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[4]

Properly grounding a VFD

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yarrrrr

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Salutations electrically minded folks. I posted a pic of my workshop, and to my surprise Richard King replied, he asked if everything was properly grounded. Hmmm, I did not know (didn't inherit the electrician gene from dad). My VFD is only grounded via the ground supplied through the 220 socket. The motors are frame grounded. Is it recommended that there be a second earth ground directly from the machine per the below diagram?

motors-fig4.jpg
 

mksj

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Mostly applies to industrial settings were the motor frame may be grounded to the steel structure, my understanding is that the optimal grounding method is to directly connect the VFD cable ground wire(s) from the motor to the ground bus/star ground at the VFD which is then grounded to earth at the electrical supply source. In addition the cable shielding is to be grounded at both the motor and the VFD enclosure to the same bus. The ground wire should not be attached by a separate pigtail. This allows the high frequency noise/current to flow via the skin affect into the shield ground as opposed to through the motor frame to the steel building frame. High voltage VFD shielded cables tend to have symmetrical construction with three ground wires, lower voltage VFD cable will have 3 wires with a single ground, the shield is typically 100% with a braid and foil. The wire insulation must tolerate voltage spikes far in excess of the motor voltage, typically not a problem with 240VAC drives, but an issue with higher voltages. The longer the motor cable the more of a problem with voltage spikes because of reflective waves. The science goes far beyond my pay grade.

If you are doing short (low voltage i.e. 240 VAC) VFD to motor connections then you can probably get away with unshielded 600V wiring. Anything over a few feet, I use shielded 1000V rated cable. The current literature directs one to ground the shield at both ends with a circumferential clamp or to pull the shield to one side, cover with insulation and connect to the ground wire/ground terminal. See below, I use locking crimp forks for the VFD connections and ringed crimp connectors for the ground and motor connections. I typically crimp the connections and then wick some solder into the tips of the wires to prevent any corrosion and provide a second means of securing the wire. This is done for cables in the marine environments, never solder a wire and then crimp it as the solder will cold flow. If solder is allowed to run down the wire it can cause a fracture/failure point in a high vibration setting. I also use fusible shrink tubing. Using a professional crimper and the properly sized crimps, on a pull out test, the wire will fail before the crimp will fail.

20180804_123939.jpg
 

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yarrrrr

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That is a clean looking cable. Thank you very much for this info. I did use proper (3 wires, 1 ground) vfd cable, and the shielding is grounded as you describe. So, the second ground (in addition to grounding at the source) is not needed?
 

JimDawson

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So, the second ground (in addition to grounding at the source) is not needed?
Adding an additional ground like shown in the diagram would likely introduce ground loops. The diagram is correct, kinda, but it is shown poorly. Normally grounds are all tied back to a single point.
 
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