Why doesn't a Single-Phase Induction Motor Self-Start?
A single-phase induction motor consists of a stator with a single-phase winding and a squirrel cage rotor. It is not self-starting like a three-phase induction motor and requires some external means to get it started.
When a single-phase supply is applied to the stator winding of a single-phase induction motor, it produces a magnetic field that pulsates in strength in a sinusoidal manner. The polarity of this magnetic field reverses after each half cycle, but the field does not rotate in space.
As a result, the alternating flux produced by the stator winding cannot produce rotation in a stationary squirrel cage rotor. This is because the magnetic flux can be resolved into two components that rotate in opposite directions at the same speed. The net flux produced by these two components is zero, and therefore, the induced current in the rotor bars is also zero. This means that the resulting torque on the rotor conductors is zero, and the motor will not start on its own.
To overcome this problem, some external means, such as a capacitor or a second winding, must be used to provide the necessary starting torque and get the motor running. Once the motor reaches a certain speed, these external means can be removed, and the motor will continue to run on its own.
How to Make a Single-Phase Induction Motor Self-Starting?
One way to make a single phase induction motor self-starting is to produce a rotating stator magnetic field by converting a single phase supply into a two phase supply using an additional or auxiliary winding. There are several ways to do this, and the method used depends on the type of motor.
Split phase induction motors use an auxiliary winding in addition to the main winding to produce a two phase supply and provide the necessary starting torque. Once the motor reaches a sufficient speed, the auxiliary winding is disconnected, and the motor continues to run on the main winding alone.
Capacitor motors use an auxiliary winding and a capacitor to produce a two phase supply and provide the necessary starting torque. Once the motor reaches a sufficient speed, the capacitor is disconnected, and the motor continues to run on the auxiliary winding alone.
Shaded pole motors use a shading coil around a portion of the pole structure to produce a small, rotating magnetic field. This field provides the necessary starting torque to get the motor running. Once the motor reaches a sufficient speed, the shading coil is no longer needed, and the motor continues to run on the main winding alone.
In all cases, the starting means is removed once the motor reaches a sufficient speed, allowing it to continue running on its own.
The various starting methods of a Single Phase Induction motor are as follows:
- Split Phase Motor
- Capacitor Start Motor
- Capacitor Start Capacitor Run Motor or Two value capacitor motor
- Permanent Split Capacitor (PSC) or Single value capacitor motor
- Shaded Pole Motor.
1. Split Phase Motor:
A split phase induction motor is a type of single phase induction motor that uses two windings, a main winding and a starting winding, displaced 90 degrees in space to produce a rotating magnetic field. The main winding has low resistance and high inductive reactance, while the starting winding has high resistance and low inductive reactance.
At the start of the motor, both windings are connected in parallel and a resistor is inserted in series with the starting winding. As a result, the current in the two windings is not equal, and the rotating field produced is not uniform. This results in a low starting torque, typically 1.5 to 2 times the rated running torque.
As the motor reaches a speed of around 70 to 80% of the synchronous speed, the starting winding is automatically disconnected from the supply mains. This is achieved through the use of either a centrifugal switch or a relay. If the motor is rated at 100 watts or higher, a centrifugal switch is used to disconnect the starting winding. For smaller ratings, a relay is used. The relay is connected in series with the main winding, and as the motor starts, the heavy current flowing in the circuit causes the relay contacts to close, connecting the starting winding to the circuit. As the motor reaches the predetermined speed, the current in the relay starts to decrease, causing the relay to open and disconnect the starting winding from the supply, allowing the motor to run on the main winding alone.
2. Capacitor Start Induction Motor:
A capacitor start motor is a type of single phase induction motor that uses a capacitor in the auxiliary winding circuit to produce a greater phase difference between the current in the main winding and the auxiliary winding. This allows the motor to produce a rotating magnetic field and provides the necessary starting torque to get the motor running.
