Special Machines Questions and Answers

Answer: A

Correct answer: increasing the number of rotor teeth or stator poles

\[ \theta = \frac{360}{\mathrm{steps\ per\ revolution}}, \] and the step count is fixed by the geometry - the number of rotor teeth and stator phases. Voltage, current and pulse frequency affect torque and speed but leave the step angle untouched; microstepping is the only electronic way to subdivide it.

Answer: B

Each phase has only a fraction of a millisecond to establish its current, and the inductance limits how far it gets. Chopper drivers with a high supply voltage and current regulation are what recover the high-speed torque.

Answer: C

Correct answer: force the current to rise quickly and then regulate it

The winding’s inductance limits the rate at which current rises, and at rated voltage the current cannot reach its working value before the next step is due. A chopper applies a much higher voltage to establish the current rapidly, then switches rapidly to hold it at the set level - which is what extends the machine’s usable torque to high stepping rates.

Answer: D

A reluctance motor is the best choice for signalling and timing devices. It runs at constant synchronous speed, has a simple rotor with no winding, and does not need DC excitation, which suits clocks, timers and signalling.

A DC series motor and a DC compound motor change speed with load and are used where high torque is needed, and a synchronous motor with DC excitation is used for larger power. So the fourth option is correct.