Single Phase Motor : Concepts -- 1 ( sai saikumar jn)

Single Phase Motor : Concepts

Introduction of Single Phase Motors:

These are small motors having an output power less than one horse power and are generally operated on single phase AC supply. These motors perform varieties of service in the home, office, business concerns, factories and farms and in a number of other applications where single phase supply is available.

Single phase motor is not self-starting. Hence, it is provided with an extra winding known as auxiliary or starting winding in addition to main or running winding. These two windings are spaced 90° electrically apart and are put in parallel, so that a rotating field is produced.

The explanation of single phase motor is made from double revolving field theory. The pulsating field produced in single phase AC motor is resolved into 2 components of half the magnitude and rotating in opposite directions at the same synchronous speed.

Let φ_m be the pulsating field which has two components each of magnitude φ_m/2. Both are rotating at the same angular speed ω_r rad/sec but in opposite direction as shown in the Figure given below.

The resultant of the two fields is φ_mcosθ . Thus the resultant field varies according to cosine of the angle θ. The wave shape of the resultant field is shown in Figure given below.



Thus an alternating field can be represented by the fields each of half the magnitude rotating at same angular speed of ω_s radians/sec but in opposite direction. The two revolving fields will produce torques in opposite directions. Let the two revolving fields be field No. 1 and field No. 2. Let the field No. 1 rotate in the clockwise direction and field 2 rotate in anticlockwise direction. Clockwise torque is plotted as positive and anticlockwise as negative. At stand still, slip for both fields is one. Synchronous speed in clockwise direction will give condition of zero slip for field 1 but it will give slip = 2 for field No. 2. Similarly synchronous speed in a counter clockwise direction will give condition of zero slip for field 2 but slip = 2 for field No. 1. Now in the two curves produced by the two revolving fields have been drawn and the resultant i.e., algebraic sum of the two fields will give the net developed torque or resultant torque. Now if we look at the resultant torque we see that the starting torque (torque at slip = 1) is zero. And except at starting there is always some magnitude of resultant torque, which shows if this type of motor once started, in any direction it will develop torque and will function as motor. Hence single phase motor with single winding develops no starting torque but if the machine is started in any direction by some auxiliary means, it will develop torque in the same direction in which it is started.

TYPES OF Fractional Horsepower Motor (FHP) MOTORS:

1. Split phase motor:

a.capacitor start
b.capacitor run
c.capacitor start capacitor run motor.
 

2. Shaded pole motor.

3. Reluctance motor.

 

 

1. Split phase motor:

The main winding has low resistance but high reactance and starting winding has high resistance but low reactance.

Current in the auxiliary winding called starting winding lags the supply voltage by lesser angle. The current in the main winding being highly inductive lags the supply voltage with greater angle. Now there are two currents which are not in phase with each other. As shown in the figure I_m is the current in the main winding lagging the voltage by angle φ_m and I_s, the current in the starting winding lagging the voltage by angle φ_s. The angle between the two currents is θ as shown in the figure. The torque produced is such that it gives circular movement to the rotor and is proportional to the sine of angle θ. A centrifugal switch which is normally closed is incorporated in series with the starting winding. When the motor comes up to speed about 75% of synchronous speed it opens automatically with the help of centrifugal force and puts the starting winding out of circuit.



Typical torque speed characteristics is shown here.

Capacitor Start Motor:

The starting winding has a capacitor in series with it. The motor gives high starting torque due to large value of C.





Torque speed characteristics is shown below.

Capacitor run motor:

There is no switch in the starting winding and capacitor is permanently connected.
Starting torque is lower about 50 to 100% of full load torque.

 

Capacitor start and capacitor run Motor:

Two capacitors are used. One for starting and the other for running purpose. These motors give best performance in terms of efficiency and power factor.

2. Shaded pole motor:

Each shaded pole has its own exciting coil. About 1/3 rd portion of each pole core is surrounded, by a strap of copper forming a shading coil. When the flux is increasing in the pole, a portion of the flux attempts to pass through the shaded tip or shaded portion of the pole. 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 the flux passes through un shaded side of each pole. When the flux reaches its maximum value, its rate of change is zero thereby the e.m.f. and hence current in the shading coil becomes zero. Large amount of main flux then links with the coil. After the main flux tends to decrease, the current induced in the shading coil now tends to prevent the flux linking with shading coil from decreasing. The shading coils thus causes the flux in shaded portion as shown to lag the flux in shaded portion of the pole.



Speed torque characteristics is shown here starting torque is very small about 50% of full load torque.



from ur's -- Bellapuri saikumar
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