Types of single phase induction motor
Types of single phase induction motor aren’t new to your ears. When we talk about induction motor we illustrated that the single phase motor can classify into many types. Depending on what we classify this motor it’s the most important.
Later we said that the single phase induction motor isn’t a self-starting motor why? Here you the answer. The Ac supply is a sinusoidal wave; we all know that and we know it produces a pulsating magnetic field in the uniformly distributed stator winding.
If this pulsating magnetic field becomes a two oppositely rotating magnetic field, we will have a no resultant torque at the starting so the motor doesn’t run and classified as not a self-starting motor.
To solve this problem we made the stator windings into two winding the main winding and the auxiliary winding.
We connect a capacitor in series with the auxiliary winding to make a phase difference when a current flow through the two coils.
The phase difference will make the rotor generates a starting torque and the motor starts to rotate.
That’s the theory which leads us to types of single phase induction motor.
Types of single phase induction motor:
According to that; we classify the single phase motor upon the additional means used to make it self-starting to:
1- Split phase induction motor:
It’s the most known type of the types of single phase induction motor.
It has a running winding, secondary start winding, and centrifugal switch; It’s usually operating at 1/20 HP to 1/3 HP so we can find it in ceiling fans, blower motors for oil furnaces, grinder, lathes, washing machine tubs, air conditioning fans, and small pumps.
We use a normally closed centrifugal switch as a control device to take out the motor start winding from the circuit once the motor reaches 75 to 80% of its rated speed.
To start the split motor we connect the start winding in parallel with the run winding.
And when the motor reaches 75% of the full speed the centrifugal switch opened to disconnect the start winding.
And the motor continues operating with the run winding. When we want to power off the motor we close the centrifugal switch when the motor reaches 40% full-load speed.
And this motor is very economical, reliable, simple in construction, and robust. But in return, it has a low power factor, very poor starting torque, very low efficiency, and high copper losses.
2-Capacitor start induction motor:
This motor has a main winding that arranged for direct connection to the power source.
And auxiliary winding connected in series with a capacitor and a starting switch.
We use this starting switch to disconnect the auxiliary winding from the power source after starting of the motor.
Even we use a solid state switch, or current-sensitive and voltage-sensitive relays as a starting switch we set the switch to stay close to maintaining the auxiliary winding circuit in operation; when the motor starts and accelerates to approximately 80% of the full-load speed the starting switch opens to disconnect the auxiliary winding circuit. And the motor continues running with the main winding as an induction motor.
3-Capacitor start capacitor run induction motor:
Capacitor start capacitor run motor or two-value capacitor motor has different values of capacitance for starting and running. And we automatically change the value of capacitance from starting to running conditions.
It doesn’t matter what is the switch we use we only care to provide two capacitors.
For starting conditions, we usually use a high (electrolytic type) value of capacitance to provide the needed high capacitance per unit volume. For the running conditions, we usually use a lower (metalized polypropylene unit) value of capacitance that is rated for continuous operation.
As the figure illustrates we connect capacitors in series with the auxiliary winding.
When we open the starting switch it disconnects the starting capacitor from the auxiliary winding circuit.
At the same time, the running capacitor connected in series with the auxiliary winding still connected to the power source.
In this case, when the motor is running both the auxiliary and main windings are energized and contribute in the motor output.
Adding a running capacitor in the auxiliary winding circuit increase breakdown torque by 5_30%, increase the lock-rotor torque by 5_10%, improve the full-load efficiency by 2_7 points, improve the full-load power factor by 10_20 point, reduce the full-load running current, and reduce magnetic noise cooler running.
This motor has a high starting torque so it is suitable for air conditioners, conveyors, compressors, grinder and other applications that require high starting torque.
4-Permanent split capacitor (PSC) motor:
The permanent split motor also called capacitor motor uses the same value of capacitance for both starting and running operations.
We can find the permanent motor in limited applications as with fans, pumps, heaters, air conditioners, blowers and other applications which don’t require normal or high starting torque.
At starting it has only a running capacitor connected in series with the auxiliary winding which reduces the starting torque (the starting torque of this motor is usually 20_30 % of the full-load torque). And we may use a high-resistance rotor to improve stable speed operation and increase the starting torque.
5-Shaded pole induction motor:
The shaded motor is a special of the types of single phase induction motor.
It is extremely popular in low starting torque applications because it’s designed in several ways to develop different values of starting torque. We can find this motor in small cooling fans, in computers, inside refrigerators.
The shade windings are additional windings that occupied in each corner of the stator poles. These windings don’t have any electrical connection for starting but they use induced current to produce a rotating magnetic field.