ST300 | Single-phase Motor

Frequency inverters are designed for operation with a three-phase motor. These can be asynchronous or synchronous motors. They load the three phases symmetrically, which is why the inverter detects asymmetry as a fault.

Most single-phase motors operate based on a different principle. Only capacitor motors are technically asynchronous motors, in which two terminals are connected directly to the mains supply and the third is phase-shifted via a capacitor. This phase shift is required for the motor to start. Some of these motors are actually standard 120° symmetrical three-phase motors that can be converted back to such by removing the capacitor. Others have a 90° phase shift on the auxiliary phase via the capacitor, making them asymmetrical and thus unsuitable for use with a frequency inverter.

Other types of single-phase motors, such as universal motors and shaded-pole motors, are fundamentally not suitable for operation with a frequency inverter. Universal motors are controlled solely by voltage, not by frequency. They inherently rectify current during commutation and can even operate on direct current. They are often used in power tools such as hand drills.

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