How to Specify a Variable Frequency Drive

By Luke Lancaster on 20th Mar 2014

How to Specify a Variable Frequency Drive

Table of Content

  • Load Type for Variable Frequency Drives
  • Motor Information
  • Supply Power
  • Control Methods for Variable Frequency Drives
  • Speed Reference Alternatives
  • Efficiency and Power Factor
  • Protection and Power Quality
  • Harmonics

Load Type for Variable Frequency Drives

Does your Variable Frequency Drive application require a variable torque or constant torque drive?

If the equipment being driven is centrifugal, such as a fan or pump, then a variable torque drive will be more appropriate.

One primary VFD Benefit and usually the primary motivation for installing Variable Frequency Drives for centrifugal applications is energy savings, and variable torque drives offer the greatest energy savings.

For example, a fan needs less torque when running at 50% speed than it does when running at full speed. Variable torque operation allows the motor to apply only the torque needed, which results in reduced energy consumption. Conveyors, positive displacement pumps, punch presses, extruders, and other similar type applications require constant level of torque at all speeds. In which case, constant torque variable frequency drives would be more appropriate for the job.

A constant torque drive should have an overload current capacity of 150% or more for one minute. Variable torque variable frequency drives need only an overload current capacity of 120% for one minute since centrifugal applications rarely exceed the rated current. When buying a VFD for a system needing tight process control, then you may need to look into purchasing a sensorless vector, or flux vector Variable Frequency Drive, which allow a high level of accuracy in controlling speed, torque, and positioning.

Motor Information

The following motor information will be needed to select the proper variable frequency drive:

  • Full Load Amp Rating: Using a motor's horsepower is an inaccurate way to size variable frequency drives.
  • Voltage Rating
  • Speed Range: Generally speaking, a motor should not be run at any less than 20% its specified maximum speed allowed. If it is run at a speed less than this without auxiliary motor cooling, the motor will overheat. Auxiliary motor cooling should be used if the motor must be operated at slow speeds.
  • Multiple Motors: To size a variable frequency drive that will control more than one motor, add together the full-load amp ratings of each of the motors. All motors controlled by a single drive must have an equal voltage rating.

In order to gather such motor data, it is crucial to know How to Read a Motor Nameplate.

Supply Power

If continuous operation is a must, then the following should be specified:

  • +/- 10% voltage fluctuation
  • +/- 3% frequency variation
  • Voltage sag ride-through for the following:
    0% voltage for 1 cycle
    60% voltage for 10 cycles
    87% voltage continuous

If you need to supply a 3-phase drive with single-phase power, then the drive should be derated by 25% to 50% of its current-handling ability, which may require you to specify a larger-sized drive.

Control Methods for Variable Frequency Drives

With 2-wire control, only one switch is used to run the variable frequency drive. An open switch stops the drive, and a closed switch starts the drive. Two-wire control is predominately used in HVAC applications since it is able to maintain the RUN command to the drive during a loss of power, which enables Variable Frequency Drives to automatically restart when power is restored. Plus, 2-wire control allows drives that have "power loss ride-through" to operate during a power drop that is 2 seconds or less in duration.

With 3-wire control, two switches are used to run the drive. One switch is needed to stop, and another to start the variable frequency drive. This allows an auxiliary contact from the start to "seal in" the RUN command, just like your more conventional motor starters.

Speed Reference Alternatives

  • Speed Potentiometer: Allows the operator to set motor speed with pot.
  • Digital Programming / Display Unit: Allows the operator to program and troubleshoot the drive by inputting values through a keypad with an LED or LCD display unit. Drive operation can also be monitored through this display.
  • Analog Signal Follower: 4-20mA or 0-10VDC; must provide variable frequency drives with an isolated input; must use a twisted/shielded pair and keep wire away from 3-phase AC.
  • Selector switch speed selection: Allows operator to select from several preset speeds. Can also be used if the speed is being set via a PLC, and an analog output is not available.
  • Serial Communications: Allow variable frequency drives to communicate on a network, such as MODBUS, PROFIBUS, DEVICENET, or METASYS, enabling drive operation to be coordinated and monitored from a PC.

Efficiency and Power Factor

A drive should have an efficiency rating of 95% or better at full load. Variable frequency drives should also offer a true system power factor of 95% or better across the operational speed range, to avoid penalties from the power utility, save on energy bills, and to protect you equipment (especially motors).

Protection and Power Quality

  • Motor overload for instantaneous trip and motor over current.
  • Additional Protection: Over voltage, under voltage, over temperature, ground fault, control or microprocessor fault. These protective circuits should provide an orderly shutdown of the VFD, provide indication of the fault condition, and require a manual reset (except under voltage) before restart. Under voltage from a power loss shall be set to automatically restart after return to normal. The history of the previous three faults shall remain in memory for future review.
  • If a built-up system is required: there should also be externally-operated short circuit protection, door-interlocked fused disconnect, circuit breaker or motor circuit protector (MCP) rated at a minimum of 65,000 AIC, and a door-interlocked handle capable of being locked.


  • The drive/system should be compliant with IEEE 519-1992. The IEEE 519-1992 Table of Current Distortion Limits for general distribution systems (120V through 69,000V) is given below:

Current Distortion Limits for General Distribution Systems

(120V through 69,000V)

Maximum Harmonic Current Distortion in Percent of IL