Single phase Motor Working principle & MCQ

SINGLE PHASE MOTOR Working Principle

Single phase motors are very widely used in home, offices, workshops etc. as power delivered to most of the houses and offices is single phase. In addition to this, single phase motors are reliable, cheap in cost,simple in construction and easy to repair.

Single phase electric motors can be classified as:
Single phase induction motor (Split phase, Capacito and shaded pole etc)
Single phase synchronous motor
Repulsion motor etc.
 This article explains the basic construction and working of single phase induction motor.

Single Phase Induction Motor

Construction of a single phase induction motor is similar to the construction of three phase induction motor having squirrel cage rotor, except that the stator is wound for single phase supply. Stator is also provided with a 'starting winding' which is used only for starting purpose. This can be understood from the schematic of single phase induction motor at the left.
Working Principle Of Single Phase Induction Motor

When the stator of a single phase motor is fed with single phase supply, it produces alternating flux in the stator winding.  The alternating current flowing through stator winding causes induced current in the rotor bars (of the squirrel cage rotor ) according to Faraday's law of electromagnetic induction. This induced current in the rotor will also produce alternating flux. Even after both alternating fluxes are set up, the motor fails to start (the reason is explained below). However, if the rotor is given a initial start by external force in either direction, then motor accelerates to its final speed and keeps running with its rated speed. This behavior of a single phase motor can be explained by double-field revolving theory.
Double-Field Revolving Theory

The double-field revolving theory states that, any alternating quantity (here, alternating flux) can be resolved into two components having magnitude half of the maximum magnitude of the alternating quantity, and both these components rotating in opposite direction.
Following figures will help you understanding the double field revolving theory.


Why Single Phase Induction Motor Is Not Self Starting?

The stator of a single phase induction motor is wound with single phase winding. When the stator is fed with a single phase supply, it produces alternating flux (which alternates along one space axis only). Alternating flux acting on a squirrel cage rotor can not produce rotation, only revolving flux can. That is why a single phase induction motor is not self starting.
How To Make Single Phase Induction Motor Self Starting?

As explained above, single phase induction motor is not self-starting. To make it self-starting, it can be temporarily converted into a two-phase motor while starting. This can be achieved by introducing an additional 'starting winding' also called as auxillary winding.
Hence, stator of a single phase motor has two windings:
(i) Main winding and
(ii) Starting winding (auxillary winding).
 These two windings are connected in parallel across a single phase supply and are spaced 90 electrical degrees apart. Phase difference of 90 degree can be achieved by connecting a capacitor in series with the starting winding.
Hence the motor behaves like a two-phase motor and the stator produces revolving magnetic field which causes rotor to run. Once motor gathers speed, say upto 80 or 90% of its normal speed, the starting winding gets disconnected form the circuit by means of a centrifugal switch, and the motor runs only on main winding.

1. In a split phase motor, the running winding should have
(a) high resistance and low inductance
(b) low resistance and high inductance✔
(c) high resistance as well as high inductance
(d) low resistance as well as low inductiance


2. If the capacitor of a single-phase motor is short-circuited
(a) the motor will not start✔
(b) the motor will run
(c) the motor will run in reverse direction
(d) the motor will run in the same direction at reduced r.p.m.


3. In capacitor start single-phase motors
(a) current in the starting winding leads the voltage✔
(b) current in the starting winding lags the voltage
(c) current in the starting winding is in phase with voltage in running winding
(d) none of the above


4. In a capacitor start and run motors the function of the running capacitor in series with the auxiliary winding is to
(a) improve power factor✔
(b) increase overload capacity
(c) reduce fluctuations in torque
(d) to improve torque


5. In a capacitor start motor, the phase displacement between starting and running winding can be nearly
(a) 10°
(b) 30°
(c) 60°
(d) 90°✔

6. In a split phase motor
(a) the starting winding is connected through a centrifugal switch✔
(b) the running winding is connected through a centrifugal switch
(c) both starting and running windings are connected through a centrifugal switch
(d) centrifugal switch is used to control supply voltage


7. The rotor developed by a single-phase motor at starting is
(a) more than i.he rated torque
(b) rated torque
(c) less than the rated torque
(d) zero✔


8. Which of the following motor will give relatively high starting torque ?
(a) Capacitor start motor✔
(b) Capacitor run motor
(c) Split phase motor
(d) Shaded pole motor


9. Which of the following motor will have relatively higher power factor ?
(a) Capacitor run motor✔
(b) Shaded pole motor
(c) Capacitor start motor
(d) Split phase motor


10. In a shaded pole motor, the shading coil usually consist of
(a) a single turn of heavy wire which is in parallel with running winding
(b) a single turn of heavy copper wire which is short-circuited and carries only induced current✔
(c) a multilayer fine gauge copper wire in parallel with running winding
(d) none of the above


11. In a shaded pole single-phase motor, the revolving field is produced by the use of
(a) inductor
(b) capacitor
(c) resistor
(d) shading coils✔


