H. William Oh Â Electro Static Technology
In recent years, manufacturers of common mode chokes or cores (CMCs) have increased their efforts to market these devices as a means of preventing bearing failures in PWM VFD/inverter-driven motors. CMCs are not new; they have been applied to power electronics for decades. This article will discuss the basic principle of how CMCs work and their effectiveness in mitigating bearing currents.
Today, it is well known that motors driven by Variable Frequency Drives (VFDs) exhibit two types of bearing currents[1-4]: capacitively-coupled bearing current and inductively-coupled bearing current (depending on motor frame size and assuming that the motor frame and the drive frame are well grounded together to avoid any possibility of the rotor ground current[5-6]). In many cases, capacitively-coupled bearing current is referred to as EDM bearing current and inductively-coupled bearing current as circulating bearing current, since these two terms are more descriptive of bearing current damage each causes and how each type of current causes it.
EDM bearing current is generated when a motor itself acts as a capacitor between the rotor and the stator. Circulating bearing current, on the other hand, is induced by high frequency (HF) ground current that flows from the stator to the drive ground point. While small frame size motors (typically below 100 HP) exhibit only EDM bearing currents and negligible circulating currents, large frame size motors (more than 100 HP) exhibit both EDM currents and the circulating currents. As motor frame size increases, both EDM bearing currents and the circulating currents increase proportionally.
EDM Bearing Currents
EDM bearing current is induced by the high dv/dt and capacitances in motors due to the presence of common mode voltage created by a VFD. While more detailed and complete descriptions of this process can be found in other technical papers[7-8], for the sake of simplicity, we have chosen to explain EDM current based on the simple capacitor charging principle to indicate the underlying process.
The charge build-up can be described by the following formula:
i = dQ/dt = C x dVcom/dtÂ Â Â Â Â Â Â Â Â Â (1)
where, i = current, Q = electric charges, Vcom = common mode voltage, C = Capacitances within the motor.
As this equation shows (1), EDM bearing current is influenced only by the capacitances of the motor and the dVcom/dt of its windings. Here, capacitance (C) is a combination of all parasitic capacitances between windings and the stator as well as the rotor and the stator, etc. If the lubrication of the bearings can withstand them, charges on the rotor surface will build up resulting in a voltage potential between the rotor and the stator. This voltage is not the same as the voltage potential between one end of the shaft and the other end of the shaft. Since the industry does not distinguish between these two definitions of shaft voltage, the voltage potential between the rotor and stator can, therefore, be misunderstood as the voltage potential from one end to the other end of the shaft. The voltage potential between the rotor and the stator can be discharged through bearings when the lubrication film breaks down due to the nature of electrostatic discharges. When the charges discharge through bearings, they produce EDM bearing currents. EDM bearing currents increase proportionally with motor frame size because of the increased surface area which results in increased capacitance. Therefore, EDM bearing currents exist from small motors (below 1 HP) to very large motors (more than 1,000 HP) and must be mitigated to prevent a premature bearing failure.
Circulating Bearing Currents
Typically, as the motor frame size increases to greater than 100 HP, another type of bearing current can occur, circulating bearing currents. These currents circulate from one end of the motor to the other via the rotor, stator, and bearings. Induced by the inductive coupling of ground current, circulating bearing currents have been well researched and analyzed in technical literatures[1, 4, 9]. In order for circulating currents to occur, there must be a voltage potential between one end of the motor and the other.
Circulating bearing currents are governed by the inductance of the motor and induced by a circumferential flux, which is excited by high frequency (HF) common mode current. The common mode current enters the motor via the stator windings and leaves through the grounding connections of the motor to the drive. Detailed analysis of this process can also be found in technical literatures [1, 4, 9]. LenzÂs law states that the voltage potential (ΔV) from one end of the motor to the other end of the motor is governed by the following equation:
ΔV = L dicom/dtÂ Â Â Â Â Â Â Â Â Â (2)
where, icom is the common mode current, and L is the total inductance of the ground current path through the stator lamination. Therefore, circulating bearing currents can be minimized by reducing common mode current fluctuations.
Use of Common Mode Chokes or Cores
Several manufacturers of common mode chokes have proposed that these chokes can be used to minimize bearing currents in VFD-driven motors. Figure 1 shows the proposed use of a CMC for this purpose.
As can be seen from Figure 2, the HF components of common mode current are attenuated by the CMC based solely on the CMCÂs inductance and dicom/dt since ΔV = LcmcÂ xÂ (dicom,0/dt) assuming that there is no capacitance coupling between wires. Therefore, the CMC only affects the amplitude of the HF components of common mode current, but has no effect on the common mode voltage or dV/dt. In other words, the use of a CMC only attenuates common mode current and thereby affects circulating bearing current, but has no effect on EDM bearing current. CMCs cannot cancel out common mode current itself; they can merely reduce its amplitude and ripples, i.e., its HF components.
