Causes of Harmonics in Drives

Conventional adjustable speed drives (ASDs) produce large amounts of harmonics as they rectify AC voltage to DC and then invert it to a quasi-square wave to power the motor. During rectification, the AC power is chopped into pulses of DC voltage which results in flat spots on the AC waveform. Similarly, inverters produce a stepped waveform to power the motor rather than a smooth sine. These non-sinusoidal currents and voltages at frequencies other than the fundamental supply frequency are what define harmonics.

Harmonics Impacts and Issues

Harmonics can negatively impact systems in several ways if not controlled. They increase losses in conductors and transformers from skin effect, causing I2R heating. This reduces efficiency and lifetime. Harmonics also produce voltage distortion that can cause issues like overvoltages tripping circuit breakers, nuisance tripping, and even failure of sensitive equipment. Large third or fifth harmonics can cause torque pulsations and acoustic noise in motors. Excessive fifth and seventh harmonics increase bearing currents which accelerate wear. This reduced reliability and increased maintenance costs over time.

Low Harmonic Drive Technology

To combat these issues, Low Harmonic Drives  technologies were developed. Rather than simply rectifying and chopping the input voltage, these drives actively filter and condition the input current and output voltage to produce a quasi-sinusoidal waveform. Some common low harmonic drive topologies used are:

Active Front End Drives

Active front end (AFE) drives have an active rectifier consisting of IGBTs at the input. This allows precise control of the input current to make it smooth and virtually distortion-free. The DC link then feeds a standard PWM inverter for the motor. AFE drives can achieve input current THD under 5% which is a huge improvement over conventional drives.

Passive Input Filters

Passive input filters, consisting of inductors and capacitors, are used to filter higher order frequencies from the input current. They provide a cost-effective solution for lower power ratings. However passive filters only reduce certain harmonics and may still allow others depending on the filter design.

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