DC Motors Questions and Answers

Answer: C

Correct answer: 7.5 kW

The converted power is \( EI_a \), and the rotational losses come out of it:

The input to the armature was 8.8 kW, so the efficiency of the armature circuit is about 85 per cent, the losses being 800 W of copper and 500 W of rotational loss.

Answer: D

Correct answer: 7.5 kW

Shaft power is torque times angular velocity:

The speed must be converted from revolutions per minute to radians per second before multiplying - the 72 kW answer comes from using 1200 directly, and the 1.2 kW answer from mishandling the same conversion the other way.

Answer: A

Correct answer: traction and crane hoisting duty

Traction and hoisting need very high torque at starting and can accept, indeed prefer, a speed that falls as the load rises. The series motor gives exactly that: torque proportional to the square of the current at starting, and a steeply drooping speed characteristic that limits the power drawn on a heavy load. What it cannot do is hold a steady speed, which rules it out of the other three duties.

Answer: B

Correct answer: lathes, fans and pumps needing a steady speed

The shunt motor’s speed falls only a few per cent from no load to full load, so it approximates a constant-speed machine and suits any duty where the speed must not wander with the load. Traction and hoisting need the series machine’s starting torque, and welding needs a differential compound machine’s falling characteristic.

Answer: C

The falling speed under a load peak lets the flywheel effect of the rotating mass supply part of the surge, and the shunt winding keeps the light-load speed in bounds. It is the classic machine for a load that comes in bursts.

Answer: D

Simple armature voltage and field control gave a wide, smooth speed range long before the variable-frequency drive existed. Every other consideration favours the cage machine, which is why the dc motor has largely been displaced.

Answer: A

The magnets set the flux once and for all, so only armature voltage control is available and there is no field weakening region. In return the machine is compact, efficient and needs no field supply, which suits small servo and toy applications.

Answer: B

Torque follows the product of flux and armature current, so reversing one factor reverses the torque while reversing both leaves it unchanged. Reversing the armature is quicker because the field's inductance is far larger.

Answer: C

Correct answer: the no-load speed less the full-load speed, as a fraction of full load

Speed regulation is

and it measures how much the speed droops as load is applied. A shunt motor manages a few per cent, a series motor a great deal more. It describes the machine’s inherent behaviour, not the range over which it can be controlled, which is a separate specification.

Answer: D

Correct answer: 1 and 2 only

Statements 1 and 2 are the two governing relations, \( T = k\Phi I_a \) and \( N \propto E/\Phi \). Statement 3 is the dangerous one: a series motor on no load has almost no field current, so its flux collapses and its speed rises without limit until the armature bursts. It must always be coupled directly to its load, never through a belt that could break.