AC Drive - What is an AC Drive Inverter or VFD

AC drives are used to control the speed and torque of a three phase motor.

PWM Drive (VFD) Block Diagram

Your trainer has been equipped with an AC drive.  The other unique feature of a drive that your trainer utilizes is it can take single phase power and make 3 phase power out of it.

It does this by running the incoming AC power  through diodes arranged in an electrical bridge that converts the AC sine wave into DC power.  This DC bus filters and smooths out the noise.  The diodes actually reconstruct the negative halves of the waveform onto the positive half. In a 460V unit, you'd measure an average DC bus voltage of about 650V to 680V. You can calculate this as line voltage times 1.414. The inductor (L) and the capacitor (C) work together to filter out any AC component of the DC waveform. The smoother the DC waveform, the cleaner the output waveform from the drive.

The DC bus feeds the final section of the drive: the inverter. This section inverts the DC voltage back to AC. It can vary the voltage and frequency output. 

The output of the drive doesn't provide an exact replica of the AC input sine waveform. Instead, it provides voltage pulses that are at a constant magnitude.

 

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