Shaded pole induction motor is constructed with salient poles in the stator. Each pole has its own exciting coil. A 1/3rd portion of each pole core is surrounded by a strap of copper forming a closed loop called the shading coil. The rotor is a usual squirrel cage type.
 

shaded pole induction motor working principle

 

Shaded Pole Induction Motor Working Principle

    shaded pole motor construction
  1. During the portion OA of the alternating-current cycle, the flux begins to increase and an EMF is induced in the shading coil. This flux induces voltage and hence current in the copper ring, and by Lenz’s law, the direction of current is such that it opposes the flux entering the coil. Hence, in the beginning, the greater portion of flux passes through the unshaded side of each pole.
  2.  

  3. During the portion AB of the alternating-current cycle, the flux has reached almost maximum value and is not changing. Consequently, the flux distribution across the pole is uniform since no current is flowing in shading coil.
  4.  

  5. As the flux decreases (portion BC of the alternating-current cycle) the current induced in shading coil now tends to prevents the flux linking with shading coil from decreasing and hence the greater portion of flux passes through the shaded side of each pole.
  6.  

  7. Thus the flux reaches its maximum value first at the shaded side of the pole and later reaches its maximum on the unshaded side of the pole. This gives a motion of flux across the pole face in the direction of arrow (in the direction of rotation of rotor) as shown in the figure above.
  8.  

  9. Under the influence of this flux small starting torque is produced. As soon as this torque starts to revolve the rotor, additional torque is produced for running because of the single phase induction motor action. The shaded pole motor accelerates to a speed slightly below synchronous speed and runs as single phase motor.

Characteristics of Shaded Pole Induction Motor

 

  • Shaded pole induction motor produces very small starting torque about 50% of full load torque.
  • Efficiency of shaded pole motor is low because of continuous power loss in shading coil.
  • Used for small fans and small appliances.
  • The direction of rotation of shaded pole motor depends upon the position of shading coil i.e. which half of pole is wrapped with shading coil. Therefore, the direction of rotation cannot be reversed unless the machine is constructed so that the shading coil can be shifted to another half of the pole.

 
Thanks for reading about shaded pole induction motor working principle & construction.
 

Single Phase Motors – 3 | Objective Type Question Answers

 

#1 If the centrifugal switch does not open at 70 to 80 percent of synchronous speed of motor, it would result in

damage  to the starting winding

#2 Speed torque characteristic of a repulsion induction motor is similar to that of a D.C.

series motor

#3 . In a ceiling fan employing capacitor run motor

secondary winding surrounds the primary winding

#4 The shaded pole motor is used for

low starting torque

#5 The rotor slots, in an induction motor, are usually not quite parallel to the shaft because it

helps in reducing the tendency of the rotor teeth to remain under the stator teeth

#6 The speed/load characteristics of a universal motor is same as that of

D.C. series motor

#7 The motor used for the compressors is

capacitor start capacitor run motor

#8 In repulsion motor, zero torque is developed when

either brush axis coincides with the field axis or brush axis is 90 degrees electrical to field axis

#9 Centrifugal switch disconnects the auxiliary winding of the motor at about……………. percent of synchronous speed.

70 to 80

#10 Starting winding of a single phase motor of a refrigerator is disconnected from the circuit by means of a

magnetic relay

#11 If a single phase induction motor runs slower than normal, the most likely defect is

worn bearings

#12 Two-value capacitor motor finds increased application as compressor motor in small home air-conditioners because

it has low starting as well as running currents at relatively high power factor.

#13 lf the centrifugal switch of a two-value capacitor motor using two capacitors fails to open then

electrolytic capacitor will in all probability, suffer break down

#14 In a universal motor, the most common cause of brush sparking is

all of the above

#15 If starting winding of a single-phase induction motor is left in the circuit, it will

draw excessive current and over heat

#16 Most of the fractional horsepower motors have either

plain or sleeve bearings

#17 Which of the following statements regarding reluctance-start motor is incorrect ?

It is similar to reluctance motor.

#18 To reverse the direction of rotation of a capacitor-start motor while it is running we should

disconnect motor from the supply  till it stops then reconnect it to supply with reversed connection of main or auxiliary winding

#19 When a ceiling fan employing a capacitor run motor is switched on it hums but does not burn. When it is driven by external means, it runs in whatever direction it is made to run. The trouble is due to

an open in capacitor

#20 Which of the following motors has two separate windings on the motor ?

Repulsion induction motor

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