Dual AC variable frequency drive cabinet
Dual AC variable frequency drive cabinet built for American Chillers (photo by Joliet Technologies VSD and Controls)

Variable Speed Control

The simplest and least expensive way to control the speed of a process or piece of equipment is to operate all the equipment at full speed.

Many applications require the speed of a process or piece of equipment to be varied. Prior to the advent of the AC Adjustable Speed Drive, many technologies have been used, although each has its inherent advantages and disadvantages.

There are few types of Variable Speed Control:

  • Control Valves, Dampers and Vanes
  • Fossil Fuel Engines
  • Eddy Current Clutches
  • Hydraulic Couplings
  • Variable Pitch Sheaves
  • DC Solid State Controllers
  • AD Adjustable Speed Drives (ASDs)

AC Adjustable Speed Drive (ASD)

AC Adjustable Speed Drive (ASD) have become very popular variable speed control devices used in industrial, commercial and some residential applications.

These devices have been available for about 20 years and have a wide range of applications ranging from single motor driven pumps, fans and compressors, to highly sophisticated multi-drive machines. They operate by varying the frequency of the AC voltage supplied to the motor using solid state electronic devices.

These systems are fairly expensive but provide a higher degree of control over the operation and in many cases, reduce the energy use enough to a least offset if not more than pay for the increased cost.

ASDs allow precise speed control of a standard induction motor and can result in significant energy savings and improved process control in many applications. ASD can also control the speed of a standard squirrel cage NEMA type B induction motor and they are suitable not only for new applications, but also for retrofit on existing motors.

Adjustable Speed Drive (ASD) Function

AC Adjustable Speed Drives can be thought of as electrical control devices that change the operating speed of a motor. ASDs 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:

Synchronous Speed = 120 x Frequency / Number of Poles


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

Changing the Motor Speed

Motors speed can be changed by altering the electrical frequency, the number of poles, or both. Motor speed can be also changed by altering the number 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.

We really CHANGED speed rather than varied speed!

Motor speed can be changed by altering the frequency of the electrical supply:

4 pole motor operating on 50 hertz = 1500 rpm.
4 pole motor operating on 40 hertz = 1200 rpm.

By varying frequency, we can adjust the speed over a wide range or vary the speed precisely using precise changes in the electrical frequency input to the motor.

Power and Torque

ASDs actually control both frequency and voltage simultaneously to maintain a constant volts/hertz ratio which keeps current flow similar to full speed conditions. This allows the motor to draw full current at any speed and produce full torque as motor speed changes.

What happens to the Horsepower when we lower the speed and torque using frequency?

Horsepower = Speed (in RPM) x Torque (in pound feet) /5,252

Reduced Horsepower = Reduced Energy Use = Energy Savings!!

What About INCREASING Speed?

Some specially designed motors meant for use with ASDs are designed to operate at higher than normal speeds at frequencies above 60 hertz.

Increasing frequency above 60 hertz makes the motor run faster than normal and creates two primary concerns:

  1. Was the motor or the load it drives designed to operate at these increased speeds?
    Many motors and devices were not mechanically balanced to operate at increased speeds and will create vibration, mechanical and safety problems.
  2. ASD’s are not capable of increasing voltage so as frequency increases above 60 hertz, the torque produced starts to decrease. 
    In order to maintain a constant horsepower output to drive our load, if speed is increased, torque must decrease!

At some point of increased speed we may not be able to produce enough torque to drive the load and at this point, the motor will slow even with increasing frequency. This point is different for each manufacturer’s motor and dependant on the torque required by the load.

Only the manufacturer can help determine when this occurs.

ASD Advantages and Disadvantages

Adjustable Speed Drives have a number of advantages and disadvantages compared to other types of variable speed controls including the following:


– Energy Savings
– Improved Process Control
– Reduced Voltage Starting
– Lower System Maintenance
– Bypass Capability
– Multi-motor Control


– Initial Cost
– Motor Heating at low Speeds
– Maintenance
Output Harmonics
– Induced Power Line Harmonics

Adjustable Speed Drives can be used to save significant amounts of energy in process operations compared to traditional control methods where the load or speed of the operation varies.

AC Adjustable Speed Drives are among the most efficient types of speed control when used on axial (variable torque) loads like centrifugal fans, pumps and compressors. As the motor reduces the operating speed of the fan, pump or compressor the horsepower required to operate the system is greatly reduced.

This is a major advantage of the ASD and one of the primary reasons ASDs have become so popular in many process operations.

Resource: PAControl – AC Adjustable Speed Drives (ASDs)

About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on


  1. eddy kasim
    Apr 20, 2016

    Could you explain about using of electric motor in electric train with dc power supply of 1500 VDC?
    What kind of control system for speed and braking..

  2. mehdi
    Jan 06, 2016

    which motor with vfd work better,2-pole or 4-pole ( 6.6 kv) ?

  3. Franklin Luna
    Jan 27, 2014

    what is the maximum % of frequency that you can apply to an induction motor so us not to damage the winding of the motor?

  4. […] Wiring)AC Motor Drive InterfaceA common PLC application is the speed control of AC motors with variable speed (VS) drives. The diagram in Figure 1 shows an operator station used to manually control a VS drive.The […]

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