Page 97 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Construction of a 3-phase squirrel cage induction motor
- relation between slip, speed, rotor frequency, copper

loss and torque

Objectives: At the end of this lesson you shall be able to:
• describe the construction of a 3-phase, squirrel cage induction motor
• describe the construction of double squirrel cage motor and its advantage
• explain slip, speed, rotor frequency, rotor copper loss, torque and their relationship.

Three-phase induction motors are classified according to their rotor construction. Accordingly, we have two major
types.
– Squirrel cage induction motors
– Slip ring induction motors.
Squirrel cage motors have a rotor with short-circuited bars whereas slip ring motors have wound rotors having
three windings, either connected in star or delta. The terminals of the rotor windings of the slip ring motors are
brought out through slip-rings which are in contact with stationary brushes.
Stator of an induction motor: There is no difference between squirrel cage and slip-ring motor stators.

The induction motor stator resembles the stator of a revolving field, three-phase alternator. The stator or the
stationary part consists of three-phase winding held in place in the slots of a laminated steel core which is
enclosed and supported by a cast iron or a steel frame as shown in Fig 1. The phase windings are placed 120
electrical degrees apart, and may be connected in either star or delta externally, for which six leads are brought
out to a terminal box mounted on the frame of the motor. When the stator is energised from a three-phase voltage
it will produce a rotating magnetic field in the stator core.
Rotor of a squirrel cage induction motor: The rotor of the squirrel cage induction motor shown in Fig 2 contains no
windings. Instead it is a cylindrical core constructed of steel laminations with conductor bars mounted parallel to
the shaft and embedded near the surface of the rotor core. These conductor bars are short circuited by an end-
ring at either end of the rotor core. On large machines, these conductor bars and the end-rings are made up of
copper with the bars brazed or welded to the end rings as shown in Fig 3. On small machines the conductor bars
and end-rings are sometimes made of aluminium with the bars and rings cast in as part of the rotor core.

The rotor or rotating part is not connected electrically to the power supply but has voltage induced in it by
transformer action from the stator. For this reason, the stator is sometimes called the primary, and the rotor is
referred to as the secondary of the motor. Since the motor operates on the principle of induction; and as the
construction of the rotor, with the bars and end-rings resembles a squirrel cage, the name squirrel cage induction
motor is used. (Fig 3)

The rotor bars are not insulated from the rotor core because they are made of metals having less resistance than
the core. The induced current will flow mainly in them. Also, the bars are usually not quite parallel to the rotor shaft

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CITS : Power - Electrician & Wireman - Lesson 70-75 CITS : Power - Electrician & Wireman - Lesson 70-75