What is the Difference between capacitor Start and Capacitor Run Motors?

All electric motors run under the effect of torque provided by a rotating magnetic field. In DC, it could be the interaction of magnetic field of magnetic fields of permanent magnets and windings on stator (or rotor), or it could be interaction of magnetic fields of stator and rotor windings.

Single phase AC motors have a number of ways to create the rotating magnetic field. In the absence of any mechanism to create the rotating field, windings on rotor and stator create simple fluctuating field as in transformer, and rotor and stator windings will just act as transformer, without any spin. The first induction motors, when developed, were in fact, called “rotating transformers”, since the stator was seen as primary and secondary was effectively a ‘shorted secondary’, free to rotate under the interaction of their magnetic fields. Operating principle is basically similar to that of a transformer.

What drives the induction rotor is the rotating magnetic field created by interaction of sets of stator windings and its effect on rotor core. A transformer is an electromagnetic static machine, while motor is electromechanical energy conversion device. Basic requirement is to create rotational torque on rotor using magnetic fields of windings.

In single phase induction motors, this is done using two sets of windings on stator. In old designs, this was done by using one winding with high resistance placed 90° electrically apart from main winding, by using thin and longer wire, which meant it has higher resistance than the main winding. This achieves a phase difference of 30° between the two currents, thereby creating 30° phase difference between their magnetic fields. These two sets of windings, spaced alternatingly around stator, creates rotating field for the rotor. Peaks of waveforms of the two sets of windings are reached at different times governed by electrical phase angle between them. Peak of magnetic field created by them is thus seen as advancing sequentially, or rotating, thereby making the rotor to spin.

Auxiliary winding can be disconnected once full speed was achieved, as the motor can then turn with one set of windings. This method created a moderate starting torque, and has a drawback that the resistance consumes significant amount of energy, reducing efficiency of motor through watt loss. The arrangement works well with motors up to 1/3 HP, or 250 Watts. Current in starting winding is so high that frequent restarting at short intervals is not advised.

Effect of Capacitor on motor

Capacitors were introduced in later designs, and have become common now, as a means of creating the phase difference. This has the advantage that capacitors do not consume any energy, and also enable much higher phase difference- both factors adding to efficiency of system, while giving higher torque. Capacitors can be used in three ways in single phase motors. The capacitors are named accordingly depending on their usage. Capacitor start or capacitor run motors enable motors to run at near unity power factor.

1. Capacitor run motor

These have electrostatic capacitors permanently connected in series with auxiliary winding. They provide the running torque all the time, maintaining the phase difference between windings. The loss factor is low, being mostly MPP film capacitors from 1.25 μF tom36 μF for different applications. (Till 1990s, paper capacitors were common before the advent of film capacitors). The capacitor is called motor run capacitor. Capacitor value is a compromise between good staring torque and full speed load torque.

2. Capacitor start motor

This is commonly used in single phase FHP motors to give high starting torque. This is usually an aluminium electrolytic capacitor placed in series with auxiliary winding. Electrolytic capacitors are preferred as large capacitance values are possible than available with electrostatic paper/film capacitors. This enables much higher torque requirements in large motors in FHP range, by creating wider phase angle difference with main winding, and allowing higher current through auxiliary winding.  The capacitor is called motor start capacitor. Capacitor value is so large as to give a phase difference of nearly 90° between the main and auxiliary windings, with high auxiliary current, and maximum possible torque. Starting torque can be as high as 300% of full load torque.

A centrifugal switch is connected between the capacitor and auxiliary winding.  The capacitor remains in circuit till motor reaches up to about 70% of its rated speed, when the centrifugal switch (placed on shaft of motor) operates and cuts off the capacitor (and the auxiliary circuit). Thereafter the motor runs on single winding, and auxiliary winding is not needed any more. These electrolytic capacitors are basically made of two electrolytic capacitors connected back-to-back internally. Loss factor is therefore very high, and capacitor remains in circuit for just under 3 seconds. MPP capacitors rated for 250 V AC and short time duty are replacing the electrolytic capacitors in recent years. They have advantage of low loss and hence can withstand longer AC voltage for longer time. They also have much longer life.

Values of motor start capacitor are quite higher than the run capacitors in order to be able to achieve high torque. They are used for motors from 1/8 HP to 2.5 HP, and have large tolerances. Typically, values from 40-60 μF to 200-250 μF, and voltage ratings of 230 V are common for 220 V 50 HZ supply. High values of these capacitors enable sufficiently high starting torque required in these motors. Occasionally values of 300μF or higher may be used.

3. capacitor-start-capacitor-run motor

A third type of motor, capacitor-start-capacitor-run motor uses both motor start and motor run capacitors in parallel. Combined high capacitance gives a high torque to motor. Large capacitor (electrolytic) is cut off when motor gains speed, and the run capacitor (paper/PP capacitor) continues in circuit. When both capacitors are connected, phase difference between windings is greater than 90°, and becomes 90° when electrolytic capacitor is disconnected.

This construction, though expensive, gives high starting torque and also a good running torque. This is the most efficient design, but is also the costliest, and is used for particularly demanding applications. Auxiliary winding remains permanently in circuit, and thereby gives benefits of both capacitor-run and capacitor-start motors. Individual capacitor values are a compromise between best starting and running characteristics.

Capacitors are not needed in three phase motors since the three sets of phase windings create their own fields separated by 120 degrees in time space, thereby creating the necessary rotating torque.