Complaints of Delta Supplied Three Phase

[altarengineering]

We recently designed an installation in which a 40 HP Submersable motor was served by a two pot transfomer bank... Each bank was 25 KVA resulting in a total of 50 KVA. Acknowledging that the 40 HP can be assumed equivalent to a 30 KW load (PF assumed to be unity) and the 50 KVA two pot bank derated to 86.6%, should the customer have any complaints. He is complaining about not having true three phase, and unless Im missing something, he's mistaken. He claims that had he know he was to be served from a two pot bank he would have set his equipment up differently, but now the current set up will cause him problems

Let me know your thoughts

 

[hillbilly1]

I don't know why he would have to change anything different between an open delta and a full delta, the voltages are the same, unless he was expecting a wye connected bank, you didn't say whether this was a 240 volt or 480 volt system, if 240, maybe his controls are set up for 208 instead of 240.

 

[jim dungar]

We recently designed an installation in which a 40 HP Submersable motor was served by a two pot transfomer bank... Each bank was 25 KVA resulting in a total of 50 KVA. Acknowledging that the 40 HP can be assumed equivalent to a 30 KW load (PF assumed to be unity) and the 50 KVA two pot bank derated to 86.6%, should the customer have any complaints. He is complaining about not having true three phase, and unless Im missing something, he's mistaken. He claims that had he know he was to be served from a two pot bank he would have set his equipment up differently, but now the current set up will cause him problems

Let me know your thoughts

Your customer does have 'true' three phase.

There is nothing inherent in an open-delta that would require him to set up his equipment differently from a closed-delta. Of course there is an issue if he had been expecting a wye connection.

But, how is your system grounded? A center-tapped delta is not directly interchangeable with a corner grounded one.

Your transformer bank output is 43.3kVA 3-phase, personally, I think this is a little small for staring a 40HP submersible pump.

Many people wrongly assume that an open-delta is synonymous with a wild-leg/high-leg/center-tapped system, especially when the voltage is 240V line-line.

 

[hillbilly1]

Many people wrongly assume that an open-delta is synonymous with a wild-leg/high-leg/center-tapped system, especially when the voltage is 240V line-line.

Am I misunderstanding you? Most open deltas are wild leg/high-leg/center-tapped systems, at least all of the open deltas I've seen any way, may be different in industrial areas or up north.

 

[jim dungar]

Most open deltas are wild leg/high-leg/center-tapped systems, at least all of the open deltas I've seen any way, may be different in industrial areas or up north.

Most does not mean all.

While I cannot remember getting an open-delta utility service that was not 'center-tapped' for 240/120 3Ph4W, I can tell you I have designed many customer owned banks that were not (I think my largest was +1000A from 480V up to 600V to feed an assembly line).

 

[Jraef]

...
Your transformer bank output is 43.3kVA 3-phase, personally, I think this is a little small for staring a 40HP submersible pump.

That jumped right out at me too!

altarengineering,
Even if you are using a VFD for this pump you are a little too small; assuming 480V, a 45kVA transformer is 54A and you only have 43.3kVA so that makes it 52A, a 40HP motor is 52A FLC. Any voltage drop down the well and you are sunk but if you are starting it with anything other than a VFD, fuggedaboudit. It likely won't even finish accelerating.

 

[augie47]

Since Jraef, a most learned motor guru, didn't pick up on this, I'm a bit hesitant.
It seems this was discussed here previously but I can't located the thread. A few years back, I had a customer with serious problems on current inbalance and loss of submersible pumps. After a lengthy back-and-forth with the utility, TVA sent engineers to the site. There specifics were way over my head, but the conclusion was that submersible pumps, more than most other motors, were subject to give more problems on an open-delta supply than a 3 pot delta or wye supply.
I can testify that once the services were changed from open delta, the problems disappeared.
I can put you in touch with the POCO engineer involved for specifics if you wish. Just PM me.

 

[hillbilly1]

Since Jraef, a most learned motor guru, didn't pick up on this, I'm a bit hesitant.
It seems this was discussed here previously but I can't located the thread. A few years back, I had a customer with serious problems on current inbalance and loss of submersible pumps. After a lengthy back-and-forth with the utility, TVA sent engineers to the site. There specifics were way over my head, but the conclusion was that submersible pumps, more than most other motors, were subject to give more problems on an open-delta supply than a 3 pot delta or wye supply.
I can testify that once the services were changed from open delta, the problems disappeared.
I can put you in touch with the POCO engineer involved for specifics if you wish. Just PM me.