The capacitor start motor has a cage rotor and two windings on the stator, a main winding and an auxiliary or starting winding. These two windings are placed 90 degrees apart and are connected in such a way that the current in the main winding lags the current in the auxiliary winding by 90 degrees. A capacitor is connected in series with the starting winding, and a centrifugal switch is also included in the circuit.
As the motor approaches its rated speed, the auxiliary winding and the starting capacitor are automatically disconnected from the circuit by the centrifugal switch, which is mounted on the shaft of the motor. This allows the motor to continue running on the main winding alone.
Overall, the capacitor start motor acts as a balanced two phase motor, with the main and auxiliary windings producing magnetic fields that are equal in magnitude but 90 degrees out of phase. This allows the motor to produce the necessary starting torque and get itself up to speed, after which it can continue running on its own.
3. Capacitor Start Capacitor Run Motor:
A capacitor start capacitor run motor is a type of single phase induction motor that uses two capacitors, a starting capacitor and a run capacitor, to produce a rotating magnetic field and provide the necessary starting torque. The main and auxiliary windings in the stator of the motor are displaced 90 degrees in space, and the two capacitors are used to create a two phase supply.
At the start of the motor, both capacitors are connected in parallel. The starting capacitor (Cs) is a short time rated, electrolytic capacitor that has a large value, which allows it to produce a low capacitive reactance (X) and provide the necessary starting torque. The run capacitor (CR) is a long time rated, oil-filled paper capacitor that has a small value, which allows it to produce a high capacitive reactance and reduce the line current at normal operating conditions.
As the motor approaches synchronous speed, the starting capacitor (Cs) is disconnected from the circuit by a centrifugal switch (SC). The run capacitor (CR) remains connected permanently in the circuit and provides the necessary phase difference between the main and auxiliary windings to keep the motor running.
Overall, the capacitor start capacitor run motor is a type of single phase induction motor that uses two capacitors to produce a rotating magnetic field and provide the necessary starting torque. Once the motor reaches its rated speed, the starting capacitor is disconnected, and the motor continues to run on the run capacitor alone.
4. Permanent Split Capacitor (PSC) Motor:
A permanent split capacitor (PSC) motor is an electric motor that uses a capacitor to help start the motor and then keep it running. It is called a "permanent split" capacitor motor because the capacitor is always in the circuit, both at the start and run stages.
The PSC motor has two windings: a main winding and an auxiliary winding. The main winding is used for running the motor, while the auxiliary winding is used for starting the motor. The auxiliary winding is connected in series with a capacitor, which helps to create an out-of-phase current that helps start the motor.
Once the motor has started, the auxiliary winding is no longer needed, and the capacitor remains in the circuit to help improve the motor's efficiency. The PSC motor operates as a balanced two-phase motor because the auxiliary winding is always in the circuit, and this helps to produce a uniform torque and noise-free operation.
Overall, the PSC motor is a simple and reliable design that is commonly used in a variety of applications, including air conditioning systems, refrigeration systems, and small appliances.
5. Shaded Pole Induction Motor:
A shaded pole induction motor is a type of single-phase motor that is designed for low power applications. It gets its name from the copper ring or "shading coil" that is placed around a portion of one of the poles of the motor's stator. The shading coil creates an out-of-phase current in the main winding, which helps to start the motor.
The stator of a shaded pole motor consists of a series of laminated poles with a short-circuited copper coil placed in a slot near the edge of each pole. The rotor of a shaded pole motor is a squirrel cage design, with skewed bars that help to improve the starting torque of the motor.
One of the main reasons why shaded pole motors are used for low power applications is because they have relatively high power losses and a low power factor. This leads to a lower overall efficiency compared to other types of motors. In addition, the starting torque of a shaded pole motor is relatively low, which means it may not be suitable for applications that require a lot of torque at start-up.
Despite these limitations, shaded pole motors are widely used in small appliances and other low power applications because they are simple and reliable. They do not have any brushes, commutators, or other moving parts, which means they have a lower risk of failure. They also do not have a centrifugal switch, which is a type of switch that is activated by the centrifugal force of a rotating shaft. This makes them well-suited for applications where a continuous, smooth rotation is required.