Drives on Delta

[Coachmike]

Have a client with a machine shop who swears his 3 phase machines are negatively impacted by the delta 240v (high leg) feeding his 3 phase equipment. Can a power supply/ drive be affected by the high leg? (They are 3 wire 3 phase machines)

 

[zbang]

What does "negatively affected" mean?

If the machine is designed for a wye connection with 120v phase-to-ground, it might have difficulty with the higher phase-to-ground of the high-leg (usually if there are either surge-suppressors connected to ground or it uses the ground as a bootleg neutral for some controls).

This is another case of "Ask the manufacturer" .

 

[Dsg319]

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Have a client with a machine shop who swears his 3 phase machines are negatively impacted by the delta 240v (high leg) feeding his 3 phase equipment. Can a power supply/ drive be affected by the high leg? (They are 3 wire 3 phase machines)

If there are no Line-Neutral loads within the drive nothing should be seeing the high leg voltage 208volt to ground.

 

[petersonra]

What negative impacts does he think are happening?

 

[LarryFine]

A couple of years ago, I was hired to add a third B-B transformer and re-wire, to convert an existing open-delta boost set-up to a wye boost set-up for a 240v machine that wouldn't accept the uneven voltages, even though there was no load neutral.

 

[petersonra]

A couple of years ago, I was hired to add a third B-B transformer and re-wire, to convert an existing open-delta boost set-up to a wye boost set-up for a 240v machine that wouldn't accept the uneven voltages, even though there was no load neutral.

what is an open delta boost setup?

 

[LarryFine]

what is an open delta boost setup?

A boost, and a buck.

 

[winnie]

The normal rule of 'line-line loads don't care about line-ground voltage' gets a bit broken when you talk about motors on drives. The reason is because of 'parasitic' line-ground parts of the circuit.

The big one will be the TVS devices which are often connected between line and ground on the input of the VSD. This is far more of a problem on 480V drives, where the TVS devices are designed for nominal 277V to ground, and will pop on a corner grounded delta, and will likely try to create a ground for an ungrounded system. Not sure that this will be a problem for a high leg 240V system, but as noted above 'ask the manufacturer'.

Another aspect of parasitic line-ground connections is the capacitance of the motor winding to the frame. The high frequency modulation of the VSD creates a high frequency current from winding to frame, and this current is trying to get back to the transformer source via the transformer ground point. I don't know if asymmetric line to ground voltages meaningfully alter this current, but I could imagine that such would introduce a large 120Hz ripple in this ground current that might impact other equipment.

-Jon

 

[petersonra]

A boost, and a buck.

View attachment 2560435 View attachment 2560436

I don't know that what you are showing is an open delta circuit.

 

[winnie]

Commonly, 'open delta' is used to describe a power company supply configuration where only two legs of the delta are used for a supply.

But the term is also used to describe an autotransformer configuration used to buck or boost a three phase supply using two transformers. The second use of the term is what post 7 shows.

-Jon

 

[LarryFine]

I don't know that what you are showing is an open delta circuit.

Do you not get the electrical connections, or do you disagree with the term?

 

[petersonra]

Do you not get the electrical connections, or do you disagree with the term?

The term. This arrangement brings one of the input lines to the output so calling it open seems kind of off.

 

[LarryFine]

The term. This arrangement brings one of the input lines to the output so calling it open seems kind of off.

Well, an open-delta source has three lines out, too. And the arrangement does fit the description, in my opinion.

I didn't make it up. See: https://www.google.com/search?q=ope...i59i450l8.281136j0j7&sourceid=chrome&ie=UTF-8

 

[kwired]

The term. This arrangement brings one of the input lines to the output so calling it open seems kind of off.

Well, an open-delta source has three lines out, too. And the arrangement does fit the description, in my opinion.

I didn't make it up. See: https://www.google.com/search?q=ope...i59i450l8.281136j0j7&sourceid=chrome&ie=UTF-8

The buck or boost portion of the setup is itself an open delta configuration.

 

[GoldDigger]

It is also possible to use three autotransformers to create a "full delta" in which all three output circuits are symmetric. But if you have acorner grounded input, the output will be corner ground referenced but none of the output lines would actually be at ground potential.
If you have a delta circuit fed from a wye source, then the boosted circuit would have the same center point ground reference supplied by the source end.

 

[Jraef]

Getting back to the subject, this issue has nothing to do with it being a “high leg” system, it’s about being a delta system. In the case of a 240V delta system, Jon’s last paragraph is the issue for the most part. Almost all drives are made expecting a solidly grounded Wye source. They will usually have “common mode capacitors” to handle the ground noise issues, but in a delta system with no direct reference to ground on one phase, that noise builds up and affects other things, including the drive electronics. The safe bet if you already know there is an issue is to use a “drive isolation transformer” to feed the drives. It will be 240v input, 240/139V grounded Wye output. You are not using the 139V for anything, it’s just to establish a solidly grounded neutral feeding the drives.

 

[wwhitney]

It is also possible to use three autotransformers to create a "full delta" in which all three output circuits are symmetric.

So let's see, this would cause the group of phase angles to rotate relative to the supply, yes?

I.e. Say you have 3 identical single coil transformers A, B, C, and each has connections A0, AX, and A1, where AX is X fraction of the turns from A0 to A1. Then for bucking you wire the supplies to A0=C1, B0=A1, and C0=B1. Then AX, BX, CX are your bucked voltages, and a little geometry tells you the bucked voltage and rotation relative to the supplies (if you care). Conversely for boosting you swap the roles of the X connections and the 1 connections.

Cheers, Wayne