3 Phase OPEN DELTA and VFD's

[g-and-h_electric]

I need some help (and no not that kind). I just finished wiring up a YORK rooftop HVAC unit with electric heat strips and a VFD controlled blower motor. The building is fed with a 3 Phase 4 wire 120/240 Volt open delta service which has a high B phase.

The drive has an annoying habit of tripping out on "phase loss or imbalance". I added a drive isolation transformer (which I had to do on a TRANE unit installed on the same building, and on the Trane the transformer fixed the problems). This unit still trips out regularly. I have measured the voltages on both sides of the transformer, and the phases are not exactly balance from phase to phase as in say 242, 250 and 256 on the primary side, this of course affects the balance on the secondary side.......

Tech info :

Drive: DELTA MS-300 series
Motor: BALDOR 3Hp air over motor SF=1.15
Service notes: 4 single phase items on the transformer bank..... a) masonic Lodge no A/C units visible from the exterior, b) street light and special event receptacle panel c) 2 special event services with receptacle panels on the building. The special event stuff had no loads on them at this time.

Any ideas how to get this working? I hate to do buck boost transformers on each leg, ans there is no guarantee that come summer I wont have to change things. The power company is saying for about 20k, they can give us a wye service...... Seeing as this is a municipal building that isnt really in the budget


de, Howard

PS: why do I get all the "dirty dogs" at the shop lately????????

 

[ptonsparky]

You didn’t say what the current was on each phase. Rotate the phases until you get the most balanced current load. Typical fix for open delta.

I’ve never had to install an isolation transformer so IDK what the voltage is in the secondary. Is it Wye?

 

[synchro]

... The building is fed with a 3 Phase 4 wire 120/240 Volt open delta service which has a high B phase.

... I have measured the voltages on both sides of the transformer, and the phases are not exactly balance from phase to phase as in say 242, 250 and 256 on the primary side, this of course affects the balance on the secondary side.......

Any ideas how to get this working? I hate to do buck boost transformers on each leg, ans there is no guarantee that come summer I wont have to change things.

This might be explained by just a mismatch in the ratios of the two open delta transformers. For example one transformer secondary would be 242 with the other transformer secondary at 256, and 250 is across the "open" leg.

The "law of cosines" with the 242 and 256 legs at 60° gives 249.3 across the remaining leg, which is only 0.3% shy of your measured 250. And so this ratio mismatch is a possible explanation of your measured values.

If the transformer ratio mismatch is the issue, then putting a 5% buck on the 256 leg would bring it down to 244 and the "open" leg would then be 243 from the law of cosines with a 60° angle. So this should give you 242, 243, 244 as a result which is better than a 1% balance. You could also choose to boost the 242 leg up 5% instead.

You would need accurate phase angle measurements to verify upfront that this would work, but I think trying out just one buck-boost this way could be worthwhile.

By the way, what ptonsparky said about rotating the phases should certainly be done first to see how that works out.

 

[augie47]

I was doing a moderator fast proof read this morning and saw " The "law of cosines"": and stopped for a minute and thought we were going to discuss southern marriages...
cosines, Gus, not cousins

 

[kwired]

Is the "third phase" derived from a transformer that is possibly a little undersized? If so can POCO bump it up a little in size, hopefully for much less than they can build a new wye system? Or maybe they can turn it into a full delta, even if one large light pot and two smaller pots?

 

[Jraef]

Like other Delta drives, the only way it detects a phase loss is by monitoring the DC bus ripple and that is affected by load. So the higher the load, the quicker it will trip on excess DC bus ripple. The fact that it is an open delta system is the likely cause, but if you put a drive isolation transformer ahead of it, that's puzzling, because that is usually the cure. But hopefully your transformer was a delta-wye, and you grounded the X0 terminal on the secondary, creating a 240Y136 system to feed to the drive. If you left the secondary floating, it might as well not be there. If it is a delta-wye with a grounded neutral on the secondary, then you might have a defective drive.

Supposedly there are a couple of parameters for tweaking the bus ripple trip times and values, 6-50 and 6-52, but the manual doesn't actually list those parameters so there is no way to know the default settings and exactly what they do.

 

[StarCat]

I thought it was me who always got the most convoluted, most bizarre, most nonsensical, most complicated situations to solve in the field, so I see I am not alone. York is much more of a bad word in HVAC than Trane, but all of them can run you around the bend. Sounds like there is some good advice running here, but you need to make York aware of it and complain to Tech support about how useless their gear actually is in the field.This also goes for anything that does not work. We are in this position because all this nonsensei Tech is accepted rather than rebelled against fully. This is again also because everything has been monetized and weaponized. Its gotten people thinking the wrong way around and it wastes a colossal amount of time that we do not get payed for.

