Chapter 13 – AC Motors

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Chapter 13 – AC Motors

AC motors convert AC electrical energy to Mechanical energy.

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Chapter 13 – AC 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 an Elementary Motor (1)

• Consider the AC voltage at 0 degrees, then, no current will flow, and there is no magnetism.

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• As voltage increases, current starts to flow and electromagnets gain strength 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 to rotate.

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• When voltage decreases, the current decreases also, the electromagnet loses the strength, and when V=0 there is no magnetism.

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• 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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• 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 whole picture

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Limitation of the Elementary Motor

• 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 AC Motor

• 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 voltage source?

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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 the current in the run winding (ICE).• Similar effect as with the inductor, but it creates a

motor with higher starting power.• Refrigerators, compressors, air conditioners

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Three Types of Capacitor Start Motors

1. Capacitor Start (disconnects capacitor after motor speed picks up)

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

3. Capacitor Start-Run (uses two capacitors, one for starting and one for running. This further improves Power Consumption)

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Synchronous Speed

• AC motors always rotate with the speed of their revolving magnetic field.

• The speed of the revolving poles is the maximum possible speed of rotation of the motor.

• It is called “Synchronous Speed”.

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Motor ConstructionThe Stator

• The stator forms a hollow cylinder with coils of insulated wire inserted into slots of the stator core.

• The coils, plus the steel core form the electromagnets.

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Motor ConstructionThe 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

aluminum electrically connected at each end with conducting rings.

• As the rotor rotates inside a magnetic field, it receives electromagnetic 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 three phases of the power supply.

• It provides a lot higher power that what single phase motor can deliver.

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AC Motor Data Plate• Each motor has a plate mounted on its frame,

with electrical and mechanical information.

Chapter 13 – AC Motors

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