12. A centrifugal switch is used to dis- connect ‘starting winding when motor has
(a) run for about 1 minute
(b) run for about 5 minutes
(c) picked up about 50 to 70 per cent of rated speed✔
(d) picked up about 10 to 25 per cent of rated speed


13. If a particular application needs high speed and high starting torque, then which of the following motor will be preferred ?
(a) Universal motor✔
(b) Shaded pole type motor
(c) Capacitor start motor
(d) Capacitor start and run motor


14. The value of starting capacitor of a fractional horse power motor will be
(a) 100 uF
(6) 200 uF
(c) 300 uF✔
(d) 400 uF


15. In repulsion motor direction of rotation of motor
(a) is opposite to that of brush shift
(b) is the same as that of brush shift✔
(c) is independent of brush shift

16. In a single phase motor the centrifugal switch
(a) disconnects auxiliary winding of the motor✔
(b) disconnects main winding of the motor
(c) reconnects the main winding the motor
(d) reconnects the auxiliary winding of the motor


17. The running winding of a single phase motor on testing with meggar is found to be ground. Most probable location of the ground will be
(a) at the end connections
(b) at the end terminals
(c) anywhere on the winding inside a slot
(d) at the slot edge where coil enters or comes out of the slot✔


18. A capacitor-start single phase induction motor is switched on to supply with its capacitor replaced by an inductor of equivalent reactance value. It will
(a) start and then stop
(b) start and run slowly
(c) start and run at rated
(d) not start at all✔


19. Which of the following motors is used in mixies ?
(a) Repulsion motor
(b) Reluctance motor
(c) Hysteresis motor
(d) Universal motor✔


20. Which of the following motors is inherently self starting ?
(a) Split motor
(b) Shaded-pole motor✔
(c) Reluctance motor
(d) None of these


21. The direction of rotation of an hysteresis motor is determined by
(a) interchanging the supply leads
(b) position of shaded pole with respect to main pole✔
(c) retentivity of the rotor material
(d) none of these


22. Burning out of winding is due to
(a) short circuited capacitor✔
(b) capacitor value hiving changed
(c) open circuiting of capacitor
(d) none of the above


23. Direction of rotation of a split phase motor can be reversed by reversing the connection of
(a) running winding only
(b) starting winding only
(c) either (a) or (b)✔
(d) both (a) and (b)


24. Short-circuiter is used in
(a) repulsion induction motor
(b) repulsion motor
(c) repulsion start induction run motor✔
(d) none of the above


25. The range of efficiency for shaded pole motors is
(a) 95% to 99%
(b) 80% to 90%
(c) 50% to 75%
(d) 5% to 35%✔


26. In a capacitor start single-phase motor, when capacitor is replaced by a resistance
(a) torque will increase
(b) the motor will consume less power
(c) motor will run in reverse direction
(d) motor will continue to run in same direction✔


27. The power factor of a single-phase induction motor is usually
(a) lagging✔
(b) always leading
(c) unity
(d) unity to 0.8 leading


28. A shaded pole motor can be used for
(a) toys
(b) hair dryers
(c) circulators
(d) any of the above✔


29. A hysteresis motor works on the principle of
(a) hysteresis loss✔
(b) magnetisation of rotor
(c) eddy current loss
(d) electromagnetic induction


30. Which of the following motor will give the highest starting torque ?
(a) D.C. shunt motor
(b) Schrage motor✔
(c) Repulsion start and induction run motor
(d) Universal motor


31. For which of the applications a reluctance motor is preferred ?
(a) Electric shavers
(b) Refrigerators
(c) Signalling and timing devices
(d) Lifts and hoists
Ans: c

32. The motor used on small lathes is usually
(a) universal motor
(b) D.C. shunt motor
(c) single-phase capacitor run motor✔
(d) 3-phase synchronous motor


33. Which of the following motors is preferred for tape-recorders ?
(a) Shaded pole motor
(b) Hysteresis motor✔
(c) Two value capacitor motor
(d) Universal motor

34. A single-phase induction motor is
(a) inherently self-starting with high torque
(b) inherently self-starting with low torque
(c) inherently non-self-starting with low torque✔
(d) inherently non-self-starting with high torque


35. A schrage motor can run on
(a) zero slip
(b) negative slip
(c) positive slip
(d) all of the above✔


36. A universal motor can run on
(a) A.C. only
(6) D.C. only
(c) either A.C. or D.C.✔
(d) none of the above

37. Which of the following single-phase motors is suitable for timing and control purposes ?
(a) Reluctance motor✔
(b) Series motor
(c) Repulsion motor
(d) Universal motor


38. Single phase induction motor usually operates on
(a) 0.6 power factor lagging✔
(b) 0.8 power factor lagging
(c) 0.8 power factor leading
(d) unity power factor


39. In split-phase motor auxiliary winding is of
(a) thick wire placed at the bottom of the slots
(b) thick wire placed at the top of the slots
(c) thin wire placed at the top of the slots
(d) thin wire placed at the bottom of the slots