 

[kwired]

I thought it was me who always got the most convoluted, most bizarre, most nonsensical, most complicated situations to solve in the field, so I see I am not alone. York is much more of a bad word in HVAC than Trane, but all of them can run you around the bend. Sounds like there is some good advice running here, but you need to make York aware of it and complain to Tech support about how useless their gear actually is in the field.This also goes for anything that does not work. We are in this position because all this nonsensei Tech is accepted rather than rebelled against fully. This is again also because everything has been monetized and weaponized. Its gotten people thinking the wrong way around and it wastes a colossal amount of time that we do not get payed for.

Sounds a little like AFCI's in the electrical trade

HVAC equipment was more reliable way back before they started making said equipment more energy efficient.

About that same time they also started finding ways to cut costs so it made for a double whammy on longevity and reliability of the final product.

Many newer air handlers (at least the ones that designed to accept electric heat strips) have such thin sidewalls they won't stand up straight and square if you take the front panel off.

 

[synchro]

Many newer air handlers (at least the ones that designed to accept electric heat strips) have such thin sidewalls they won't stand up straight and square if you take the front panel off.

Yes a lot of things no longer are as rugged as they once were. My first car which I had in the mid 70s was a '65 Cadillac, and my friends could all pile on top of the hood without a problem. Try doing that on cars now.

 

[ptonsparky]

Yes a lot of things no longer are as rugged as they once were. My first car which I had in the mid 70s was a '65 Cadillac, and my friends could all pile on top of the hood without a problem. Try doing that on cars now.

A car with 100k miles was wore out back then. Todays are just getting started.

 

[synchro]

A car with 100k miles was wore out back then. Todays are just getting started.

Yes, there have been a lot of improvements. Hardened valve seats, chrome plated piston rings, better heat treating, much better gaskets, seals and hoses (the cork gaskets were the worst), electronic fuel injection instead of carbs, elasomeric joints in the suspension instead of metallic bearings, plastic body parts that don't rust out, etc. I think the universal adoption of closed loop fuel injection was driven mostly by emission requirements, but it also keeps engines from running too rich and getting oil dilution that accelerates engine wear. Also, electronic ignition and spark plugs with platinum electrodes have greatly reduced the required maintenance which was often ignored and therefore shortened engine lifetimes.
I think modern synthetic oils with improved additive packages would've prolonged the life of even the older cars.

I believe that some of the reduction in the strength and stiffness of structures in today's cars compared to those of many years ago is so they can give way and crush in specific areas to absorb energy in an accident. The car may be worse off for that but the owner is more likely to survive. Also weight reduction for fuel economy is another factor driving that trend.

 

[kwired]

Yes a lot of things no longer are as rugged as they once were. My first car which I had in the mid 70s was a '65 Cadillac, and my friends could all pile on top of the hood without a problem. Try doing that on cars now.

some cases you don't even want a 5-10 year old to be on top the car.

 

[kwired]

Yes, there have been a lot of improvements. Hardened valve seats, chrome plated piston rings, better heat treating, much better gaskets, seals and hoses (the cork gaskets were the worst), electronic fuel injection instead of carbs, elasomeric joints in the suspension instead of metallic bearings, plastic body parts that don't rust out, etc. I think the universal adoption of closed loop fuel injection was driven mostly by emission requirements, but it also keeps engines from running too rich and getting oil dilution that accelerates engine wear. Also, electronic ignition and spark plugs with platinum electrodes have greatly reduced the required maintenance which was often ignored and therefore shortened engine lifetimes.
I think modern synthetic oils with improved additive packages would've prolonged the life of even the older cars.

I believe that some of the reduction in the strength and stiffness of structures in today's cars compared to those of many years ago is so they can give way and crush in specific areas to absorb energy in an accident. The car may be worse off for that but the owner is more likely to survive. Also weight reduction for fuel economy is another factor driving that trend.

Demolition derby is a big thing aournd here at county fair time. Older cars are getting harder to find and in recent years they only have one heat of older cars (if there is enough to even do so) and the rest of the event is usually about 1980 and newer cars. Compared to the old cars they fall apart and leave debris everywhere, and do so pretty quickly. Majority are front wheel drive and do seem to maneuver around pretty well compared to the old cars.

