Siskind DC Machines Q&A Answers

Neil John Perez 16, 2014 EE411/TTH/9:00-10:30

Sept.

1. What is meant by load on a generator? A load on a motor? In a generator, load means the electrical machine that the generator supplied electrical energy. In a motor, load is the object that the motor is carrying to accomplish its function. 2. List several types of loads applied to motors. Examples of motor loads are: Fan Blades, Pumps, grinder, boring mills, crushers, excavators, elevators, turntables, churns drills, food mixers and hoists. 3. When the load changes, what tends to change in a generator? In a motor? In the generator, voltage changes when the load is changed while in motors, the speed of revolution will change. 4. What methods are usually employed to adjust the voltage of a generator? The speed of a motor? In the generator, voltage is controlled by: changing the speed or/and changing the magnetic field strength. In the motor, speed is controlled by: changing the magnetic field strength or/and changing the voltage supplied across the armature. 5. Generally speaking, what factor is kept constant when a generator is in operation? When a motor is in operation? In a generator, it Is speed that is usually kept constant while in motors, it is speed that is kept constant. 6. Are generators started under no load? Explain. Generators are started under no load since the procedure is to bring them up to speed, adjust the voltage then close the main switch that permits the machine to deliver current. 7. Are motors started under no load? Explain. Motors may or may not be started at no load because the motor armature will still rotate even if there is no load or not. 8. Is it possible to operate a dc-generator as a motor and vice versa? Explain. Yes, it is possible to operate one to the other since the dc-motor and dcgenerator has more or less identical construction. When a motor’s back emf is greater than the supplied voltage, it means that the motor is already acting as a generator. 9. Name the three types of dc-motor. The three types of dc-motor are: series wound motor, shunt wound motor and the compound wound motor. 10.Indicate in a general way, how the speeds of the three types of dc motor is affected by an increase in load. When the load is increased, the decrease in speed is different among the three types. The dc shunt motor slows down the least while the dc series

generator slows down drastically while the dc compound generator slows moderately when load is increased. 11.What is meant by a constant speed motor? What types of motor exhibits this characteristic? Constant speed motors are motors where the speed only reduces less than the arbitrary value when a load is applied. DC shunt motors are under this category. 12.What is meant by variable speed motors? What type of motor exhibits this characteristic? Variable speed motors are motors where the speed drastically changes when there is a change of load. Series wound motors are under this category. 13.What is meant by adjustable speed motors? Adjustable speed motors are motors where the operator can control the motor’s speed. 14.Under what condition a motor would be called a constant-speed-adjustablespeed motor? A variable-speed-adjustable-speed motor? A motor will be called constant-speed-adjustable-speed motor when the shunt motor has a field rheostat control while the variable-speed-adjustable-speed motor is possible when the series motor contains a line rheostat. 15. Why is the generated emf in the motor called a counter emf? It is called counter emf because the generated voltage of the motor opposes the supplied flow of current and usually has a lesser voltage than the voltage supplied. 16.Can the counter emf be equal to the impressed voltage in a motor? Give reasons for your answer. Counter emf will never be equal to the impressed voltage because of the voltage drop due to the armature winding and the opposing direction it has from the supplied current. 17.How does the counter emf control the value of the armature current? The counter emf controls the value of the armature current in a way that it subtracts the emf being supplied across the armature due to its opposite direction. If we do KVL in the Armature, we can see the the armature current is really dependent on the counter emf produced, Ia=(VS-Vb)/Ra 18.Upon what two factors does the counter emf depend in a given motor? The counter emf depends on: the flux per pole and the speed of rotation of the armature. 19.When the load upon the shunt motor is increased, what electrical factor affects the speed? It is the armature current that affects speed when there is an increase in speed 20.When the load upon the series or compound motor is increased, what electrical factor affects the speed? In compound and series motors it is the armature current and the flux that is affected due to the increase in load.

