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Advantages of Variable Speed Motors
Advantages of Variable Speed Motors

An overview of variable speed motors

Most motors are designed to operate at a constant speed and provide a constant output. While in many cases this may be more than adequate, it is not in all. Two-speed induction motors can improve efficiency for refrigerators, air conditioners, and blowers.

Although in theory this can be done with any induction motor application, a greater value is obtained with appliances that run frequently. With a two-speed mode of operation, long time periods that would normally use full power can be replaced by long periods of substantially less power with short periods when full power may be needed.

Currently, residential central air conditioners, blowers (furnaces), and clothes washers take advantage of this technology since small changes in speed can drastically cut down on power usage (power consumption is approximately proportional to the cube root of shaft speed, e.g., a shaft reduction of 10% corresponds to at 27% reduction of power).

There are many ways to control the shaft speed of a motor. The most common way is via throttling devices such as valves and inlet vanes.

However, this type of control is comparable to driving a car at a high speed and controlling the speed by using the brake.

Another way is by using ASDs. This type of drive controls the speed by regulating the voltage, current, and/or frequency sent to the motor until the approximate load speed is obtained. Several types of ASDs are available, each with its own characteristics and practical applications. Even in these devices, there are many different kinds.

Pulse width modulation (PWM) ASD

Pulse width modulation (PWM) ASDs work by chopping pulses of varying widths to create the desired output voltage. They do this by using computer software which in turn is controlled by complex algorithms monitoring timing, duration, and frequency.

This type of ASD has a rather high power factor, good response time, as well as low harmonic distortion. They also have the capability to contol many different motors from the same system.

Their downfall is higher heat dissipation and a limited data cable length from the control to the motor.

Voltage source inverter (VSI) ASD

Voltage source inverter (VSI) ASDs can also control many motors from a single drive and have the advantage of simple circuitry (an advantage that does not exist in PWM ASDs). They normally have a capacitor before the inverter to help store energy and keep the voltage stable.

Their control ranges from about 10 to 200% of rated motor speed; however, below 10% it breaks down and becomes very inefficient.

Current source inverter (CSI)

The last common type of ASD is the current source inverter (CSI). It uses the inductive characteristics of the motor to stabilize DC as it reaches the inverter. Because this induction has to be rather large, this type of drive can only be used in medium to large motors.

Advantages include short-circuit protection, quiet operation, and high efficiency at a wide range of speeds (normally above 50%). However, disadvantages include the inability to test the drive while not connected to a motor and complexity in connecting multiple motors to a single drive.

Table 1 summarizes the characteristics of these four types of drives. Although many of these applications are beyond utilization for the small-scale motors being analyzed, it is mentioned to point out the potential energy saving for similar systems on an achievable smaller scale.

Table 1 – Drives characteristics

CharacteristicsPulse with modulationVoltage source inverterCurrent source inverter
Ease with retrofittingYesYesNo
Soft startYesYesYes
RegenerationOption (a)Option (a)Ingerent
Motor heatingHighLowLow
Motor noiseHighLowLow
Partial loadingYesYesUnstable
Low-speed operationSmoothNoNo
Low-speed torque pulsationNoYesYes
Frequency above 60HzYesYesNo
Open-circuit protectionInherentInherentRequired (b)
Short-circuit protectionRequired (b)Required (b)Inherent
Overload protectionRequired (b)Required (b)Inherent
Multimotor driveYesYesOption (a)
Controller and logicComplexSimpleSemicomplex
60-Hz power factorHighPoorPoor
60-Hz harmonicsLowHighHigh
Motor harmonicsHighModerateModerate
Voltage stresses on motorYesNoYes
DC filter sizeSmallLargeLarge
Invertor noiseHighMediumMedium
Transistor/GTO technologyYesYesNo
Inverter switchesHigh frequencyLow frequencyLow frequency
Size and weightSmallMediumSmall

(a) – Feature is available at extra cost
(b) – Feature must be provided by the system design


  1. With a high enough applied voltage, electrons can be freed from the atoms of insulating materials, resulting in current through that material.
  2. The minimum voltage required to ”violate” an insulator by forcing current through it is called the breakdown voltage, or dielectric strength.
  3. The thicker a piece of insulating material, the higher the breakdown voltage, all other factors being equal.
  4. Specific dielectric strength is typically rated in one of two equivalent units: volts per mil, or kilovolts per inch.

Resource: Energy-Efficient Electric Motors by By Ali Emadi

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Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. Luke Smith
    Nov 19, 2019

    It’s interesting to know that residential AC units and furnaces use variable electric motor because it cuts down the power usage. I’d assume that plants with big motors want their motors maintained to prevent more expenses. I’ll recommend hiring an outsourced motor repair and maintenance for thorough inspection and replacement of parts, if necessary.

  2. aiitabz
    Jun 14, 2012

    i am interested on this topic. it is very useful. is it possible that I could make this as my topic on my thesis?

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