Synchronous Motor Drives Questions and Answers

Answer: A

Correct answer: near synchronous speed with the poles favourably placed

The machine is accelerated as an induction motor to just below synchronous speed, and the field is then applied at the moment the rotor poles are approaching alignment with the stator field, so that the machine is pulled into step with the smallest disturbance. Applying it at standstill or at half speed produces alternating torques and no pull-in, and above synchronous speed the machine would be generating rather than motoring.

Answer: B

Whether the machine is wound field or magnet, position-fed inverter control is the same idea, and the waveform decides whether it is called brushless dc or a sinusoidal synchronous drive. The family is wider than the names suggest.

Answer: C

Correct answer: a higher efficiency and a controllable power factor

At several megawatts these two advantages are worth a great deal. The synchronous machine has no rotor copper loss from induced currents, so its efficiency is a point or two better, which is a large sum in continuous service; and over-excited it supplies reactive power, correcting the power factor of the whole installation. It costs more, not less, and it cannot start against load without help.

Answer: D

Any slip at all accumulates into a positional error over a long run, which a synchronous machine avoids by construction and a closed-loop position drive avoids by feedback. Slip-dependent methods cannot hold such a ratio.

Answer: A

Correct answer: an induction motor

Both machines have a three-phase distributed winding in a slotted laminated stator producing a rotating field, and the same stator can serve either. All the difference lies in the rotor: a squirrel cage or wound rotor in which current is induced, against a rotor carrying its own direct current field or magnets. This is why the two machines share so much of their manufacture and why the same frame sizes are used.

Answer: B

A smooth rotor withstands the centrifugal stress of two- or four-pole operation, which salient poles would not. Slow many-poled machines are built with salient poles, which are cheaper and give some reluctance torque as well.

Answer: C

The difference between the direct and quadrature reactances gives a torque that tries to align the low-reluctance axis with the field. It is the basis of the synchronous reluctance machine, which dispenses with excitation entirely.

Answer: D

Correct answer: constant power with weakening field

Below base speed the voltage is raised with the frequency to hold the flux constant, so the machine can develop rated torque throughout. At base speed the converter voltage reaches its ceiling and cannot rise further, so beyond it the excitation is reduced - field weakening - and the flux falls as \( 1/f \). Torque falls in proportion while the product of torque and speed stays roughly constant, giving the constant power region.

Answer: A

Correct answer: a converter capable of four-quadrant operation

Rapid reversal requires torque in both directions at speeds of both signs, and it requires the kinetic energy of the load to be taken out of the machine during deceleration - that is, operation in all four quadrants of the torque-speed plane, with power flowing back to the supply or into a braking resistor. A converter able only to motor in one direction would have to coast to a stop, which is precisely what a fast reversing drive cannot afford.

Answer: B

Correct answer: The synchronous reluctance machine

The synchronous reluctance rotor is simply a shaped and slotted lamination stack with no winding, no magnets and no cage, its geometry giving a low reluctance along one axis and a high one along the other. Torque arises from the rotor’s tendency to align its low-reluctance axis with the stator field. The result is a cheap, robust rotor with no rotor loss, at the cost of a lower power factor than a magnet machine.