Limitations on the Use of CMCs
Since the PWM waveform of the output signal from a drive contains many frequencies[11,12], the improper selection of CMC for a drive/motor system could result in resonance and could amplify EDM potential in the motor and Ldi/dt voltage amplitudes. The amplitude of HF ground current could also be magnified if the motor and cable system were underdamped. For this reason, system engineers must pay close attention to the proper selection of CMCs when designing drive systems with relatively low power ratings.
For ungapped common mode cores (the majority of feed-through CMCs), several cores must be used in series to obtain the cross-sectional area required to achieve the desired CMC inductance. High common mode current requires a minimum core diameter to avoid saturation. The core diameter should provide a core window area large enough to accommodate the motor cable.
Large Frame Motor:
Test results were taken from http://www.magnetec.de/fileadmin/pdf/cb_appl_1.pdf. Referring to Picture 7 through Picture 9 in the reference, shaft voltages were measured without and with CMC using a large frame motor greater than 100 HP (the authors did not disclose the exact motor frame, but we assume the motor was greater than 100 HP based on the shaft diameter in the picture). As can be seen in Picture 9, shaft voltage was reduced from 37 V to 14 V using three CMCs. By shaft voltage, we mean the voltage potential from one end of the motor shaft to the other. The measured shaft voltage, 37 V, is a combination of both EDM voltage (associated with EDM bearing current) and end-to-end shaft voltage (associated with the circulating bearing current). When CMCs were applied on the output lines of the drive, the measured shaft voltage dropped by 23 volts to 14 V. Assuming that the CMCs reduced the common mode current by 75 percent based on the Magnetec.de internal report, combined with the reduction in shaft voltage of 23 volts, the EDM shaft voltage and the end-to-end shaft voltage can be calculated using Ohms Law of V=IR (assuming no change in resistance of the motor system).
End-to-End Shaft Voltage
23/0.75 = 30.7 volts
The two components of this 30.7 volts are:
ÂÂ The 75 percent reduction of common mode current = 23 volts
ÂÂ The 25 percent common mode current that was unaffected = 7.7 volts
Total EDM Shaft Voltage
The ÂafterÂ readings of 14 V make sense at 6.3 volts of EDM shaft voltage plus the 7.7 volts of remaining end-to-end shaft voltage. These results show that the CMC had no effect on the EDM voltage, leaving the bearings for this motor still subject to EDM bearing discharge damage. Referring to Picture 10 through Picture 12 from the same report, currents from one end of the shaft to a ground point were measured. It is also extremely important to understand the true nature of the measured current in this measurement. Referring to Figure 3, any current generated within the shaft will flow to both the bearings and the wire connected for current measurement as depicted in Figure 3. Therefore, the measured current shown in the reference must have been less than the induced current. If we assume that the induced current of 12 A was divided into 7 A to the shaft end and 5 A to the bearings, there is possibility that the bearing current was still 5 A and the shaft end current was 2 A after the CMCs were applied. Therefore, the measurement of the shaft end current is somewhat meaningless unless the bearings were insulated for the measurement.
Small Frame Motor
Referring to Figure 4, a 10 HP motor with an inverter and CMCs was set up to measure the voltage potential between the shaft and the stator. CMCs reduced the HF ripples but did not reduce the dV/dt and the charge build-up on the rotor via the motor capacitance. Test results prove the theory that CMCs are only associated with inductance, not the capacitance of the motor. With CMCs installed, the bearing discharge could still be seen from the measurement. Therefore, it is important to understand that CMCs should only be used for large frame motors along with other mitigation techniques such as shaft grounding. Small frame motors up to 100 HP do not need CMCs, because CMCs are not effective in reducing EDM bearing currents.
This article describes the two types of bearing currents that can harm PWM VFD-controlled motors and how each type of bearing current is created. EDM bearing currents are generated by a capacitance effect and circulating bearing currents are produced by an inductance effect. Common mode chokes only reduce ground current, and therefore can only reduce circulating bearing current.
In summary, PWM VFD-controlled motors are subject to two types of bearing currents: EDM bearing current and circulating bearing current. Circulating bearing current is induced only by HF common mode current. For small frame motors, circulating bearing current is negligible. For large frame motors, typically over 100 HP, circulating bearing current must be mitigated. CMCs may be used in reducing circulating currents in large frame motors, but careful selection is necessary to avoid a resonance within the motor drive system or saturation of cores based on the motor frame size. EDM bearing current is generated only by motor capacitance. The magnitude of EDM bearing current increases as motor frame size increases. Even for large motors, CMCs should be used only with other mitigation techniques such as shaft grounding or insulated bearings, since CMCs alone have no effect on EDM bearing current and therefore cannot prevent bearing damage.
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