Variable Frequency Drives

Variable Frequency Drive (VFD) / Inverter Drive Operation

The converter section rectifies the 3Ø Alternating Current (AC) voltage to Direct Current (DC) voltage via rectifying diodes. Then DC voltage is stored and smoothed in the DC Bus capacitors.

The stored DC voltage is then feed to the Inverter section. Which when called for pulses the DC out to the motor windings. Creating a Pulse Width Modulated (PWM) AC wave form as seen below.

This PWM AC wave can be shaped and changed to any frequency besides the AC line frequency. The output voltage can also vary between 0 ~ AC line Voltage.

Since the motors revolutions per minute (RPM) are directly related to the frequency they are ran on. Varying the frequency then changes the RPM of the motor.

This then allows the driven machines to run at variable rates to improve performance based on manufacturing needs.

A VFD’s full output voltage is achieved at the Base frequency. It is adjustable in most drives to match the driven motors frequency and voltage. Most motors in the USA have a Base frequency of 60 Hz for their rated RPM and voltage.

The control algorithm in the drive can adjust for different load types.

In constant torque mode as seen below. A reduction of half speed or 30 Hz out of the VFD, decreases the output voltage by half. Usually used in machine tools, spindles, conveyors, centrifuges.

For standard 460 VAC motor the voltage to hertz ratio is 7.67V AC per 1 Hz.

In reduced torque operation. The output voltage is reduced at lower frequencies below the constant torque curve. This is due to the load requiring less power at lower speeds. This is used in Fan and pump applications to save energy.