From a design standpoint, it is most desirable to operate all motors within a plant at full rated speed. Due to product considerations, however, there are many applications requiring variable speed operation. When a process requires the quality of speed variability, variable speed control is necessary.

The most common types of variable speed controls in the process industry today are: eddy current clutches, hydraulic couplings, variable pitch sheaves, DC solid state controls, AC variable frequency drives, and control valves. The type of variable speed control finally selected depends on the initial cost, application parameters, and environmental operating characteristics.

AC drives allow loads driven by AC induction motors (such as fans and pumps) to operate in a wide range of speeds compared to the motor fixed speed. AC drives are also called variable frequency drives, adjustable speed drives, frequency inverters, or frequency converters.

AC drive installations can increase energy efficiency (in some cases energy savings can exceed 50%), improve power factor and process precision, and provide other performance benefits such as soft starting and over-speed capability. They also can eliminate the need for expensive and energy-wasting throttling mechanisms such as control valves and outlet dampers.

Typical applications of AC drives
AC drives can provide significant savings in applications such as:

  • Variable air volume air conditioning systems
  • Chilled water pumping
  • Exhaust air systems, such as dust extraction, paint shop exhaust, and fume cupboards
  • Refrigeration systems
  • Some modern compressors (including air and refrigeration compressors)

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