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PHY 152(ELECTRICITY AND MAGNETISM)

•ELECTRIC MOTORS (AC & DC)

•ELECTRIC GENERATORS (AC & DC)

AIMS

Students should be able to

• Describe the principle of magnetic induction as it applies to DC and AC generators.

• Describe the differences between the two basic types of ac generators.

INTRODUCTION

Regardless of size, all electrical generators,whether dc or ac, depend upon the principle ofmagnetic induction. An emf is induced in a coil asa result of (1) a coil cutting through a magneticfield, or (2) a magnetic field cutting through acoil. As long as there is relative motion betweena conductor and a magnetic field, a voltage willbe induced in the conductor. That part of agenerator that produces the magnetic field iscalled the field. That part in which the voltage isinduced is called the armature.

For relative motion to take place betweenthe conductor and the magnetic field, allgenerators must have two mechanical parts - arotor and a stator. The ROTor is the part thatROTates; the STATor is the part that remainsSTATionary. In a dc generator, the armature isalways the rotor. In alternators, the armaturemay be either the rotor or stator.

QUESTION: Magnetic induction occurswhen there is relative motion between whattwo elements?

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DC MotorA simple DC motor has a coil of wire that

can rotate in a magnetic field. The current inthe coil is supplied via two brushes that makemoving contact with a split ring (commutator).The coil lies in a steady magnetic field. Theforces exerted on the current-carrying wirescreate a torque on the coil.

The force F on a wire of length L carrying a current I in a magnetic field B is

(i.e ILB times the sine of the angle between B and I, which would be 90° if the field were uniformly vertical.

The direction of F comes from the righthand rule.

Fig 1: DC Motor

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The two forces shown above are equal and

opposite, but they are displaced vertically, so

they exert a torque.

The commutator is that device/component

that periodically reverses the direction of an

electric current.

NOTE

A DC motor requires often anexternal resistor or rheostat to limitthe current. The value, in Ohms, ofthat resistor is reduced in steps as thespeed of the motor increases, untilfinally that resistor is removed fromthe circuit as the motor reaches closeto its final speed.

DC Generator

A DC generator is also a DC motor (withthe same components of a motor), but the coilis turned to generate an e.m.f.

Fig 2: DC Generator

If you use mechanical energy to rotate thecoil (N turns, area A) at uniform angularvelocity ω in the magnetic field B, it willproduce a sinusoidal e.m.f in the coil. Let θ bethe angle between B and the normal to thecoil, so the magnetic flux φ is NAB.cos θ.Faraday's law gives:

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As in the DC motor, the ends of the coil connect

to a split ring, whose two halves are contacted by

the brushes. Note that the brushes and split ring

'rectify' the e.m.f produced: the contacts are

organized so that the current will always flow in

the same direction, because when the coil turns

past the dead spot, where the brushes meet the

gap in the ring, the connections between the ends

of the coil and external terminals are reversed.

AC Motor

As in the DC motor case, a current is passed

through the coil, generating a torque on the coil.

Since the current is alternating, the motor will run

smoothly only at the frequency of the sine wave. It is

called a synchronous motor. More common is the

induction motor, where electric current is induced in

the rotating coils rather than supplied to them

directly.

Fig 3: AC Motor

One of the drawbacks of the induced AC motor is the

high current which must flow through the rotating

contacts. In common AC motors the magnetic 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 which rotates is called

the "armature". In an AC motor the magnetic field is

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

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AC motors are generally divided into twocategories, induction and synchronousmotors. The most common AC motor is the"Squirrel cage motor", a type of inductionmotor. Most AC motors require a starter, ormethod of limiting the inrush current to areasonable level.

AC Generator

The turning of a coil in a magnetic fieldproduces motional emfs in both sides of thecoil which add. Since the component of thevelocity perpendicular to the magnetic fieldchanges sinusoidally with the rotation, thegenerated voltage is sinusoidal or AC. Thisprocess can be described in terms of Faraday'slaw when you see that the rotation of the coilcontinually changes the magnetic flux throughthe coil and therefore generates a voltage.

AC generators are generally called analternators.

Fig 4: AC Generator

AC GENERATOR AND MOTOR

Fig 5: AC motor and generator

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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 that created

it.

If the motor is not driving a load, then the

generated back e.m.f 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 e.m.f 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.

Types of Alternator

1. Rotating – amarture alternators: the rotating-armature

alternator is similar in construction to the dc generator

in that the armature rotates in a stationary magnetic

field.

2. Rotating field alternators: the rotating-field alternator

has a stationary armature winding and a rotating-field

winding. The advantage of having a stationary armature

winding is that the generated voltage can be connected

directly to the load. Alternators

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QUESTIONS

• What is the part of an alternator in which the output voltage is generated?

• What are the two basic types of alternators?

• What is the main advantage of the rotating field alternator?