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
Motor controllers receive supply voltages and provide signals to motor drives that are interfaced to motors. They include a power supply, amplifier, user interface, and position control circuitry.
Single Phase Uninterruptible Power Supplies (UPS)
Single phase uninterruptible power supplies (UPS) sit between an AC outlet and an electronic device to provide power conditioning, back-up protection, and distribution for electronic equipment loads. They also prevent power disturbances from affecting the performance and life of the electronic device and vital data. 
Power Rectifiers
Power rectifiers are electrical devices that convert alternating current (AC) to direct current (DC). They are an integral part of telecom rectifiers for the telecommunications industry, and in battery chargers, DC power systems, and other power system devices.
Three Phase Uninterruptible Power Supplies (UPS)
Three phase uninterruptible power supplies (UPS) operate in conjunction with existing electrical systems to provide power conditioning, back-up protection, and distribution for electronic equipment loads that use three-phase power. 
Adjustable Speed Drives
Adjustable speed drives alter the speed of a driven shaft to a speed selected by an operator.

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