21.Approximately, what percentage of the impressed voltage is the counter emf in a dc motor? The counter emf is usually 80 to 95 percent of the impressed voltage. 22.Explain why the power developed by a dc motor is determined by the value of counter emf. The power output of the motor is determined by the counter emf because our power output is just the product of the counter emf and the armature current. 23.What limits the armature current in a dc motor in the instant of starting? It is the rheostat or the external resistance in series with the armature that limits the armature current during startp of the motor. 24.How is it possible to keep the armature current down to a reasonable value when a dc motor is started? It is possible by inserting an external resistance in series with the armature winding in order to limit the starting current that passes through the armature. 25.Why is it not particularly serious to start a small motor directly from the line without the use of external resistors? It is okay to start a small motor directly because: (1) the resistance and the inductance of the armature winding is already sufficiently high that it already limits the initial current to a safer value and (2) the inertia of the small motor is very small that the speed increases quickly. 26.What is the primary function of a starter in a dc motor? The primary function of a starter is to limit the starting current in the armature during startup and acceleration. 27.What are the two general types of manual starter for dc motor? The two geral types of starters are: the three point type and the four point type. 28.How are starters rated? Motor starters are rated by sizes. These sizes are grouped by rated current and horsepower. 29.When starting a dc motor with a manually operated starter, why is it not permissible to hold the handle on an intermediate stud for a considerable length of time? Give reasons for your answer. The starter arm should not be kept long in an intermediate stud because the holding coil will no longer be able to act on the iron keeper thus damaging the motor if the power comes back again in case of a power failure or the motor rotating dangerously fast once the shunt field circuit is opened accidentally. 30.Explain exactly how a dc motor should be started properly with a manual starter. To start the motor, one hand is held on the handle of the open main switch while the starter arm is moved to the first stud with the other hand; then the main switch is closed. After the armature has accelerated sufficiently on the first stud, the starter arm is moved slowly to studs 2, 3, 4, 5, etc. until the soft iron keeper rests firmly against the iron poles of the holding coil.

31.How many electrical circuits are there in a three point starter? a four point starter? A three point starter contains two electrical circuits while the four point starter contains three electrical circuits. 32.What is the disadvantage of the three point starter? How is this disadvantage overcome by the four point starter? The disadvantage of three point starters is that the current through the field winding and the holding coil is the same thus a great increase in the resistance in the field rheostat will prevent current to pass through which results the holding coil will not be able to produce sufficient electromagnetic induction to the springing coil and the starter arm goes back to the off position. The problem in three point starters was overcome in four point starters by placing the holding coil separately from the field rheostat in order to maintain current in the holding coil. 33.What is the function of the holding coil? As the name suggests, holding coil holds the starter arm’s spring coil through magnetic induction. When there will be power failure or accidental removal of the field circuit, the holding coil allows he starter arm to go back to off position through spring action thus preventing any further damage to the motor. 34.What is a controller? What functions can it perform other than starting? Controllers are starters that are equipped with some means of varying the speed of the motor to which it is connected. The controller can also be used to permit reversing the direction of rotation. 35.What two important advantages are possessed by a manual controller for a shunt or compound motor? One advantage is that controllers are more economical since the controller already functions as a speed control as well as a starter instead of using two rheostats for starting and speed contol. Another advantage is that it prevents damage to the motor by limiting the starting current in faulty starting procedures. 36.Explain the operation of a four point controller for a shunt or compound motor. When the motor is started, the two arms move forward simultaneously, the long one pushing the short one. Resistance is first inserted and then cut out of the armature circuit in the usual way as the armature accelerates. The field is excited without and rheostat resistance in the field circuit because current directly passes from line to armature circuit to the holding coil circuit to the field circuit. When the final position of the armature resistor arm is reached, the arm is held by the electromagnet. When the field rheostat arm is moved backwards, the resistance is cut into the field circuit, which causes the motor to speed up. 37.In an automatic starter distinguish between a relay and a contactor; normally open and normally closed contacts.

