Question

With reference to a 3speed a.c. cage motor driven cargo winch

Sketch a circuit diagram for a pole change motor

Describe how speed change and braking are achieved

Answer.

Three speed pole changing winch motor employ dual motors in which one cage rotor operating in a stator wound for say 24 poles gives a low speed where as other cage rotor operating in a pole amplitude modulated stator wound for say 8 poles to give medium speed and 4 pole to give high speed

The low speed 24 pole stator winding has a single winding per phase and for low speed operation the stator contactor closes and provides power supply to the winding when all the contactors are kept open

For medium and high speed operation the pole amplitude modulated stator has two windings in each phase with a centre lap. One stator contactor gives supply such that phase winding are in service and 3phase winding are in delta which makes 8poles and medium speed operation

For high speed operation another two contactor comes ‘ON’, such that phase winding are in parallel and 8phase windings are in star, which makes 4poles.

For reversing operation two phase winding are interchanged by other contactors.

Braking of winch motor

For a 3 phase , 50 Hz supply the speed of induction motor at 4pole would be about 1400RPM, at 8poles 700RPM, and 24 pole about 200 RPM.

First step slow speed is used for breaking out and decking second step medium speed is used for handling heavy loads and third step high peed for normal loads

The lowering speed are slightly higher than those for lifting as the speed of an induction motor rises slightly when changing over from motoring to regeneration

For braking winch motor disc operated electromagnetic brakes are employed. The disc brake is D.C. operated through srlenium cell rectifiers and is provided with a hand release so that loads may be released under control in case of a power failure

An independent foot pedal operated brake is also provided

When switching back to the ‘OFF’ position the 24 pole step is brought in before the brake is applied so that when braking heavy loads the wear on the magnetic brake is reduced to a minimum

However for luffing and slewing operation sometimes dynamic braking is employed before magnetic brakes are used

In dynamic braking D.C. supply is injected into the stator winding just prior to switching off

This has the effect of producing a static field in the stator

Rotation of the stator induces an emf in the rotor winding and the interaction of the stator field and rotor current produces torque which opposes the rotating braking power being dissipated in the form of heat in the rotor winding and in stator from losses

The effectiveness of the method can be adjusted to the requirement of the carne by adjusting the D.C. voltage

When the crane comes to rest the magnetic brakes are applied and the D.C. fed to the motor is switched off.