3 phase Screw Compressor on an Add-A-Phase.

11bgrunt

11bgrunt

Pragmatist
Location
TEXAS
Occupation
Electric Utility Reliability Coordinator
Customer has a shop on the same single phase, 50kVA POCO transformer as his house.
The shop does tire repair with an autoclave. This requires a large cu. ft. of compressed air to 85psi.
An Ingersoll Rand SSR-EP25SE 25HP Rotary Screw Air Compressor served by an Add-A-Phase, provides part of the air requirement.
The POCO three phase line extension was quoted $40K+. A rotary phase converter would also be expensive.
The LRA on the screw is 540A @ 3P, 240V.
I believe this starting current is the root of all his power quality complaints over the last five years.
I thought the automatic unloader might be an issue and asked him to start @ 0psi. He reported this was not going to work for his operation.
There are a lot of different things that could be done, but all are expensive.
Can a soft start be used on this compressor, that is served by the Add-A-Phase? If the Add-A-Phase was not functioning correctly, what would the symptoms be?
The POCO is willing to increase the transformer to a 75kVA.
 

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11bgrunt said:
No need to control speed. Ramping the startup is the objective.
Click to expand...

Actually, this is a variable speed load.

You have a positive displacement compressor, meaning that it produces so many cubic inches of compressed air per rotation.

This is filling a tank. Then compressed gas gets used from this tank at so many cubic inches per second.

Currently the compressor gets cycled on and off so that the average input to the tank matches the average output. This is a form of variable speed.

A VFD will provide clean startup with none of the 'locked rotor' current loads of an across the line start, and then (with more complexity in the control) permit variable speed operation so that the compressor runs continuously at moderate speed rather than turning on and off.

On the other hand, the motor in the compressor may not be happy with the switching noise from a VFD. The switching can create voltage spikes that damage old insulation. Also the compressor may have mechanical limitations (lubrication issues, resonances, etc.) that prevent changing speed. Finally the input rectifier to the VFD may cause power quality issues for other loads on the transformer.
 
I would see what the new transformer does. That may help a lot especially if the existing one is older.

What exactly are the "power quality" issues he is having?

I helped a guy install a rotary phase converter to run a 30 HP motor. The service was just a residential 200 amp service in a residential area, not sure what that transformer is. I was a bit worried but it seems to be fine.
 
If "add a phase" means one of those capacitor box type converters - they are not a very good solution for this type of load, which I assume will vary as system pressure varies. These converters are good for loads that remain fairly constant, you can fine tune them by adding/subtracting capacitors until you get fairly balanced phase currents, but motor load needs to be constant or the capacitor values need changed to match the new load.

I was asked once to check into overload tripping on an air compressor that someone had fitted with such a converter. I discovered a lot about these from that experience, the thing ran fine until it started getting closer to setpoint then the demand from motor kept going up as pressure went up, but phase current's were so imbalanced there was no way to keep motor current within range for the overload to actually protect the motor. Adjust capacitors so it is balanced at that load level and it is way off at lower levels.

Rotary phase converter will be better but still has limitations as it likely will not have balanced current to the driven load and you really need to de-rate the motor or lessen the driven load. They usually recommend changing pulley sizes when driving an air compressor from a rotary phase converter - which lowers overall compressor output.

A VFD is likely the best option, will deliver balanced output and deliver full torque. For motors over about 3 HP you typically won't find off the shelf drives that are marked single phase in three phase out, but pretty much all drives will still work with only two input lines, you need to de-rate the drive though. Technically by a 1.73 factor, though as a general rule most go with a drive that has at least twice the output rating as the motor they are going to drive with it.
 
If there is no existing Add-a-Phase, a VFD could be use to supply three phase power to the motor from a single-phase supply. It would have to be oversized, but might still be a cheaper solution. POCO probably should install a larger transformer anyway. If it can't be started from a VFD, a soft starter won't be able to start it either.
 
What is the problem we are trying to solve? It sounds like the compressor starts and runs, but it is (suspected of) causing power quality issues (on startup?) in the residence on the same transformer. If so, the increase in transformer size is probably where I’d start.
 
If that’s the only piece of three phase equipment, why not have the poco quote.adding only one line brought in instead of two? Two is all that is required for an open delta three phase bank with one additional transformer.
 
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