Starting Methods / Types of Single-Phase Induction Motor

An induction motor can be designed to operate either on single phase AC supply or on three phase AC supply. In its single-phase form means single phase induction motor, these are non self starting whereas, in its three-phase form means 3 phase induction motors these are self-starting. However, in both the motors we require external circuitry at the time of starting. (reason discussed further)

We have already created a dedicated article about the starting methods of three phase induction motors. Here, in this article we will focus on the core concept  of the starting methods of single-phase induction motors. 

Why do we require Starting Methods of Single-Phase Induction Motors?

In single phase induction motors, the net torque generated in the rotor is zero therefore, the motor cannot start on its own. So, to overcome this, we need an external circuitry that generates starting torque in the single-phase induction motors. However, once the rotor start rotating, we do not need this starting circuitry anymore.

On the other hand, 3 phase induction motors naturally generate starting torque, but at the time of starting they draw a large current, which can be problematic. So, to prevent this we need external methods that limit the starting current and ensure smooth operation.

Why is the Starting Torque in Single Phase Induction Motors Zero?

In single phase induction motors, the stator winding produces a double revolving pulsating magnetic field (two rotating magnetic fields which rotate in opposite directions). These fields generate equal and opposite torque in the rotor, which cancel out each other. As a result, the net torque developed in the rotor is zero, and the rotor remains stationary.

Starting Methods / Types of Single-Phase Induction Motor

There are four different types of starting methods of single-phase induction motors. Based on these starting methods a single-phase induction motor can be classified as:

 

• Split Phase / Resistance Split Phase Single Phase Induction Motor

• Capacitor Start / Capacitor Start and Run Single Phase Induction Motor

• Capacitor Start and Capacitor Run / Permanent Capacitor Single Phase Induction Motor

• Shaded Pole Single Phase Induction Motor 

Split Phase or Resistance Split Phase Single Phase Induction Motor

As the name implies, “Split Phase” in this starting method, the single-phase winding of a single-phase induction motor is split into two phases at the time of starting. This temporarily converts a single-phase induction motor into a two-phase induction motor, which helps to start it.

 

To achieve this, an auxiliary winding (also called starting winding) is introduced in parallel with the main winding. This winding has a higher resistance to reactance R/X ratio than the main winding. However, both the windings are powered by the same single phase. supply. 

 

The schematic diagram of this setup of two windings is shown in the given figure below.

The starting winding or auxiliary winding of higher R/X ratio is mounted on the different slots from the main winding of the stator of the single-phase induction motor. It is mounted in such a way that it is 90^o space displaced from the main winding. 

The higher R/X ratio of the starting winding causes the auxiliary winding current (let us say I_a) leads to the main winding (let us say I_M) current by angle α as illustrated in the phasor diagram given below.

Since, the current in the main winding and the auxiliary mining have a phase difference α, the magnetic field they generate also have the same phase difference. This results in a net rotating magnetic field, which rotates at a synchronous speed. As there is only one rotating field unlike a single-phase induction motor, the rotor gets accelerated and starts rotating. 

 

In this case the starting torque produced in the single-phase induction motor is directly proportional to the starting winding current and main winding current and sine of the phase difference between these two currents.

 

T_st  α  I_st. I_m. Sinα

 

In the above figure, a centrifugal switch is connected in series with the auxiliary winding. This switch is used to disconnect the starting winding from the supply. Because once the motor speed reaches up to 70-80% of synchronous speed there is no need for this auxiliary winding. So, we can remove this auxiliary winding by operating this centrifugal switch. So that the motor continues to rotate on the main winding or single-phase AC supply as its name implies “a single-phase induction motor”.

 

The torque speed characteristics of this motor is shown in the given figure which also shows the speed no at which the centrifugal switch operates and thereafter the motor runs only on the main winding. Therefore, the auxiliary winding needs then be designed only for short term use whereas the main winding is designed for continuous use.

The phase difference between these two current Alpha can at best be about 30 degrees which results in poor starting torque. Hence this type of starting method is not suitable for high inertia or hard to start loads however, this type of starting method is suitable for small rating motors where high starting torque is not necessary such as exhaust fan, small washing machine, agriculture machine polishing machines etc.

Capacitor Start and Run Single Phase Induction Motor

 

The problem of poor starting torque discussed in the split phase starting method can be resolved by adding the capacitor in series with the auxiliary winding. The application of capacitor in series with auxiliary winding causes a good displacement between starting winding current and main winding current (around 90 degree). 

 

Since the torque in a single-phase induction motor is directly proportional to the phase difference between these two currents. So, a larger phase difference between these two currents tends to a greater starting torque. 

 

T_st  α  I_st. I_m. Sinα

 

The schematic diagram of a capacitor start motor is shown in the given figure.

As its name implies “Capacitor Start and Run Motor” it uses the capacitor only for the purpose of starting. The value of the capacitor is chosen such that it provides a 90^o phase difference between the starting winding current and the main winding current as shown in the given figure phasor diagram.

The torque slip characteristics of this type of motor is shown in the given figure, in which we can clearly see the starting torque of this type of motor is high. 

This starting method or we can say this type of motors are best suitable for high inertia, high rating or frequently starting and stopping motors such as air compressors, refrigerators, air conditioners etc. Under running conditions this type of motor is similar to basic split phase motors and operates at low power factor.

