Special Machines Questions and Answers

Answer: A

Correct answer: magnetising current larger and its power factor poorer

A linear machine must tolerate mechanical clearances along its whole length, so its gap is several millimetres rather than a fraction of one. A larger gap means a higher reluctance, so more magnetising current is needed for the same flux, and that current is almost purely reactive - hence the poor power factor characteristic of linear induction machines.

Answer: B

Correct answer: gives thrust with no contact between vehicle and track

A linear synchronous machine produces thrust across the gap by magnetic action alone, so there is no wheel, no rail contact and no adhesion limit. This is what allows very high speeds, and it fits naturally with the magnetic suspension that holds the vehicle off the track in the first place.

Answer: C

The revolving field magnetises the ring, and the lag of the magnetisation behind the field gives a torque that is constant from standstill right up to synchronous speed. That constancy is what makes such machines so smooth and quiet.

Answer: D

The hysteresis loss per cycle is a property of the material rather than of the slip, so the torque does not vary with speed. It makes the machine ideal for driving a load of high inertia smoothly up to speed, such as a tape reel or a gyroscope.

Answer: A

Correct answer: its salient rotor locks to the revolving field

A reluctance motor starts as an induction machine on its cage and then pulls into synchronism, after which the saliency of the rotor holds it locked to the revolving field. It needs no rotor excitation of any kind, which makes it simple, but the reluctance torque available limits it to modest ratings.

Answer: B

Correct answer: a far higher energy product for the same volume

Neodymium iron boron offers several times the energy product of ferrite, so the same flux comes from a much smaller magnet and the rotor is lighter and lower in inertia. That is exactly what a servo machine needs. The costs are price, a lower working temperature and a strong tendency to corrode.

Answer: C

Correct answer: cheap and resistant to corrosion

Ferrite is an oxide, so it does not rust, and it is made from abundant materials at a small fraction of the price of a rare earth magnet. Where the volume is available and the last measure of performance is not needed, those two properties decide the choice - which is why ferrite still dominates by number of machines made.

Answer: D

Correct answer: excessive temperature or an excessive opposing field

Both mechanisms drive the magnet past the knee of its demagnetisation curve, after which the loss is permanent. Heat lowers the coercivity so that a field which was safe when cool is not, which is why the two causes so often act together - a stall current in an already hot machine being the classic case.

Answer: A

Energising the transmitter's rotor induces voltages in its three stator windings that depend on its angle, and connecting them to an identical receiver makes its rotor take up the same position. It is an electrical shaft rather than a measurement.

Answer: B

Correct answer: gives an output voltage proportional to the angular error

A control transformer’s rotor is not free to follow; it stays where the servo has put it, and the voltage induced in it measures the difference between its own angle and the angle transmitted. That error voltage is the input to the servo amplifier, which drives the load until the voltage falls to zero.