When are Capacitors used with single phase motors?

Answer 1

The capacitor is used to produce a phase shift to create a rotating magnetic field so the motor will start turning. 3-phase motors do not need one because of the phase difference inherent between the phases.

While a capacitor connected external to the motor does affect the power factor of the overall system (as mentioned below), an internal start capacitor is not used for that reason in a single-phase motor.

Another answer

There is inductive reactance and capacitive reactance. (Look these terms up, it is good stuff). Think of them as opposites in a sense. The operation of the induction motor is inefficient in a power factor sense due to electro magnetic field properties that affect the power factor in an inductive sense. The capacitor is an "offset" to the inductive reactance which, when sized accordingly, affects the power factor as a correction towards unity.

Answer 2

Some types of single-phase motor do not have a rotating magnetic field, so the rotor does not start rotating automatically. A "motor start and run" circuit is needed, which includes a capacitor to produce a rotating magnetic field. That rotating field pulls the rotor and it starts to turn. Once the motor has been started, it will continue to spin as long as enough power is supplied to it to overcome the force of the load it is driving. In homes such motors are commonly used in lawn-mowers, for example.

However not all designs of single-phase motor require a "motor start and run" circuit: a "commutator and brushes" motor, which has its field winding in series with its rotor windings, will start turning on its own without needing a capacitor-start circuit. Such motors are commonly used in small domestic hair-dryers, power drills and vacuum cleaners, for example.

However a different kind of capacitor-based circuit is usually fitted to such motors: a "spark-suppression circuit". This is fitted to help suppress the production of high levels of interference at radio frequencies that is sent out from the sparks produced when a "commutator and brushes" motor system is running.

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A three-phase motor has a rotating magnetic field so that the motor's rotor is pulled and starts rotating automatically. Hence no "motor start and run" capacitor is needed to start the rotor turning.

However, depending on the type of three-phase motor, to limit the amount of current drawn at motor startup, a different type of starter circuit may be needed, such as a "star-delta starter". Once the motor has been started, it will continue to spin as long as enough power is supplied to it to overcome the force of the load it is driving.

Three-phase motors are not typically used in household appliances but are very common in factories, workshops and other industrial sites because of their higher power-to-weight ratio and better electrical efficiency compared to single-phase motors.

Answer 3

One method of starting a single phase motor is to connect a capacitor through a switch to a second winding to induce a rotating magnetic field in the stator iron. Once the rotor has reached about 75% of synchronous speed, the switch opens removing the capacitor from the circuit. This is a 'capacitor start' motor. In other cases, the capacitor is used across the motor leads to improve the power factor of the motor. Finally the capacitor may be connected to the secondary winding without a switch and this is a 'cap start cap run motor'. These motors generally do not have as much operating torque as 'cap start' motors. Still another use is to use one capacitor to start the motor, then another while the motor is running.

Answer 4

In single phase motors capacitors are used to provide a rotating magnetic field. There is generally two windings in the motor the main winding and the auxiliary winding. The main winding provides the most torque to spin the rotor of the motor. The auxiliary winding is used to create a phase shift between these two windings. This is where you get your rotating magnetic field. This is called the Cross Field Theory and it is actually quite complex. This phase shift can be done a few number of ways. Resistance can be added to the aux winding (but shortly after start up the winding will need to be switched out to avoid burning up), or a capacitor can also be used. These motors are called resistance split phase, capacitor start and capacitor run motors.

Answer 5

It has nothing to do with efficiency; it's all about providing a starting torque.

The function of the capacitor is to provide an out-of-phase current through the motor's starting winding, in order to set up the rotating magnetic field necessary to start the motor turning. In other words, its function is to provide the motor with its starting torque. Once the motor is turning, the capacitor/starting winding is (usually) disconnected, as the motor can continue to run with its starting winding disconnected.

Answer 6

The only time that a capacitor and a three phase motor are combined is for power factor correction. For individual motor correction the capacitor is connected usually at the load side of the motor contactor before the overload protection. If this position can not be utilized and the capacitor has to be installed after the overload protection then the overload heaters have to be increased to compensate for the additional capacitor current.

Answer 7

to make single phase induction motor a self staring motor