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 φm be the pulsating field which has two components each of magnitude φm/2. Both are rotating at the same angular speed ωr rad/sec but in opposite direction as shown in the Figure given below.
The resultant of the two fields is φmcosθ . 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 ωs 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 the 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.