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AC Motor

As in the DC

motor case, a

current is passedthrough the coil,

generating atorque on the coil.

Since the current

is alternating, themotor will run

smoothly only at

the frequency of 

the sine wave. Itis called a

synchronousmotor. Morecommon is the

induction motor ,

where electriccurrent is induced 

in the rotating

coils rather than

supplied to themdirectly.

One of the drawbacks of this kind of AC motor is the high current which mustflow through the rotating contacts. Sparking and heating at those contacts can

waste energy and shorten the lifetime of the motor. In common AC motors themagnetic field is produced by an electromagnet powered by the same AC

voltage as the motor coil. The coils which produce the magnetic field are

sometimes referred to as the "stator", while the coils and the solid core whichrotates is called the "armature". In an AC motor the magnetic field is

sinusoidally varying, just as the current in the coil varies.

How does a motor work? AC Generator  Generator and Motor   DC Motor 

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DC

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AC Generator

The turning of a coil in a magnetic field produces motional emfs in both sides

of the coil which add. Since the component of the velocity perpendicular to the

magnetic field changes sinusoidally with the rotation, the generated voltage issinusoidal or AC. This process can be described in terms of Faraday's law 

when you see that the rotation of the coil continually changes the  magneticflux through the coil and therefore generates a voltage.

Voltage generation detail AC Motor  Generator and Motor  DC Motor 

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DCCircuits

 

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Generator and Motor

A hand-cranked generator can be used to generate voltage to turn a motor. This

is an example of energy conversion from mechanical to electrical energy and

then back to mechanical energy.

As the motor is turning, it also acts as a generator and generates a "back  emf ".

By Lenz's law, the emf generated by the motor coil will oppose the change thatcreated it. If the motor is not driving a load, then the generated back emf will

almost balance the input voltage and very little current will flow in the coil of the motor. But if the motor is driving a heavy load, the back emf will be less and

more current will flow in the motor coil and that electric power being used is

converted to the mechanical power to drive the load.

Demonstration AC Motor  AC Generator  DC Motor 

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DCCircuits

 

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DC Motor Operation

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Magnetic interactions with charge

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Current in DC Motor

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DC Motor Operating Principles

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Magnetic Field in DC Motor

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DC Motor Operating Principles

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Force in DC Motor

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DC Motor Operating Principles

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Torque in DC Motor

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DC Motor Operating Principles Torque on a Coil

Variation of torque with coil angle

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How Does an Electric Motor Work?

DC Motors AC Motors

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Induction Motor Action

Induction motors use shorted wire loops on a rotating armature and obtaintheir torque from currents induced in these loops by the changing magnetic

field  produced in the stator (stationary) coils.

At the momentillustrated, the current in

the stator coil is in the

direction shown andincreasing. The induced

voltage in the coil

shown drives current

and results in aclockwise torque.

 Note that this simplified

motor will turn once it isstarted in motion, but

has no starting torque.

Various techniques areused to produce some

asymmetry in the fields

to give the motor a

starting torque.Simple induction motor 

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Induction in Armature Coils

Induction motor action involves induced currents in coils on the rotating

armature.

This is an active graphic. Click on the bold text for further detail.

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Simple AC Induction Motor

A large percentage of small AC

motors are classed as inductionmotors. This implies that there is

no current supplied to the rotating

coils. These coils are closed loopswhich have large currents

induced in them because of their 

low resistance.

An induction motor must achieve a rotating magnetic field to continue to exert

a torque on the armature coils. In this example, the rotating field is achieved

 by the extra coils on the pole pieces.

More detail Induction motor principle

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Simple AC Induction Motor

Induction motor principle

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AC Generator

The turning of a coil in a magnetic field produces motional emfs in both sidesof the coil which add. Since the component of the velocity perpendicular to the

magnetic field changes sinusoidally with the rotation, the generated voltage issinusoidal or AC. This process can be described in terms of Faraday's law 

when you see that the rotation of the coil continually changes the  magnetic

flux through the coil and therefore generates a voltage.

Voltage generation detail AC Motor  Generator and Motor  DC Motor 

Index

DCCircuits

 

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How Does an Electric Generator

Work?

AC Generator  Generator/motor demonstration

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Generator and Motor

Identical hand-cranked generators can be used to show that a generator can

function as a motor  and vice versa. If mechanical energy is put into the oneof the devices by turning the crank,

electrical energy can be produced. If 

that electrical energy in the form of voltage is applied to the other, then

mechanical energy can be produced

(motor action).

In the video below, the crank of one of the devices is turned to put in

mechanical energy, resulting in electrical energy at the output. The handle

of the other device is turned because electrical input is operating it as amotor. The last segment of the film lines up the handles of the devices and

then turns the generator 1 turn. You can observe that the handle of the"motor" turns about 60% of a turn, indicating an efficiency of about 60%

for the generator/motor exchange. But that is almost twice the efficiency of 

the electric generation system of the U.S., which yields electricity at about33% efficiency! The reason is that this generator/motor pair does not have

to go through the "thermal bottleneck " imposed by the second law of 

thermodynamics on the common electrical generation plants which operateas heat engines.

The high efficiency of the transfer of energy from a generator to a motor finds widespread application in the operation of diesel-electric locomotives.

It is difficult to mechanically transfer energy from the powerful dieselengines to the wheels of the locomotive which have low-friction metal-to-

metal contact with the steel rails. So the power of the diesel engines is used

to run a generator, and the generated electric power is used to operate

electric traction motors to drive the wheels. The generated electric power can be very slowly and smoothly transferred to the wheels by the electric

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concepts

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motors which drive the axles. You don't "pop the clutch" on a big

locomotive with steel-on-steel wheels! You would just sit and spin thewheels until you melted the rails.

How does a generator work? How does a motor work?

Discussion of a generator/motor pair 

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