DC Generators Questions and Answers

Answer: A

A small residual flux generates a small emf, which drives a small field current, which increases the flux, and the process runs up the magnetisation curve until it meets the field resistance line. Without residual magnetism nothing starts.

Answer: B

Correct answer: the intersection of the magnetisation curve and the field resistance line

Two conditions must be satisfied at once: the emf must be that which the magnetisation curve gives for the field current flowing, and the field current must be that which the field resistance passes at the emf generated. Build-up therefore continues until the two agree, that is until the straight field resistance line crosses the magnetisation curve, and the machine settles there. Residual magnetism only starts the process; the speed and the field resistance determine where it ends.

Answer: C

Correct answer: the field resistance line becomes tangent to the magnetisation curve

The field resistance line is a straight line through the origin whose slope is the field circuit resistance. As the resistance is increased the line steepens, and at the critical value it just touches the initial straight part of the magnetisation curve. Below that value the line cuts the curve and the machine builds up; above it there is no intersection except the origin, and the machine fails to excite. There is a corresponding critical speed, since lowering the speed depresses the magnetisation curve to meet a given resistance line.

Answer: D

Since the magnetisation curve scales with speed, reducing the speed lowers it until it no longer crosses the resistance line. Below the critical speed the machine will not build up whatever the field resistance is set to.

Answer: A

Correct answer: lost its residual magnetism

Without residual flux there is no initial emf, so no field current flows, so no flux is produced - the process has nothing to start it. The cure is to flash the field from a battery, restoring the residual magnetism. The other common causes of failure to build up are a field resistance above critical, a speed below critical, and field connections reversed so that the initial current destroys the residual flux instead of reinforcing it.

Answer: B

If the field current opposes the residual flux, the machine drives itself towards zero rather than towards voltage, and swapping the leads fixes it. Getting it wrong the other way round destroys the residual magnetism and calls for the field to be flashed.

Answer: C

Correct answer: momentarily exciting it from a battery to restore residual magnetism

Flashing the field passes a direct current from a battery through the field winding for a moment, magnetising the poles enough to leave residual flux behind. The machine can then generate the small initial emf on which self-excitation depends. Polarity matters: the connection must magnetise the poles in the direction the machine will subsequently reinforce, or the residual flux will be destroyed rather than restored.

Answer: D

Reversing the rotation reverses the armature emf relative to the residual flux, so the field current now opposes that flux. Reversing the field leads as well restores the self-exciting condition, with the terminal polarity unchanged.

Answer: A

The armature's field sits across the main one, distorting it and shifting the magnetic neutral axis, and saturation makes the net effect slightly demagnetising. Interpoles and compensating windings exist to deal with it.

Answer: B

Distortion of the resultant field carries the neutral axis forward in a generator and backward in a motor. Brushes set on the geometric axis then commutate at the wrong instant, which is one reason interpoles are preferred to shifting the brushes.