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OVERVIEW OF VFD INDUCED BEARING CURRENTS
One of the fastest growing applications of pulse width modulation (PWM) variable frequency drive (VFD) inverters is in commercial and industrial heating, ventilation, air conditioning and refrigeration (HVAC/R) equipment. VFD's allow system designers to realize substantial energy savings and motor control capability provided by PWM drives. As the use of VFD's to control motors in air handlers, heaters, fans, blowers, pumps, air conditioning units, chillers and clean rooms has increased over the years, there has also been an increase in motor failure from bearing currents as the VFD's induce voltage onto the shaft of the driven motor which ultimately may cause pitting, fluting and finally bearing and motor failure.
This cause of motor failure has become an increasingly important reliability issue and the prevention of bearing failure in VFD driven motors an important design consideration to ensure motor reliability and to reduce operating costs. Technical articles and actual testing results show the severity of bearing damage that may occur in motors when drives are used - often in a relatively short period of time.
From its first minute of operation, a VFD induced destructive voltage is present on the motor shaft until an alternate discharge path, usually the motor's bearings, is found to discharge the voltage to ground. Inside the bearing, once voltage is sufficient to exceed the breakdown potential of the oil film layer, bearing currents cause an electrical discharge machining (EDM) effect which pits the bearing race and rolling elements. This destructive phenomenon continues until the motor bearings become so severely pitted that fluting such as that shown in Figure 1 occurs. This causes excessive vibration and noise in the motor, which may be audibly transmitted through the HVAC system ductwork. Finally motor failure occurs, sometimes in as little as only a few months.
CAUSES OF BEARING CURRENTS IN MOTORS DRIVEN BY VARIABLE FREQUENCY DRIVES
The root cause of bearing current discharges that damage motor bearings is the high speed switching from pulse width modulation (PWM) drives that use insulated gate bipolar transistors (IGBTs). Switching events may occur at a rate of over 12,000 Hz. The generated voltage pulses induce an AC voltage onto the motor shaft via parasitic capacitive coupling between the rotor shaft and the stator windings. This may even occur in a properly grounded and suitable electrically shielded motor (6). Because the IGBT's fast rise time of only 50 ns or less, this capacitive coupling may cause the peak motor shaft voltage to reach as high as 60 volts, or higher in some cases, unless a discharge path exists. Typically, when the voltage reaches 20-30 volts or more, the oil film in the bearing breaks down and a discharge takes place. These discharge events occur continuously while the motor is operating, causing the electrical bearing damage to increase over time. This process is also known as electrical discharge machining (EDM) effect. This extremely fast bearing discharge, measured at around 40 ns, instantaneously heats and melts the surface of the bearing race and causes a small pit at the discharge point. Over time, the size of such pits can increase. It has been reported that the stray capacitive currents can generate shaft voltages in the following three different ways:
1. High motor frame voltage due to common mode current return path circuit inductance. A well-grounded motor frame can minimize the motor frame voltage by employing auxiliary high frequency bonding connections for ground potential equalization.
2. High frequency axial shaft voltage induced by a circumferential magnetic flux around the motor shaft. This type of circulating currents can be interrupted with insulated sleeves or ceramic bearings. For motors less than 11 kW, it has been shown that no circulating bearing current flow occurs.
3. Coupling of Common Mode Voltage via bearing capacitance between the shaft and the motor frame and capacitance between the stator and rotor. This mechanism is considered as the most predominant factor in bearing failures when the motor frame is adequately grounded (4). Bearing currents caused by stator-to-rotor capacitive coupling must be diverted from the motor shaft by providing a least resistance path to ground other than the bearings themselves. Ceramic bearings or insulated sleeves around the bearing stator break the electrical current path through the bearings, but in many cases cannot protect the bearings of connected equipment.