A Quick Look at Variable Frequency Drives

Variable Frequency Drives

A variable frequency drive (or VFD) is a device used to control the speed of a three phase AC motor. Variable frequency drives are also known as AC drives, or variable speed drives (VSD) They achieve motor control by converting sine wave power into a digitally-optimized signal, which is manipulated by a controller.

VFD’s save energy, and reduce mechanical shock. Applications include fan speed and pump flow. VFD’s enable control over speed, flow, pressure, acceleration, torque and more. A VFD can reduce operating costs by increasing system reliability, reducing energy costs and reducing equipment downtime.


An example of a VFD unit that could be found mounted on a wall or enclosure.

An input reactor (also known as line reactor) protects a variable frequency drive from input power line disturbances (such as electrical noise, harmonics, and transients), thereby reducing occurrences of nuisance tripping and preventing damage to the drive. In addition, and input reactor reduces harmonics created by the VFD reflected back onto the line

An output (load) reactor can be implemented to protect the VFD-controlled motor when the wiring distance between the VFD and motor is significant. The VFD-generated waveform is susceptible to generating noise spikes, which are amplified with long cable lengths and cable capacitance, which can result in insulation breakdown.

For very long cable runs, an improved type of protection can be used in place of an output reactor, known as a dV/dT filter. These filters are better suited for protection at very long distances.

Signal protection methods such as reactors and filters must be considered with respect to on-site power quality, cable shielding, noise suppression, distance between VFD and motor (resulting in capacitance), and cable specifications. To suppress electrical noise and reduce harmonics, an isolation transformer can be implemented among other noise suppression techniques.

A high efficiency motor with a 1.15 service factor is recommended when used with a VFD. Due harmonics, the 1.15 service factor is often reduced to 1.0 when the motor is used for a VFD application.

Starting a Motor with a VFD

When started with a VFD, a NEMA B motor can benefit from a reduced inrush current requirement. Recall that the typical inrush current for a NEMA B motor is 600% for a full-voltage start. This is necessary due to the nature of the full-voltage start; the motor must immediately get to full rated RPM. However, with a VFD, the motor now has the ability to start from 0 RPM and slowly ramp itself to the required RPM, all while maintaining 150% starting torque (just like a full-voltage start).

Operation of a Variable Frequency Drive

The volts per hertz ratio (V/Hz) is the ratio of the applied voltage to applied frequency of a motor. For example, a 600V three phase AC motor may operate at 60Hz, resulting in a volts per hertz ratio of 10V/Hz.

Controlling a Motor with Volts per Hertz

With constant torque loads, the torque loading is not a function of speed. As the speed changes, the load torque remains constant. Horsepower changes linearly with speed. Applications include positive displacement pumps, conveyors, and compressors.

In order to fulfill constant torque operation requirements, a VFD must operate a motor such that the volts per hertz ratio is maintained. To increase the speed at which a motor runs, the frequency must be increased. However, the applied voltage must increase as well to maintain the volts per hertz ratio.

In contrast, a variable torque load operate non-linearly. Applications include centrifugal pumps and fans. For these loads, horsepower typically varies as the cube of motor speed while torque varies as square of motor speed.

There are limitations to V/Hz control. With V/Hz control, the VFD generates a waveform calculated under ideal conditions, without any consideration for the real-world load conditions of the motor. The motor is limited to receiving a waveform determined solely by the VFD program. True torque control cannot be achieved since the VFD is blind to the motor’s real-world output torque.

Controlling a Motor with Vector Control

Vector control utilizes real-world motor performance information to fine tune the waveform the VFD sends to the motor. Feedback information from the motor is used to calculate voltage and frequency vectors. With this control method, the VFD monitors the motor and constantly updates vector to maintain motor operation and fulfill any torque or speed requirements. The drive generates a waveform for the motor, checks the motor performance and operation, and corrects the waveform depending on how the motor performs.

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