- Single Phase Induction Motor Definition: A single-phase induction motor is a type of electric motor that operates with a single alternating current phase, requiring additional mechanisms to initiate rotation.
- Split Phase Operation: The split phase motor utilizes an auxiliary winding with higher resistance and a centrifugal switch that disengages at 75-80% of the synchronous speed, facilitating the motor start.
- Capacitor Start and Run: These motors use capacitors to create a necessary phase difference, producing a strong starting torque and improving the power factor during operation.
- Permanent Split Capacitor Advantages: The PSC motor maintains a constant capacitor connection, which eliminates the need for a starting switch and enhances efficiency.
- Shaded Pole Characteristics: Shaded pole motors use a copper ring to induce a phase shift in part of the pole, creating a rotating magnetic field suitable for small, low-power devices.
Single-phase induction motors are not self-starting from the main winding alone. An extra flux, from an auxiliary winding or a shaded pole, is used to start them.
They are classed by how that extra flux is produced:
- Split phase induction motor.
- Capacitor-start inductor motor.
- Capacitor-start capacitor-run induction motor (two-value capacitor method. Used to both start and run the motor).
- Permanent split capacitor (PSC) motor.
- Shaded pole induction motor.
Split Phase Induction Motor
Besides the main winding, the stator of a single-phase induction motor has an auxiliary or starting winding. A centrifugal switch opens that winding at about 75-80% of synchronous speed.
The switch drops the auxiliary winding out of circuit once the motor has reached about 75 to 80% of synchronous speed.
The running winding is inductive. A useful phase split between the two windings needs the starting winding to have high resistance.
In the figure the symbols are:
- Irun is the current flowing through the main or running winding,
- Istart is the current flowing in starting winding,
- VT is the supply voltage.
In a highly resistive winding, current stays close to the voltage in phase. In a highly inductive winding, current lags the voltage by a large angle.
The starting winding is highly resistive, so its current lags the applied voltage by only a small angle. The running winding is highly inductive, so its current lags the applied voltage by a large angle.
The resultant of these two currents is IT. That resultant produces a rotating magnetic field in one direction.
In a split-phase induction motor, the starting current and the main current are split in phase, which is how the motor got the name.
Applications of Split Phase Induction Motor
Split phase induction motors have moderate starting torque and a relatively high starting current for that torque.
Split phase motors, typically from about 1/20 to 1/2 kW, drive fans, blowers, washing machines and lathes.
Capacitor Start IM and Capacitor Start Capacitor Run IM
Capacitor-start inductor motors and capacitor-start capacitor-run induction motors both use a capacitor to produce the starting phase split.
A single-phase induction motor is not self-starting because the main-winding field is pulsating, not rotating. A rotating field needs a phase difference between two currents.
A split-phase motor uses resistance to create the phase split. These motors use a capacitor instead. The current through a capacitor leads the voltage.
So, in a capacitor start inductor motor and a capacitor start capacitor run induction motor, there are two windings: the main winding and the starting winding.
A capacitor is connected with the starting winding, so the current in that branch Ist leads the applied voltage by some angle φst.
The running winding is inductive, so its current lags the applied voltage by an angle φm.
The two currents then have a large phase split, and their resultant produces a rotating magnetic field. Starting torque is high when that split is near 90o.
These motors therefore produce a high starting torque. On a capacitor-start induction motor a centrifugal switch drops the start winding at about 75 to 80% of synchronous speed. On a capacitor-start capacitor-run motor the run capacitor stays in circuit.
There is no start-only switch on a two-value or PSC run capacitor arrangement: the run capacitor stays in circuit and raises the power factor and the running torque of the single-phase induction motor.
Application of Capacitor Start IM and Capacitor Start Capacitor Run IM
High starting torque suits conveyors, grinders, air conditioners and compressors. Typical ratings run up to about 6 kW.
Permanent Split Capacitor (PSC) Motor
It has a cage rotor and a stator with two windings, main and auxiliary. One capacitor sits in series with the starting winding. There is no starting switch.
Advantages of Permanent Split Capacitor Motor
No centrifugal switch is needed. Efficiency and pull-out torque are higher than a split-phase machine of similar size.
Applications of Permanent Split Capacitor Motor
PSC motors drive fans and blowers in heaters and air conditioners, and they also drive office machines.
Shaded Pole Single Phase Induction Motors
The stator of the shaded pole single-phase induction motor has salient or projected poles. A copper band or ring, inductive in nature, shades part of each pole.
The poles are divided into two unequal halves. The smaller portion carries the copper band and is called the shaded portion of the pole.
ACTION: When a single-phase supply is given to a shaded pole induction motor’s stator, an alternating flux is produced.
That changing flux induces emf in the shaded coil. Because the shaded portion is short-circuited, current flows there in a direction that opposes the main flux.
Flux in the shaded pole lags flux in the unshaded pole. That phase split produces a resultant rotating flux.
Stator current is alternating, and so is the flux it produces. Three parts of a half-cycle show how a shaded-pole motor works:
- When the flux changes its value from zero to nearly maximum positive value.
- When the flux remains almost constant at its maximum value.
- When the flux decreases from a maximum positive value to zero.
REGION 1:
When the flux changes its value from zero to nearly maximum positive value – In this region, the rate of rising flux and current is very high.
According to Faraday’s law, whenever there is a change in flux emf gets induced. Since the copper band is a short circuit, the current starts flowing in the copper band due to this induced emf. This current in the copper band produces its own flux.
Per Lenz’s law, the current induced in the copper band counteracts the change that generated it, here the rise in flux.
So the shaded ring flux opposes the main flux, which leads to the crowding of flux in the non-shaded part of the stator, and the flux weakens in the shaded part.
This non-uniform distribution of flux causes the magnetic axis to shift in the middle of the non-shaded part.
REGION 2:
When the flux remains almost constant at its maximum value- In this region, the rate of rising current and hence flux remains almost constant.
Hence there is very little induced emf in the shaded portion. The flux produced by this induced emf has no effect on the main flux, and hence the distribution of flux remains uniform, and the magnetic axis lies at the center of the pole.
REGION 3:
When the flux decreases from a maximum positive value to zero – In this region, the rate of decrease in the flux and hence current is very high. According to Faraday’s law, whenever there is a change in flux emf gets induced.
Since the copper band is a short circuit, the current starts flowing in the copper band due to this induced emf. This current in the copper band produces its own flux. According to Lenz’s law, the direction of the current in the copper band is such that it opposes its own cause, i.e., a decrease in current.
So the shaded ring flux aids the main flux, which leads to the crowding of flux in the shaded part of the stator, and the flux weakens in the non-shaded part. This non-uniform distribution of flux causes the magnetic axis to shift in the middle of the pole’s shaded part.
This shifting of the magnetic axis continues for the negative cycle and leads to the production of a rotating magnetic field. This field’s direction is from the non-shaded part of the pole to the shaded part of the pole.
Advantages and Disadvantages of Shaded Pole Motor
The advantages of shaded pole induction motor are
- Very economical and reliable.
- Construction is simple and robust because there is no centrifugal switch.
The disadvantages of shaded pole induction motor are
- Low power factor.
- The starting torque is very poor.
- The efficiency is very low as the copper losses are high due to the presence of the copper band.
- The speed reversal is also difficult and expensive as it requires another set of copper rings.
Applications of Shaded Pole Motor
Shaded pole motors are used where starting torque can be low:
Low starting torque and low cost suit small instruments, hair dryers, toys, record players, small fans and electric clocks. Typical sizes are about 1/300 to 1/20 kW.
