How do VSDs improve system Power Factor?

VSD impact on Power Factor

A Variable Speed Drive (VSD), delivers power from the source (typically the utility) to a motor in three basic steps:

1. The Rectifier converts AC power to DC power;
2. The DC Bus receives, smooths, and stores the power;
3. The Inverter converts the DC power back into AC with the necessary frequency and voltage via Pulse Width Modulation (PWM).

The Power Factor is essentially the measure of how effectively the piece of equipment uses the electricity to produce the necessary work. A high Power Factor indicates effective utilisation of electrical power, while a low Power Factor is considered poor. In basic terms, the Power Factor is the ratio of Real Power (kilowatts – kW) to Apparent Power (kilovolt-amperes – kVA). A desirable ratio of near unity results when the Real Power and Apparent Power are nearly equivalent.

Utility companies monitor Power Factor and often charge customers a penalty (higher rate) if their Power Factor falls below a specified threshold (because of the extra burden imposed on the distribution system). Not only does the distribution system have to accommodate the Real Power being consumed, but it must also accommodate the Reactive Power component (non-work producing component) flowing within the system.

The purpose of the DC Bus capacitors is to supply the motor load Reactive Power, not to improve the distribution system Power Factor, although this is a beneficial consequence. VSDs improve Power Factor primarily because the motor’s Reactive Power is supplied by the VSDs DC Bus, thereby protecting the utility power supply from being the source of the Reactive Current (improves the displacement Power Factor). A VSD draw almost sinusoidal current from the power supply, so Power Factor on the source side can be controlled up to unity and the generated harmonics are minimal – so the system distortion Power Factor is not greatly affected either.

In other words, VSD input current remains in phase with the power supply voltage under load conditions (ensuring high Power Factor). As alluded to before, the Power Factor can be impacted negatively by current Total Harmonic Distortion (THD) caused by the rectification process.  If the VSD however incorporates an Insulated Gate Bipolar Transistor (IGBT) based PWM rectification scheme the THD could be significantly low (IGBT based rectification configurations typically include substantial input impedance so that the PWM converter switching is not reflected onto the supply).

Engineers mostly opt for motors that provide nearly no excess headroom for the required application rather than operating a larger motor at low Power Factor (due to penalties imposed by utility companies). To avoid such penalties, engineers are very diligent when sizing motors, firstly to prevent oversizing, but also to ensure the selected motor is not undersized for the application. In extreme cases, reduction in machine productivity may result due to selection of a motor that is only marginal for the load. In these cases, selected motors may not support future productivity increases due to the tightly selected motor size. VSDs can however be used to improve system Power Factor when oversized motors or motors that operate at low Power Factor are used.

In conclusion, as energy charges increase, improving the Power Factor can help keep energy expenses low. VSDs can be used to improve Power Factor as well as provide flexibility in motor selection (effective utilisation of energy is maintained when using oversized motors to accommodate future production increases).

If even more Power Factor improvement is desired, consider using a PWM based VSD that offers a DC Bus/Link Choke/Reactor or add an AC Input Choke/Reactor.