DC Motor Drives Questions and Answers

Answer: C

Correct answer: cannot be varied

The flux of a permanent magnet machine is fixed by its magnets, so there is no field winding to adjust and no field weakening in the ordinary sense - speed above base can only be obtained by injecting a demagnetising armature current, which is inefficient. The compensations are real: no field copper loss, no excitation supply, higher efficiency and a smaller frame for the same output. The magnets do weaken somewhat with temperature, but reversibly, not steadily.

Answer: D

Correct answer: the losses of both the converter and the machine

The overall efficiency is the product of the two: the converter loses a little in conduction and switching, typically one to three per cent, and the machine loses in its copper, iron, brushes, friction and windage. Neither can be ignored, and a drive quoted only on machine efficiency overstates what the installation delivers. The same applies to any converter-fed drive, ac or dc.

Answer: A

The armature current sets up its own field that shifts and distorts the main one, which is why interpoles and compensating windings are fitted on larger machines. Poor commutation shows as brush sparking and shortens brush life.

Answer: B

Small poles between the main ones, carrying the armature current, cancel the reactance voltage in the coil being commutated. Their winding is in series with the armature so that the correction tracks the load automatically.

Answer: C

Switching at hundreds or thousands of hertz gives far less ripple than switching at supply frequency, which reduces the extra heating in the machine. The chopper needs a dc source, which is usually a rectifier and a link capacitor.

Answer: D

Correct answer: the resistance wasted a great deal of energy

Resistance control reduces the voltage reaching the armature by dissipating the surplus as heat, so at half speed roughly half the input is thrown away in the resistor bank. On a train stopping and starting constantly that is an enormous waste, and the heat itself is a problem in a confined space. Chopper control replaced it by varying the mean voltage with a switch that is either fully on or fully off, and so loses almost nothing. The series motor’s starting torque was never in question - it is what made it the traction motor of choice.

Answer: A

Wide speed range, four-quadrant capability and precise torque control were all available from a dc drive long before they were available from an ac one. Fixed-speed loads never needed anything more than a direct-connected induction machine.

Answer: B

Correct answer: an induction or synchronous motor from the ac supply

A Ward-Leonard set has an ac motor driving a dc generator at constant speed, the generator’s field being varied to control the voltage applied to the drive motor. That gives smooth, stepless, four-quadrant control with regeneration, and it was the standard high-performance drive for half a century. Its drawbacks - three machines, poor efficiency, large floor space - are what static converters removed.

Answer: C

With the generator voltage reduced below the motor's back emf, current reverses and the drive motor generates into the set, which returns the power to the ac supply through its own motor running as a generator. That capability was a large part of the system's appeal.

Answer: D

Correct answer: hold the link voltage steady against pulsating current

The rectifier delivers a pulsating current and the inverter or chopper draws current in bursts, so without a capacitor the link voltage would swing with them. The capacitor acts as a low-impedance reservoir, absorbing those pulsations and presenting a stiff voltage source to the stage that follows. Note that this same stiffness is why a diode-fed link cannot accept regenerated energy: the voltage simply rises until protection operates.