VFD Converter and Inverter stages VFD Converter and Inverter stages VFD Converter and Inverter stages

The Converter Stage

The first stage of a Variable Frequency Drive is the converter. This component consists of six diodes, which function similarly to check valves in a plumbing system, allowing current to flow in only one direction. The diodes are arranged to create a three-phase rectifier. Here’s how it works:

When the A-phase voltage is higher than the B or C-phase voltages, the corresponding diode opens, allowing current to flow.

As the B-phase voltage becomes higher than the A-phase, the B-phase diode opens while the A-phase diode closes.

The same principle applies to the three diodes on the negative side of the bus.

This arrangement results in six current “pulses” as each diode opens and closes, which is why this setup is known as a “six-pulse VFD”—the standard configuration for current VFDs.

DC Bus and Smoothing

Assume the VFD is operating on a 480V power system. The 480V rating is “rms” (root-mean-square), with peak voltages reaching approximately 679V. The DC bus within the VFD has a DC voltage with an AC ripple, fluctuating between roughly 580V and 680V.

To smooth out this AC ripple, a capacitor is used. The capacitor functions like a reservoir or accumulator in a plumbing system, absorbing the AC ripple and delivering a stable DC voltage. Typically, the AC ripple on the DC bus is less than 3V, resulting in an approximate DC bus voltage of 650V. The actual voltage can vary based on factors such as the AC line voltage, voltage imbalance in the power system, motor load, power system impedance, and the presence of reactors or harmonic filters.

The Inverter Stage

The DC-AC conversion process in a VFD involves two main converters:

Diode Bridge Converter: Converts AC to DC.

Inverter: Converts DC back to AC.

The inverter uses transistors as switches. When a top switch in the inverter is closed, the corresponding motor phase connects to the positive DC bus, making that phase positive. Conversely, closing a bottom switch connects the phase to the negative DC bus, making it negative. By controlling these switches, any motor phase can be made positive, negative, or zero, allowing the generation of any desired frequency.

Pulse Width Modulation (PWM)

The output from the VFD is a rectangular waveform rather than a sinusoidal one. Although this rectangular waveform is not suitable for general-purpose distribution systems, it works well for motor control.

To reduce the motor frequency to, for instance, 30 Hz, the inverter output transistors are switched more slowly. However, reducing the frequency to 30 Hz necessitates reducing the voltage to 240V to maintain the V/Hz ratio. This is achieved through Pulse Width Modulation (PWM).

PWM involves switching the output transistors on and off at a high rate of speed. During the first half cycle, if the voltage is on half the time and off the other half, the average voltage is half of 480V, or 240V. By adjusting the pulse width, any average voltage can be achieved at the VFD output.