That may be the issue, since open deltas are used to save the expense of a third transformer, they probably undersized the transformer to begin with to save money, on the application that you had experience with, they probably just threw another transformer of the same size up there instead of changing the whole bank, which greatly increased the capacity. Just a thought.

 

[kwired]

As long as the power delivered starts the motor and there is no other problems the undersizing of the transformer is likely POCO's problem. Many of them do like to size them close to load and even a little under the load at times - especially if the load is not continuous, and by that I am not talking 3 hours like NEC calls continuous, 8 hours on and 16 off is something they do consider for time for things to cool down.

Open delta still has all voltage possibilities on the secondary as a full delta. It can be corner grounded, mid point of one phase grounded or ungrounded.

Most farm loads I run into if the total HP is greater than 25 or 30 the POCO usually prefers to supply with full delta or wye transformer bank. Occasionally they do have times where some distance is involved to get all three phases to the site and they may settle for an open delta. I have run into open delta services supplying 60 and 75 HP motors, but is a little rare. On all of those I am aware of with that big of a motor there is not primary line with all three phases supplying the installation so full delta or wye is not an option unless rebuilding the primary to carry all three phases.

 

[broadgage]

As others post, the transformer capacity is marginal for the pump size if used with a VFD, and almost certainly to small without.

An open delta supply is not ideal for motors that are large in relation to the service capacity/transformer size.
OFF LOAD the voltage between the 3 phases should be equal.
However when a load is applied the voltage will drop, and NOT EQUALLY on the three phases.
The voltage drop on the "fake" phase will be greater than on the other two, remembering that the current passes through two transformer windings, with voltage drop in each.

Most induction motors will tolerate a supply of from 90% up to 110% of the nominal or nameplate rating. A much closer tolerance is however required as to voltage difference between phases.
Some motors will only tolerate 1% or 2% difference in voltage between phases.
That is, a supply of 90% of nominal, but the same on each phase, is OK, but A supply of 90% nominal on one phase and 96% of nominal on another phase is not acceptable.

Presuming that the motor will run, I would suggest measuring the on load voltage of each phase and confirming that they are not only within tolerance, but also within 2% of each other.
Also measure the on load current in each phase and compare with the nameplate rating.

I suspect that you will find find either low voltage in general, or an excessive difference between phases, or both.
In case of a deep well pump you can not readily measure the voltage at the motor, measure elswhere and calculate/estimate the actual voltage at the motor from the length and size of the cable.

 

[kwired]

We have had 2 people mention the motor may not even start.

Outside of failing to start because of tripping overcurrent devices, I find it a little hard to believe it will not start. Will there be voltage unbalances that maybe shorten the life of the motor - yes that could be very true, I still think it will start.

Overcurrent protection on a utility transformer will not normally be so tight to rating of transformer that it will open when starting this motor. If it were a high inertia load that takes a long time to accelerate there would be a better chance of starting problems. I see that a lot with crop drying fans.

If voltage imbalance is going to make that big of a difference in motor performance then why do I find so many motors powered by phase converters that have horrible voltage imbalance and still work? They possibly have shorter life than if they had a good balanced source but they do work.

The OP has a 40 hp motor, but what is the load on it? That will make some difference.

 

[renosteinke]

It's not 'true' three phase, and the customer is right to be disappointed.

I say it's not 'true' in that the power derives from but two transformers. Does anyone here really think that the third transformer in a 3-transformer arrangement does nothing?

Nor is a 'delta' system wise in this day. You need a neutral- even if that neutral exists only at the point where the transformers connect together. Let me explain ...

All the different 'delta' arrangements work, sort of, when your loads are nothing but pure resistance and pure inductance. Heaters and motors. Period. Add one microchip, one transformer, and the house of cards starts shaking. All it takes in one little thing to act up and you're lost.

That's a pretty big hit for the momentary convenience of having one less transformer and one less wire.

