VFD on Open Delta Service

[JoeStillman]

POCO says you can't feed VFD's from an open delta transformer. This is news to me. Anybody else?

 

[LarryFine]

POCO says you can't feed VFD's from an open delta transformer. This is news to me. Anybody else?

It seems like the VFD maker would tell you that, not the POCO. Is it a warning that the line voltages to neutral will not be equal, or that they don't want that kind of load?

 

[Russs57]

The POCO could be concerned with harmonic distortion if the VFD is a high percentage of the total load. You may be required to provide a harmonic study. Likely you will need to over size the drive and add line reactor/DC link choke. I would not expect the drive to last as long as one installed in a system with equal impedance between phases. Of course neither would a motor without the drive.

 

[ptonsparky]

The POCO could be concerned with harmonic distortion if the VFD is a high percentage of the total load. You may be required to provide a harmonic study. Likely you will need to over size the drive and add line reactor/DC link choke. I would not expect the drive to last as long as one installed in a system with equal impedance between phases. Of course neither would a motor without the drive.

Open delta fed motors are all over this State. VFDs are a different animal. Most have limitations in their installation instructions.

 

[Hv&Lv]

POCO says you can't feed VFD's from an open delta transformer. This is news to me. Anybody else?

Easier to read...
lots of times we don’t know about it until something is burnt up a time or two...

https://www.yaskawa.com/delegate/ge...PS.01&cmd=documents&documentName=PR.PS.01.pdf

 

[Russs57]

Ptonsparky, I would ask what is your average life expectancy of a 3 phase 50+ HP motor on said open delta services. If it isn't a minimum of 20 to 30 something years you are proving my point.

Adding a VFD just move the motor's problem to the VFD.

 

[Jraef]

1

 

[ptonsparky]

Ptonsparky, I would ask what is your average life expectancy of a 3 phase 50+ HP motor on said open delta services. If it isn't a minimum of 20 to 30 something years you are proving my point.

Adding a VFD just move the motor's problem to the VFD.

Some of them have been in place since the 50s. 30 HP and below. Ag, not manufacturing. Grounded deltas, open deltas, and a few ungrounded deltas are disappearing as age requires replacement of the utility distribution but they are still there.

 

[kwired]

Ptonsparky, I would ask what is your average life expectancy of a 3 phase 50+ HP motor on said open delta services. If it isn't a minimum of 20 to 30 something years you are proving my point.

Adding a VFD just move the motor's problem to the VFD.

Motor doesn't care what is grounded or even if anything is grounded, just wants to see rated volts and frequency, or same ratio as rating if you change one of those values, which is a big factor in why you can use VFD's on them in the first place.

The problem is not all VFD's are designed to operate on delta systems. As mentioned many of them are designed to operate with equal voltage to grounded point like you would find on a typical wye source.

We have a lot of high leg delta systems around these parts. Even 480 volt high leg delta, and some corner ground delta. Motors don't care and some last for very long time if not compromised by some other factor.

 

[JoeStillman]

VFDs don't like any kind of delta service. That's because VFDs are designed for use on solidly grounded wye systems, with MOV protection on the line side of the rectifier to protect them from surges on the line. Those MOVs are typically configured in a wye pattern with a ground reference through a 4th device. Some also have Common Mode Noise filter capacitors on the DC bus, also referenced to ground.


In a delta system, open or not, there is no ground reference. If those devices are reference to ground and your system is not, the potential across them is the line voltage, but they are typically rated for the wye L-G reference. Then the first time there is a ground fault anywhere upstream of the VFD, the fault tries to flow through the wye connection to ground, until the devices vaporize, usually taking out other things around them or coating the entire insides of the VFD with conductive metal oxides. In the above drawing (from A-B drives), it shows that they give you a jumper on the ground reference that can be removed, making them no longer referenced to ground and thereby "safer" to use on delta system. This also however removes some of the protection these devices provided. So even those that provide the jumper will warn you that you must then add back in some surge protection appropriate for your delta system configuration.

Unfortunately many drive mfrs do NOT provide easy ways to remove the ground reference points, because North America is the only place on the planet where delta power systems are used and for some mfrs, it's just not a big enough deal to worry about; they just put a statement in their manual saying "This drive is designed for use on a solidly grounded wye system." or words to that effect, leaving it up to you to understand what that means. So the "safe bet" is to either not use VFDs on delta systems, or use a 1:1 drive isolation transformer ahead of them that is delta-wye, where you ground the wye point just on the circuit feeding the drive(s).

Thanks Jraef, I should have clarified, it's open 4-wire delta, so there is a ground reference.

 

[synchro]

Thanks Jraef, I should have clarified, it's open 4-wire delta, so there is a ground reference.

So it's just like a high-leg delta but without one "side" leg. Therefore the voltage to ground would be highly asymmetrical and this could cause the problems Jraef mentioned above.

It might be worth considering a single-phase VFD driven from the center-tapped leg of the open delta if the service would support this load. Line reactors might be needed with single-phase in order to reduce the conducted harmonics to an acceptable level, but they would also be beneficial by reducing the peak rectifier diode current in the VFD.

 

[ptonsparky]

So it's just like a high-leg delta but without one "side" leg. Therefore the voltage to ground would be highly asymmetrical and this could cause the problems Jraef mentioned above.

It might be worth considering a single-phase VFD driven from the center-tapped leg of the open delta if the service would support this load. Line reactors might be needed with single-phase in order to reduce the conducted harmonics to an acceptable level, but they would also be beneficial by reducing the peak rectifier diode current in the VFD.

I was wondering if this would be an acceptable alternate method. Depending on the size of the motors, I'm guessing.

 

[JoeStillman]

So it's just like a high-leg delta but without one "side" leg. Therefore the voltage to ground would be highly asymmetrical and this could cause the problems Jraef mentioned above.

It might be worth considering a single-phase VFD driven from the center-tapped leg of the open delta if the service would support this load. Line reactors might be needed with single-phase in order to reduce the conducted harmonics to an acceptable level, but they would also be beneficial by reducing the peak rectifier diode current in the VFD.

So a 1:1 delta-wye transformer between the drives and the utility should solve the problem?

 

[ptonsparky]

So a 1:1 delta-wye transformer between the drives and the utility should solve the problem?

That would be my understanding. Grounding and bonding the secondary center point per NEC 250.