Why Single-Phase Induction Motors Need Capacitors

The single-phase induction motor is a popular workhorse motor with the advantages of being cheap, reliable, and able to connect directly to single-phase power, making them especially common in domestic and small commercial appliances. However, unlike three-phase motors, they are not self-starting and require an additional winding driven by a capacitor to accelerate from a standstill.

Rotating Magnetic Fields

For an induction motor to start running, a rotating magnetic field (RMF) must be produced in the stator, which induces rotation and torque in the rotor. Since the stator does not physically move, the rotation of the magnetic field is produced by the interaction between electromagnetic forces produced in the stator windings. In a three-phase motor, with each winding supplied by a voltage that is 120 degrees out of phase with the other windings, the sum of the forces produced is a vector that continuously rotates. This means that three-phase power can induce torque in the rotor at a standstill, and three-phase motors can self-start without additional components.

However, a single-phase induction motor is fed by a single-phase power supply that runs through a single stator winding. One stator winding on its own cannot produce an RMF – it merely produces a pulsing magnetic field that is made of two opposing fields spaced 180 degrees apart.

This creates two problems:

1. First, the motor is not self-starting because the magnetic field produced by the stator is not rotating.
2. Secondly, although a single winding can drive the motor once it gets up to speed, it does not produce a consistent torque in the rotor during a full revolution, which leads to a loss of efficiency and performance. The rotor experiences maximum torque at approximately 10% slip (the difference in rotation between the rotor and the stator winding). Therefore the rotor will spend a large part of each revolution experiencing very low torque.

Auxiliary Winding

Single-phase induction motors use a second stator winding to overcome these problems, called an ‘auxiliary winding’ or ‘start winding.’ This winding is rotated 90 degrees away from the main winding, and, by means of a capacitor that changes the phase of the supply voltage, it is fed by a voltage that is out of phase with the voltage supplied to the main winding. This means that the interaction between the two windings produces a rotating magnetic field, and the motor can self-start.

There are two capacitors with different characteristics used by single-phase induction motors for different parts of their operation.

Start Capacitors

A start capacitor is one that is used to provide starting torque to the motor. They are electrolytic capacitors with a capacitance value of between 50 uf all the way up to 1500 uf. They have relatively high losses and low efficiency and are not designed for continuous duty; it is necessary to disconnect them once the motor gets up to speed using a centrifugal switch or relay of some kind.

Run Capacitors

A run capacitor is used to smooth the motor's torque during each revolution, increasing efficiency and performance. It is usually much smaller than a start capacitor, often less than 60 uf, and of the oil-filled type to reduce energy losses.

Limitations of Single Phase Motors

Even with the additional auxiliary winding, a single-phase induction motor suffers from several limitations compared to a three-phase motor. The phase shift provided by a run capacitor changes with the motor's speed, which means efficiency is not consistent as the motor changes speed. Efficiency is also affected by RMF being produced by two stator windings. It is not as close to a perfect circle as a three-phase RMF, meaning that torque still varies considerably during each revolution, reducing performance and increasing vibration. The components required to make single-phase induction motors self-starting, including the capacitors and centrifugal switch, provide an opportunity for thermal and mechanical wear to create maintenance problems.

For larger industrial applications requiring high efficiency, operating in areas where three-phase power is available, a three-phase motor may be better suited.

How to Test Your Capacitor

Summary

Single-phase induction motors are commonly found anywhere single-phase power is being used. When fitted with a start capacitor, they can develop sufficient starting torque to self-start, and a run capacitor improves their efficiency and performance during operation.