Motors & Generators

Electronics Motors and Generators Project Jean-Louis C Haywood 5A1 Albena Lake Hodge Comprehensive School 1

Contents: Introduction....................................................................................................................................3

Motors Introduction to Motor....................................................................................................................3 Motor parts....................................................................................................................................4 DC Motors......................................................................................................................................5 AC Motors......................................................................................................................................6 Motor Equations; Slip & Synchronous speeds...............................................................................7 Motor Applications........................................................................................................................9 Electric Motor family tree.............................................................................................................10

Generators Introduction to Generator............................................................................................................11 Generator Parts............................................................................................................................12

Conclusion Gallery References

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Introduction This report has been written within this electronics class to present relevant and powerful information for the student mind in need of this piece of written work. This report shall cover the basics of a motor at the very least. It shall contain the introduction to several types of motors, their equations as well with measurement of efficiency.

Motors

Introduction to Motor

The movement or rotation of your electrical device parts, heat sinks, the warm emission of radiated heat and the ability to make a robotic arm diverge from the shoulder and vice versa; an electrical motor has that potential. This ingenious device that could be so small . . . and sometimes so large in comparison to a 3-storey building, functions in one certain way, always; Electrical motors convert electrical energy into mechanical energy and utilises power. Of course, an electrical component of this application could not be used alone with only a structure composed of minorities, such as; the Resistor (which only comprises of length, diameter thickness and resistivity,) Capacitor (Two plates and a dielectric) and the Inductor (with only a coil of wire). The electrical motor functions on several parts, noticeable ones to boot. Since that this device functions on electricity, it does use the power consumed from a DC power supply. Not only that, another version from the DC motor know as the AC motor, functions on an appreciable efficiency when consuming AC electrical power. The motor uses two applications of speeds which are supply frequency and rotor speed where the difference

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between their speeds is measured as a percentage under the name “Percentage Slip.” Also, the simultaneous speed between the two forces is known as synchronous speed.

Motor Parts Parts of motors usually vary with the different types. There are Servo, Induction, Synchronous, Electrostatic, Brushed & Brushless, and Core & Coreless motors of Direct Current. As for Alternating Current, there’s the Squirrel cage and Wound rotor examples as well. [1] The typical electric motor has six parts;

Parts of an electric motor 

The Armature – the structure attached to the centre piece that conducts the electromagnetic flux reaction from the material to the field magnet.

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

Commutator – a rotational switch curved around the centre piece or axle that switches the flow of current on the armature so the electromagnetic flux alternates as it makes contact with any one of the poles, to encourage repulsion in one direction only.



Brushes (Brush motors only) – the best conductor used for rotating shafts where collecting electrons from a negative pole would be difficult using a solid conductor, whereas this one would bend, flex, and catch the commutator rings without stopping the rotation. However, the on and off contacts with the commutator promotes sparking and then heat.

 

Axle – Quite simply, the centre piece of the rotating shaft that turns on its axis. Field Magnet – The field magnet’s role is to encourage the rotation of the armature where the electromagnetic impulses from the rotor decides the polarity and swaps through the commutator rings. Opposing polarities forces the magnets to push the armature in one direction where the other pole would attract it and the cycle continues via a swap in polarity in the armature.



DC power supply – In any electrical device that functions, this is extremely necessary. As a matter of fact, without the power supply, this whole apparatus would be impossible. If there was no electrical energy, there would be no conversion to mechanical energy via the electromagnetic effects.

DC Motors The diagram above showed a brush motor in action. Brush motors are actually one of the many types of motors you would find that run on Direct Current (or DC). Brushed motors are internally commutated, meaning that the switch of the polarity of a commutator’s contact will only take place between the two permanent magnets. [2] Each DC motor has a certain response to the change of load or electrical resistance. For example, a shunt wound motor responds to increased load by trying to maintain speed and this leads to an increase in armature current. The disadvantage of this is overheating when

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supplying power across different loads. However, other devices such as the series wound motor have different responses towards these resistance increases.

