux.

At point ‘C’, the rate of change of flux and of current is zero, and there is no voltage induced in the shaded coil. Consequently, the flux is uniformly distributed across the poles. When the flux decreases, the current reverses in the shaded coil to maintain the flux in the same direction, as at ‘D’. The result is that the flux crowds in the shaded segment of the pole.

An examination of ‘B’, ‘C’ and ‘D’ will reveal that at intervals ‘B’, ‘C’ and ‘D’, the net effect of the in theface polerepresenting has been toaproduce a sweeping motion of flux flux distribution across the pole clockwise rotation. The flux in the shaded segment is always lagging the flux in the main segment in time as well as in physical space (although a true 90º relation does not exist between them). The result is that a rotating magnetic field is produced and the rotor always turns in the direction of the rotating field. For the type of shaded-pole motor shown, the rotation is clockwise since the flux in the shaded segment lags the main flux. In order to reverse the

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direction of rotation, it would be necessary to unbolt the pole structure and reverse it physically. The shaded-pole motor is rugged, inexpensive, and small in size, and it requires little maintenance. Unfortunately, it has a: 

very low starting torque.



low efficiency and



low power factor.

The last two considerations are not serious in a small motor. Hysteresis Motors A Hysteresis motor works on this principle: 

In a material with a large Hysteresis loop, the magnetic flux lags behind the current which produced it by almost 90°.



In a material with a small Hysteresis loop the two are almost in phase.



A stator of small Hysteresis loop material is supplied with a multi-phase input, as is the rotor which is made of large Hysteresis loop material



(usually cobalt steel). The result is that the flux in the rotor lags that in the stator by almost 90°.



The rotor will then move in an attempt to line up its field with that of the stator.



Thus, as the stator field rotates, the rotor follows it.



The effect on the rotor of the rotating stator field is that if the rotor is stationary, or turning at a speed less than the synchronous speed, every point on the rotor is subjected to successive magnetising cycles. As the stator field reduces to zero during each cycle, a certain amount of flux remains in the rotor material, and since it lags on the stator field it produces a torque at the rotor shaft which remains constant as the rotor accelerates up to the synchronous speed of the stator field. This latter feature is one of chosen the principle advantages of Hysteresis motors and for this reason they are for such applications as;



Autopilot servomotors, which produce mechanical movements of an aircraft’s flight control surfaces.

When the rotor reaches synchronous speed, it is no longer subjected to successive magnetising cycles and in this condition it behaves as a permanent magnet.

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Notes:

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3.12: Direct Current Generators/Motors 3.12.1 : Theory A loop of wire rotated in a magnetic field has a continuously changing flux through it and so long as the rotation continues, an induced voltage will be maintained in the wire. The magnitude of this induced voltage depends on the rate at which the flux changes. This principle forms the basis of any rotating electrical generator, (AC or DC). The method by which the generator electricity is actually connected into the external circuit will determine the ultimate generator function. This method will be Commutator (DC generator) and Slip Rings (AC generator), with ‘collection’ provided by carbon brushes. A generator converts mechanical energy into electrical energy. It does this by producing relative motion between loops of wire and magnetic flux so that an induced voltage is set up in the loops of wire.

A Simple dc Generator The simplest form of dc generator is shown and consists of a single loop of wire able to rotate freely between the poles of a permanent magnet. Connection is made from the loop to the external circuit (or ’load’) by carbon brushes pressing on a commutator, which is connected to the ends of the loop and rotates with it. It should be noted that any generator in the first instance produces ac; however, it is the method in which the output is picked off that determines whether it is ac or dc, (i.e. commutator or slip rings).

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Production of Direct Current Direct current can be obtained in the external circuit by substituting a form of automatic reversing switch, known as a ‘COMMUTATOR’, for the slip rings. The commutator automatically reverses the connection between the loop and the external circuit as the voltage in the loop reverses, thus maintaining the direction of current in the load, as shown.

Production of DC by Commutator Action Each end of the loop is connected to a segment of the commutator and the load is connected to the loop by brushes on opposite sides of the commutator.  As the loop rotates, an alternating voltage is induced in it, but, because of the action of the commutator, a ‘rippled dc’ is produced as opposed to a genuine ac waveform.

36



Because the commutator rotates with the loop, the brushes bear on opposite segments of it during each half cycle.



This results in the left hand brush always being in contact with the segment that is positive, with the change-over taking place at the instant when the voltage induced in the loop is zero.



The current in the external circuit is therefore always in the same direction and is called a UNI-DIRECTIONAL current. It is also the first step towards obtaining a true dc output such as we get from a battery. 3.12.1

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The voltage at the brushes, and therefore the current in the external circuit of a simple example single loop dc generator, falls to zero twice during each complete revolution. As has already been mentioned, this variation of dc is called ‘ripple’ and can be reduced by the addition of more loops as shown. Remember, an operational generator will not return to zero after switch-on until it is switched ‘off’.

Multi-Loop dc Generator As the number of loops is increased, the variation between maximum and minimum values of voltage is reduced and the output voltage of the generator approaches a steady dc value, as can be seen.

