DC Generators Questions and Answers

Answer: A

Correct answer: applying a mechanical brake and measuring the torque and speed

In a brake test a rope brake or dynamometer absorbs the mechanical output, and the torque is found from the net force on the brake and its radius. The output is then \( 2\pi NT/60 \), and the efficiency follows from the electrical input. It is a direct measurement rather than a computation, which is its merit; its drawbacks are that all the output is dissipated as heat, which limits it to small machines, and that the brake must be cooled.

Answer: B

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

Voltage regulation is defined as

usually expressed as a percentage. A small figure means the terminal voltage holds up well as load is applied, which is what a lighting or battery-charging load wants. A cumulatively compounded machine can be arranged to give zero or even negative regulation, its voltage rising with load to compensate for the drop in the feeder.

Answer: C

Correct answer: 248 V

The armature carries the load current and the field current:

Forgetting the field current and using 400 A gives 248 V as well to this accuracy, but the habit matters: in a shunt machine the armature always carries the sum, and in a machine with a larger field current the difference would show.

Answer: D

Correct answer: 3.3 kW

The armature copper loss uses the armature current, not the load current:

This is the variable loss, rising with the square of the current, and it is what the maximum efficiency condition sets against the constant losses.

Answer: A

Correct answer: 91 per cent

Efficiency is output over input, and the input is the output plus the losses:

Dividing the output by itself plus losses is the generator convention; for a motor the input is measured and the output is input less losses. The 90 per cent answer comes from dividing the losses by the output rather than by the input.

Answer: B

Hysteresis follows roughly the 1.6 power of flux density and the first power of frequency, while eddy current loss follows the squares of both. Thin laminations of silicon steel reduce the second and a good grade reduces the first.

Answer: C

Thin sheets separated by insulation break up the paths available to circulating currents, and the loss falls with the square of the thickness. Hysteresis loss depends on the grade of steel rather than on how it is divided.

Answer: D

Correct answer: share the load and allow one to be taken out of service

Machines are paralleled for the same reasons in any power system: the load can be divided between them, a machine can be shut down for maintenance without interrupting supply, and the plant can be run near its efficient loading by bringing sets in and out as demand varies. Paralleling does not raise the voltage - that would require a series connection - and it does nothing to armature reaction in the individual machines.

Answer: A

Correct answer: altering their field excitations

Both machines are held at the same terminal voltage by the busbar, so the current each delivers depends on its own generated emf, which is set by its excitation. Increasing the field current of one raises its emf and makes it take a larger share; reducing the other’s completes the transfer. Speed adjustment has an effect only through the emf and is far coarser, and reversing a field connection would simply cause the machine to be driven as a motor.

Answer: B

If one machine takes a little more current, its series field strengthens, its emf rises and it takes more still, which is unstable. A heavy conductor joining the two series field ends forces them to share the current and stops the runaway.