AC Motors

An AC motor comprises of two basic parts;

induction motor

 

The stator The rotor

The stator is situated on the outside, around the rotor. AC electrical power is supplied to it to propel the rotor which is within the stator, and this rotation travels in one direction only. [3] Unlike the DC motor which has a series of different versions of itself with 3 self excited

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motors; Series, Compound and Shunt, the available AC motors are either one of the two; Synchronous or Induction motors. The induction motor normally runs on induced current to operate the rotor and can use either single or three phased power. The advantages of this motor include; Simple design, inexpensive, high power to weight ratio, easy maintenance and the direct connection to an AC power source

Motor Equations; Slip & Synchronous speeds

Every AC motor uses the frequency from an electrical power supply to run the rotor in a particular speed. However, the efficiency of this speed, according to the supply frequency, can sometimes never be 100% accurate. Every AC motor has its percentage slip. The percentage slip is usually the slight deficiency of speed from the actual supply frequency. Induction motors are known to have a higher percentage slip than synchronous motors. In opposition to percentage slip, the synchronous speed is almost exactly the same speed as the frequency supplied by the power source. The number of pole pairs, situated on the stator winding also has an effect on the efficiency of the rotor speed. [4]Therefore;

F = NsP Where F = Supply Frequency Ns = Synchronous speed P = Number of pairs of poles

So finding Synchronous speed is expressed as;

Ns = F/P

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Because the slip is a percentage of deficiency towards the Synchronous speed, it is expressed as;

S = (Ns – Nr)/Ns x 100 Where Ns = Synchronous speed Nr = Speed of the rotor S = Percentage Slip

To lay down an example to these principles, the following question can be used.

A six-pole cage induction motor runs at 4% slip. Calculate the motor speed if the supply frequency is 50Hz. Find the motor speed with P = 3 pairs, S = 4%, F = 50Hz, Nr = ?, Ns = ? In order to find the Rotor speed, the Synchronous speed must be found first through this equation;

F = NsP  50Hz = Ns x 3 pairs

So the equation can be rearranged as. . .

Ns = F/P  Ns = 50Hz/3 pairs Our answer becomes 16.66 revolutions per second (Revs/s)

From here we input this answer into the percentage slip equation to find the rotor speed;

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S = (Ns – Nr)/Ns x 100%

Again, the equation (where “/Ns x 100%” have been swapped to the left hand side) can be rearranged to find the specific item;

SNs/100% = Ns – Nr  4% x 16.66/100% = 16.66 – Nr  16.66 – 0.67 = Nr

We derive our answer that Nr is approximately equal to 16 revolutions per second.

Motor Applications

Nearly everywhere in our modern world, motors are used for convenience and business to carry out basic functions in our everyday lives. Motors are found in rotating machines such as; Fans, Turbines, Drills, the wheels on electric cars, locomotives and conveyor belts. Motors are popular in the construction of robotics where moving parts are extremely necessary instead of digital computation alone. The trigonometric functions and relations of a robotic arm also contribute to the knowledge of how big the motor should be or the relative speed to the body part. [5] An example of this is the “Canadarm2,” a space arm used for positioning astronauts. It uses three special motor bases that comprise of the “Mobile Servicer Base System”, “Remote Manipulator System” and the “Special Purpose Dexterous Manipulator,” which are just a number of other examples that makes use of motors in robotics and engineering.

