AC Adjustable Speed Drives


An adjustable speed drives (ASD) is a device used to provide continuous range process speed control (as compared to discrete speed control as in gearboxes or multi-speed motors).An ASD is capable of adjusting both speed and torque from aninduction or synchronous motor

AC adjustable speed drives (ASD) have become very popular variable speed control devices used in industrial,
commercial applications and some residential applications. These devices have been available for approximately 20
years and has a wide range of applications ranging from pumps driven by a single motor, fans and compressors, a highly sophisticated multi-drive machines. They work by varying the frequency of the AC voltage supplied to the motor with a solid state electronic devices. These systems are quite expensive but they provide a greater degree of control over the operation and in many cases, reduce your energy usage enough to make up for it at least if not more than you pay for it. increased cost. ASD allows precise speed control of a standard induction motor and can result in a Energy saving and improved process control in many applications

How it works?

AC Adjustable Speed Drives can be thought of as electrical control devices that change the operating speed of a motor. ASD’s are able to vary the operating speed of the motor by changing the electrical frequency input to the motor

The speed an AC induction motor operates is given by the following equation:

Frequency = Electrical frequency of the power supply in Hz.
Number of poles = Number of electrical poles in the motor stator

  • Motors speed can be changed by altering the electrical frequency, the no. of poles, or both
  • Motor speed can be changed by altering the # of poles in a motor from 4 to 2:

4 pole motor operating on 60 hertz = 1800 rpm.
2 pole motor operating on 60 hertz = 3600 rpm

Types of AC ASD’s

There are three different types of ASD’s on the market that primarily differ in the type of rectification they use to convert AC to DC and back to AC.

  • VVI – Variable Voltage Input
  •  CSI – Current Source Input
  •  PWM – Pulse Width Modulated



  • Energy Savings
  • Improved Process Control
  • Reduced Voltage Starting
  • Lower System Maintenance
  • Bypass Capability



  • Initial Cost
  • Motor Heating at low Speeds
  • Maintenance
  • Output Harmonics

Also read 

What are Auxiliary Current Transformers ?

Electrical safety in power distribution

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