Warning: Electrical and mechanical hazards may be present. These can cause injury or death. Make sure that voltage and other electrical conditions are known and proper precautions are taken to prevent exposure to electrical hazards. Putting any material such as a probe in contact with a rotating shaft or other moving parts is inherently dangerous. If not properly done this could cause serious injury or death. Safe procedure must be established for each test and performed only by properly qualified and trained personnel.
The safe practical field measurement of shaft-to-frame voltage in running equipment can be obtained by using a specially modified shaft grounding system as a pickup on the running shaft, a battery powered oscilloscope capable of sensing the rapid voltage changes in capacitive build up and discharge and a shielded probe extension.
There are a number of technical issues that need to be considered that can greatly affect shaft-to-frame voltage readings. These include but are not limited to the following:
a. Often there is confusion from other voltages or frequencies present. These may be found in the air or from various sources such grounding of equipment, fluorescent lights, the motor itself, plastic pipe that is carrying water, fiberglass components such as pumps and some fans that are blowing very cold air.
b. The probe that contacts the shaft needs to be shielded from the attachment of the scope probe to where the probe contacts the pickup on the shaft. A probe that is not properly shielded will act as an antenna. Voltage readings can be obtained with a scope probe that is not in contact with the shaft or frame. This can result in readings ranging from 5 volts to 300 volts.
c. Alternative current paths such as bearings can cause erroneous readings between shaft and frame.
Procedure for Setting Scope for Testing Motor Shaft-to-Frame Voltages:
Equipment: Fluke 199 scopemeter.
See scopemeter screenshots below.
- Set the voltage reading on channel A to 5 volts per division.
- Set the time to 50 milliseconds per division.
- Press the yellow scope button. Make sure readings are highlighted to ON by toggling f1.
- Push f2 and make sure scope probe A is on.
- Use the four directional arrows in the center of the scope to highlight the peak and press enter.
- Using the arrow keys again select peak-peak. Highlight by pressing enter key.
