Variable-frequency drives (vfd’s) convert ac input voltage into dc in a rectifier and switch the dc voltage to the output in a manner that simulates a sine wave of voltage to the motor. The electronics controlling the switching permit the output voltage to be varied in frequency, thus controlling the speed of a standard ac motor, which is proportional to frequency. For a detailed explanation of drive and motor operation, refer to my August 1999 column (page 72).

Power quality impacts can occur on both the input and output sides of the vfd. We will start this month with the input side, which has the greater potential to affect other equipment.

Figure 1. Typical vfd input current waveform rich in harmonics.

Switched Input Current

Whenever current flow in an ac circuit is switched on and off rather than permitted to follow the voltage waveform, high-frequency currents at integer multiples of the power system frequency (harmonics) are generated. The switching required to control the rectifier in a vfd results in an input current waveform containing a summation of current at the fundamental frequency (60 Hz) and higher order harmonic currents (see Figure 1). The harmonics produced by a standard three-phase vfd are the 5th (300 Hz), 7th (420 Hz), 11th (660 Hz), 13th (780 Hz), and so on, following the pattern of 6N ± 1, with magnitude decreasing as the harmonic order increases.

Figure 2. Distorted ac voltage waveform showing effects of harmonic currents.

Voltage Drop – Again!

The reason that these harmonic currents are a concern goes back to an old familiar concept: voltage drop. Any current drawn from the power system results in voltage drop equal to the product of the current and the impedance of the system, and we must limit system impedance to prevent voltage drop from reducing the voltage at the loads to unacceptable levels. Unfortunately, power systems are highly inductive – their impedance to current flow increases with frequency. This means that relatively low magnitude high-frequency currents can produce significant voltage drops. When these high-frequency voltage drops are subtracted from the “clean” source voltage, the result is a distorted sine wave at the load (Figure 2).


Voltage distortion due to harmonics is thus a concern when either harmonic currents, system impedance, or both, are high. But how high is high? A general rule of thumb is that if the capacity of vfd’s or other power conversion equipment (dc drives, UPS systems, etc.) exceeds 20% of the system capacity, harmonics should be addressed. Vfd’s operated from a standby generator always present a concern due to the high impedance of the generator compared to the utility. Another situation with potential for problems is operating vfd’s from a system having power factor correction capacitors connected at the same voltage level; resonance between the system inductance and the capacitors at harmonic frequencies can cause excessive currents and blown fuses.

The magnitude of harmonic distortion is defined in RMS terms as Total Harmonic Distortion (THD) and is usually expressed as a percentage. IEEE 519 – 1992 “Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems,” provides voltage THD limits for commercial electrical systems. Dedicated systems which supply only vfd’s or other power conversion equipment are permitted to have as much as 10% THD. General systems, the category most buildings fall into, are limited to 5%, and special systems such as hospitals and air traffic control facilities are limited to a maximum of 3%. The document also includes limits on current distortion, which we will not discuss here, but is important when vfd’s are served from a standby generator.

Specification Tips

If harmonic distortion is a concern, the vfd specification should require compliance with the appropriate IEEE 519 limits. Drive manufacturers can supply computer analysis predicting harmonic performance as a shop drawing submittal; because the results depend on system impedance, the manufacturer will require the layout of the power system as shown on the electrical drawings and the utility impedance to do this analysis. Also include a requirement for certified field measurements verifying compliance as part of startup.