Synchronous Motor Drives Questions and Answers

Answer: A

Because the inverter is commanded from where the rotor actually is, the machine cannot fall out of step however the load or the reference changes. The arrangement behaves much like a dc machine with electronic commutation.

Answer: B

Correct answer: takes an open-loop frequency reference

In true synchronous mode the inverter frequency is commanded independently, and the rotor is expected to follow, exactly as if the machine were connected to a supply of that frequency. The arrangement is simple but retains the fixed-supply weakness: a sudden load change can pull the rotor out of step. Self-controlled mode removes that by measuring rotor position and commutating the inverter from it, so the machine cannot lose synchronism.

Answer: C

A single frequency reference sets the speed of every machine, so the ratio between them is exact without any mechanical connection. Textile and paper machines used the arrangement long before electronic shaft schemes existed.

Answer: D

Running the machine at a leading power factor makes its emf turn the thyristors off, so no forced commutation circuit is needed. That simplicity suits very large drives, though the machine cannot commutate the inverter at standstill.

Answer: A

Correct answer: the machine’s emf is then too small to commutate the thyristors

A load commutated inverter relies on the motor’s own back emf to reverse-bias the outgoing thyristor and turn it off, and that emf is proportional to speed. Below roughly ten per cent of rated speed it is too small to do the job, so the drive starts in a pulsed dc link mode, interrupting the link current to force commutation until the machine is fast enough to commutate naturally.

Answer: B

Constructing the output directly from the supply gives good waveforms only well below supply frequency, which suits mine hoists, ball mills and ship propulsion. Above about a third of supply frequency the distortion becomes unacceptable.

Answer: C

Correct answer: one third of the supply frequency

A cycloconverter synthesises its output by selecting segments of the incoming supply waveform, so it needs several segments per output cycle to produce an acceptable shape. In practice this limits the output to about a third of the input frequency - roughly 16 Hz from a 50 Hz supply. That confines it to very low-speed drives, but there it excels: large mill and mine hoist drives use it precisely because they need low speed and very high torque.

Answer: D

The magnets fix the flux, which makes the machine compact and efficient but removes power factor control and complicates field weakening. Weakening is still possible by driving current on the direct axis to oppose the magnets.

Answer: A

Since the magnet flux cannot be turned down, the only route is a stator current component that opposes it, which costs current without producing torque. The available speed range above base speed is therefore more limited than in a wound field machine.

Answer: B

A trapezoidal back emf commutated in six steps from Hall sensors is simple and cheap, while a sinusoidal machine with continuous position feedback gives smoother torque. Both are synchronous machines with magnet rotors.