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Shaded-Pole Single-Phase Motors

written by: shankar • edited by: KennethSleight • updated: 8/31/2009

We know that single phase induction motors are not self-starting. Inorder to make single phase motor a self-starting one, a certain arrangement must be provided so that the stator flux produced becomes a rotating one rather than alternating one.Such an arrangement is provided by shaded pole motors.

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    Introduction

    Shaded pole motor is one of the types of single phase induction motors, which are used for producing a rotating stator flux in order to make the single phase induction motor a self starting one. Let us discuss the constructional details, diagrams and working of shaded pole motors in detail.

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    Shaded-Pole SIngle-Phase Motors

    Like any other motors the shaded pole induction motor also consists of a stator and rotor. The stator is of salient pole type and the rotor is of squirrel cage type.

    The poles of shaded pole induction motor consist of slots, which are cut across the laminations. The smaller part of the slotted pole is short-circuited with the help of a coil. The coils are made up of copper and it is highly inductive in nature. This coil is known as shading coil. The part of the pole which has the coil is called the shaded part and the other part of the pole is called unshaded part.

    Shaded pole motor 

    Now let us consider that an alternating current is passed through the excited winding which surrounds the pole. Due to the presence of shading coil, the axis of the pole shift from unshaded part to shaded part. This shifting effect is equivalent to the physical movement of the poles, which is nothing but the rotation of poles. So the rotor starts rotating in the direction of the shift from unshaded part to shaded part.

    Shaded pole with shading coil 

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    Why Magnetic Axis Shift Takes Place?

    Now let us discuss why the axis shift occurs when current is passed through the winding and how the shading coil aids in producing the shift.The current carried by stator winding produces alternating flux. The distribution of flux through the poles is greatly influenced by the shading coils.

    When the alternating current through the coil increases, it induces a current in the shading coil. The direction of current in shading coil is such as to oppose the cause producing it (from Lenz law). The cause is the alternating current. So the flux in the shading coil decreases and it opposes the main flux. Hence the flux mostly crowds or shifts towards the unshaded part of the pole. So the magnetic axis lies along the middle of the unshaded part.It is denoted as NC in the picture below.

    Case 1: current increases rapidly Now consider that the alternating current has reached its peak (or) somewhere near the peak. Here the rate of change of current is low, as it has already reached the peak (or) it is very close to peak value. Since the change is current is so small, the induced current at shading ring is also small and negligible. So the shading ring does not affect the distribution of main flux. The flux is distributed uniformly and the magnetic axis lies at the center of pole face.The magnetic axis is denoted as ND in the picture.

    Case 2: Peak value of current 

    The alternating current, after reaching the peak starts to decrease rapidly and in turn decreases the main flux. The change in current induces a current in shading coil. According to Lenz law the direction of this current is so as to oppose the cause producing it (the decreasing alternating current). So the flux in shading coil opposes the decrease in main flux and strengthens it. This increases the strength of main flux in the shaded part. SO the magnetic axis shifts itself to the middle part of shaded pole.The magnetic axis denoted as NE in the picture below.

    Case 3: current decreases rapidly 

    So it is quite clear that during the positive half cycle of the alternating current, North Pole shifts from unshaded part to shaded part and during the negative half cycle, the South Pole shifts along from unshaded part to shaded part. This effect is nothing but the rotation of poles from left to right.

    Thus shaded coils aids in producing the rotating flux and thus the single phase Induction motor is converted into self starting one using the Shading coil. Due to fixed of position of shading coils, the direction of rotation of such motors cannot be changed.

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    Advantages, Disadvantages and Applications

    The various advantages of Shaded pole motors includes

    • Very cheap and reliable
    • Easy to construct
    • Extremely rugged in nature

    Disadvantages includes

    • Low efficiency
    • Low starting torque
    • Since the shading coil is made of copper, the copper loss is high.

    Uses

    Due to their low starting torques they are mostly employed in small instruments, toys, small fans, electric clocks, hair dryers, ventilators, circulators etc.

    Thus we have discussed in detail the working, constructional features and applications of Shaded pole Single Phase motors.

    Image and content Courtesy:

    A Text Book of Electrical Technology by “B.L.Theraja”

    Fundamentals of Electrical Engineering and Technology by "Stanley, Hackworth and Jones"