Voltage pulses from the variable speed drive can couple between the motor stator and rotor, resulting in voltage on the rotor shaft. When this rotor shaft voltage exceeds the insulation voltage of the bearing grease, high-voltage arcing or electrical discharge machining (EDM) may occur, leading to pitting and groove corrosion in the motor bearing races. This damage can cause premature motor failure. This application case demonstrates how to use the MDA-550 MotorDriveAnalyzer and shaft voltage probes to measure motor shaft voltage discharge events.
Bearing raceway groove corrosion caused by bearing current (Photo courtesy of ElectroStatic Technology)
shaft voltage discharge
Capacitive coupling between the stator and rotor of a motor generates a voltage on the motor shaft. Therefore, the bearings in the motor will wear not only from the rotation of the shaft but also from the current flowing from the motor shaft through the bearings to the ground. A motor powered by a sinusoidal AC power supply may have a shaft/bearing-to-frame voltage of approximately 1 to 2 V. However, a motor powered by the rapidly changing waveforms of a variable frequency drive (VFD) may have a shaft/bearing-to-frame voltage as high as 8 to 15 V. This level of voltage can overcome the insulating properties of the bearing grease, and the resulting sparks can lead to pitting, grooved corrosion, and cratering, ultimately causing premature failure of the bearings and the motor.
Measuring motor shaft voltage using a shaft voltage probe kit
Axis voltage carbon brush probe
Capacitive coupling between the stator and rotor of a motor generates a voltage on the motor shaft. Therefore, the bearings in the motor will wear not only from the rotation of the shaft but also from the current flowing from the motor shaft through the bearings to the ground. A motor powered by a sinusoidal AC power supply may have a shaft/bearing-to-frame voltage of approximately 1 to 2 V. However, a motor powered by the rapidly changing waveforms of a variable frequency drive (VFD) may have a shaft/bearing-to-frame voltage as high as 8 to 15 V. This level of voltage can overcome the insulating properties of the bearing grease, and the resulting sparks can lead to pitting, grooved corrosion, and cratering, ultimately causing premature failure of the bearings and the motor.
FlukeMDA-550 Motor Drive Analyzer
Measuring equipment
The MDA-550 is designed for quick and easy detection and troubleshooting of various common problems in three-phase and single-phase inverter-type motor variable frequency drive systems. In addition to its 4-channel portable oscilloscope and recorder functions, specific motor drive analysis functions provide programmed step-by-step wiring setup guidance with on-screen information. This facilitates analyzer configuration and enables measurements from the power input to the installed motor's variable frequency drive, including shaft voltage measurements.
Axis voltage peaks can change very rapidly, even within the microsecond range. The FlukeMDA-550 features a high bandwidth of 500MHz and a fast sampling rate (up to 5G samples/second), making it ideal for measuring rapid changes in axis voltage.
Measurement results
The default motor shaft voltage measurement displays the voltage waveform on the motor shaft itself. The voltage peak displayed at the top of the screen indicates the maximum level of the captured waveform, suggesting a high voltage level on the shaft. However, the frequency of these rapid discharges cannot be determined in this way. With the "Event On" function, the MDA-550 can display discharge waveforms recorded based on predefined minimum voltage difference and maximum time difference values.
The display updates the results for each captured waveform with a steeper rise or fall time, as well as the number of events detected per second. Discharge waveforms show an increase in voltage to ground, followed by a clear, vertical voltage line to ground at the moment of discharge. For detailed analysis of the last captured waveforms, the MDA-550 offers a "replay" function, with a screen buffer capturing the last 100 waveforms. The screen can be selected individually or displayed as an animation.
A voltage discharge exceeding 15V with a transient time less than 50 nanoseconds indicates that the discharge may damage the bearing. However, this alone is insufficient to determine the potential damage to the bearing, as many other factors can influence this.
If the detected shaft voltage is too high, it is recommended to reduce the discharge voltage by adjusting the cable, grounding, driver parameters, or lubricant. If the above suggestions are not feasible or do not help alleviate the problem, a shaft grounding device or an insulated shaft can be used. The effectiveness of these shaft voltage mitigation measures can be easily understood by comparing the number of events recorded before and after the changes.
Fluke Corporation
Founded in 1948, Fluke Corporation is a world leader in electronic test tools. For many years, it has created and developed a specific technology market, providing high-quality measurement and troubleshooting products for various industrial sectors. Fluke's users are diverse, including technicians, engineers, and metrologists who utilize Fluke test tools for the installation, troubleshooting, and management of industrial electrical equipment and process calibrations, thereby controlling quality. In the past five years, Fluke test tools have received numerous awards, winning over 50 annual product awards, including the Best Test Tool Award from Test & Measurement World and the Engineer's Choice Award from Control Engineering, earning widespread user praise.
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