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Fluke MDA-550-III Manual

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190 Series III
Users Manual
72
6. Use one of the ground leads to contact the motor chassis that is the reference ground.
When it is not possible to connect near the shaft, an extension cord with 4 mm connections
on both ends is included for extending the connection to the ground lead. The measurement
can be done on the driven and the non-driven end of the motor.
7. Turn on the motor.
8. Do the measurement after the motor warms up to normal operating temperature.
With this function you can determine the number of flash-overs that occur between shaft and
motor chassis, also referred to as electrical discharge machining. When motor shaft voltages
exceed the bearing grease insulation capability, flash-over currents occur and cause pitting
and grooving of the bearing races.
Tips:
• Under direct 50/60 Hz mains operation, the shaft voltage is normally <1 V.
• Due to the fast edges of the switching voltage from a motor drive, the shaft voltage for a
motor powered by a motor drive can be much higher. High Voltage can cause large
breakdown currents across the grease barrier causing damage to the bearings.
• Normal, unavoidable shaft voltage due to asymmetry in the air-gap magnetic field result in
peak voltages <5 V and slower than 100 ns, which, in general, are not damaging.
• Voltage discharges of >15 V and transition times faster than 50 ns can point to grease flash-
over currents that can damage bearings. However, no fixed values can be given that can be
considered damaging for the motor, as there are many factors that influence this value.
After selecting the Motor Shaft Voltage measurement, the display shows the voltage waveform.
The voltage peak-peak reading shows at the top of the display. Select 2 EVENTS ON to
view the discharge event and count the amount of discharge events. Only the discharge events
show. The readings at the top of the display show the Volt Peak-Peak, dV/dt, and falltime or
risetime as well as the number of events/second. Expect about 20 seconds before the number of
events/second shows on the display.
Use 4 DEFINE EVENTS to define what is considered a discharge event.
In this screen, select the maximum voltage change and maximum rise or fall time that will be
counted and displayed as an event.
Tips:
• No waveform is visible when no events are detected.
• If excessive shaft voltage is measured, check that the voltage discharges can be reduced by
adapting the cabling, grounding, drive parameters, or lubricant. If this is not possible or does
not help, use the shaft grounding devices or the isolated shaft.
• If a bearing runs hot or is noisy and high shaft voltages are measured, bearing flash-over
currents can be the primary source of excessive bearing wear.
• Check other sources of bearing wear, such as, coupling misalignment or looseness.
… 78 …


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