DC Generators Questions and Answers

Answer: C

Correct answer: the electrical output plus all the machine’s losses

The prime mover supplies the whole mechanical input, which must cover the electrical output delivered to the load together with the iron loss, the friction and windage, the armature and field copper losses and the brush contact loss. The ratio of output to that input is the efficiency. It is a useful check on any efficiency calculation that the input must always exceed the output.

Answer: D

Correct answer: the armature resistive drop and armature reaction

Two effects combine. The load current flowing through the armature resistance produces the drop \( I_aR_a \), which subtracts directly from the generated emf; and the magnetomotive force of that same current distorts and weakens the main field, reducing the emf itself. In a shunt machine there is a third effect, since the falling terminal voltage reduces the field current and weakens the flux further. The prime mover speed and the flux do not rise with load, and the brush drop is very nearly constant.

Answer: A

Poor contact raises the drop and concentrates the current into a smaller area, which sparks and damages the commutator surface. Regular inspection of brush length, spring pressure and commutator condition is the main maintenance task on such a machine.

Answer: B

Correct answer: an open-circuited or high-resistance armature coil

A single blackened or burnt segment is the classic sign of a fault in the coil connected to it. If that coil is open-circuited or has a bad joint, the current cannot pass through it and arcs across the gap as the brush leaves the segment, burning it locally. Uniform blackening across all segments points instead to brush grade, spring pressure or atmospheric conditions, and a regular pattern of marked segments to a fault repeating once per pole pitch.

Answer: C

Correct answer: a reverse current relay or a blocking arrangement

If the generator’s voltage falls below that of the battery - because its speed drops or its prime mover stops - the battery will drive current back into it and run it as a motor, which may damage the machine or the prime mover. A reverse current relay senses the direction of current and trips the circuit breaker, and in modern practice a blocking diode achieves the same end passively. The equalizer connection addresses a different problem, the sharing of load between compound machines.

Answer: D

Relative motion between conductor and flux is what induces the emf, and rotating the conductors through a stationary field is the simplest arrangement to commutate. An ac machine reverses that choice, putting the armature on the stator.

Answer: A

Correct answer: 50 Hz

A conductor passes under one north and one south pole per pair of poles, so the emf in it reverses \( P/2 \) times per revolution:

The emf in the conductor is therefore alternating; it is the commutator that rectifies it mechanically so that the brushes see a unidirectional voltage. This is also the frequency at which the armature iron is magnetised, and so the frequency that determines its hysteresis and eddy current loss.

Answer: B

Correct answer: 20 kW

The output is the product of terminal voltage and load current:

This is the useful electrical output; the mechanical input from the prime mover is larger by the sum of the copper, iron, friction and windage losses, and the ratio of the two is the efficiency.

Answer: C

Varying the resistance in the field circuit moves the field resistance line and therefore the intersection with the magnetisation curve, which sets the voltage. It is the operator's principal control on such a machine.

Answer: D

A few amperes in the field can be adjusted with a small and cheap rheostat, whereas controlling hundreds of amperes in the armature would waste a great deal of power. That asymmetry is why field control is the preferred method throughout dc machine practice.