Output Waveform of a Multi -loop dc Generator

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It should also be noted that the number of segments on the commutator is increased in direct proportion to the number of loops; There are; 

Two segments for one loop.



Four segments for two loops.



Eight segments for four loops.



The loops are not just loops of wire but are made up like coils and so the construction of them can be a big determining factor in the output obtained.

The voltage induced in a single-turn loop is quite small, and although an increase in the number of loops does not increase the maximum value of generated voltage, an increase in the number of turns in each loop will. Within narrow limits, the output voltage of a dc generator is determined by the product of the number of turns per loop, the total flux pair of poles in the machine and the speed of rotation of the armature. Whether it is an ac or dc generator, they are identical as far as the method of generating voltage in the rotating loop is concerned. However, if the current is taken from loop by slip rings, it is an alternating current and if it is collected bythe a commutator, it is direct current. The variation in the output of a dc generator is reduced to a very small amount by having a large number of loops and a commutator with a correspondingly large number of segments. The construction is such that each loop is connected between adjacent segments, the end of one loop being connected to the same segment as the beginning of the next loop, as shown.

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Connection of Multiple Loops with Commutator Segments and Resultant Output Loop A is connected between segments 1 and 2, loop B between segments 2 and 3 and so on. With this arrangement, the emf induced in each loop will reach its maximum value when the emf in the preceding loop is already decreasing, and that in the succeeding loop is still increasing. Thus, the emf in  loop ‘E’ is at maximum. 

loop ‘F’ is decreasing.



loop ‘D’ increasing.



The voltage at the brushes equals the sum of the emf induced in the loops connected in series between the brushes.



Loops ‘A, ‘B’ and ‘C’ are in series between the brushes on the right.



Loops ‘D’, ‘E’ and ‘F’ with the brushes on the left.



The two branches are parallel with each other.



The graph shows the resultant voltage between the brushes. Only three loops need to be considered as the arrangement is symmetrical and both branches (A, B and C and D, E and F) give the same voltage at the instant shown. As the number of loops is increased, the ripple in the brush voltage becomes smaller and the magnitude of the dc output voltage increases

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Notes:

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3.12.2: Construction and Purpose of Components in DC Generator In a practical dc generator we obtain high voltage outputs by: 

Using a large number of coils of many turns instead of single loops.



Rotating the coils at high speed.



Using electromagnets to provide a strong magnetic field and mounting the coils in which the voltage is to be induced on a soft iron core: the air gap between this core and the electromagnet pole pieces is very small.

The electromagnets used to provide the magnetic field require a dc voltage source to pass current through the winding. In small machines such as those used in aircraft, the design of the machine is simplified by using the output voltage of the generator itself to provide this current.

DC Generator

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Construction A dc generator consists of two main assemblies: 

THE STATOR OR FIXED PORTION . This carries the FIELD MAGNET SYSTEM, the BRUSH GEAR and the BEARINGS. The Brush Gear Assembly and end frame may be considered as a separate major sub assembly.



THE ROTOR OR ARMATURE ASSEMBLY. This carries the COILS, COMMUTATOR and often COOLING FAN BLADES.

Since the generator converts mechanical energy into electrical energy, mechanical energy must be supplied to the generator to turn it. The ‘prime mover’ used to drive aircraft generators is usually the engine. The frame or yoke is the main chassis of the generator and it also serves to complete the magnetic circuit between the pole pieces. The pole pieces are laminated to reduce eddy current losses, and the field coils or windings are mounted on the pole pieces. The end housings contain the bearings for the armature which rotates at high speed, and one of these housings also holds the brush gear. The armature (the rotating part of the machine) is made up of shaft, armature core, armature windings coils, and commutator. The armature core is in laminated to reduce eddyorcurrent losses, and the armature windings rest slots cut in the core, but insulated from it. The commutator is made of copper segments insulated from each other, and from the shaft. The ends of the armature windings are hard soldered to their appropriate commutator segments. The brushes ride on the commutator and carry the generated voltage to the load. They are usually made of carbon and are held in brush holders in such a way that they can slide up and down against a spring so as to follow the small irregularities in the surface of the commutator.

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The Yoke Field Magnet System

Typical Generators Except for very small machines in which permanent magnets are used, the magnetic field is produced by electromagnets in such a way that the armature conductors pass under North and South poles alternately. The poles may be salient, in which case the armature emf wave form has a flat top, or may be flush pole, low reluctance which gives an almost sinusoidal wave form. Salient poles are the most common in aircraft DC generators. The salient pole piece may be laminated to prevent eddy current heating, or it may be solid, with a laminated pole ‘shoe’ fitted to the end. It will be noted from the diagram that the yoke is an essential part of the magnetic circuit, and must therefore combine permeability with structural strength. It is normally of cast or rolled steel. Field Assembly The heavy iron or steel housing that supports the field poles is called the field frame. It not only supports the field poles but also forms part of the magnetic circuit of the field. Small generators usually have two to four poles while larger generators can have as many as eight main poles and eight interpoles. (Interpoles will be dealt with later). The pole pieces are rectangular and in most instances are laminated to prevent Eddy Current losses.

