Why is rectifier used in variable frequency drive?

• You want a specific type of control
• You can’t get it directly from AC line voltage or frequencies
• You need DC to power the control circuits

This is a common circuit theme in nearly all switching converters. A power supply I’m currently working for a product has:

AC → DC → AC → DC1, DC2, DC3, etc.

The specific reason for the first step is because the second step requires DC and all the power must be passed as DC. This second step converts that power to AC. AC is desired because it allows the use of a transformer for ground isolation and it reduces the ripple voltage (this AC is at a far higher frequency than the input line AC). Then the third step generates multiple DC supplies. DC1 is 5V, DC2 is ±15V, DC3 is ±250V