ST500 | Single-phase motor

Frequency converters are designed for operation with a three-phase motor. These are asynchronous or synchronous motors. They load the three phases symmetrically. Therefore, the inverter recognises an asymmetry as a fault.

Most single-phase motors work according to a different principle. Only a capacitor motor is actually an asynchronous motor, where two terminals are connected directly to the mains supply and the third is phase-shifted by means of a capacitor. This is necessary for the motor to start. Some of these motors are actually normal 120° balanced three-phase motors and can be converted back into such by removing this capacitor. Others have a phase offset with the auxiliary phase via the capacitor at 90°, so they are unbalanced and cannot be operated on the frequency converter.

Other single-phase motor types such as series-wound motors and shaded-pole motors are generally not suitable for operation on a frequency inverter. Incidentally, series-wound motors are controlled solely by voltage, not by frequency. Implicit rectification takes place during commutation, and they can even be operated directly with direct current. They are often used in power tools such as hand drills.

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