high leg and VFD's on roof top units

[timtom58]

We have three rooftop units with a delta 4 wire, high leg system. currently grounded with conduit. that is rusted and not a good connection. Ran an open-air ground wire, And units are still messing up. I was reading that. VFD 's, for some reason, do not like Stinger legs with 3 phase. Would that be the reason that the three rooftop units are constantly showing error codes? being that we have a ground now to all three units.

 

[LarryFine]

I was reading that. VFD 's, for some reason, do not like Stinger legs with 3 phase. Would that be the reason that the three rooftop units are constantly showing error codes?

For sure. They look for balanced input voltages.

 

[ptonsparky]

Looks like a wye system for the rooftop units are in your future.

Some VFDs have a jumper that can be removed or cut to help with that problem. Read the fine manual for details. YMMV.

How big are the drives? Replace then with single phase input units tied to the non hi-leg.

 

[Jraef]

It’s not the high leg they don’t like, it’s the delta power system and the fact that only two of the phases are referenced to ground.

As Tom said, RTFM, there is likely a procedure describing what you need to do if you have a delta power system. But also, some Asian based drives do NOT address it, they just say it MUST be used on a solidly grounded wye system, or words to that effect using IEC grounding terminology that means nothing to most American electricians. In that case, your only choice is to put in a Drive Isolation Transformer, 240 delta in, 240Y138 out (you don’t actually use the 138, it just makes the drive happy).

 

[tortuga]

Possibly the ultimate win-win for utilities and old hi-leg delta customers with VFD's is for the utility to reconfigure the bank to a 220Y/127 transformer bank, it can be done with a notched up 208Y/120 bank so no special transformers.
Since incandescent lighting is a thing of the past I can't think of anything in a commercial build that could not handle 127 vs 120, then no drive isolation transformer, and 220 is within spec for a 240V motor.

 

[ptonsparky]

Possibly the ultimate win-win for utilities and old hi-leg delta customers with VFD's is for the utility to reconfigure the bank to a 220Y/127 transformer bank, it can be done with a notched up 208Y/120 bank so no special transformers.
Since incandescent lighting is a thing of the past I can't think of anything in a commercial build that could not handle 127 vs 120, then no drive isolation transformer, and 220 is within spec for a 240V motor.

Do they need to add a transformer if it was open delta to begin with?

 

[tortuga]

Do they need to add a transformer if it was open delta to begin with?

Yeah I think it would be a complete change out.
And you'd need 3-phase primary, which you technically don't need for open delta.

 

[ptonsparky]

Yeah I think it would be a complete change out.
And you'd need 3-phase primary, which you technically don't need for open delta.

That was another question.

 

[kwired]

Possibly the ultimate win-win for utilities and old hi-leg delta customers with VFD's is for the utility to reconfigure the bank to a 220Y/127 transformer bank, it can be done with a notched up 208Y/120 bank so no special transformers.
Since incandescent lighting is a thing of the past I can't think of anything in a commercial build that could not handle 127 vs 120, then no drive isolation transformer, and 220 is within spec for a 240V motor.

Unless it is supplying say a single or limited loads chances are they also have a certain amount of 120 volt loads supplied by the system and therefore a decision would need to be made, separately derive what you fee the drive with or separately derive what you supply the 120 volt loads from.

 

[winnie]

Have you recorded the error messages? How many HP are we talking, are the VFDs the dominant load, or do you have small VFDs for blowers combined with heat strips?

 

[kwired]

Have you recorded the error messages? How many HP are we talking, are the VFDs the dominant load, or do you have small VFDs for blowers combined with heat strips?

I'm with you, fault code may be key factor in what the issue is - possibly isn't even anything related to the supply.

Most general purpose drives in this voltage class will work on 208-240 nominal volts. They may require disconnecting a link that essentially disables the front side surge protection if used on high leg, corner ground or even ungrounded delta as the surge protection is set up for 120 volts nominal to ground and will continuously be clamping if any line is over say 170 or so volts to ground. I could also see if it is a drive intended only for 208 volt input that a 240 volt input could maybe trigger a fault for too high of voltage on the DC bus, especially if it is operating on the high end of tolerance for 240 nominal.

 

[petersonra]

The pro dominant fault code for vfds that have the wrong power input is for the input MOVs to catastrophically fail. You will see bits and pieces of them where they exploded. This may or may not cause the drive to fail.

 

[kwired]

The pro dominant fault code for vfds that have the wrong power input is for the input MOVs to catastrophically fail. You will see bits and pieces of them where they exploded. This may or may not cause the drive to fail.

I think many drives care less about what is on the supply side of the rectifier and mostly start monitoring anything starting with DC bus and beyond. This also part of why you can input single phase on just two of the three input terminals on most of them and it has no issues as long as you don't load the output enough to drag the DC bus down to too low of a voltage.

 

[ptonsparky]

I think many drives care less about what is on the supply side of the rectifier and mostly start monitoring anything starting with DC bus and beyond. This also part of why you can input single phase on just two of the three input terminals on most of them and it has no issues as long as you don't load the output enough to drag the DC bus down to too low of a voltage.

There is a parameter for this.

 

[kwired]

There is a parameter for this.

I just connected an Automation Direct drive about a week ago. Was not installing on high leg but recall seeing info on this topic had it applied to me in the instructions. Here is what you do to disconnect on that series to disconnect from the ground reference if needed:

 

[winnie]

The pro dominant fault code for vfds that have the wrong power input is for the input MOVs to catastrophically fail. You will see bits and pieces of them where they exploded. This may or may not cause the drive to fail.

I was lucky I wasn't drinking anything when I saw the catastrophic failure of an MOV described as a 'fault code'

 

[Jraef]

The pro dominant fault code for vfds that have the wrong power input is for the input MOVs to catastrophically fail. You will see bits and pieces of them where they exploded. This may or may not cause the drive to fail.

The collateral damage is what usually takes out the drive.

 

[Jraef]

I think many drives care less about what is on the supply side of the rectifier and mostly start monitoring anything starting with DC bus and beyond. This also part of why you can input single phase on just two of the three input terminals on most of them and it has no issues as long as you don't load the output enough to drag the DC bus down to too low of a voltage.

Actually when you have a single phase input, it’s the high DC bus ripple that they look for. But you are right in that this is affected by the actual load on the drive, along with the amount of DC bus capacitance and/or whether there is a DC bus choke. So over sizing the drive is what gets you that additional DC bus capacitance. 230V 3HP and below however, they just put the extra capacitance in anyway, which is why you see 3HP as the upper limit of not having to de-rate.

Asian based drive designs use all capacitors since caps are cheaper for them, American and EU designs tend to use a DC bus choke and fewer caps. That DC bus choke results in being able to use a 50% de-rate for single phase input, as opposed to a 65% de-rate for all-cap designs.

 

[wwhitney]

a 65% de-rate for all-cap designs.

Do you mean 35% * original rating or 65% * original rating?

Cheers, Wayne

 

[Elect117]

The pro dominant fault code for vfds that have the wrong power input is for the input MOVs to catastrophically fail. You will see bits and pieces of them where they exploded. This may or may not cause the drive to fail.

This might be a bit of a odd ask, but do you know of any documentation for the effects VFDs might have on MOVs? Has there been a study?

I ran into a situation where the MOVs where failing, shorting, and eventual failure was an abrupt explosion. I couldn't get to the bottom of why the MOVs keep failing like this. It was an ungrounded service so I assumed it was a restriking ground that was pushing the voltage higher than the MOV was rated and eventually it flashed over and failed.