Solutions To Examination Questions In Electrical Engineering.

  Explain the effect of voltage reduction (due to supply cuts or voltage drop on consumer's wiring) on standard squirrel-cage motors with respect to:

(a) Burnouts,
(b) Starting current,
(c) Starting torque,
(d) Speed.

Answer 07:

(a) Generally speaking, induction motors seldom run fully loaded for comparatively long spells, therefore, although the current would increase for a given mechanical load unless the voltage reduction was very large, the increase in current would not be sufficient to over-heat the windings.

(b) At starting, the current would be reduced proportionately to the reduced voltage, but would not fall as rapidly to its normal running value, as it would if the voltage was kept at its correct value.

(c) As, for given values of the rotor reactance and resistance, the torque is proportional to the square of the flux, and as the flux is proportional to the voltage, it follows that the torque is reduced in proportion to the square of the voltage reduction.

(d) The speed of a squirrel-cage induction motor is approximately constant, the difference between the running speed and the synchron¬ous speed, termed the slip, being dependent for a given frequency and a fixed -number of poles upon the value of the current taken by the motor. As, for a given mechanical output, the current is increased when the voltage is reduced, the slip also is increased, so that the motor would run at a slightly slower speed, than it would when running with the same mechanical load, but at its normal voltage.

Question 08:

List the tests and checks you would consider necessary for the compilation of a report upon the condition of a 50-hp three-phase slip¬ring induction motor together with its associated control gear.