Lecture 6

Three-phase Induction Motor

I. Construction

(Stator slots: House three set of insulated

electrical windings to achieve a 3-phase stator)

Rotor Types

(Copper or aluminum bars embedded

into the slots, which are connected to

shorting rings at the end of the rotor)

Squirrel cage

Stator

Wound

Rotor currents are accessible at the brushes,

where extra resistance can be inserted to

modify the motor torque-speed characteristic

Wound-rotor induction motors are more expensive

they require much more maintenance because of the

wear associated with their brushes and slip rings.

II. Operation Principle(a) When the motor is excited with three-phase supply, three-phase stator winding produce a rotating magnetic field at

synchronous speed.

(b) The stator's magnetic field is therefore changing or rotating relative to the rotor. Hence, according to the principle of

Faraday’s laws of electromagnetic induction, a voltage is induced at the rotor. Thus, when the rotor is short-circuited or

closed through an external impedance, a current is induced in the induction motor's rotor.

(c) Finally, a flux will be produced in rotor due to induced rotor current forcing the rotor to rotate in the same direction

of the stator rotating magnetic field.

Since rotation at synchronous speed (speed of stator rotating field) would result in no

induced rotor current, an induction motor always operates slower than synchronous

speed. Due to this relative speed difference between the stator field and the rotor, the 3-

phase motor is called as asynchronous machine. The difference, or "slip," between actual

rotor speed and synchronous speed is given by;

III. The Electrical Frequency on the Rotor

An induction motor works by inducing voltages and currents in the rotor of the machine, and for that reason it has

sometimes been called a rotating transformer. Like a transformer, the primary (stator) induces a voltage in the

secondary (rotor), but unlike a transformer, the secondary frequency is not necessarily the same as the primary

frequency.

If the rotor of a motor is locked so that it cannot move, then the rotor will have the same frequency as the stator. On

the other hand, if the rotor turns at synchronous speed, the frequency on the rotor will be zero.

where, fre and fse are the rotor and stator electrical frequencies respectively

Since, and Hence,

IV. Equivalent Circuit Per Phase

An induction motor relies for its operation on the induction of voltages and currents in its rotor circuit from the stator

circuit (transformer action). Hence, the equivalent circuit of an induction motor will tum out to be very similar to the

equivalent circuit of a transformer

,

The largest induction occurs when the rotor is stationary, called the locked-rotor or blocked-rotor condition, so the largest voltage and

rotor frequency are induced in the rotor at that condition. The smallest voltage (0 V) and frequency (0 Hz) occur when the rotor

moves at the same speed as the stator magnetic field, resulting in no relative motion. The magnitude and frequency of the voltage

induced in the rotor at any speed between these extremes is directly proportional to the slip of the rotor. Hence,

• The frequency of the induced voltage at any slip will be given by

• The magnitude of the rotor induced voltage at any slip (ER ) will be given by

where ERO is the induced rotor voltage at locked-rotor conditions

The final per-phase equivalent circuit of the induction motor referred to the stator

It is possible to make measurements that will directly give the rotor resistance and reactance referred to the stator

(R2 and X2 ), even though are not known separately

OR

V. Power Flow

(hysteresis

and eddy

currents

losses)

Power transferred to

the rotor across the

air gap between the

stator and rotorDeveloped Power

Input Electrical Power

Output Mechanical Power

VI. Torque-speed charactersitics

S = 1 S = 0

The induced torque of the motor is zero at

synchronous speed since no current will be

induced in the rotor in this case

The starting torque on the

motor is slightly larger than

its full -load torque since

the current induced in the

rotor is maximum at the

case of zero rotor speed.

• Varying the synchronous speed, which is the speed of the stator and rotor magnetic fields.

The synchronous speed of the machine can be varied by;

1. Changing the electrical frequency

By using variable frequency control induction motor drive. Input power can be either single-phase or three-phase,

either 50 or 60 Hz, and the output from this drive is a three-phase set of voltages whose frequency can be varied from

0 up to 120 Hz.

2. Changing the number of poles on the machine

The number of poles in the stator windings of an induction motor can easily be changed by a factor of 2: 1 with only

simple changes in coil connections

• Varying the slip of the motor for a given load

Slip control may be accomplished by 1. Varying the terminal voltage of the motor.

Motor speed may be controlled over a limited range by varying the line voltage.

2. Varying the rotor resistance

In wound-rotor induction motors, it is possible to change the shape of the torque-speed curve by inserting extra

resistances into the rotor circuit of the machine.

VII. Speed Control of induction motor

There are two techniques by which the speed of an induction motor can be controlled

Due to the robust construction and ease of control, three-phase induction motors are widely preferred

over many other motors in AC industrial drives. This motor is suitable for large load operations in several

applications like lifts, conveyer belts, drilling machine, compressors, pumps, ventilation systems,

blowers, industrial fans, etc.

VIII. Applications

• Discuss construction of three-phase induction motor

• Discuss the theory of operation of three-phase induction motor

• Draw the equivalent circuit of three-phase induction motor

• Draw power flow diagram of three-phase induction motor

• Discuss the torque-speed characteristics of three-phase induction motor

• Show how to control the speed of three-phase induction motor

• State some application of three-phase induction motor