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The Brush Gear Brushes are made of specially treated carbon which is self lubricating; therefore causing little commutator wear. They are carried in small open ended boxes called BRUSH HOLDERS. Brush pressure is maintained on the commutator by SPRINGS. Connection to the external circuit is made by copper braid. Electro-graphitic brushes of normal design, although generally reliable in performance when used in ground equipment and low-altitude aircraft generators, tend to wear very rapidly at high altitudes. This wear can be of the order of 12mm per hour and is because of the following factors: 

At ground level and low altitudes the moisture content of the atmosphere gives a substantial degree of lubrication between the contact surfaces of the brushes and the commutator or slip-rings on which the brushes are bearing.



At high altitudes the moisture content of atmospheric air is negligible, and with little or no lubrication at the ‘rubbing contacts’ there is considerable friction. Rapid wear of the soft electro-graphitic brushes is, in consequence, inevitable.

Normally the contact resistance between brush-faces and commutator (or slip-ring)onsurfaces is fairly high because of the existence of a resistive formed the metallic surfaces by the electrolytic decomposition of thefilm moisture content of the atmosphere. At high altitudes this film is removed by frictional wear, and cannot be made good because of the dryness of the atmosphere. Hence the contact-resistance between brush surfaces and metallic surfaces becomes small. This reduction in contact resistance, in the case of a DC generator, gives rise to heavy reactive sparking which, in turn, accelerates brush erosion. Lack of lubrication of the brush-to-commutator contact surfaces at high altitudes and the reduction of brush-contact resistance experienced at increasing altitudes, are largely eliminated by using brushes which have been especially developed for high-altitude operation. Two distinct categories of high-altitude brushes are in general use:

44



Brushes which form a constant resistance semi-lubricating film on the commutator or slip-rings.



Brushes which are self-lubricating and do not form a film.

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Film Forming Brushes The make-up of these brushes includes such chemicals as barium fluoride which builds up, progressively, a constant-resistance semi-lubricating film on the surfaces of the commutator or slip-rings. 

Brushes of this category do not wear abnormally at altitudes of up to some 35,000 ft providing that generators to which such brushes are fitted are previously run at low altitude for some hours to allow the formation of the protective film.



This film, once it has been formed, is very dark in colour and to the inexperienced eye it may well give the impression of a dirty commutator or slip-rings.

Non Film Forming Brushes Brushes in this category contain a lubricating ingredient such as molybdenum disulphide: this lubricant is often packed in cores running longitudinally through the brush. 

Since the brush is itself self-lubricating there is no question of preliminary formation of film, hence there is no necessity for running generators fitted with these brushes at low altitude before entering into high-altitude



Against this advantage of immediate availability for high-altitude operation must be set the disadvantage of appreciably shorter life, due to somewhat more rapid wear when compared with film-forming brushes.

operation.

The following precautions MUST be observed when using high-altitude brushes: 

Film-forming brushes must not be used at high altitudes until the generator has been in operation for a specified period after fitting the brushes to a machine with a ‘Clean’ commutator or slip-rings – this period is essential to allow the film produced by brush action on the commutator or slip-ring surface to attain a serviceable thickness.



Under no circumstances should non-film forming brushes be run on films created by film-forming brushes, nor should film forming and non-filmforming brushes be used simultaneously in the same machine. When changing from film-forming to non-film forming brushes the existing film must be completely removed by cleaning the commutator or slip-ring with a rag moistened in lead free gasoline, or other approved cleaning agent.

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The Armature The rotor or Armature Assembly consists of the shaft, the iron core, the output windings and the commutator, as shown. 

The Iron Core provides a low reluctance path between the field pole pieces giving increased flux density, ensuring that the largest emf possible is induced into the output windings.



The core is constructed as a laminated soft iron drum with longitudinal slots into which the output windings are fitted.



The core is laminated to reduce eddy currents and thus heat.



The Output Windings are placed in longitudinal slots in the iron core to reduce the magnetic circuit air gap. The armature and coil windings are vacuum impregnated with silicone varnish to maintain insulation resistance under all conditions with the coils also insulated with p.t.f.e. [Poly-tetrafluoro-ethane].

The windings are wedged into the slots with insulating material to prevent them from being thrown out by centrifugal force. All coil connections are silver soldered to withstand local hot spot temperatures.

A Typical Armature Assembly

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Wave Winding Another feature of multi-pole machines is the manner in which the Coils of the armature winding are connected together to provide the required output conditions. One method, called wave winding, provides increased output voltage by arranging for the voltages induced by each pair of poles to be added in series. 

Therefore, the output voltage is twice (four pole) and three times (six pole) that of the equivalent two pole machine. With wave winding the output voltage may be obtained across one pair of brushes.

Lap Winding The other armature winding method is called lap winding and this method is most useful when high output current is required. 

