Migrating EMI, Stray Ground Currents and Voltage Reflections in VFD Systems
Over a dozen industry studies have been conducted regarding the role of motor-supply cables in the safe and reliable operation of VFD (variable-frequency drive) systems. In one of the larger studies, extensive testing of 150-foot lengths of eight different cable constructions was performed on a 50-horsepower VFD system. In another major study, the magnitude and effect of reflected waves in the cable were studied in detail. Overall, the studies focused primarily on three concerns of motor-supply cable: EMI, stray ground currents and voltage reflections.
The semiconductor devices used in VFDs increase the potential for EMI (electromagnetic interference). The fast switching of the drive’s IGBTs (insulated gate bipolar transistors) generates RF energy, which can be radiated by the drive’s inverter-to-motor power cable and picked up by nearby circuits. This interference can cause malfunctions in sensitive electronics. As a result, VFD manufacturers usually recommend the use of a shielded power cable between the drive electronics and the drive motor to contain this RF energy.
Satisfactory shielding can be obtained with cable technology using impervious corrugated metal armor as the shield. This type of armor is often called CCW for continuously corrugated and welded. Other shield types, such as copper tape, tinned-copper braid or aluminum/polyester foils, can also be used with slightly reduced effectiveness. The effectiveness of cable shields is typically determined by measuring the transfer impedance according to the procedure defined in NEMA WC 61. A cable shield with transfer impedance of less than 100 mΩ/m (milliohms per meter) is generally considered to be a “good” shield, and one of less than 10 mΩ/m is considered to be an “excellent” shield. Impervious corrugated aluminum armor typically has transfer impedance in the range of 1 mΩ/m to 10 mΩ/m from DC to 30 MHz, the frequency range of interest for VFD applications. Most other common shield types provide transfer impedance in the range of 10 mΩ/m to 100 mΩ/m.
Stray Ground Currents
A second concern is that stray electrical currents in the VFD’s grounding system may flow though the drive motor’s main bearings. The flow of electrical current through motor bearings in the milliamp range can cause damage to the races of bearings (known as fluting), which can result in excess noise and premature bearing failure. Motor bearing currents can be minimized by using an inverter-to-motor power cable with three symmetrically positioned grounding conductors in combination with a low-impedance shield. This design minimizes the total current that is induced in the grounding system by current flowing in the phase conductors and, ultimately, the amount of stray current that flows through the motor bearings.
A third concern is that the combination of high frequencies generated by the VFD and long cable runs between the VFD and the drive motor can result in reflected waves in the cable. Reflected waves can produce abnormally high voltages in the cable. Peak voltages of up to 2.5 times the nominal system voltage have been measured on VFD power cables. Because of these high voltage pulses, the use of 2,000-volt-rated cables is recommended on some 600-volt drive systems. Due to their lower capacitance, proper VFD cable designs also reduce the magnitude of voltage reflections.
For most VFDs operating at 480 volts or below, the recommended cable rating is 600 volts. However, 575-volt VFDs sometimes require a cable with a higher voltage rating depending on cable length and drive switching time. To determine the recommended cable voltage rating for a 575-volt drive system, refer to Figure 1 to determine if reflections are likely. If a reactor or a low-pass filter is used with the drive, the resulting longer switching times can reduce the likelihood of reflections. If reflections are likely, a 2,000-volt-rated cable is recommended. If not, a 600-volt-rated cable is usually adequate. If inverter switching time or cable length is unknown at the time the cable is specified, a conservative approach is to specify a 2,000-volt-rated cable in either case.
Figure 1: Voltage Reflection Likelihood
For the best mitigation against EMI, stray ground currents and voltage reflections, specify a proper VFD cable. A proper cable includes three conductors, three symmetrically positioned ground conductors and a shield or armor with low transfer impedance.
“Evaluation of Motor Power Cables for PWM AC Drives” by J. M. Bentley and P. J. Link, ABB Industrial Systems Inc., IEEE Pulp and Paper Industry Conference, IEEE publication # 0-7803-3148-6-5/96, pages 55-69.
“Riding the Reflected Wave - IGBT Drive Technology Demands New Motor and Cable Considerations” by L. A. Saunders and G. L. Skibinski, Allen-Bradley Company; S. T. Evon, Reliance Electric; and D. L. Kempkes, Chevron USA Offshore, IEEE Petroleum and Chemical Industry Conference, IEEE publication # 96-CH35988-6/96/0000-0075, pages 75-84.
 In small sizes, such as 10 AWG, 12 AWG and 14 AWG, only one ground wire may be needed because the effect of stray ground currents is small.