Capacitor motor
The capacitor motor is an AC motor which is fed with single-phase alternating current and belongs to the group of asynchronous motors and induction motors . Like other asynchronous motors, it has a short-circuit rotor , usually in the form of a cage rotor executed in which by the stator generated rotating field , a torque is generated. The rotating field is generated by an additional, eponymous capacitor that is essential for the function .
principle
For reasons of cost, capacitor motors are designed for two-phase alternating current with lower powers , i.e. they have two windings in the stator that are rotated by 90 ° to each other, as shown in the adjacent circuit diagram. However, there are also types with several windings which, from a certain number, are arranged in a circle in the stator package. Asynchronous motors for three-phase alternating current are used for higher outputs . The three windings in the stator are then rotated by 120 ° and the underlying, differently implemented circuits for single-phase operation are referred to as Steinmetz circuits .
In all cases, the capacitor is used to generate the phase shift required for the rotating field in the form of the so-called auxiliary phase. In the simplest case with a two-phase motor, one of the two stator windings is supplied directly from the alternating current network, while a capacitor is connected in series to supply the second winding arranged at 90 ° for this purpose . The reactive current through the capacitor causes a phase shift in the second winding and forms the auxiliary phase. There are different circuit variants: Instead of the capacitor, an additional choke can be used to generate the auxiliary phase, which reverses the direction of rotation compared to the capacitor circuit, which is possible without switching the winding.
The rotating field generated in this way is sufficient to move the rotor, but it is also load-dependent and leads to a low starting torque. The starting torque can be increased by briefly connecting a further, 2 to 3 times larger so-called starting capacitor during the start - up period, parallel to the existing one. The higher starting current requirement, which can be a multiple of the operating current at nominal speed, must be taken into account.
Since an asynchronous motor can also run with one phase, i.e. completely without a capacitor, provided that the drive speed is not undershot due to excessive torque, an operating capacitor can also be dispensed with. The start-up winding is then often only suitable for short-term use.
The dimensioning of the capacitor is optimized for a single load case of speed and torque. With an optimal capacitor design, around 65% of the mechanical power can be achieved compared to a three-phase asynchronous motor of roughly the same size. For safety reasons, self-healing MKP capacitors with typical capacitance values of approx. 20 - 25 µF per kW motor output are used for the phase shifter or starting capacitor . In the case of a heavy start-up using a starting capacitor, the capacitance value can be up to approx. 60 - 100 µF per kW motor output. The capacitors generally have to withstand a voltage higher than the mains voltage.
Types
Above all, capacitor motors are designed to be cost-optimized for specific, specified applications and therefore the main and auxiliary windings as well as the auxiliary capacitor are only optimized for one direction of rotation and are usually not interchangeable. The main and auxiliary windings have a different structure.
For general applications, more expensive capacitor motors can also have two windings of the same type. They are then suitable for reversing the direction of rotation using a single-pole changeover switch. However, this changeover switch must have a certain delay between the two switch positions in order to avoid an arc fault when the direction of rotation is changed .
Areas of application
The advantages and disadvantages of the capacitor motor determine its areas of application: Its higher mass compared to three-phase motors, the lower starting torque and the additional capacitor required prevent its wider use. The main advantage is the possibility of being able to operate it with a single-phase alternating voltage with good efficiency. It can be reversed in a simple manner or its direction of rotation is not dependent on the phase sequence of the three-phase current, which is why a wrong direction of rotation due to a wrong phase sequence is avoided. Windings must be switched for speed changes. Its smooth running, maintenance-free, long service life and the higher efficiency compared to the shaded-pole motor, which is also used for single-phase operation, have a wide range of applications as a drive, e.g. B. for larger refrigeration machines , as a tubular motor for roller shutter and awning drives, for lawn mowers, bench grinders, small machine tools, powerful room fans and for circulation pumps in heating systems. Higher-quality specimens have sintered bearing bushes or ball bearings for the motor axis instead of hardened steel.
Due to the increasing availability of inexpensive electronic frequency converters , capacitor motors, especially in the upper power range, are increasingly being replaced by three-phase three-phase motors. Frequency converters can generate the three phases with variable frequency and amplitude required for the three-phase motor from single-phase alternating current or direct current.
literature
- Rolf Fischer: Electrical machines . 16th edition. Carl Hanser Verlag, 2013, ISBN 978-3-446-43813-2 .
- Günter Springer: Fachkunde Elektrotechnik , 18th edition, Verlag Europa-Lehrmittel, Wuppertal, 1989, ISBN 3-8085-3018-9
- Fritz Henze: Multi-colored circuit diagrams for heavy current technology, Vol. 2 , Leipzig specialist book publisher 1953
Web links
Individual evidence
- ↑ Lawnmower TROLLI from GDR production
- ↑ Manfred Rudolph, Ulrich Wagner: Energy application technology, ways and techniques for more efficient energy use . Springer, VDI, 2008, ISBN 978-3-540-79021-1 .