In lap winding, groups of series connected coils are connected in parallel by the provision of additional brushes at points around the commutator which are equal in potential.



In a four pole machine this results in the provision of four parallel current paths from the two positive brushes to the two negative brushes.



In a six pole machine six parallel current paths from the three positive brushes to thethere threeare negative brushes.

The provision of additional parallel paths makes the lap wound generator suitable for high output current. Wave winding is used for DC generators of high output voltage. Lap winding is used for DC generators of high output current. The Commutator This is a cylinder mounted at one end of the armature and consists of a large number of copper segments. The segments are wedge-shaped and a large number are assembled side by side to form a ring, each being insulated from the other by a mica insulating strip. Each segment forms the junction between two armature coils, the wires being soldered into risers at the ends of the segments. Generator Cooling The maximum output of any generator, assuming no limit to input mechanical power, is largely determined by the facility with which heat (arising from HYSTERESIS, thermal effect of currents through armature and field windings, etc) can be dissipated. With large bulky generators of relatively low output the

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natural processes of heat radiation from the extensive surfaces of the machine case may well provide sufficient cooling effect, but such ‘natural’ cooling is hopelessly inadequate for the lightweight high output generators used for aircraft electrical supply, and must, therefore, be supplemented by forced cooling. The majority of aircraft generators in current use are blast-cooled by slipstream air. Generators fitted to the modern aircraft are oil cooled. Adequate cooling may, therefore, be self induced, separately induced, a ram air function, or oil heat exchanger system.

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Generator Drives The fundamental requirements of the element through which torque is transmitted to the rotor shaft of the generator may be summarized as follows: 

Effective transmission of torque up to a specified maximum.



Effective interruption of torque transmission if the torque-demand of the generator exceeds the permitted maximum, this condition can arise as a result of seizures of the generator rotor, etc.



Quick and simple removal and replacement of the torque-transmission element.

The requirements quoted above are satisfied almost entirely by ‘weak-link’ devices known as quill drives. The device is basically a ‘necked’ metal shaft with serrations or splines (These may be either male or female) at one or both ends. The serrations or splines mate with corresponding formations on the driven rotor shaft to transmit the torque delivered by the drive unit, and the ‘necked’ portion is designed to shear in the event of rotor seizure, etc, thus interrupting the drive and protecting the components against further possible danger.

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Notes:

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3.12.3

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Operations of and Factors Affecting Output and Direction of Current Flow

The commutator and brush gear of a dc machine have two distinct functions: 

Collection - the transference of current between the moving armature and the fixed external circuit.



Commutation - the periodic reversal of current during transfer between the armature and the external circuit to produce dc.



These two operations are independent, but faulty collection or incorrect commutation produce similar results, i.e. the formation of a destructive spark or arc between the trailing edges of the brushes and the commutator surface.

Faulty Collection This is normally the result of poor brush fittings and maintenance. Sparking occurs between the brush trailing edge and the commutator surface and is very destructive. Electromagnetic Problems In addition to the problems associated with actual collection, two problems which are associated with the electromagnetic functions in the generator also exist. Though having similar effects, they are created by different things, may be compensated for by different design features and should therefore be understood as separate entities. These are:  Armature Reaction. 

Reactive Sparking.

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Armature Reaction/Reactive Sparking Since an armature is wound with coils of wire, a magnetic field is set up in the armature whenever a current flows in the coils. This is called the armature flux and its field is right angles to the generator field, (also known as the field flux). This is called cross magnetisation of the armature. The effect of the armature flux is to distort the field flux and shift the magnetic neutral axis as illustrated. This effect is known as armature reaction and is proportional to the current flowing in the armature coils.

Resultant Magnetic Fields due to Armature Reaction

52



The magnetic neutral axis (MNA) is the resultant of the armature flux and the field flux interacting with one another.



The Geometric Neutral Axis (GNA) is the axis running through opposite poles.

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The brushes of a generator must be set in the MNA which means that they must contact segments of the commutator that are connected to armature coils having no induced emf. If the brushes were contacting commutator segments outside the MNA, they would short-circuit ‘live’ coils and cause arcing and loss of power (reactive sparking). In an ideal machine, the MNA will be equal to the GNA, which means there would be no distortion of the field flux and so no shifting of the MNA away from the brushes. This would result in no armature reaction or reactive sparking. However, the ideal machine has never been invented and armature reaction is something that has to be accepted and compensated for, and there are three principle methods with which it is overcome. 

The first method is to shift the position of the brushes so that they are in the MNA when the generator is producing its normal load current.



The second method is by using special field poles, called INTERPOLES.



The third is by the use of COMPENSATING WINDINGS, both of which counteract the effect of armature reaction.

The brush-setting method is only satisfactory in installations in which the generator operates under a fairly constant load. If the load varies to abemarked degree,position the MNA shift proportionally, the brushes will not in the correct at will all times. This method and is most commonly used in smaller generators (those producing 1kW or less) because it is less expensive. Larger generators require the use of interpoles.

