3 Phase Incoming Power 1

[ozchevy]

I was testing and doing some maintenance to the main power supply board at work yesterday, and tong tested each of the three phases, no load on the factory as it was Saturday and no production machines were running. I was very suprised to find such a variation of amperage on each of the phases.

255 amps, 206 amps, and 218 amps

I am planning on commissioning Power factor correction capacitors in the very near future, but this will not iron out the amperage levels, any suggestions on why the variations are occurring, and how can I get this back to a level that is some what uniform.

 

[jraef]

Parasitic 1 phase loads such as lighting, PC power supplies, battery chargers, sump pumps? Could be all kinds of things. With that much imbalance, a site survey would appear to be in order.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[waross]

I'm surprised when I do get balanced currents particularly when the plant is not running.
If you had a 200 amp panel balancing would not help. If the panel is 400 amps or more, what's the problem?
You most likely have single phase loads that are not balanced. You may even have a local magnetic field that is affecting your tong meter more on one phase than the others.
If a lot of the lighting is turned off on the weekend, it's possible that the weekend lighting load is unbalanced.
A couple of single phase air conditioners in the offices, and/or the loads that jraef has suggested can be responsible.
You have a 49 Amp difference on a base current of 255 Amps. Thats about 19%. On Monday morning, much of the unbalance may go away. Even if it does not change you may be adding another 1000 Amps of balanced motor load. Now you're looking at a 49 Amp difference on a base current of 1255 Amps. That's less than 4% imbalance.

Unbalanced supply voltages can sometimes result in a disproportionate current unbalance.
When you're planning an installation, you strive for a balanced system. When you measure the currents at an industrial facility that is not operating, what you got is what you get. Is this overloading your main transformer bank?

What is important is first, the voltage balance when the panel is loaded.
Second, you should not draw more current on any one phase than your breakers conductors and transformers are rated at.
Third, heating in the conductors, or I^2 R losses. If one phase is loaded heavier than the others the heating losses will be a little more than if the currents are balanced. This is usually a small loss.
Check your currents when the plant is running. then check the voltages. If you have unequal voltages, check your motors for running hot. If nothing's hot and nothing's overloaded, don't fix it.
yours

 

[ozchevy]

Thanks guys,
I will be checking the loads at regular intervals come Monday, I do hear what you are saying re: single phase loads from lighting etc. I haven't noticed any hot motors of late, but have had a lot of problems recently with burn outs of relay timers on some of our older machines, and erratic PLC signals on the newer additions. As part of the maintenance to the main board and sub-boards, I found many, many loose terminals which would probably have heated things up a little in the panels, these are checked every six months and it's quite surprising how loose some were.

Thanks again.

 

[jraef]

Also, have you considered a harmonics problem? That would not necessarily cause an imbalance reading, but it certainly would contribute to heating problems in some loads and transformers, exacerbated by imbalances.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[davidbeach]

Unbalance, what unbalance. Yeah, if you had one motor drawing those currents you might well have something to worry about, but a service entrance to a building with multiple loads, no. That looks surprisingly well balanced actually.

 

[VEBill]

Wouldn't one want to check the voltages if you're concerned about phase imbalance?

 

[ozchevy]

Voltages are good, 440 volts consistantly.

I checked the load today at least six times, as the plant was fully loaded. Current was different in all phases at all times I checked. Nothing was consistant there - I'm guessing with other big manufacturing plants in the industrial estate, that this is the cause and drawing big amps from the sub-station down the road. Anyway I will continue to monitor the situation, and will try to balance out the single phase circuits a little better.

Thanks for your input, appreciate the replies.

 

[davidbeach]

What is your concern? So far you have not described anything to be worried about.

 

[ScottyUK]

Totally agree with David - it's surprisingly well balanced for a facility load. One or two heavy single-phase loads can upset the most careful efforts to achieve balancing across phases. I also agree with waross - I get puzzled when currents do balance! I usually get another instrument to cross-check the first reading. Don't worry about it, you're just seeing normal loading conditions.


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I don't suffer from insanity. I enjoy it...

 

[Powerman83]

Unless you know exactly what loads are connected to each phase at any given moment and/or what loads may be wired as single phase or multi-phase then you have no way of knowing what the loads should be at the moment you happen to measure. Remember, many items regularly cycle such as fans, heaters and motors in various types of equipment and appliences. Measuring balanced loads would be almost impossible in most cases. I would not worry about those readings. Only substantial imbalances or if one or more phases were zero would caause me to be concerned.