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| Troubleshooting the Source of Stray Voltage on Motor Leads when a VFD is used. | |
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In the pump system shown at
left, two variable frequency drives are fed from a common 3 phase power
connection.
The output of each drive is connected to its respective motor leads. The motor leads are installed in one metal conduit, that goes from the drives, to the motors. The lead length is about 70 feet. VFD #1 is disconnected from input power, and the M1 motor leads are disconnected from its output terminals. VFD #2 is connected as usual, and M2 is ready for operation. The voltage waveform in Fig. 1 below is from the test point at the disconnected M1 motor leads while VFD #2 has power applied, but is not running its motor. The voltage waveform in Fig. 2 below is from the test point at the disconnected M1 motor leads while VFD #2 is ON at nearly full output. Figure 3 below shows the new wiring configuration for the VFD's. Motor leads from each VFD are now in their own conduit, and separated from each other. Figure 4 below displays the waveform result after the leads for M1 motor are removed from the common conduit, and run in their own conduit. There is still a small voltage present when M2 motor is running at 50 Hz. |
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| Fig. 1. M1 leads. No Output on VFD #2 | Fig. 2. M1 leads. VFD #2 at 52 Hz Output (almost full speed) |
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| Fig. 3. Re-wired Motor Leads from VFD's | Fig. 4. M1 Leads after Re-Wiring. 50Hz output on VFD #2. |
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