Permanent Capacitor or Capacitor Start and Capacitor Run Single Phase Induction Motor

 

As the name implies, in this type of motor, the capacitor connected in series with auxiliary winding is permanent. Unlike the capacitor start and run induction motors, where the capacitor is used only for the starting purpose and later disconnected with the help of the centrifugal switch, once the motor starts. The permanently connected capacitor is called run capacitor and since it remains in the operation throughout, there is no need for the centrifugal switch.

 

The schematic diagram of permanent capacitor single phase induction motor is given below.

The significance of making the capacitor permanent is improving the motor performance under running conditions. As we see previously in capacitor start and run motors, there is a dip in its torque speed characteristics at the time when the centrifugal switch is operated or we can say the capacitor and auxiliary winding disconnected. This dip represents the turbulence in the operation of the single-phase induction motor. 

 

So, by keeping the capacitor permanently connected this dip in the torque speed characteristics can be removed, which means the running performance of the single-phase induction motor will improve.

 

The torque speed characteristics of this type of motor is illustrated in the given figure.

The value of the run capacitor used in this method is selected carefully to ensure the phase difference between the auxiliary winding current and the main winding current will become 90°, as shown in the given phasor diagram below. So that a balanced two-phase system can be maintained which sets a balanced two-phase field at a specified speed, in which case the backward rotating field does not exist. 

The absence of backward rotating field is making these motors exhibit best running characteristics at optimum efficiency among all the types of single-phase induction motor. Although, these motors are not so popular for the applications where high starting torque is required because the starting torque in this type of induction motor is less as compared to the starting torque in capacitor start and run induction motors.

 

The starting torque in single phase induction motors depend on the phase difference between the auxiliary winding current and the main winding current as discussed previously. This phase difference depends on the value of the run capacitor. So, if we increase the value of the run capacitor then the starting torque of the motor will increase but then the balanced two-phase system cannot be maintained because phase difference will be greater than the 90^o. Therefore, the value of the run capacitor is limited up to which the phase difference between these two currents will be 90^o.

 

To increase the starting torque of this type of motor, an additional capacitor called start capacitor is connected in parallel with the run capacitor through a centrifugal switch as shown in figure below. The motor shown in the given figure is also known as the permanent split capacitor single phase induction motor or two value capacitor induction motor. 

By adding the start capacitor, the overall capacitance of these two capacitors increases at the time of starting which tends to a larger phase difference between the main winding current and the auxiliary winding current. This results in a greater torque at the starting. 

Once the motor starts up to a nearly synchronous speed then this start capacitor will be removed by the help of the centrifugal switch. After removing the start capacitor, the single-phase induction motor runs only on the run capacitor, which maintains a balanced two-phase system. Hence this type of motor has best starting torque (due to start capacitor) and best running performance (due to run capacitor) with optimum efficiency among all the single-phase induction motors.

 

The torque speed characteristics of this type of motor is shown in the given figure. As you can see there is a small dip in the given figure at a speed No this is due to the centrifugal switch operation. This represents a small turbulence in the running of single-phase induction motor.

Shaded Pole Single Phase Induction Motor

 

In this type of starting method, we do not require to convert the single-phase induction motor into two phase induction motor by splitting the phase, as we discussed previously in other starting methods. Hence this type of motor does not contain starting winding or auxiliary winding, capacitor and centrifugal switch. Therefore, this type of motor possesses a simple and very compact design and requires less maintenance.

Instead, in this type of induction motor, a small portion of each pole on the stator winding is covered with a short circuited, single turn copper coil called shading coil, as illustrated in the given figure.

So, in this case, a portion of the sinusoidal varying flux created by the single-phase AC excitation of the main winding links with the shading coil and induces EMF as well as current in it. This induced current in the shading coil produces its own flux in the shading portion of the pole.

Let us say, 

• ɸ_m is the main flux produced by the single-phase Ac supply, 
• A small portion of this flux (ɸ_m^sc) is linked to the shading coil which induces EMF and current in it. 
• And the major portion of this flux (ɸ’_m ) pass down the air gap of the rest of the pole.

 

The induced current in shading coil produced its own flux (ɸ_sc), which in phase with the induced current and lags the induced EMF in the shading coil (E_sc) by an angle theta, whereas the flux links to the shading coil (ɸ_m^sc) leads to E_sc by 90^o, as shown in the phasor diagram in the given figure. It is obvious that the flux links with the shading coil ɸ_m^sc is in phase with the flux passes through the unshaded portion (ɸ’_m). 

In the above figure, we can clearly observe that the resultant flux in the shaded pole (the resultant of the flux linked with the shading coil and the flux produced by the shading coil (ɸ_sp)) lags by the flux passes through the unshaded portion (ɸ’_m) by angle α.

And as according to the theory the two sinusoidal varying flux ɸ’_m and ɸ_sp having phase difference (alpha in this case) and are displaced in space as well creates a net torque. Consequently, the motor will start rotating.

 

A typical torque speed characteristic of the shaded pole induction motor is shown in the given figure. It may be seen that the motor is self-starting unlike a single winding, single phase induction motor. Although the starting torque in this type of motor is low compared to other starting methods.

As discussed above, the net flux in the shaded portion of the pole lags behind the flux in the unshaded portion of the pole which causes the motor to rotate from the unshaded to the shaded portion of the pole. The motor thus has a definite direction of rotation which cannot be reversed.