A VFD, or Variable Frequency Drive, does not change the output voltages up or down. The AC arrives from the Mains. The AC if rectified with a full-bridge rectifier. That DC is fed to a device(inverter circuit) that creates 3-phase AC at specified output voltage, usually at 120VAC. The VFD changes motor speed by changing the AC on each phase using a PWM, or Pulse Width Modulation device, thereby changing the effective outputs controlling the 3-phase motor from a typical AC wave to the new PWM wave.
Nowadays VVVFD's are used means for 'Variable Voltage Variable Frequency Drives.’
One can change the sample frequency, but not the output voltage. If the voltage on any phase were to be be less than 120VAC, the motor would generate excess heat and burn out pretty quickly.
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