Relay is a electromagnetic device that serves as a “switch” of the contactors. The contactors, as the name suggests, is where the load is usually connected to. Normally open contactors are open when the relay is off and closed when the relay is on. Normally closed contactors are closed when the relay is off and open when the relay is on. 38.What is a timing relay? Explain its operation. Timing relays are relays that are timed to operate at preset intervals of time by means of devices that function mechanically, pneumatically or electrically. To help understand, some definitions are important: (1) Input Voltage-control voltage applied to the input terminals. Depending on the function, input voltage will either initiate the unit or make it ready to initiate when a trigger is applied. (2)Trigger Signal-on certain timing functions, a trigger is used to initiate the unit after input voltage has been applied. As noted above, this trigger can either be a control switch (dry contact switch) or a power trigger (voltage). (3) Output (Load)-every time delay relay has an output (either mechanical relay or solid state) that will open & close to control the load. Note that the user must provide the voltage to power the load being switched by the output contacts of the time delay relay. 39.In what respects do the counter emf, time limit and current limit automatic types of starter differ from one another in operation? What advantages are possessed by each? In counter emf, a number of relays are connected across the armature where the counter emf increases as the motor accelerates, and the former are adjusted to pick up at predetermined values of voltage. Time limit is composed of timer relays that operate at a certain interval of time. Current limit, the relays are designed that they are sensitive to changes in the armature current. 40. Describe the operation of the counter emf automatic starter Thus shunt motor is started by pressing the start button. This energizes the main contractor, which instantly closes the auxiliary contacts. The motor then starts with resistors in series in the armature circuit. Relays 1AX, 2AX and 3AX are connected across the armature terminals where the voltage drop changes as the motor accelerates and these relays activate at preset of increasing values of voltage until all resistors connected in series to the armature are shorted. The motor then runs normally until the stop button is pressed. 41.Describe the operation of the time limit automatic starter. A time-limit acceleration starter is connected to a compound motor. In the design there are a group of three contactors which each has one pair of instantaneously closing contacts across a block of armature resistance and another pair of timed contacts that close with a time delay after the coil is energized. 42.Describe the operation of the current limit automatic starter. The current-limit acceleration starter functions in still another way, depending for the

motor’s increase in speed upon the current taken by the armature circuit; this permits the motor to start more slowly when the load is heavy and more rapidly under light-load condition. 43.In the push button automatic starter, is the start button normally open or closed? Is the stop button normally open or closed? Are the overload relay contacts normally open or closed? The start button is normally open, the off button is normally closed and the overload relay contact is normally closed. 44.Explain why the armature of a dc motor automatically draws more current from the source when the load is increased? When the load is increased, our motor naturally slows down from its speed at no load. This slowing down of rotation results to a decrease in our back emf. A decrease in back emf results in a lesser voltage drop in the armature side thus increasing our armature current. 45.What is meant by normal speed of a motor? Normal speed is the speed at which a motor operates when it Is driving its rated load. 46.What general statements can be made with regard to the change in speed with load for shunt motor? Compound motor? Series motor? In shunt motors, a change in load results to a little change in speed. For compound motor, a change in load results to a moderate change in speed. In series motors, a change in load results to a drastic change in speed. 47.What is meant by torque? In what units is it usually expressed? Torque is a tendency of a mechanical body to produce rotation. It is usually expressed in Lb-ft or Nm. 48.In general, upon what 2 factors doe the torque of a motor depend? The torque of a motor depends on the flux created by the main poles and the current flowing in the armature winding. 49.How much power must a motor develop? The motor must develop power that is necessary to overcome the motor’s own rotational losses as well as able to drive the mechanical load. 50.Why is the torque of the motor directly proportional to the armature current? Torque is directly proportional to the armature current because when the torque, for instance, is already increases, it will result to a drop in speed which results to the decrease in counter emf which subsequently results in an increase in armature current. 51.Explain how the torque varies with increased load upon a compound motor, series motor and shunt motor. In shunt motors, torque will vary linearly with an increase in load since the load only depends on the armature current. In series motor, an increase in load will tend to produce a exponential increase that eventually becomes linear. In compound motor, the variation tends to become linear since its characteristics are a combination of the series and shunt motor. 52.Under what operating conditions is it desirable to use a shunt motor?? A series motor? A compound motor?

It is desirable to use shunt motor when the load requires little starting torque. The series motor is desirable when the load requires a very high starting torque. Lastly, compound motors is desirable when the load requires moderate starting torque. 53.Is shunt and compound types of motor more stable at no load? Shunt and compound motors are more stable at no load because the speed regulates at higher rpm. 54.What precautions must be taken in operating a series motor, the load on which varies over with? In operating series motor, great care must be taken not to permit the load to be reduced to such extent that the speed becomes excessive. 55.Two similar shunt motors are changed to compound motors. If one of them is wound with twice as many series field turns per pole than the other, which will have (a) the greater speed change with the load? (b) the greater starting torque? (c)the greater overload torque? (a). The one with the most number of series field turns has a greater speed change with load. (b) The one with the most number of series field turns has a greater starting torque. (c) The one with the most number of series field turns has a greater overload torque. 56.Define speed regulation. Speed regulation is the natural or inherent change of speed in motors between full load and no load. 57.What approximate values of speed regulation can be assigned to shunt and compound motors? For shunt motors, the percent speed regulation is about 2-8% while in compound motors, the percent speed regulation is about 10-25% 58.Why is it improper to speak of speed regulation in a series motor? There is no speed regulation in series motor because there is no such thing as speed in no load for series motor since by the time the series motor has no load, it will be under the “runaway” state or the no load speed is already approaching infinity which will most likely result to a damaged motor. 59.Distinguish between the terms speed regulation and voltage regulation Speed regulation, as the name suggests, focuses more on the change of speed between no load and full load. Speed regulation is more frequently used for motors. Voltage regulation, on the other hand, is used more on generators and focuses more on the change of voltage when full load or no load. 60.Why are shunt motors referred to as constant speed motors? Shunt motors are called constant speed motors because they still maintain more or less the same speed when the load in removed. Shunt motors have the least speed regulation percentage among the three types. 61.Why compound and series motors are generally referred to as variable speed motors?

