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Motor Listrik ACAC motors convert AC electrical energy to Mechanical energy.

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Chapter 13AC Motors

AC motors:

1. the armature of rotor is a magnet (different to DC motors).

2. the stator is formed by electromagnets (like in DC motors).

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Effects of AC Supply on Magnetic Poles

Consider the rotor to be a permanent magnet. Current flowing through conductors energize the magnets and develop N

and S poles.

The strength of electromagnets depends on current.

First half cycle current flows in one direction.

Second half cycle it flows in opposite direction.

As AC voltage changes, the poles alternate.

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Using AC Supply to Make anElementary Motor (1) Consider the AC voltage at 0 degrees, then, no current will flow, and

there is no magnetism.

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Using AC Supply to Make an

Elementary Motor (2)

As voltage increases, current starts to flow and electromagnets gainstrength and North and South poles appear.

(Use left hand rule to find poles).

The rotor magnet is pushed CW, and the rotor and motor starts torotate.

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Using AC Supply to Make anElementary Motor (3) When voltage decreases, the current decreases also, the

electromagnet loses the strength, and when V=0 there is nomagnetism.

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Using AC Supply to Make anElementary Motor (4) Now, AC voltage builds up as part of the negative cycle.

Then, current flows in opposite direction, and the magnets reverse

polarity.

Therefore, the CW rotation continues.

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Using AC Supply to Make anElementary Motor (5) This process is repeated over and over, as AC voltage goes through

its cycles, and we have continuous rotation.

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AC Motor Rotation The wholepicture

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Limitation of the ElementaryMotor The initial position of the rotor determines the direction of the

motor rotation.

Indicate the rotation in the figures below:

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Practical AC Motor

By adding another pair of electromagnets the limitation mentioned

before is removed. Two electromagnets = Vertical & Horizontal

Two phases with phase difference = 90 deg.

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Effect of Two Pole-Pairs(Observe the pole rotation)

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Operation of the Practical ACMotor Fig. of page 124 shows a CCW rotation

Can you see it?

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Magnetic Poles Revolve in AC

Motors From the previous slide we can see that the poles rotate around the

circumference of the motor. The rotor, no matter how it is positioned at rest, will be locked-in

with the magnetic field and will turn in one direction only.

(Same rotation as the poles).

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Phase Splitting Method (1)

So, two voltage sources with 90 degree phase connected

to electromagnets make the rotor turn.

Question is: Can we do the same using only one voltagesource?

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Phase Splitting Method (2)

The answer is yes!

Because we can use inductors and capacitors to produce

a voltage out of phase with the source!

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Reactor Start AC Motor

(One phase + Inductor) Two parallel branches connected to the power supply.

First branch: Start winding through a centrifugal switch.

Second branch: Run winding (through an inductor).

The current in the second branch lags the current in the first branch(Remember ELI).

This phase difference makes motor work.

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Reactor Start AC MotorThe Centrifugal Switch

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Capacitor Start AC Motor

(One phase + Capacitor)

Here the capacitor provides the phase difference. The difference is that the current in the star winding leads thecurrent in the run winding (ICE).

Similar effect as with the inductor, but it creates a motor with higherstarting power.

Refrigerators, compressors, air conditioners

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Three Types of Capacitor StartMotors1. Capacitor Start(disconnects capacitor after motor

speed picks up)

2. Capacitor Run(Keeps the capacitor connectedduring the operation of the motor, in order to keep theelectric power consumption low)

3. Capacitor Start-Run(uses two capacitors, one forstarting and one for running. This further improvesPower Consumption)

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Synchronous Speed

AC motors always rotate with the speed of their revolving magneticfield.

The speed of the revolving poles is the maximum possible speed ofrotation of the motor.

It is called Synchronous Speed.

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Motor Construction

The Stator

The stator forms a hollow cylinder with coils of insulated wireinserted into slots of the stator core.

The coils, plus the steel core form the electromagnets.

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Motor Construction

The Rotor There are two types of motor rotors:

The wound rotor

The squirrel cage

The wound rotor has coils of wire wound in the slots of the rotor(Similar to generator coils).

The Squirrel cage consists of bars of copper or aluminumelectrically connected at each end with conducting rings.

As the rotor rotates inside a magnetic field, it receiveselectromagnetic induction, then current flows and form the

rotor electromagnet.

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Types of Motor Enclosures

1. ODP Open Drip Proof

2. TENV Totally Enclosed Non-Ventilating

3. TEFC Totally enclosed Fan Cooled

4. XP Explosion Proof

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ODP Open Drip Proof

Air flows through motor (fan blades help flow)

Used in environments free from contaminants

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TENV Totally Enclosed Non-Ventilating

Protect motor from corrosive and harmful elements

Frame fins help to dissipate heat

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TEFC Totally enclosed Fan Cooled

Similar to TENV except has external fan for cooling

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XP Explosion Proof

Similar to TEFC but enclosures are cast iron

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Slip Slip is associated with synchronous speed.

If the motor turned at the same RPM as the magnetic field, there

would be no relative motion between the rotor and the field. Therefore, no current would be induced into the rotor, and no

magnetic field would exist.

Rotor speed < synchronous speed

Slip = synchronous speed rotor speed

% slip = ( Ns Nr / Ns ) 100

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Three Phase AC Motor

It has three pairs of electromagnets, connected to one of the threephases of the power supply.

It provides a lot higher power that what single phase motor candeliver.

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AC Motor Data Plate Each motor has a plate mounted on its frame, with electrical and

mechanical information.

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08 Motor AC