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Why do some VFDs need reactors?

There are two types of reactors used with VFDs: load reactors (or DC bus chokes) and AC line reactors.

The goals of these reactors are to reduce the harmonic content of the input current (improve distortion power factor) and to reduce the conducted EMC noise on the supply.

Some VFDs have either of these reactors built in as standard, and others don't . The variable frequency drives that don't include the reactors are smaller and cheaper, but may require them to be mounted separately in order to comply with CE or similar EMI/EMC regulations.

Look at the input current waveform through an oscilloscope (see image below). You will notice that it's made up of short high-amplitude current pulses on the crest of the waveform (left). This is due to the capacitive-input filter. The addition of the reactor will stretch these pulses (right).

VFD input current waveform

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