Compound and series motors are considered as variable speed motors because their percentage speed regulation is relatively high compared to shunt motors. This means that these motors have more or less higher change in speed when the load is removed as compared to shunt motors. 62.Give several practical applications for shunt motors. Shunt motors are seen mostly used as wood planers, circular saws, grinders and line shafts. 63.Give several practical applications for compound motors. Compound motors are widely used as compressors, pumps and pressure blowers. 64.Give several practical applications for series motors Series motors are used as streetcars, turntables, cranes, bucket and mine hoists, and the operation of large valves. 65.Why are differential compound motors unstable at heavy loads? Explain carefully. At heavy loads, the series field current is comparatively high, which means that the demagnetizing action in the shunt field may be considerable 66.Under what conditions is it permissible and desirable to use differential compound motors? It is desirable to use differential compound motors when a better constant speed characteristic is needed since is speed characteristic graph is almost flat within the good part of the operating range. 67.What precaution must be made in starting a differential compound motor? Differential compound motors should be started with the series field short circuited because a large starting series field current may be sufficient to reverse the normal magnetic polarities and cause the motor to start up in the wrong direction. 68.Distinguish between speed control and speed regulation. Speed control is the process of adjusting the speed of a motor to some value other than which it normally operates its load. Speed control is usually done by external forces like mechanical or electrical intervention. Speed regulation on the other hand is the change of speed due to the removal of load. 69.How does the speed vary when the shunt field rheostat is adjusted? When the field rheostat is adjusted, speed will vary due to a change of flux since the addition/reduction of resistance in the field rheostat will result in a change of field current. 70.How does the speed vary when the armature rheostat is adjusted? When the armature rheostat is adjusted, speed will vary due to a change in armature current brought to by a change in voltage drop in the armature circuit. 71.How does the speed vary when the armature voltage is adjusted? When armature voltage is adjusted, speed will vary due to a subsequent change in armature current brought by a change in the voltage supplied in the armature circuit. 72.Using the fundamental equation for a dc motor, justify the answers in 69-71.

S=

V A −I A R A kφ

a.)Since the speed and the flux (φ) is inversely proportional, changing the resistance in the field circuit will change the flux generated thus changing the speed of the motor. b.)The speed will also vary by changing the total resistance in the armature (RA) since the voltage drop in the armature (IARA) will change. c.) The speed will obviously vary by changing the voltage supplied in the armature (VA). 73.Describe the Ward Leonard system of control for shunt motor. The shunt field must be connected to a constant potential supply so that flux of unvarying intensity is created, while the armature is permanently connected to the armature terminals of a special generator whose voltage can be varied. ; the latter is usually separately excited from the same source that feeds the controlled motor. To drive both the controlling generator and the exciter, a prime mover is necessary. 74.Compare the power losses in the rheostats of field and armature resistance methods of control. The field resistance method of control involves less power loss because the shunt field current is just 2-8% if the total motor current while the armature resistance method of control involves a high power loss since the armature current is more or less equal to the line current. 75.What is the advantage of the Ward-Leonard system of control? What are its disadvantages? The chief advantage of the Ward-Leonard control system is that it provides excellent step less speed control for a motor which must have a very wide range of speed. Its disadvantages are: high first cost and low overall efficiency. 76.List some practical applications of the Ward-Leonard control system. Ward-Leonard control system is frequently used as electric excavators, freight handing ships, blooming and paper mills, and for operation of large elevators in tall buildings. 77.Describe the operation of the modified Ward Leonard system of control in which two exciters are found. The fundamental interconnection of the two armatures of the main machines is identical to the original Ward-Leonard control system however, the important departure from the original is that a controlled intermediate exciter, connected directly across the field of the main generator, is used not only to adjust the voltage in the loop but also to vary the flux produced by the field of the controlled motor; the latter is accompanied by having the two exciters in series bucking and directly across the field of the controlled motor. 78.What advantages are possessed by the modified Ward Leonard control system?

