Variable Frequency Drive

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Variable frequency drive, abbreviated as VFD, also called variable speed drive, abbreviated as VSD, or adjustable frequency drive, abbreviated as AFD, is a device used to control the speed and rotational output of an alternating current (AC) electric motor by adjusting the frequency and voltage supplied to the motor.  By increasing the frequency, the motor can speed itself up and down as required by the control process.  This enables efficient control of motor driven systems, such as pumps, fans, compressors, and conveyor systems, by varying the motor's speed according to the requirements of the application.

Here's how Variable Frequency Drive work

VFDs are particularly useful for optimizing energy consumption, reducing wear and tear on machinery, and improving process control.

  • Basic Function  -  Electric motors operate at a fixed speed determined by the frequency of the electrical power supply.  A VFD changes the frequency and voltage supplied to the motor, allowing the motor's speed to be adjusted over a wide range.
  • Dynamic Braking  -  VFDs can provide dynamic braking, which is the ability to rapidly decelerate the motor by converting its kinetic energy back into electrical energy.  This feature is useful in applications where quick stops are required.
  • Energy Efficiency  -  One of the primary benefits of using a VFD is energy savings.  Many motor driven systems do not need to run at full speed all the time.  By adjusting the motor's speed to match the actual load requirements, the VFD reduces energy consumption compared to running the motor at a constant speed.
  • Frequency Control  -  By increasing or decreasing the frequency supplied to the motor, the VFD can increase or decrease the motor's rotational speed.  This means that the motor can operate at different speeds without the need for mechanical gearboxes or other speed-changing mechanisms.
  • Precise Process Control  -  VFDs enable fine tuned control over processes that require varying speeds, such as in manufacturing, HVAC systems, and industrial automation.  This control can lead to improved product quality and better overall system performance.
  • Soft Start and Stop  -  VFDs allow for smooth acceleration and deceleration of the motor, reducing mechanical stress on the motor and the connected machinery.  This feature is particularly important in applications where abrupt starts and stops could cause damage.
  • Torque Control  -  VFDs can also provide precise control over the torque output of a motor.  This is especially important in applications where maintaining a specific level of torque is critical.

VFDs come in various types and configurations, depending on the specific application and motor type.  They are widely used in industries such as manufacturing, energy, HVAC, water treatment, and more, wherever motor driven systems are utilized and require flexible speed control for improved efficiency and performance.

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

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Tags: Pump Electrical Motor