Make the Most of Variable Frequency Drives

Optimum performance depends upon proper installation and control.

By Robert Heider, Washington University, and Clay Lynch, French Gerleman Electric Co.

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The first task is to gather as much information about the application as possible. How will it be started and stopped? Will a two-wire or three-wire control start the drive? Will it undergo a controlled ramp to stop, will load determine how long it will stop by using a coast to stop, or will the drive stop very fast and dissipate the energy through a dynamic brake? How will the drive be controlled? Will it need an analog signal or a digital signal commanding a preset speed? Will a 4-to-20-mA signal or a 0-to-10-V signal control the drive? Will the drive need to be configured to give an analog output to feed another process or another drive? Will the application require the use of a programmable digital output? Is it necessary to program the drive to close a set of “dry contacts” once it’s achieved a certain speed, using that as an input to feed a start command to another drive or another process?

Next, access the motor and accurately record all of the data from its nameplate:

1. the rated volts — most industrial AC motors are either 230 or 460 V;

2. the full load amps (FLA), the maximum current the motor is expected to draw under fully loaded conditions — this is probably the most important piece of information;

3. the frequency, which normally is 60 Hz but can vary depending on where the unit will be installed; and

4. the rated rpm — this is the load speed at which full load torque is delivered. A very useful formula to keep at hand is hp = (torque) × (speed)/ 5,250, with torque in lb-ft and speed in rpm. Slip affects this, so, an AC motor rated for 1,800 rpm may really only run at 1,750 rpm.

Finally, you’re set to program the drive. Assuming that it’s been installed properly by qualified individuals, the drive will have the appropriate AC power and control voltages. Status lights on the front of most drives will indicate if the drive is in a ready or faulted state. They also will tell you the status of the communications of the drive.

Some AC drives will have assisted startup routines. These types of programs actually will prompt you for the information needed for most applications and commonly adjusted parameters.

VFD training. All AC drive manufacturers offer drive programming training courses or “lunch and learns.” These, when presented properly, can provide a great foundation to build upon.

Understanding the basics of how a VFD works is crucial. This should start with the fundamentals of magnetism, include details about how AC and DC motors operate, the role of speed and torque, and then how the VFD controls the stator and rotor of an AC motor by rectification and using transistors to invert DC voltage back to a voltage an AC motor can use to rotate.

Robert Heider is an adjunct professor in the Department of Energy, Environmental and Chemical Engineering at Washington University, St. Louis. Clay Lynch is VFD and MCC product manager at French Gerleman Electric Co., Maryland Heights, Mo. E-mail them at and


1. “Variable Speed Pumping: A Guide to Successful Applications,” Executive Summary, DOE/GO-102004-1913, U. S. Dept. of Energy, Washington, D. C. (May 2004). Downloadable via
2. Krause, P. C. et al., “Analysis of Electric Machinery and Drive Systems,” 2 ed., Wiley Interscience, Hoboken, N.J. (2004).
3. Spear, Mike, “Get a Fix on ASD Problems,” p. 25, Chemical Processing, December 2005,


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