Single Phase Induction Motors Questions and Answers

Answer: D

A universal motor cannot be used as a power factor corrector. A universal motor is a series motor that runs on AC or DC and gives high speed and high starting torque, which suits a food mixer, a hand drill and a hair drier.

A power factor corrector needs a machine that can draw leading current, such as an over-excited synchronous motor (a synchronous condenser), and a series motor cannot do that. So the fourth option is correct.

Answer: B

Frequency control cannot be used for the speed control of a universal motor. The speed of a series motor depends on the voltage applied and the field strength, not on the supply frequency, since it has a commutator and is also run on DC.

Field flux control (by tapping the field winding), variable resistance control in series with the motor and centrifugal governor control are the methods in use. So the second option is the answer.

Answer: B

The backward slip is 1.9. For a single phase induction motor, the slip with respect to the backward rotating field is 2 minus the forward slip.

\[ s_b = 2 - s_f = 2 - 0.1 = 1.9 \]

The value 0.1 is the forward slip itself, 0.9 would be 1 minus the slip, and 2.1 would be 2 plus the slip. The backward field rotates in the opposite direction, so a rotor turning at 0.9 of synchronous speed forwards is seen by it as moving at 1.9 times the relative speed. So the second option is the answer.

Answer: A

The best suited motor is the capacitor-start motor.

A compressor must start against pressure, so it needs a high starting torque. A capacitor-start induction motor, with a capacitor in its starting winding giving nearly 90° between the winding currents, develops a starting torque of three to four times full-load torque and runs at a nearly constant speed, which suits compressors, pumps and refrigerators in the fractional to one horsepower range.

Resistance split-phase motors have only moderate starting torque, shaded-pole motors very low starting torque, and single-phase series motors run at a speed that varies widely with load.

Answer: D

The current in the main winding is 27 A.

At starting the main winding is connected directly across the 230 V supply, so its current is the voltage divided by its impedance.

\[ \begin{aligned} Z_{m} &= \sqrt{4^{2} + 7.5^{2}} = \sqrt{72.25} \\ &= 8.5\ \Omega \\ I_{m} &= \frac{230}{8.5} = 27.06\ \text{A} \end{aligned} \]

The starting winding has the same impedance (8.5 Ω), so it also draws 27 A, but with a different phase angle; the line current is their phasor sum, which is less than twice that. 11.5 A, 22 A and 9 A do not follow from the winding impedance.

Answer: D

The largest starting torque is that of the repulsion motor.

A repulsion motor has a commutator armature whose brushes are short-circuited and displaced from the field axis. It develops a very high starting torque, about three to four times full-load torque or more, with moderate starting current, which is why repulsion and repulsion-start motors were used for heavy single-phase drives.

Capacitor-start motors have high starting torque but generally below that of the repulsion motor, split-phase motors have moderate starting torque, and shaded-pole motors very low starting torque, suitable only for small fans.