Shaft grounding - Leine & Linde - #1

Text version of the page

www.leinelinde.com Figure 1. VARIABLE FREQUENCY DRIVES INDUCE SHAFT CURRENTS IN AC MOTORS Shaft grounding for inverter driven motors The use of variable-frequency drives (VFDs) to control AC motors has increased dramatically in recent years. In addition to their low operating cost and high performance, they save energy. Today, the challenge facing system designers and engineers is to minimize damage to AC motors from shaft current. From its fi rst minute of operation, a VFD induces destructive voltages that build up on the motor shaft until they fi nd discharge paths to the frame (ground). In most cases, the motor bearings present the path of least resistance. Once voltage is suffi cient to overcome the resistance of the oil fi lm layer in the bearing, shaft current discharges, causing electrical discharge machining (EDM) pits and fusion craters in the race wall and ball bearings. This phenomenon continues until the bearings become so severely pitted that fl uting, excessive noise, and failure occur. Mitigation of this damage is possible through various strategies. Some are narrow in application, and most are costly. Many are not technically feasible. However, a new technology employs a circumferential ring of conductive micro fi bers to discharge harmful currents and provide a low-cost solution to the problem. Due in large part to an increased focus on energy savings, the use of pulse-width-modulated (PWM) variable-frequency drives (VFDs) to control AC motors has grown dramatically over the last few years. While they offer low operating costs and high performance, VFDs are not without their problems. Shaft currents induced by VFDs can lead to motor failures. Without some form of mitigation, shaft currents travel to ground through bearings, causing pitting, fusion craters, fl uting, excessive bearing noise, eventual bearing failure, and subsequent motor failure. Due to the high-speed switching frequencies used in PWM inverters, all variable frequency drives induce shaft current in AC motors. The switching frequencies of insulated-gate bipolar transistors (IGBT) used in these drives produce voltages on the motor shaft during normal operation through electromagnetic induction. These voltages, which can register 70 volts or more (peak-to-peak), are easily measured by touching an oscilloscope probe to the shaft while the motor is running [Figure 1]. ELECTRICAL DAMAGE TO BEARINGS