 

[g-and-h_electric]

Ok, so here is the latest......

I spoke with the local YORK branch, and the man in tech support was absolutely fantastic. Walked me through unlocking the drive, and trying some parameter changes ( which overnite it was found didnt help). He also walked me through an "approved" way to run the motor single speed without the VFD.

Back to the VFD...... Yes, the transformer is a delta-wye unit, IIRC, I did ground X0. I am seeing voltages around 133V to ground on the secondary.

As to the primary voltages... these were taken Wednesday afternoon, around 2 PM.....
A-B = 250
A-C = 249
B-C = 256 (this is an issue?????)

A-Grnd = 124.2
B-Grnd = 220
C-Grnd = 123.8

Secondary voltages:
A-B = 234.1
A-C = 241.6
B-C = 242.6

The transformer is a Jefferson Electric
413-6ACC-000
(https://quote.jeffersonelectric.com/Products/SpecSheet/Details?catalogNumber=413-6ACC-000)

I forgot to write the amperages at the motor down, but one leg of it was noticeably higher than the other 2. Would have to "clear" it with my boss to reconnect the drive an re-amp the motor if we deem it necessary...

Hope this info is helpful We had a similar problem with another unit and the transformer cleared things right up, BUT that was IIRC, and ABB drive, and this one is a DELTA brand.


Howard

 

[synchro]

As to the primary voltages... these were taken Wednesday afternoon, around 2 PM.....
A-B = 250
A-C = 249
B-C = 256 (this is an issue?????)

On your first post you mentioned:

I have measured the voltages on both sides of the transformer, and the phases are not exactly balance from phase to phase as in say 242, 250 and 256 on the primary side, this of course affects the balance on the secondary side.......

Was the 242 a typo, or is one leg of the open delta changing from 242 to around 249 or 250 over time?

 

[g-and-h_electric]

It was a typo.... I was doing the number from memory

 

[Jraef]

There is nothing wrong with those numbers. Unless they change, the drive is defective.

 

[g-and-h_electric]

Just a little "final info"...... We have the blower running single speed 100% on a starter ( with the suggestion and blessing of the manufacturer), and are "dumping" the current VFD issue on the designing engineer, seeing as he "over designed" the system to begin with, and was warned about the electrical service before he finished the design.

I have 1 last question in case I really have to VFD the blower...... Would a single phase in 3 phase out VFD , connected to the "balanced" legs of the service solve the problem? Just curious (mainly in case we need to get the "last laugh " on the engineer that seems to be book smart and have no field knowledge)


Howard

 

[Jraef]

Just a little "final info"...... We have the blower running single speed 100% on a starter ( with the suggestion and blessing of the manufacturer), and are "dumping" the current VFD issue on the designing engineer, seeing as he "over designed" the system to begin with, and was warned about the electrical service before he finished the design.

I have 1 last question in case I really have to VFD the blower...... Would a single phase in 3 phase out VFD , connected to the "balanced" legs of the service solve the problem? Just curious (mainly in case we need to get the "last laugh " on the engineer that seems to be book smart and have no field knowledge)


Howard

THAT drive? Probably not, because I think the drive is defective, probably a loose or failing DC bus capacitor. Feeding it single phase will make it WAY worse. When you have a single phase input, the DC bus caps experience a LOT more ripple, so you need MORE capacitance than what is designed in for a 3 phase input and the diodes need to be bigger. That's why most drive mfrs will tell you to double the size of the VFD, maybe triple it. But at 230V 3HP and down, SOME drives are already designed to accept single phase input without de-rating, you would need to check, do not assume.

If you use that transformer the way you had is hooked up, a good 3 phase VFD would irk fine, because as I said, your input numbers were perfectly acceptable.

 

[kwired]

THAT drive? Probably not, because I think the drive is defective, probably a loose or failing DC bus capacitor. Feeding it single phase will make it WAY worse. When you have a single phase input, the DC bus caps experience a LOT more ripple, so you need MORE capacitance than what is designed in for a 3 phase input and the diodes need to be bigger. That's why most drive mfrs will tell you to double the size of the VFD, maybe triple it. But at 230V 3HP and down, SOME drives are already designed to accept single phase input without de-rating, you would need to check, do not assume.

If you use that transformer the way you had is hooked up, a good 3 phase VFD would irk fine, because as I said, your input numbers were perfectly acceptable.

I think he wants to use a different drive, designed/sized for single phase input. But I agree something probably wrong with the original and some other three phase input drive should also work.