Don't try telling me that delta systems 'work, they're all over the place.' As I post I'm sitting in a steel mill that is delta-powered. They've had the induction welding equipment make miles of bad welds, simply because an HID fixture had a high-impedance fault to ground in the ballast. The interaction of the transients from the many frequency driven motors ("harmonics") causes all sorts of unique problems. Start up Mill 1 and sections of Mill 6 - miles away - go haywire.

There's the issue: we're not using Grandpa's pumps anymore. Frequency drives and control circuits abound. The best way to prevent problems is to have a solidly grounded "wye" system. I'd no sooner design an open, corner-grounded delta system today than I'd design a house with 2-prong plugs and penny fuses.

 

[kwired]

It's not 'true' three phase, and the customer is right to be disappointed.

I say it's not 'true' in that the power derives from but two transformers. Does anyone here really think that the third transformer in a 3-transformer arrangement does nothing?

Nor is a 'delta' system wise in this day. You need a neutral- even if that neutral exists only at the point where the transformers connect together. Let me explain ...

All the different 'delta' arrangements work, sort of, when your loads are nothing but pure resistance and pure inductance. Heaters and motors. Period. Add one microchip, one transformer, and the house of cards starts shaking. All it takes in one little thing to act up and you're lost.

That's a pretty big hit for the momentary convenience of having one less transformer and one less wire.

Don't try telling me that delta systems 'work, they're all over the place.' As I post I'm sitting in a steel mill that is delta-powered. They've had the induction welding equipment make miles of bad welds, simply because an HID fixture had a high-impedance fault to ground in the ballast. The interaction of the transients from the many frequency driven motors ("harmonics") causes all sorts of unique problems. Start up Mill 1 and sections of Mill 6 - miles away - go haywire.

There's the issue: we're not using Grandpa's pumps anymore. Frequency drives and control circuits abound. The best way to prevent problems is to have a solidly grounded "wye" system. I'd no sooner design an open, corner-grounded delta system today than I'd design a house with 2-prong plugs and penny fuses.

If you want all three phases supplying your service you can certainly have it - POCO may want you to pay for it though. If you only have a limited load they are not going to give you much of a break when they feel they can sufficiently supply you with open delta.

Open delta is still better than using rotary or static phase conversion equipment IMO. Solid state phase conversion - is probably OK - somewhat of a case by case issue. If VFD was to be used anyway you really have nothing different.

 

[jim dungar]

I say it's not 'true' in that the power derives from but two transformers. Does anyone here really think that the third transformer in a 3-transformer arrangement does nothing?

It is true three phase, unlike a manufactured three phase system.
For the most part the third transformer simply provides capacity. Yeah there is a little bit of voltage regulation that goes along with the capacity, but that it is usually not a problem for 3-wire 3-phase loading. And remember, usually does not mean always.

Most of the facility problems you describe seem to relate to the grounding and ground current detection (GF) more than they do to the system simply being delta connected. The philosophy of "we can ignore faults on our system while we maintain production" is foolish as your ballast example illustrates.

I have noticed it seems most power electronic manufacturers now design their equipment specifically for wye connected sources (i.e. L-G connected filter circuits), thus they are not robust enough for the common overvoltages found on delta systems.

 

[mivey]

Some motors will only tolerate 1% or 2% difference in voltage between phases.

That's a little tight and the motor should be derated. The POCO is probably going to tell you that is not their problem...and it really isn't (up to a point).

If you want all three phases supplying your service you can certainly have it - POCO may want you to pay for it though. If you only have a limited load they are not going to give you much of a break when they feel they can sufficiently supply you with open delta.

That is true. Many "special circumstance" deals still exist though.

It is true three phase, unlike a manufactured three phase system.
For the most part the third transformer simply provides capacity. Yeah there is a little bit of voltage regulation that goes along with the capacity, but that it is usually not a problem for 3-wire 3-phase loading. And remember, usually does not mean always.

I agree.

The OP might ask the utility for a motor start analysis. Sometimes the results can prompt the POCO to make a no-additional-fee change that might help.

We had a customer with a large chiller that was about maxed out and thus more sensitive to voltage imbalance. While it was within acceptable levels for us, we were able to make a system change that helped slightly and brought the imbalance back to just under the wire for what the customer needed. They still need to upgrade their out-grown chiller as it will continue to be an issue during extreme heat. We can't always cater to individual customers and have to consider the system as a whole.