Chapter 13 – AC Motors ppt

Limitation of the Elementary Motor • The initial position of the rotor determines the direction of the motor rotation.. Reactor Start AC Motor One phase + Inductor • Two parallel branche

Chapter 13 – AC Motors

AC motors convert AC electrical energy to

Mechanical energy

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.

• (Use left hand rule to find poles).

• The rotor magnet is pushed CW, and the rotor

and motor starts to rotate

AC Motor Rotation

The whole picture

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:

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

Effect of Two Pole-Pairs

(Observe the pole rotation)

Operation of the Practical

AC Motor

• Fig of page 124 shows a CCW rotation

Can you see it?

• (Same rotation as the poles).

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?

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!

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.

Reactor Start AC Motor

The Centrifugal Switch

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)

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 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

Types of Motor Enclosures

• ODP – Open Drip Proof

– Air flows through motor (fan blades help flow)– Used in environments free from contaminants

Types of Motor Enclosures

• TENV – Totally Enclosed Non-Ventilating

– Protect motor from corrosive and harmful

elements

– Frame fins help to dissipate heat

Types of Motor Enclosures

• TEFC – Totally enclosed Fan Cooled

– Similar to TENV except has external fan for cooling

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

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

AC Motor Data Plate

• Each motor has a plate mounted on its frame,

with electrical and mechanical information.

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