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Electric Motor Family Tree [6]

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Generators

Introduction to Generator The Generator, this device is the exact opposite of an electrical motor in the sense that it converts mechanical energy into electrical energy instead of the other way around. Generators are also commonplace around the entire world in the mass production of electricity. According to Wikipedia, “A generator forces electric charge to move through an external electrical circuit, but it does not create electricity or charge, which is already present in the wire of its windings. It is somewhat analogous to a water pump, which creates a flow of water but does not create the water inside.” [7] The misconception that electricity is produced inside a generator is quite common. Also, the “water”(electricity) is already in every material around us with the aid of electrons, but the “water pump”(generator) circulates it throughout the “pipes”(materials/wires). The general function of the generator is to produce the electrical flow via mechanical movements (from the prime mover) where an equation applies. This equation states that Electricity = Magnets (or Magnetism of any kind) + Copper Wire (a conductor) + Motion. Without any of these elements, the production of electrical flow is impossible; there must be a movement of either the conductor around the magnet or the movement of the magnet around the conductor. The increase of any of these elements may result in higher electrical flow or voltage; faster motions, higher conductivity (or lower resistivity), increase in flux density. In order to produce a constant mechanical movement, electrical engineers use several fuels towards the movement of their generator parts along with other mechanisms that cause the processing and movement from them. These include;      

Wind - Windmills Water – Water Wheels (Dams) Tidal Harnesses Oil – Turbines Coal – Steam engine Petroleum – Internal combustion engine

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   

Natural Gas Geothermal (Heat from the earth) Solar Energy Uranium

Generator Parts Just like the AC motor, the generator only comprise of two major parts known as the stator and rotor. Both of these components have the same name and function as their motor counterparts. For the electrical parts, there’s the armature and the magnetic field coil which generates as either a permanent or an electromagnet. [8] The way a generator would process the electrical flow in a wire goes like this; (1) Say we were using a hydroelectric powered generator to generate electrical current. The flow of water from a dam would push the water wheels. (2) Attached on the other end of the water wheel is the rotor which is a solid conductor situated between two poled magnets . . . or sometimes two pairs of poled magnets. When there is movement, the conductor cuts through the magnetic field and a voltage is induced.

(3) As the armature rotates through the magnetic field, a voltage is generated in the armature which causes current to flow. (4) Slip rings are attached to the armature and rotate with it. (5) Carbon brushes ride against the slip rings to conduct current from the armature to a resistive load.

Conclusion So the two devices are in fact completely opposite in nature to their functions. One could illustrate that a generator can process electrical energy throughout a power grid and send it to

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another home several miles away, by time the electrical power reaches into the socket, that power can be used again into a fan, which is a basic motor, to blow cool air. In this report, the researcher has demonstrated the functions, applications and illustrations of these two devices.

Gallery

Induction motor parts

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DC Brushed motor

Dynamo Iron Filings distorted

Permanent magnet DC motor

DC motor

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Gasoline Generator

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References [1]http://electronics.howstuffworks.com/motor1.htm [2]http://en.wikipedia.org/wiki/Torque_and_speed_of_a_DC_motor#Characteristics_of_ DC_motors [3]Electronics Class: Motors and Generators. 15th October 2009 [4]Electronics Class: Motors and Generators. 29th October 2009 [5]http://www.nasa.gov/mission_pages/station/structure/elements/mss.html [6]http://openbookproject.net/electricCircuits/AC/02420.png [7]http://en.wikipedia.org/wiki/Electrical_generator [8]http://www3.sea.siemens.com/step/pdfs/elec_2.pdf

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End Keywords Motors DC motor – An electrical motor that runs on a DC power supply AC motor – An electrical motor that runs on an AC power supply

Generators Dynamo – The very first generator which only generates DC power Alternator – The standard generator which only generates AC power

Energy Electrical energy – Energy manifested in the form of electricity Mechanical energy – Energy manifested in the form of movement

Speeds and Quantities Power – The product of Voltage and Current. Also the rate at which energy is made Percentage Slip – The lagging of speed between rotor and frequency speeds. Synchronous speed – Nearly 100% synchronisation of rotor and frequency speeds Frequency – The number of cycles of alternating current from 0 degrees to 360 degrees Flux – The amount of magnetic force generated from electric current

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