Open Delta 3 Phase Problems

[joejensen16]

I have a large workshop in a residential neighborhood and when we built the utility installed 2 transformers to provide 600A of open delta 240V to the detached workshop. I've had a ton of problems. This is in Arizona and the utility is SRP. Late last spring I installed the first 3 phase machine which runs on a VFD. The VFD would constantly fault for input phase loss. In measuring the voltages that summer they were always spot on at 240.1V between all three phases. Oddly, on 4 different 3 phase machines the current on L2 (not the wild leg) was always low, some days 5% some days as much as 18%. This was on all machines. Not all the same drop but always L2 on all machines. I assumed that meant there was a phase problem. But when the utility installed a data logger it showed no phase problem. Phase perfect on all three legs, voltage the same on all three legs but on 4 different machines the current was always very low. This went on for months and in the end the utility kept a data logger running for months. By Sept the peak cooling loads for the city were dropping and the power was better. Fine all winter. Now this summer the VFD again faults out but this time the voltages are all over the place. They are close in the morning before peak loads and then the move apart in the temp outside climbs.

First, the single phase load on the workshop has never exceeded 50A and most of the time I took my own measurements I had all other breakers off.

I'm a BSEE but 38 years out of school and never a power engineer. The data from last year when the voltages were way off was:
L1-L2 = 240.1v
L2-L3 = 240.1v
L1-L3 = 240.1v
Current
L3 = A
L2 = 0.8 * A
L3 = 1.05 * A

Today for example (did not measure current)
L1-L2 = 246.5v
L2-L3 = 250.6v
L1-L3 = 242.6v

Is I have been unable to find any formulas of spreadsheets to help me understand how with open delta ungrounded I could get these measurements.

I suspect the Utility has a known issue a the main overhead line feeding our neighborhood and the underground in the neighborhood are scheduled for replacement in 2023 and they were installed in the early 1980s which I am told is not old by utility standards. The account manager and I am unable to get any info from the utility engineers. With the account manager I've been able to get 7 different calls with the utility engineers and each time they bring another expert in.

 

[davidbeach]

The open delta is a way of serving 3-phase motors with only two primary phase (and a neutral). Motors endure crap for voltage. Voltage imbalances make the motors run hotter, but if not maxed out they may not overheat to the point of damage. The more load the wonkier the voltages. So great voltage at no load and not so great at higher loading is to be expected.

VFDs, unlike motors, expect perfection. You might well be better off with a VFD large enough to allow it to run the 3-phase motor with a single phase source voltage.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[itsmoked]

Those voltages and currents look typical to what I always see, actually probably better.

VFDs definitely prefer balanced wye inputs. Newer ones can completely freak-out with open-delta. Some will actually be toasted as their EMI mitigation hardware is tied to case ground.

How about you more clearly describe the immediate functional problem(s) you're having then we can better help you solve the functional problem(s). You can save yourself the hassle of dealing with the utility who is not going to change anything since what they're delivering is well within the required statutes.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[joejensen16]

The immediate problem is the VFD faults and stops. It’s an ABB ACS310 5hp driving a dust collection system. FLA for the motor is 11.8A and I have the VFD programmed to adjust motor speed up and down to hold current to 11.0 A. I installed a line reactor on the input to the VFD which helped a lot. I read online about something called “stiff power”. Apparently VFDs do not like stiff power which is when the VFD load is less than 10% of the transformer feeding the service to the building. I have a 600a service and I doubt I’ve ever exceeded 100a, usually more like 30-50a. Without the line reactor the currents into the VFD were as low as 40% in line 2, with the line reactor it’s been fine but that was before peak AZ cooling season. Now the dust collector Faults and shuts down in the afternoons.

I would love to understand how the voltage relationship between the legs on open delta work. I understand how the transformers are wired but I can’t find any equations that would help me understand what’s happening. I suspect phase errors in the afternoon from the utility but without an expensive Power quality meter I can’t see.

 

[JohnMcNutt]

I wonder if the utility has uneven loading on their system and in the afternoon, their voltage balance between phases gets out of whack. I think that is what you said.

Maybe I am a simpleton, but why can't a dust collector be run direct off the line? Eliminate the VFD and all associated troubleshooting entirely?

 

[joejensen16]

The variable speed on the dust collector is so when machines with smaller ports are run the fan RPM increases, increasing the pressure which pulls more CFM. This approach doubled the CFM through smaller ports. Last year I removed the VFD so I could run. That's an option but I have 3 other 3 phase machines have VFDs internally for variable speed. I would hate to ruin any of those VFDs.

 

[davidbeach]

Talk to the utility about paying for third transformer. Your specific benefit, your cost, not a cost to be shared by all rate payers. It might be a pretty simple case of just installing a third transformer or it might require building the third phase for some distance. Or just find a VFD that will accept a single phase input and use that; stop beating up the one you have that doesn't like where it is.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[joejensen16]

They utility is going to replace the feed to the neighborhood sometime in 2023. That feed runs down my property line and it’s where they installed two transformers. They have already committed to run the third phase line and add a third transformer at their cost when they do this line replacement. But no committed date yet.

 

[Palletjack]

600 amps of three phase on an open delta? Personally that’s a little too much.
Need to close the delta here.

 

[itsmoked]

What VFD make and model have you got?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Parchie]

@joejensen16 (Electrical),
I have doubts the voltage imbalance caused your ABB VFD to trip. The manual says the ABB unit can withstand a maximum of 3% phase-to-phase voltage imbalance in the power input. My calcs tell me your voltage unbalance is just 1.6% (based on your posted values).
Be that as it may, the use of line reactors is good. Investigate more and solve your problems. could be just loose terminations on some of your equipment.

 

[joejensen16]

The VFD is made by ABB. Model ACS310

 

[iop95]

On ABB VFD change parameter 3016 from 0 to 1 (ripple alarm + output current limit) or 2 (ripple alarm only).