Generator Circuit with Interpoles The use of interpoles is a very efficient way of maintaining a constant MNA in a generator. The windings of the interpoles are in series with the load, so the

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effects of the interpoles are proportional to the load. The polarities of the interpoles are such that their effect is opposite to that of the armature field; i.e. the interpoles are of the same polarity as the next field pole in the direction of rotation. With this polarity, the interpoles are said to pull the generator field back into the correct position. A typical interpoles system is shown.

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In many generators, compensating windings are used to overcome the problem of armature reaction. These are windings placed in slots in the pole faces.

Use of Compensating Windings to overcome Armature Reaction The current flowing in them travels in the opposite direction to that in the armature conductors, and by connecting them in series with the armature, the current in the windings is the same as that in the armature. With this method, the armature flux is cancelled out by the compensating flux under all conditions of load resulting in the MNA and GNA being equal and commutation remains static.

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In some machines, interpoles are used to minimize reactive sparking and armature reaction. However, for more efficient reduction of both, interpoles and compensation windings would be used as shown. The compensating windings are in series with the interpoles and increase their effectiveness. The spark-less commutation obtained by the use of interpoles and compensating winding. 

Increases the life of the brushes and commutator.



Reduces radio interference.



Greatly improves the efficiency of the generator.

Generator with Interpoles and Compensating Winding

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Typical Generator Fault Chart Defect

Possible Cause

Appropriate Action

1. Failure to excite

Loss of residual magnetism

Remagnetise. Disconnect shunt field winding and connect battery across the winding; positive of battery to positive end of winding.

2. Voltage fails to

a. Dirty commutator

Clean as described

b. Glazed contact surface on brushes owing to prolonged ‘off load’ running.

Clean contact surface of brushes with Grade 00 glass paper.

c. Brushes not in contact with commutator

If result of sticking brushes, treat as described.

d. Incorrect brush position.

Check position and correct as necessary.

e. Disconnection in field circuit.

Check all connections, test field winding for continuity

f. Reversed field connections

Reconnect correctly

g. Incorrect direction of rotation

Reverse drive

h. Machine run up on load (shunt machines only)

Disconnect load, run up ‘off load’

Residual magnetism reversed

Remagnetise. See 1 above.

a. Excessive load

Reduce load

b. Weak field

Reduce resistance of shunt field rheostat

c. Insufficient speed

Reduce speed of prime mover

a. Excessive field strength

Increase resistance of shunt field rheostat

b. Excessive speed

Reduce speed of prime mover

a. Dirty commutator

Clean commutator

b. Excessive load

Reduce load

c. Incorrect brush position

Check position. See 2d above

build up

3. Reversed Polarity 4. Insufficient Voltage

5. Excessive Voltage

6. Uniform sparking at all brushes

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Notes:

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3.12.4: DC Motor Principle of Operation Introduction An electric motor is a machine for converting electrical energy into mechanical energy. Its function is the reverse of a generator. There is little difference between the construction of a dc motor and a dc generator. Both have essentially the same parts and they look alike. In fact, in many cases, a dc machine can be used either as a motor or a generator. Current Carrying Conductor in a Magnetic Field A current flowing through a wire placed in a magnetic field causes the wire to move; a motor works on this principle. It is the reaction of two magnetic fields that produces the motion that produces the torque that we see as the output of the motor. The force with which the conductor moves is clearly dependent upon the strengths of the two interacting magnetic field. In turn this force relates to the speed at which a motor containing the current carrying conductor will turn.

Effects of a Current Carrying Conductor in a Magnetic Field The figures show the magnetic field between the poles of a magnet and the magnetic field round a wire carrying a current. If the wire is placed in the magnetic field the overlapping field pattern would seem to be as shown in (c). Of course, as we have seen earlier, lines of flux cannot cross and this pattern cannot exist. The resultant field is as shown in (d). The lines of flux reinforce each other in the space above the conductor and oppose each other below it. Lines of flux act as if they are pushing away from each other and also tend to straighten out. In this way they apply a force to the conductor tending to move it downwards.

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It may therefore be appropriate to remember that the force is directly proportional to the Flux Density (B) of the major field, the current (  ) in the Conductor (producing the minor field), and the length of the Conductor (L). This is expressed as: Force =Flux Density B   Current F  BIL

 

Length of conductor L 

The direction in which the conductor moves depends on the direction of the current in the wire and also on the direction of the magnetic field. The direction of motion is given by Fleming’s LEFT HAND RULE for motors: ‘The first finger, the second finger and the thumb of the left hand are held at right angles to each other’ . With the first finger pointing in the direction of the field (N to S) and the second finger in the direction of conventional current, the thumb shows the direction of motion of the wire. To change the direction of rotation of a motor having an electro-magnetic field we need to reverse the direction of current in the armature OR the direction of the current in the field. Changing the supply connections to the motor will not have any effect; the current being reversed in direction in both the armature and the field, the motor continues to run in the same direction. Permanent magnetic motors are however, reversible by simply changing over the supply connections.

