Tech Tip

Increase VFD System Performance & Reliability

by John A. Houdek - MTE Corporation


Reactors have been used for many years to solve problems in variable speed drive installations. About ten years ago the use of line reactors started to become more common as they helped to solve typical problems on the input (line side) of variable frequency drives (VFD) and SCR controllers. They often have been used as low cost substitutes for 1:1 isolation transformers. The typical problems that line reactors solved were drive nuisance tripping, voltage notch reduction (for SCR controllers) and harmonic attenuation. They were called "line reactors" because they were always used on the "line side" or input of a variable speed drive. Attempts to use "line reactors" on the output side of a drive tended to fail because the line reactors typically overheated due to the harmonic content of the output waveform.

In 1989 the industry experienced the introduction of Harmonic Compensated Reactors which now offered a product that was suitable for use on either the input or output of a variable speed drive. Harmonic compensation meant the reactor was designed to handle the harmonic spectrum and high frequency carrier waves which are typical on the output side of a variable speed drive. Not only are the frequencies higher, but the RMS current is also higher whenever harmonics are present. (Example: 100% fundamental current + 100% harmonic current = 141% RMS current, via Pythagorean theorem). Harmonic compensated reactors would not only handle these conditions from a thermal perspective, but they also offered full performance and inductance in the presence of even severe harmonics. Therefore Harmonic Compensation offered an assurance of both safety and performance.

Now that reactors could be used on the output of a VFD, many more application problems could be solved. The most typical problems included motor temperature rise, motor noise, motor efficiency, and VFD short circuit protection.

Motor Temperature Reduction

Motors operated on a VFD tend to run warmer than when they are operated on pure 60hz, such as in an across-the-line stator application. The reason is that the output waveform of the VFD is not pure 60hz,, but rather it contains harmonics which are currents flowing at higher frequencies. The higher frequencies cause additional watts loss and heat to be dissipated by the iron of the motor, while the higher currents cause additional watts loss and heat to be dissipated by the copper windings of the motor. Typically the larger horsepower motors (lower inductance motors) will experience the greatest heating when operated on a VFD.

Reactors installed on the output of a VFD will reduce the motor operating temperature by actually reducing the harmonic content in the output waveform. A five percent impedance, harmonic compensated reactor will typically reduce the motor temperature by 20 degrees Celsius or more. If we consider that the typical motor insulation system has a "Ten Degree C Half Life" (Continual operation at 10 degrees C above rated temperature results in one half expected motor life), then we can see that motor life in VFD applications can easily be doubled. Harmonic compensated reactors are actually designed for the harmonic currents and frequencies whereas the motor is not.

Motor Noise

Because the carrier frequency and harmonic spectrum of many Pulse Width Modulated (PWM) drives is in the human audible range, we can actually hear the higher frequencies in motors which are being operated by these drives. A five percent impedance harmonic compensated reactor will virtually eliminate the higher order harmonics (11th & up) and will substantially reduce the lower order harmonics (5th & 7th). By reducing these harmonics, the presence of higher frequencies is diminished and thus the audible noise is reduced. Depending on motor size, load, speed, and construction the audible noise can typically be reduced from 3 - 6 dB when a five percent impedance harmonic compensated reactor is installed on the output of a PWM drive. Because we humans hear logarithmically, every 3dB cuts the noise in half to our ears. This means the motor is quieter and the remaining noise will not travel as far.

Motor Efficiency

Because harmonic currents and frequencies cause additional watts loss in both the copper windings and the iron of a motor, the actual mechanical ability of the motor is reduced. These watts are expended as heat instead of as mechanical power. When a harmonic compensated reactor is added to the VFD output, harmonics are reduced, causing motor watts loss to be reduced. The motor is able to deliver more power to the load at greater efficiency. Utility tests conducted on VFDs with and without output reactors have documented efficiency increases of as much as eight percent (at 75% load) when the harmonic compensated reactors were used. Even greater efficiency improvements are realized as the load is increased.

Short Circuit Protection

When a short circuit is experienced at the motor, very often VFD transistors are damaged. Although VFDs typically have over correct protection built-in, the short circuit current can be very severe and its rise time can be so rapid that damage can occur before the drive circuitry can properly react. A harmonic compensated reactor (3% impedance is typically sufficient) will provide current limiting to safer values, and will also slow down the short circuit current rise time. The drive is allowed more time to react and to safely shut the system down. You still have to repair the motor but you save the drive transistors.

Other Applications

Output reactors solve other problems on the load side of VFDs in specialized applications also. Some of these include: Motor protection in IGBT drive installations with long lead lengths between the drive and motor, Drive tripping when a second motor is switched onto the drive output while another motor is already running, and Drive tripping due to current surges from either a rapid increase or decrease in the load.


Whether you are using reactors on the input or output of a VFD the best performance results will always be achieved using reactors which are fully compensated for the harmonics which are present. This assures maximum inductance and thus best attenuation of harmonics with maximum current surge protection. Harmonic compensated reactors have proven to be the most cost effective solution to a wide variety of typical VFD application problems. Use them on the input or output of your drives to improve the total system performance and reliability.

Recommended Percent Impedance for Typical Applications:

3% - Current surge protection
3% - Voltage transient protection
3% - Drive nuisance tripping
3% - Voltage notch reduction (SCRs)
3% - Capacitor switching spike protection
3% - Motor short circuit protection
3% - Multiple motor applications

5% - Harmonic reduction
5% - Motor temperature reduction
5% - Motor noise reduction
5% - Motor efficiency improvement
5% - IGBT w/ long lead lengths

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REACSOLU (orig.-7/31/02)