Question

With reference to a 3-phase AC cage motor driven cargo winch:

(a) Sketch a circuit diagram for a pole change motor

(b) Describe how speed change and braking are achieved

Answer 1

Part (a)

Part (b)

The synchronous speed of an induction motor is governed by the formula:

$$N_{s}=\frac{120f}{P}$$

Where,

Ns = Synchronous speed.

f = Frequency of power supply.

P = number of poles.

Methods to Achieve Speed Change:

Multiple Stator Windings:

Two sets of windings are installed on the stator, each designed for different pole numbers. Only one winding is energized at a time, allowing a change in speed.

Consequent Pole Method:

A single stator winding is divided into coil groups. By altering the connections (series or parallel), the number of poles is changed, resulting in different speeds.

Pole Amplitude Modulation (PAM):

Used when a speed ratio other than 2: 1 is required. The winding is split into parts that can be connected in series or parallel. The current direction in specific parts of the winding determines the pole configuration, allowing finer speed adjustments.

Braking Mechanism:

Braking is used to reduce the torque and stop the motor.

Plugging:

Plugging is a braking method where the power supply to the motor is switched over in a way that two phases are interchanged. This creates a reverse torque that quickly reduces the motor speed. Once the speed becomes negligible, the power is switched off to prevent the motor from running in the opposite direction. An electromagnetic brake is then applied to stop the motor.

Rheostatic Braking:

In this method, the motor is switched off, and all three phases are shorted through rheostats. The rheostats act as resistors, dissipating the kinetic energy of the motor in the form of heat through copper losses. The resistance provided by the rheostats slows down the motor and brings it to a stop.

Regenerative Braking:

For regenerative braking, the motor is switched off from the A.C. power supply, and the stator winding is provided with a D.C. supply from batteries. The fixed magnetic flux of the D.C. tries to create a magnetic locking with the rotating rotor poles, generating a retarding torque that reduces the motor speed. When the speed drops to zero, the D.C. supply is switched off, and an electromagnetic brake is applied to stop the motor.

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