Fleming’s Left Hand Rule

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The simplest form of motor has a single loop of wire able to rotate freely between the poles of a permanent magnet. Connection is made from the dc supply voltage to the loop by BRUSHES bearing on a COMMUTATOR, the two segments of which are connected to the loop, as shown.

Fig 1 Simple DC Motor The forces on the of the loop combine to apply a force, known as aacting TORQUE , totwo turnsides the loop in an anticlockwise direction.

Action of DC Motor

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By applying Fleming’s Left Hand Rule it can be clearly seen that when: 

The loop is in position (A), side ‘P’ of the loop tends to move downwards and side ‘Q’ upwards.



As the loop passes through the vertical position (B), the direction of the current flow must be reversed to keep the loop rotating in the same direction, and it is the action of the commutator that does this.



Because the commutator is two halves of a ring separated by insulation, the result of the loop rotating is such that as one half of the commutator leaves a brush, the other half comes into contact with it.



So now, at (C), when we apply Fleming’s Rule, side ‘Q’ will move in a downward direction and side ‘P’ upward, keeping the rotation of the loop in an anticlockwise direction.



At position (D), the loop passes through the vertical and the current reverses direction again until we get to (E) where the loop is back to where it was at the start (A) and the process goes on.

A single loop dc motor would not be able to turn heavy loads, so to obtain a large smooth mechanical output; some improvements have to be made. A laminated iron core carrying a number of armature coils is used together with a corresponding number of commutator segments. The magnetic field is produced by an electromagnet and its field coils, with the spacing between the armature and the pole pieces kept as small as possible.

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3.12.5: Operation and Factors Affecting Motor Output Power Torque The movement of a conductor in a magnetic field induces in it an emf, which we know from Lenz’s Law, will OPPOSE the motion producing it. That is to say, the induced voltage will oppose the supply voltage. This is called BACK EMF. Back emf will never be as great as the supply input and the difference between them is always such that current can flow in the conductor and produce motion. The value of this current is dependent upon the value of the voltage across the conductor. This voltage, often referred to as the EFFECTIVE VOLTAGE is equal to the difference between the applied voltage and the back emf. Therefore:

EFFECTIVE VOLTAGE = APPLIED EMF – BACK EMF

Example

F

Rf

a

 120R  0.25 a

Back emf Consider the diagram shown. A 24V supply is fed to a shunt motor with an 



armature resistance 0.25 a field resistance of 12 . The motor takes a current of 6A underof‘no load’and . What is the back emf? As the back emf is produced by the armature, the first thing to be calculated is the armature current.

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Armature Current (a) = Total Current – Field Current (f) 24 f   2A 12  a  6  2  4A Back emf (Eb) = Supply Voltage – Armature Voltage E b  V  ( a  Ra )  24  ( 4  0.25) 

Eb



23V

When the motor is ‘on load’, the current it draws is 52A. What effect does this have on the back emf (Eb)? The field current ( f) will remain the same (Ohm’s Law applied to a parallel circuit), so: f 

2A

 a  52  





Eb



2  50 A

24  (50  0.25)  11.5V

It is noticeable how the back emf falls as the load is increased on the motor. When the motor is ‘loaded’ it will tend to slow down, and as generated emf is directly proportional to the rate of change of flux linkage (Faraday’s Law), the value of the back emf will be reduced.



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This will increase the Effective Voltage, and therefore the Armature Current and the motor speed will be restored. The back emf therefore determines the armature current and makes the dc motor a SELF REGULATING machine.

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Electrical Fundamentals

Motor Speed Control Back emf determines the current in the armature, making the motor a selfregulating machine in which speed and armature current are automatically adjusted to the load. 

At small values of load, the shaft torque exceeds the load torque causing the armature to accelerate and produce a larger back emf.



The increased back emf reduces the armature current, and therefore the shaft torque, until a state of balance is achieved and the speed is stabilised.



When the load torque is increased (with increasing load), it exceeds the shaft torque causing a fall in armature speed.



This results in a reduced back emf and an increased armature current.



This increase in armature current increases the shaft torque, restoring torque balance, and stabilises the speed again.



This variation of speed with armature current is known as the SPEED CHARACTERISTIC of the motor.

Although many motors run at a constant speed, it is sometimes necessary to be able to vary the speed to suit the application, and this can be carried out by varying the amount of current through the field windings. 

When the amount of current flowing through the field is increased, the field strength increases, but the motor will slow down because a larger amount of back emf has been generated in the armature.



When the field current is decreased, the field strength decreases and the motor 'speed up' because the back emf in the armature has been reduced. A motor in which the speed can be controlled is called a VARIABLESPEED MOTOR and may be either shunt or series wound.

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Shunt Motor with Variable Speed Control In the shunt motor shown, speed is controlled by a rheostat connected in series with the field windings. The speed of the motor will depend upon the amount of current flowing through the rheostat to the field windings.

66



To increase the speed the resistance in the rheostat is increased, which decreased the field current, resulting in a decrease in strength of the magnetic field and also the back emf in the armature.



