DC Machine Applications - GATE EE Electrical Machines Free MCQ Test

The control of dc machines is very much simplified when compared to other machines. So this makes it very useful when compared.

Separately excited dc generators are used in Ward leonard system of speed control.

Commutation is the process which reduces the induced emf and so the torque.

Hoists, cranes require large starting torque which can be provided by dc series motor.

 It is compound machine whose flux can varied higher as well as lower in order to have variable torques.

The dc shunt motor provides a medium values of the torque.

 Centrifugal pumps, fans-blowers use shunt as well as induction motor.

 It is usually over compounded so that degree of compounding can be adjusted by diverters across series field.

A differential compound motor has series and shunt field windings connected so that their fluxes oppose each other.

As load increases, the series field flux tends to increase and opposes the shunt flux; the net flux thus decreases and the motor speed can rise sharply, producing unstable and potentially dangerous behaviour.

Because of this instability, differential compound motors are rarely used in practical applications.

Hoists, cranes and drilling machines require high starting torque and controlled (or decreasing) speed with load; these are typically driven by DC series motors or cumulative (aiding) compound motors rather than differential compound motors.

Therefore, none of the listed applications is appropriate for a differential compound motor. Correct option: D.

Most commercial compound dc generator are normally supplied by manufacturers as over compounded.

PMDC motors have field fixed on the stator and rotor has coils.

The sparking will occur at the last stud not all the studs of the starter.

 Interpoles do not reduce the flux across the windings, hence the speed does not rise above base speed.

When Ra is maximum and Rf minimum then the current to the armature is reduced. And hence the sparkings are reduced while starting.

Dc generator if has displaced by 120 degrees physically then it will be act as ac machine.