AC motors - 02

Modern cargo winches driven by AC motors must be able to meet high cargo handling rates involving frequent starting and stopping.

(a)                Briefly describe how this is accomplished without incurring excessive losses.   (5m)

(b)        Briefly describe the run up operation of such a motor.                                         (5m)

Answer :-

(a)                AC motor suitable for cargo winches to meet high cargo handling rates and involving frequent starting and stopping would be a 3 speed AC cage motor.

It is a specially design motor with dual starter. This would minimise the starting and accelerating losses which are responsible for a large proportion of the overall losses and heat generated.

 

                                                                                     1     1     1     2     2      2       3     3      3

                  Reversing connection                                                     333 sped

                                                                                                         3 spped

 

                                                                                                        3 speed motor       

                                                         brake circuit      

            The 3 speed motor is connected as drawn. It consist of :

a)      a low speed starter with 24 poles wound controlled by contactor.

b)      A medium / high speed starter with 2 independent windings, are wound for 8 poles controlled by contactor. High speed contactor with 4 poles controlled by contactor.

c)      All windings connected and fed y reversing contactor.

(b)               Procedure of running up the motor :

1)      low speed contactor cosed, forward or reversed contactors  are on if energised. Motor will run at low speed.

2)      Electro magnetic brake simultaneously released by another contactor.

As controller is moved to step 2 and 3, motor accelerate first to medium and then to high speed.