This will momentarily increase the armature current and the torque causing the motor to automatically speed up until the back emf increases and the armature current decreases to its former value.



When this has happened, the motor will operate at a higher fixed speed than before. The opposite action takes place for a decrease in motor speed.

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Electrical Fundamentals

R Motors used for the operation of landing gear, flaps and other types of apparatus must be designed to work in either direction and are therefore called REVERSIBLE MOTORS.

Reversible DC Motor Schematic The voltage polarity applied to the field and armature windings of any motor will determine that motor’s direction of rotation (clockwise or anticlockwise). To reverse the rotation of a dc motor containing an electromagnetic field, the polarity of the voltage applied to the field or the armature must be reversed. This will reverse the magnetic field of one of the two coils, hence reversing the motor’s direction. Reversing a motor by this method is shown, where it can be seen that by moving the double-throw switch from the UP to the DOWN position the current through the field coil is reversed, thus reversing polarity, but stays the same through the armature. It would require quite a complex external circuit to achieve this, but a simpler method is normally employed that provides a double field winding known as a SPLIT FIELD.

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Split Field Reversible DC Motor The drawing shows a split field motor. It is operated by a single-pole doublethrow (SPDT) switch which, when connected to either the CW (clockwise) or CCW (counter clockwise which is another way of saying anti-clockwise), positions will cause current to flow in the respective field winding. This makes it possible to change the direction of the motor at will by placing the switch in the desired position. The motor is reversed by changing the field polarity in relation to the armature polarity when the different field windings are energised. Reversible dc motors are commonly controlled by single-pole double-throw switches, as with the split field type, but can also be controlled indirectly by the use of relays. The use of relays is dictated by the amount of current the motor draws while in operation. Any motor requiring more than 20-30 amps will operate more satisfactorily with a relay controlled circuit. The separate field coils of a reversible motor are usually wound either in opposite directions on the same poles or on alternate poles. Since the field coils are in series with the armature, they must be wound with wire of a size large enough to carry the entire motor current.

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The brushes in a reversible motor are usually held in box-type holders in line with the centre of the motor shaft. With this arrangement the brushes are perpendicular to a plane tangential to the commutator at the point of brush contact and the brushes will wear evenly regardless of the direction of motor rotation. On small motors the field and brush housing is sometimes made in one piece. Brakes and Clutches Many motor-driven devices used in aircraft must be designed so that the operated mechanism will stop at a precise point. For example, when landing gear is being retracted or extended, it must stop instantly when the operation is complete. If the driving motor is connected directly to the operating mechanism, a great amount of strain will be imposed upon the motor when it is forced to stop because of the momentum of the armature and other moving parts. In installations requiring an instantaneous stop, a clutch and brake mechanism is employed to prevent damage when the machine is stopped. Clutch and Brake Assembly Clutches of several types have been designed for the purpose of disengaging the motor from the load when the power is cut off. All such clutches are engaged by magnetic attraction when the power is switched on and disengaged by spring action. Two clutch faces are located within the clutch coil. One of the faces is mounted solidly on the armature shaft and the other is connected through a diaphragm spring to the drive mechanism. When the clutch coil is energised, the two faces are magnetised with opposite polarities, hence they are drawn together firmly. The friction thus produced causes the driven mechanism to turn with the motor. When the power is cut off, the diaphragm spring separates the faces, thus disengaging the motor. Limit Switches and Protective Devices Because of the limited distance of travel permitted in the driven mechanism, reversible actuating motors are usually limited in their amount of rotation in each direction. It is essential, therefore, that the motor circuits be provided with switches which will cut off the power when the driven mechanism has reached the limit of its travel. 

Switches of this type are called limit switches and are actuated by cams or levers linked or geared to the driven mechanism.

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

The adjustment of these switches is critical because severe damage may result if the motor continues to run after the limit of operation is reached.



Stripped gears and broken shafts are often the result of improperly adjusted limit switches.



If the driven mechanism is strong enough to withstand the torque imposed by the motor, the fuse or circuit breaker in the motor circuit will usually cut off the current to the motor.

Adjustment of the limit switches is accomplished by running the motor to the limit of travel and then adjusting the switch-actuating mechanism so that it has just opened the switch. The switches should be adjusted to open slightly before the extreme limit is reached.

Reversible Motor Circuit with Thermal Protection Some actuating motors are provided with a thermal circuit breaker, or thermal protector, to protect the motor from overload and excessive heat. This device is mounted on the motor frame, and when heat reaches a predetermined limit, the circuit breaker will open and cut off the current to the motor. After the motor has cooled sufficiently, the circuit breaker will automatically close, thus permitting normal operation. The diagram is a schematic diagram of a reversible motor circuit with a thermal protective device and a coil for operating the clutch and brake.

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

A circuit of the type shown would be used for operating cowl flaps, oil cooler shutter, air valves, and a variety of other devices.



Both the limit switches shown are normally closed. Since they open only when the motor has reached the limit of travel in one direction or the other, it is readily apparent that there will never be a time when both switches are open. Notice that the thermal circuit breaker and the clutch coil are both in the ground (negative) side of the circuit and are therefore in operation for either direction of travel.

