20 Oct

Variable Frequency Drives (VFDs) Explained

By Dez Duran-Lamanilao

Hydraulic power units (HPUs) are a great example of applications where variable frequency drives (VFDs) are used. A VFD uses power electronics to change the frequency of input power to the motor. This allows the VFD to control motor speed.

Why is VFD important? When electric motors (e.g.: induction motors, permanent magnet motors, reluctance motors) spin at a constant RPM (revolutions per minute), there is a constant need as well for hydraulic pumps to flow oil, thus the possibility of wasted energy in between system operations. Using VFDs can reduce the required energy.

Essentially, a variable frequency drive has three main sections:

  • Rectifier – converts incoming AC (alternating current) power into direct current (DC) power
  • DC Bus – contains capacitors to take power from the rectifier, store it and deliver it through the inverter section
  • Inverter – comprises transistors that deliver power to the motor

Estimates show that up to 65% of electrical energy in the US is used for motors; and 18% of energy goes to around 40 million motors operated in the country. Imagine the savings if you are able to reduce that energy. Aside from energy efficiency, other key advantages of using variable frequency drives include:

  • Longer motor life
  • Seamless integration into existing equipment and systems
  • Less possibilities of shutdowns
  • A viable solution to safety concerns
  • Elimination of water hammer issues in a liquid system
  • Protection for driven equipment that cannot tolerate excessive torque
  • Help in the efficient charging of accumulators, eventually reducing system maintenance frequency

The list is continuously growing, especially in today’s world where technology is becoming more powerful and innovative. Similarly, HVAC applications such as chillers, pumps and tower fans also benefit from variable frequency drives. Some of the advantages include energy savings, reduced thermal and mechnical stresses on motors and belts during starts, high power factor and lower KVA (kilovolt-ampere).

Variable frequency drives’ capability to improve a motor’s performance should not be seen as competition or as a threat but rather should be considered as a future possibility for the growing needs of your company. Being able to exercise control on temperature, flow or pressure may be an efficient option especially if you are looking for ways to improve a complex circuit over a relatively long time.

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