Three Phase Induction Motors Questions and Answers

Answer: A

Correct answer: the relative motion between field and rotor decreases

The rotor frequency is \( sf \), and the slip falls as the rotor catches up with the field. On a 50 Hz supply the rotor current is at 50 Hz at standstill and at only two or three hertz at full load, which is why the rotor iron loss is negligible in normal running.

Answer: B

At unity slip only the leakage impedance limits the current, and the resulting surge is what starters and converters exist to reduce. It is also why the supply has to be checked for the voltage dip such a start produces.

Answer: C

Both the flux and the rotor current follow the voltage, and torque follows their product. That square law is the whole difficulty with reduced-voltage starting, and it is why a load with a high breakaway requirement cannot be started that way.

Answer: D

Correct answer: four poles

A nameplate speed is the full-load speed, a few per cent below synchronous, so the synchronous speed here must be 1,500 rpm and \[ P = \frac{120 \times 50}{1500} = 4. \] The slip is then four per cent. Reading the pole number off the nameplate this way is a routine piece of arithmetic on site.

Answer: A

Correct answer: full load, a few per cent below synchronous

The nameplate quotes what the machine does at its rated output, which is a few per cent below synchronous speed. Unloaded it runs almost exactly at synchronous, so a machine marked 1,440 rpm will be found turning at something close to 1,498 on no load - which is not a fault.

Answer: B

Correct answer: the same as the direction of the revolving field

The rotor is dragged after the field, always trying to catch up and never succeeding. Reversing the phase sequence reverses the field and the rotation with it, which is the standard means of reversing an induction machine.

Answer: C

Flux is roughly voltage divided by frequency, so lowering one while holding the other drives the iron into saturation. That is precisely why a variable-frequency drive reduces the voltage in proportion to the frequency.

Answer: D

Torque falls with the square of the voltage, so the machine must slip further to carry the same load, and it draws more current in doing so. Sustained undervoltage is therefore a thermal hazard as well as a performance one.

Answer: A

Correct answer: A commutator with brushes running upon it

A commutator belongs to the dc machine, where its job is to reverse the armature connection mechanically. An induction machine needs no such thing: the stator field revolves of its own accord and the rotor current is induced rather than supplied. Even the wound rotor uses slip rings, which are continuous, not a segmented commutator.

Answer: B

Correct answer: lubrication and inspection of the bearings

With no brushes, no commutator and no rotor connections, the only wearing parts of a cage machine are its bearings, and they account for the great majority of failures in service. Beyond keeping them lubricated and the enclosure clean so the machine can cool, there is very little to do.