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Notes:

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3.12.6: Series Wound, Shunt Wound and Compound Motors There are three basic types of dc motors, namely series, shunt and compound and they differ largely in the way in which their field and armature coils are connected. Series DC Motor

Series Wound DC Motor In the series motor shown, the field windings, which consist of relatively few turns of heavy wire, are connected in series with the armature winding. The same current flowing through the field winding also flows through the armature winding. Therefore, any increase in current strengthens the magnetism of both the field and the armature. 

Because of the low resistance in the windings, the series motor is able to draw a large current in starting.



This large starting current, when passing through the field and armature windings, produces a high starting torque, which is this type of motor’s principal advantage. The speed of a series motor is dependent upon the load. Any change in load will result in a substantial change in speed, so it will run at high speeds with light loads and low speeds with heavy loads. If the load is removed entirely, the motor may operate at such a high speed that the armature will break apart, so it must never be run under ‘no load’ conditions. So, if a high starting torque is required under heavy load conditions, a series motor would be the best application.

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Characteristic Curve of a Series DC Motor Looking at the characteristic curve of a series dc motor it can be seen that when very little load is applied, the speed is very high and as the load is increased the speed decreases. Shunt dc Motor

Shunt Wound DC Motor In the shunt motor shown, the field winding is connected in parallel with the armature winding. The resistance in the field winding is high and, as it is connected directly across the power supply, the current through the field is constant. Unlike the series motor, the field current does not vary with motor speed, so the torque of the shunt motor will only vary with the current through the armature. Also, the amount of torque developed at starting is less than that developed by a series motor of equal size.

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Characteristic Curve of a Shunt DC Motor The characteristic curve of the shunt dc motor can be seen showing that the speed varies very little with changes in load. If the load is removed, it will assume a speed slightly higher than that when loaded. This type of motor is particularly suitable for use when constant speed is desired and when high starting torque is not needed. Compound dc Motor

Compound Wound DC Motor

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The compound motor is a combination of the series and shunt motors. There are both a shunt and a series winding in the field. A schematic diagram of a compound motor is shown where the shunt winding is composed of many turns of fine wire and is connected in parallel with the armature winding and the series winding consists of a few turns of heavy wire and is connected in series with it. The starting torque is higher than that of a shunt motor but lower than in the series motor and any variation of speed with load is less than in a series motor but more than in a shunt motor.

Characteristic Curve of a Compound DC Motor The characteristic curve of the compound dc motor clearly shows the effect of load against speed, being somewhere between that of series and shunt wound motors. Continuous and Intermittent Duty Motors Many electric motors used in aircraft are not required to operate continuously. Because the heat developed in a short time is not sufficient to cause any damage, a motor in this type of service is designed to deliver more power for its weight than a motor used for continuous service. If such a motor were used continuously, it would overheat and burn the insulation and thus become useless. Motors designed for short periods of operation are called intermittent duty motors, and those which operate continuously are called continuous-duty motors. The type of duty for which a motor is designed is sometimes stated on the nameplate and if not on the nameplate, can be found in the manufacturer’s specifications.

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Continuous Duty Motors These are usually shunt wound machines used as cooling fans, blowers, fuel pumps. Intermittent Duty Motors These are usually series wound machines used to actuate loads such as landing gear, flaps, cowls, trim tabs, valves. The machine, which is reversible, is referred to as an ACTUATOR.

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Notes:

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3.12.7: Starter/Generator Construction Several types of aircraft are fitted with starter motors which will also function as generators. These units will provide the rotary power to turn and start the engine. Instead of being disconnected when the engine is running the unit will be switched electrically and will then provide electrical power. The units are light and less bulky than a separate starter motor and separate generator. They will also require only one drive from the engine gearbox and will always remain directly connected to the engine. The basic construction is similar to a compound wound motor but the fields can be switched separately. As a starter it will operate as a series wound motor. When operating as a generator the series winding will be disconnected and only the shunt winding will be energised and controlled by the voltage regulator. There are many different means of connections, circuits and operation and one simple circuit is illustrated. Operation 

When START is selected the starter relay will be energized, supplying power to the starter to operate it as a series wound motor. An electrical supply will also be connected to the ignition through the ignition cut-off



When the engine is running and generator switched ‘on’ the starter relay will be de-energised. This will also de-energise the changeover relay disconnecting the series field and connecting the armature to the voltage regulator.



The armature will also be connected to the busbar through the Reverse Current Relay. The energised field relay will connect the shunt field to the voltage regulator.



This will allow the unit to operate as a voltage regulated shunt wound generator. The Reverse Current Relay will operate when the generator output drops below a preset value. This will prevent discharge of the battery through the generator which will also try to motor the generator.

switch.

In some circuits the start selection will be spring loaded to generator ‘on’. Other circuits will lock in the start selection until a speed switch will deenergise it when the engine reaches self-sustaining speed.

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Starter/Generator Circuit

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