Motor capacitor

The capacitor motor is an electric motor and is one of the asynchronous motors which is supplied with single-phase alternating current. Like other asynchronous motors he has a squirrel-cage rotor ( squirrel cage) in which, through a signal generated by the stator elliptical rotating field, a torque is generated. The required rotating field is generated by an additional eponymous and essential for the function of capacitor. When fed with three-phase AC induction motors, this capacitor is not necessary.


Since the measures necessary for generating a rotating field at least two phase- shifted voltages for single phase alternating current is not available, a so-called auxiliary phase must be generated: One of the two stator windings is supplied directly from the AC mains, while the supply of this arranged 90 degrees second winding, a capacitor is connected in series. This causes the phase shift of the voltage across the second winding. Instead of the capacitor can also be used, an additional throttle for generating the auxiliary phase that reverses the direction of rotation compared to the Capacitor what order winding without switching is possible.

The rotating field generated in this way is adequate to move the rotor, but it is also dependent on load and leads to a low starting torque. Therefore, capacitor motors should start without load as possible to the nominal operating speed. The starting torque can be significantly improved if connected in parallel to the existing briefly another, about 2 - 3x large so-called start capacitor for the duration of the start-up. In this case, then the higher start-up current requirement is observed, which can be a multiple of the operating current at rated speed.

Strictly speaking, the size of the capacitor to be optimized for a single load case. At optimum capacitor dimensioning about 65 % of the mechanical performance as compared to an approximately identical induction motor with three-phase power supply can be achieved. Mostly high-quality MKP capacitors are used for the phase shifter or start capacitor. Typical capacitance values ​​are approximately 20 uF per kW engine power. For heavy starting the capacitance value can be / kW up to 50 uF.

Other types of induction motors, but the work with other operating principles also be operated with single-phase alternating current, are the shaded pole motor and the Repulsionsmotor.


Usually the main and auxiliary windings, and the auxiliary capacitor are mutually optimized for only one direction of rotation.

However, capacitor motors can also have two identical windings - they are then suitable for the direction of rotation by means of a single-pole switch. This is one of its advantages over the shaded-pole motor. This switch must however have a certain amount of delay between the switching positions, in order to avoid arcing when switching the direction of rotation.

There are also capacitor motors with auxiliary winding is switched off after start - auxiliary winding and capacitor are then designed only for short operation. The capacitor is non-polarized in this case, a ( much smaller ) electrolytic capacitor. Accordingly, you run capacitor and starting capacitor is different.

Areas of application

The pros and cons of the capacitor motor determine its Applications: His opposite phase motors higher mass, the low starting torque and the need for additional capacitor prevent wider use. The main advantage is the possibility to be able to operate it at high efficiency with a single-phase AC voltage. Have features common for asynchronous motors running, maintenance-free, long life and against the also suitable for single phase shaded pole motors higher efficiency him a wide range of applications in the household sector (eg as a drive for the compressor in domestic refrigerators, as tubular motor for roller shutter and awning drives), to small machine tools and example provides for circulation pumps in heating systems or in lawn mowers.

Due to the increasing availability of inexpensive electronic frequency converters are induction motors, especially in the upper power range, being replaced by AC motors with three-phase alternating current - corresponding frequency can produce the necessary for the three-phase motor phase current of single-phase alternating current.