Variable Frequency Drive

Written by Jerry Ratzlaff. Posted in Instrumentation

Variable frequency drives are also known as variable speed drives (VSD).  A VFD is a system for controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor.  By increasing the frequency, the motor can speed itself up and down as required by the control process.

  • Abbreviated as VFD


Motor Starting

When an electrical motor is started by a switch or a PLC, the motor draws a very high current.   Until the motor is running at full speed, the current draw is very high.  When a VFD is used, this amperage inrush can be mitigated by varying the frequency to the motor.  The VFD can then gradually ramp up the frequency lowering the initial current demand.

Note a VFD should not be confused with a Soft Starter (though it can be used in its place)


Reciprocating pumps move fluids based on the speed of the piston strokes. As the speed is increased, more fluid can be pumped (to a limit).  If there is an application where the amount of fluid needs to be increased or decreased, for example, a water injection pump or level control on a tank, a VFD is capable of this.

Centrifugal pumps operate based on required pressures and will move fluid based on the system curve.  However, in dynamic proceses where the downstream pressures or required flow rates change, a back pressure control valve can be used to hold the pump at a specific point on the curve.  However, this is very inefficient as the pump motor is always running at a set point.  A variable frequency drive can increase the speed or decrease the speed of the pump and provide only the horsepower that is required, when it is required.

Throttling Valves

The use of a throttling valve in place of a PLC can often times be an inexpensive solution for controlling the discharge of a pump.  However, this is not always a good solution.  While using a throttling valve, back pressure is held against the discharge of a centrifugal pump.  The pump runs on its curve, regardless of the efficiency.  This is much like running your car at full throttle and controlling your speed with the break pedal.  Operation of a VFD is using the throttle to speed your vehicle up and down.

  • VFDs eliminate requirement for engineering, design, installation, operation and maintenance of control valves. Add the supporting pneumatics, tuning and servicing required (leaks).
  • When throughput rates are constantly changing, the process can throttle itself back, allowing the pumps to run at lower speeds. Note, even with throttling valves, there is a minimum flow rate that must go through a centrifugal pump. In some cases, flow control valves may result in reduced flow rates below pump manufacturer recommended minimums, potentially causing damage do to overheating, etc.
  • VFDs provide a single non-intrusive point for both flow and pressure control. This should be taken into consideration when the process is such that requires very expensive valves or requires the minimization of leak paths (such as acid service).
  • VFDs provide a significant reduction in hydraulic horsepower at lower flows and heads verses a using a throttling valve
  • VFDs are promoted by many utilities as a method of energy conservation and rebate programs may be available to offset the cost of the drive.
  • VFDs offer many other fringe benefits:
    • Multiple measured variables for dynamic motor monitoring/feedback
    • Higher degree of dynamic motor protection
    • Motor current (torque) control
    • Elimination of across the line starters
    • Employ less expensive fusing verses HMPCs required with starters
    • Increased motor and pump life