Have you ever tried tipping your table fan blades or your ceiling fan blades for the simple reason that it is not starting? Have you ever wondered what kind of motor we use for almost all our domestic appliances? The single answer for the above questions will be "Single Phase Motors." The motors which work on a single phase supply (or) the motors which are excited by a single phase supply are called as single phase Induction motors.Let us discuss about the single phase motors.
Types of Single Phase Motors
The single phase motors are classified depending on their construction and method of starting.
The following are the types which are based on constructional features of the motor:
· Single phase Induction motor
· Single phase repulsion motors
· AC series motor
· Unexcited synchronous motor
The following are the types based on the method of starting:
· Split Phase motors
· Capacitance start Induction motors
· Shaded pole motors
The single phase motors consists of two main parts: The rotating part and the stationary part. The rotating part is called the rotor and the stationary part is the stator.
The stator consists of various stampings which are slotted on its periphery. These slots carry the winding called stator winding or main winding. These stampings are made up of high grade silicon steel to reduce the hysteresis loss. The stator is laminated to reduce the Iron loss. In case of single phase Induction motor the rotor is of squirrel cage type and the stator is made of distributed winding.
The stator is fed by a single phase ac supply, which produces alternating flux in the stator windings. But the alternating flux produced is along one space axis only and not a rotating one as in the case of three phase motors. Therefore the alternating flux in stator cannot produce the necessary rotation for the rotor to rotate. Hence the single phase Induction motor is not a self starting one.
However the rotor can be accelerated to full speed if it is given an initial start with hand or by any other means. If the rotor is given an initiation in clockwise direction then the clockwise torque increases subsequently reducing the anti clockwise torque. So the net torque acting in clockwise direction aids the motor in accelerating to its full speed.
Making Single Phase Induction motor self starting
As discussed above, we know that the single phase motors are not self starting. To make single phase motor a self starting one, it can be converted into a two –phase motor during the starting period. For this purpose, along with main winding in the stator an auxiliary winding is provided. The two windings should be 90 degrees electrically apart and are connected in parallel to each other. The phase difference between them should be large.
Now when the motor is fed with a single phase supply, two currents with a phase difference between them produces a rotating flux and hence the motor is self starting.
There are various other methods by which a single phase motor can be made self starting such as
· In split phase machines
· In capacitance start induction run motors.
In my next article, each type of single phase motor and different methods of starting will be discussed in detail.