It’s advantage is that a controlled intermediate exciter can be used both to adjust the voltage in the loop and to vary the flux produced by the field of the controlled motor. 79.Explain why the voltages of the main and intermediate exciters in the modified Ward Leonard control system must never be equal. What will happen if they were When the voltage of the intermediate exciter is increased to raise V A, the voltage across the series opposed exciters is simultaneously lowered; the result is that the main motor is affected by two actions, both of which tend to increase the speed. When the voltage of the intermediate exciter is reduced to lower VA, the voltage across the series-opposed exciters is increased, the controlled motor is again affected by two actions, both of which tend to reduce the speed. If they were equal, the motor will not change speed. 80.Describe the operation of the simplified Ward-Leonard system of control in which towe series machines are used. What important magnetic design features must the machine possess for good operation? In another modified Ward-Leonard control system, the controlling generator is driven by a prime mover; usually a constant speed ac motor and speed control of the controlled motor is affected by shunting the series field of the generator with a variable resistance. The two series machine should be designed so that the flux are directly proportional to the loop current, i.e., that the magnetic circuits are never saturated. 81.How does the effect of armature reaction in motors differ from its effect in generators? Since the armature current in a motor is opposite to that of a generator for the same direction of rotation, it follows that the shift of the magnetic axis is opposite to the shift produced by the generator. The magnetic neutral of a motor always tend to shift in a direction opposite to that of the armature rotation. 82.If no interpoles are used, how must the brushes be shifted in generators? In motors? in the absence of interpoles, brushes must be shifted forward beyond the resultant magnetic neutral in generators. In motors, on the other hand, the brushed should be shifted backward beyond the resultant magnetic neutral. 83.What are the polarities of the interpoles with respect to the main poles in generators? In motors? In generators, the polarities of interpoles is opposite as the preceding main pole with respect to the direction of rotation while in motors, the polarities of interpoles is the same as the preceding main poles with respect to the direction of rotation. 84.Why is it possible to use half as many interpoles as main poles in some small motors? What advantage would this have? Half as many interpoles are used since the span of every coil is 180 electrical degrees so if an interpole is made doubly strong, its effect on one coil side is

equivalent to the action of two interpoles, each acting on one coil side. The main advantage of this practice would be reduction in manufacturing costs. 85.What two fundamental methods may be used to reverse a dc motor? Which is preferable in compound machines? A dc motor’s rotation can be reversed by changing the direction of current flow through the armature and by changing the direction of current flow through the field circuit. In compound machines, it is more preferable to change the direction of the armature current in reversing its rotation. 86.When a DPDT switch is used in the field circuit of a shunt motor for reversing purposes, what precaution must be taken when the motor is started? Is this necessary if the DPDT switch is placed in the armature circuit? The DPDT switch must already be closed to the contacts in order to prevent high current in passing through the armature since if the DPDT switch is open, then no current passes through the field which results to the armature monopolizing the high current thus damaging the windings. If the DPDT switch is placed in the armature circuit, then the precaution will no longer be necessary. 87.Describe the operation of the automatic reversing starter. When the For button is pressed, the F contactor is energized and the R circuit is opened at f as a safety measure; this seals the For button at F1, closes the F contacts and the current passes through the armature circuit from a to b. At the same instant, the F2 contacts close to energize the timing relay; after the latter times out, the T-T-C contacts close to energize the 1A contactor, which, in turn, closes the 1A contacts to short circuit in resistor R1. Contactor 1A also has a set of timed contacts, which, after a certain delay, close to make contactor 2A pick up; the operation of the latter closes contacts 2A to short circuit the second resistor so that the motor can run normally. To reverse the motor, the Stop button is first pressed ten the rev button is pressed. 88.Carefully explain why the control relay CR has a normally closed contact in the F contactor circuit. CR has a normally closed contactor before the F relay in order to prevent the F contactor from being operated accidentally and momentarily opens the r contacts as a further safety measure. 89.Explain why each of the push buttons, for and rev, has one normally open and normally closed set of contacts. For and rev has one normally open and normally closed set of contacts in order to prevent current flow once the two push button are pushed simultaneously. 90.What would happen if the for and rev buttons were pressed simultaneously? If for and rev buttons are pushed simultaneously, the motor will not turn on since no current flow is happening to energize the F relay.