Electric motors operating on alternating current often run at unnecessary speeds, waste power, and heat up due to excessive harmonics. Variable frequency drives resolve these issues by controlling AC motor speed and torque by varying the frequency (hertz) and voltage delivered to the motor.

What are Variable Frequency Drives?

Variable frequency drives, or VFDs, go by a number of names, including adjustable speed drives, inverters, AC drives, microdrives and variable speed drives. In all cases, they deliver a controlled voltage to electric motors to match the motor’s actual load.

How do Variable Frequency Drives Work?

The frequency, or hertz, of electrical current affects motor speed, or rpms. The higher the frequency, the faster the motor runs. Motors operating at frequencies in excess of what they optimally should heat up faster, waste energy, and must handle harmonics that, left unresolved, shorten the motor’s lifespan.

A VFD reduces frequency and voltage to match the requirements of the motor’s workload, creating smooth DC current from a variable AC power supply. Each VFD includes a converter with positive and negative diodes that open and close to control current. Most drives are six-phase drives, with six diodes.

A smoothing circuit smooths current pulsations after current travels through the diodes, while an inverter switches DC power back to AC for variable frequency demands. The system’s control circuit oversees the process.

One of the great advantages of VFDs is the ability to ramp up or down engine speed as required. Exact energy savings depend on the specific motor application and how the VFD is set up.

Value of VFDs

A VFD offers added value, energy savings, and motor protection in a number of ways:

Harmonics caused by excessively high frequencies cause motors to run hot, making them less efficient and reducing their working life. Adding a VFD helps prevent harmonic interference.

Power factor correction is built in at the service point of many facilities, but only to avoid costly PF corrections from utility providers. Power is still wasted by low power factor at the load, which a VFD drive corrects to reduce energy waste.

Soft starting is an option built into most VFDs. When properly set up, soft starts can both reduce energy use and prolong motor life. More importantly, a VFD allows for speed control at the gearbox’s point of input instead of output. You may still need the gearbox to multiply torque, but controlling speed at input allows you to save energy by reducing motor speed rather than dissipating work already generated by the motor.

Finally, a VFD allows for continuous drive rather than on/off cycling. Most motors are oversized to meet theoretical loads that they rarely, if ever, reach in practice. A VFD matches the motor’s actual load instead of continuously throttling it back, resulting in a smoother operation.


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