Properly grounding a VFD

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.



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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...

Sent from my SAMSUNG-SM-G930A using Tapatalk
 
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Very nice. I am still confused on one point, though, probably due to "The ground wire should not be attached by a separate pigtail.". My VFD has only a single ground terminal on the power in and out block, and that is on the 220V single phase 'in' side. It concerns me that there is no conductivity between that terminal and the VFD frame (cooling fins). There is also no conductivity between that terminal and the labelled "ground" ports for the control wiring. I understand that I should install a grounding bus in my VFD enclosure, and link all ins and outs to that - all wire conductors and shielding. So, should I put in a 'jumper' from the 220V 'in' ground terminal to the bus on the enclosure?
 
I do it as a single point ground. All ground connections come to a single connection which is connected to the safety ground from the electrical source. The idea is to prevent a loose connection from channeling current over the ground lines. This is somewhat less likely when neutral is not involved. I don't know why you would avoid pigtails that's how most ground connections are made in AC home wiring here.

If you are doing an enclosure, a ground bar is common and just a fancy pigtail really.
 
You should not confuse the AC ground with those used for the control (low voltage) DC ground or common. Typically you have either a ground post (screw post or star ground) attached to a metal back plate/metal frame or a ground bus bar. All AC grounds should be connected to this point including a ground wire from the VFD AC ground. On some VFDs this is a separate terminals, on others it is a screw attached to the metal fins. The AC power in ground is also attached to this ground point. When using a metal back plate, clean the metal of any paint where the ground is attached and use a star washer to secure the screw/bus. I use ring terminals so they cannot dislodge.

If you are using shielded cables, how that shield is attached varies. Typically for low voltage control wiring, the shield is grounded at the VFD end to the VFD chassis ground. With a shielded motor cable, the shield may be grounded to the metal enclosure with a metal strain relief designed to ground the shield, or you can weave out the braid and attach it to the back plate ground.
Star Ground.jpg
 

Attachments

  • VFD GROUNDING AND BONDING PRACTICES.pdf
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Got it. Thank you. One point of lingering confusion involves connecting the control cable shielding to the VFD chassis ground ... not to the AC ground bar ...? so to a DC Common/Ground terminal ...? or to the metal cooling fins ...? or a ground connection to my enclosure? Or does it matter which of the above so long as it is grounded? I haven't worked previously with low voltage shielded cable in any application.
 
I think I would try the safety ground (NOT neutral). The idea is to ground out interference from the AC line and 3 phase output, so using the same ground reference the AC uses makes sense to me.

Only grounding one side of the shield helps prevent ground loops. Again, not a likely issue in this setup, but one less thing to worry about.

My control wiring is cat5 UTP. No shielding, no issues with control signals.

If I were to shield, I would lean towards shielding the 3 phase cable. The switching harmonics are going to be on those.
 
Thanks. Using Cat 5e UTP cable for the control wiring would simplify my project, and my home shop isn't a RF-busy environment. I intend to use shielded cable on the output-to-motor.
 
Ground the shield of the control cable to the VFD ground screw which is also connected to the star ground/bus bar ground. This is the braided outer shield or drain wire. The low voltage controls are DC so the it is normally isolated from the AC ground, you do not want to attach the shield ground to the DC circuits. A bit confusing, in particular on some of the generic VFDSs with poor manuals. The motor cable shield is connected to both the motor ground and at the VFD end. Some shielded VFD cables have a separate ground wire which is also connected at both ends, others the ground wires consist of three symmetrical ground wires wrapped in copper foil. Keep your control wires at least 4-6" away from the motor cable, cross at 90 degrees if needed. With short cables noise is typically not a problem, but I have seen issues with the VFD noise contamination, hard to predict. The Teco manuals are well written, you might look at one as to wiring/terminal recommendations. Cat 6 cable does come shielded with twist pairs of 23 AWG, should work. I typically use shielded communication cable 20-22 AWG which can be picked up in short increments. The speed pot should also be shielded and grounded at the VFD end.

https://www.tecowestinghouse.com/Manuals/E510_startup_installation_manual.pdf
 
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