Properly grounding a VFD

[yarrrrr]

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?

 

[mksj]

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.

 

[yarrrrr]

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]

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.

 

[warrjon]

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.

I agree, multiple ground paths especially if there is a potential difference will cause ground loops.

Also shields should only be grounded at ONE end if you ground a shield at both ends current can flow in the shield radiating noise.

My VFD's (3hp and 5HP) are only grounded via the mains lead and I have no noise issues.

 

[tq60]

Not sure on the primary ground but there is return and safety ground.

The primary is connected to safety ground only at service entrance where the meter is.

From that point out you have for 3 phase L1, L2, L3 and for single phase L1, neutral and L2.

Everything made of conductive material is connected to safety ground with a wire sized to cause breaker to pop if short in said equipment.

Inter-device cables should be shielded at single end but can be at both but be certain the shield cannot carry current as the parts at both ends need solid and clean safety ground connections.

Inside an electrical enclosure place single large bolt and make connections from all devices to single bolt.

Jumper from machine to bolt as well as motor to bolt

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[mksj]

Current convention is to bond the VFD motor cable shield/ground at both ends. Many of the VFD specific cables have symmetrical construction of 3 current carrying conductors and 3 ground wires surrounded by a foil shield. Other VFD cables have a separate insulated ground wire in addition to the a foil and/or braided shield, convention by both the VFD manufactures and VFD cable manufactures is both are bonded at both ends (see reference material above). Both the VFD cable construction and materials/insulation/design are specific to issues encounter with VFD output voltages/switching frequency. Typically this is less of an issue with short motor cable runs and 240 VAC 3 phase output. As outlined by others you want all grounds in the enclosure to come to one post mounted to the back plate or a grounding buss bar. If using a painted back plate I always grind away the paint where I have my ground post and use a star locking washer. On some systems I do use an inline noise filter on the input power lines going to the VFD.

Signal/low voltage cable shield's are only bonded at the VFD end and insulated from shorting at the other end of the cable. I have had no noise issues with this approach, but grounding can be tricky in some scenarios. I had one case where the shielded tachometer cable picked up noise if the shield was bonded to the machine, I ended up floating the shield at both ends.

 

[ttabbal]

Not sure on the primary ground but there is return and safety ground.

The primary is connected to safety ground only at service entrance where the meter is.

For most VFD installs, there is no Neutral (return). At least, not one that is in use. They don't need 120V, so you just use L1, L2, and the safety ground. If you do use a neutral, as you mentioned it should not be bonded to the safety ground. If I had one in the wiring, I would just cap it. Obviously, the AC wiring details vary by country. For those without the split-phase setup we use in the US things will be different.

I haven't seen any noise issues on my setups from unshielded wiring. I would suspect the DRO would be the most likely thing to be affected, but it's fine. I do always keep an eye though. I suspect the DRO scales are shielded, so they don't pick anything up, and the head is on a separate 120V circuit, so it's less likely to pick up noise from the input. I also put the DRO runs on the opposite side of the machine from the VFD lines. Distance and grounded metal are your friends when electrical noise is an issue. If you can, crossing at right angles rather than running parallel with the other lines also helps reduce coupling.

If I were in a professional environment, I would use the more expensive shielded cables etc.. At home I'm willing to work on issues like noise if it comes up and the wiring I used was available locally on the weekend cheaply. I can always order in the better stuff as needed. One of these days I'll find time to take the oscilloscope out there and search for any stray noise sources. And probably drive myself nuts fixing things till they are broken.

 

[yarrrrr]

Excellent input everyone, thanks again. Hoping threads like this will help others like myself, as there is not really that much info out there on vfd's and grounding - especially for non-production/hobbyist setups.

 

[Skowinski]

Can someone point me to a write up of the very basic basics of hooking up a VFD? This thread, and most others that I read, seems to start at a level far beyond my current understanding. A couple of searches for the basic info have led to similar writeups, beyond my current level...

Thanks, and sorry for the thread hijack.

 

[ttabbal]

Can someone point me to a write up of the very basic basics of hooking up a VFD? This thread, and most others that I read, seems to start at a level far beyond my current understanding. A couple of searches for the basic info have led to similar writeups, beyond my current level...

Thanks, and sorry for the thread hijack.

The manual they come with should have that. The short version is pretty simple, power in to a few screws, power out to the motor on 3 more. Everything else is generally optional. The main terminals are generally pretty well labeled. If you have a particular VFD in mind, we can probably find an online manual to help sort it out. Here's a random one. You care about the top left and right, input on the left, output on the right. There are generally controls on the unit to control it, the other terminals are mostly to make it possible to put controls somewhere else.

 

[tq60]

Wiring is basic but always RTFM...

Before buying anything always download the manual and first check the compatibility chart to insure which exact unit you need for your specific motor.

Next go to the install section along with paper and pencil and complete a virtual install on paper.

Meaning determine your connections and programming.

From this you can determine if you understand it.

You also can ask the vendor/manufacturer for tech support and this will give you a clue to how the unit is supported.

Bad customer support is turn off usually


Control side usually has 3 inputs.

Run
Reverse
Stop

Run is usually a closure

Reverse same

Stop usually a break contact.

Most require run to close for normal then both run and reverse for reverse and simply opening the stop loop does stop.

We modified stock drum switch to do above and programmed for 3 wire control and the lathe looks like it left factory in 1945 with an added pilot light in the drum switch indicating line power on.

VFD are not difficult if one can understand the manual...

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