chapter 13 – ac motors.ppt

7/27/2019 Chapter 13 – AC Motors.ppt

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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 po les 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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an Elementary Motor (2)

• 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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an Elementary Motor (3)• When voltage decreases, the current decreases

also, the electromagnet loses the strength, and

when V=0 there is no magnetism.



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an Elementary 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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an Elementary 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 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 thelimitation 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 degreephase 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 Motors1. 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 Construction

The 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 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

aluminum electrically connected at each end with

conducting rings.

• As the roto r rotates insid e a magnet ic f ie ld, i t

receives electrom agnet ic induc t ion , then cu rrent

f lows and fo rm the rotor electrom agnet .

0



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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 themagnetic 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 < syn chronous speed

Slip = synchronous speed – rotor speed

% s l ip = ( 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.



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chapter 13 – ac motors.ppt

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