Dr. Ahmed G. Abo-Khalil

Electrical Engineering Department

Long lead effects

The carrier frequency pulsed output voltage of a PWM VFD causes rapid rise times in these pulses, the transmission line effects of which must be considered. Since the transmission-line impedance of the cable and motor are different, pulses tend to reflect back from the motor terminals into the cable. The resulting voltages can produce overvoltages equal to twice the DC bus voltage or up to 3.1 times the rated line voltage for long cable runs, putting high stress on the cable and motor windings and eventual insulation failure. Note that standards for three-phase motors rated 230 V or less adequately protect against such long lead overvoltages. On 460 or 575 V systems and inverters with 3rd generation 0.1 microsecond rise time IGBTs, the maximum recommended cable distance between VFD and motor is about 50 m or 150 feet. Solutions to overvoltages caused by long lead lengths include minimizing cable distance, lowering carrier frequency, installing dV/dt filters, using inverter duty rated motors (that are rated 600 V to withstand pulse trains with rise time less than or equal to 0.1 microsecond, of 1,600 V peak magnitude), and installing LCR low-pass sine wave filters. Regarding lowering of carrier frequency, note that audible noise is noticeably increased for carrier frequencies less than about 6 kHz and is most noticeable at about 3 kHz. Note also that selection of optimum PWM carrier frequency for AC drives involves balancing noise, heat, motor insulation stress, common mode voltage induced motor bearing current damage, smooth motor operation, and other factors. Further harmonics attenuation can be obtained by using an LCR low-pass sine wave filter or dV/dt filter.

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