From Electrical Equipment Handbook: Troubleshooting and Maintenance



What are the main components of a variable-speed drive?


What are the differences between a variable-speed drive and an uninterruptible power system?


What is the principle of operation of a pulse width-modulated (PWM) inverter?


What are the steps required to size and select an uninterruptible power system?


Are there any industrial problems with UPS and variable-speed drives?



The main components of a variable-speed drive, also known as a variable-frequency drive, are

  • A rectifier that rectifies the input three-phase, 60-Hz, ac power to dc power. It is normally called the source bridge.

  • An energy storage device, which separates the input from the output and allows each to operate independently from the other. It is called a link fitter (or a dc link). It consists of inductors in series with the rectifier and capacitors in parallel with the rectifier.

  • A dc-to-ac inverter. It generates the desired ac output voltage and frequency. The frequency in the output power is normally 10 to 120 Hz.


A UPS has all the components that make a variable-speed drive and a battery bank tied to its dc link. The frequency at the output of the UPS is normally maintained constant (for example, 60 Hz). Its function is to provide uninterruptible power when the ac power supplied to its input fails.


Pulse width modulation is known as time ratio control. From a constant dc input voltage to the inverter, we get a variable...

Products & Services
Motor Controllers
Single Phase Uninterruptible Power Supplies (UPS)
Power Rectifiers
Three Phase Uninterruptible Power Supplies (UPS)
Adjustable Speed Drives

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