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Question #: 14543

Question: the relay on the 57CNC card turns on and the VFD starts up but it won"t turn off. Relay #1 stays on all the time. Even the Estop won't turn it off. Relay #2 doesn't work all

Current Solution

Turns out it was an electrical noise problem from the VFD. Troubleshooting showed me everything worked perfect even with the drive powered up but when the VFD started it would NOT stop from Mach3. I had run shielded cable and grounded it in the VFD but I had an intermediate break in the shield wire at my Master Control Relay (MCR) that is activated by an Estop on my control panel.

This confirms the importance of shielded cable and making sure that the shield is contiguous.

My guess is the wire(s) going to the dry contacts on the relay carried enough energy to fire the base of the transistor supplying coil voltage to relay #1. Or there might have just been enough induced voltage to the relay to fire the relay coil.
I've fought similar VFD noise issues for 25 years and it still amazes me how pesky it can be. It's impossible to maintain any separation of signal and motor leads in the tiny space in the VFD.


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