DC Motors Questions and Answers

Answer: C

Correct answer: the square of the armature current

In a series machine the field carries the armature current, so below saturation \( \Phi \propto I_a \), and

This square law is what gives the series motor its enormous starting torque: doubling the current quadruples the torque, which is precisely what a tram or a crane needs when starting under load.

Answer: D

Correct answer: the armature current, in nearly direct proportion

Once the iron saturates the flux can grow no further, so \( \Phi \) becomes effectively constant and

The characteristic therefore straightens out at high currents, and the series motor behaves much like a shunt machine there. The square law applies only in the unsaturated region, which is where the machine works at starting.

Answer: A

With little current there is little flux, and the speed must rise enormously before the back emf can balance the supply. Such machines are geared or coupled directly to their load rather than driven through a belt that could break.

Answer: B

Correct answer: nearly flat, with a small droop

The shunt field is across the supply, so the flux is nearly constant and the speed falls only by the armature resistance drop as load is applied - typically a few per cent from no load to full load. That is why the shunt motor is the approximately constant-speed dc machine, suited to lathes, fans and pumps. The steeply falling hyperbola belongs to the series motor.

Answer: C

Correct answer: between those of the shunt and the series machine

A cumulative compound machine has a shunt field that fixes the no-load speed and a series field that strengthens the flux as load comes on. It therefore has a better starting torque than a shunt machine and a greater speed droop, but it will not run away on no load as a series machine does, because the shunt field holds the flux up. The result is a characteristic lying between the two, which suits presses, shears and rolling mills.

Answer: D

Correct answer: its speed rises with load and it can become unstable

In a differential machine the series field opposes the shunt field, so as load current rises the net flux falls and the speed \( \propto E/\Phi \) rises. A rising speed with load is unstable in most drives, and at heavy load the series field may overpower the shunt field and reverse the flux, reversing the direction of rotation. Its only real application is where a nearly constant speed is wanted over a narrow range.

Answer: A

Carrying the field current, the coil releases the starting arm if the field circuit opens or the supply fails, which protects against a runaway and against an unattended restart. Its drawback is that a field regulator in the same circuit weakens the hold.

Answer: B

With the coil on its own branch, the field regulator can be set for a weak field without the hold failing. The price is that a break in the field circuit no longer releases the arm, so a separate field failure relay is needed.

Answer: C

Losing the supply releases the arm, so the starting resistance is back in circuit when power returns and the machine cannot restart without an operator. It also releases if the field circuit fails on a three-point starter.

Answer: D

Correct answer: short-circuiting the no-volt coil when the current is excessive

The overload coil is an electromagnet carrying the load current; on overload it pulls up an armature that short-circuits the no-volt coil. Deprived of its current, the no-volt coil releases the starter arm, which springs back to the off position and disconnects the motor while reinserting the full starting resistance. The two coils thus work together, one sensing loss of supply and the other excessive current.