Because of the high-frequency switching characteristics of the outputs of modern variable frequency drives (VFDs), additional attention should be paid to the cables connecting a VFD to its motor. Modern pulse-width modulated (PWM) VFDs use sets of controlled transistors turning on and off at frequencies from 2 – 20 kHz to generate voltage pulses which, taken together, approximate the sine wave an AC motor requires. These transistors, typically Insulated Gate Bipolar Transistors (IGBTs), switch very rapidly, and are capable of reaching 90% of rated output voltage in less than 0.1 microsecond. This results in very steep wave fronts on voltage pulses sent down the cables at very high frequency. In turn, this places additional design demands on cable capacitance, impedance, electro-magnetic (EM) shielding, and length in order to ensure a high-quality, EM-compliant installation that is safe for the equipment and dose not create interference with other connected loads. In Part I of this series, we are going to discuss cable capacitance and impedance characteristics and their impact on the VFD and motor.
All cables have a characteristic capacitance determined by insulation type and thickness and shielding material, and influenced by conductor configuration. At fundamental frequencies (50 or 60 Hz) this capacitance is typically sufficient to prevent leakage through the cable insulation to ground via capacitive coupling. Capacitance is simply a measure of electrical storage, and in a cable reflects the amount of charge that can be stored between the conductors themselves, and between the conductors and shielding. When supplied by the output of a drive, with its significant high frequency component, that capacitance results in rapid, high-amplitude charging currents and quickly saturates, providing common mode current paths and increasing the potential for cable heating, insulation breakdown, and failure. Depending on the dielectric constant of the cable insulation and the length of the cable, charging currents can be tens of amps, none of which is available to supply the motor. The amplitude of charging current is also affected by the drive supply voltage; 460 volt applications will generate higher charging currents than 230 volt ones, all other things being equal. These currents can be minimized mainly by limiting cable length, because the amount of capacitance in a length of cable is proportional to length. But they can also be reduced by specifying low-capacitance cable. Although manufacturers rarely publish capacitance values for their cables, it is generally true that XLPE insulation provides a lower capacitance than the more common PVC-containing materials.
The characteristic impedance of the cable should also be taken into account in VFD applications. While this impedance can play a role in reducing the magnitude of high-frequency voltage transients, there are trade-offs when it comes to cable length and motor impedance. Where possible, cables should be specified with impedances which most closely match the motors they supply. This is to reduce the magnitude of reflected waves – voltage peaks sent back along the cables resulting from an impedance mismatch between motor and cables. If the cable is sufficiently long, these peaks can combine with later voltage pulses sent by the drive and result in cable voltages in excess of 2 – 3x the supply voltage. This not only increases stresses on motor winding insulation, but increases stray currents and stresses cable insulation. It is not always possible to match cable and motor impedance, particularly with smaller motors, so the main mitigating strategies where cables are concerned are to, again, minimize length and/or specify a cable construction with an impedance as close as possible to that of the motor.
Next time, we’ll provide an overview of cabling electro-magnetic interference issues and compliance strategies. In the meantime, please feel free to share your thoughts with other readers in our Comments section, visit us at www.vtdrive.com and www.vtdrive.tk, or email us at simon.fan@vtdrive.com. We look forward to hearing from you. Thanks for reading!
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