Generator balance

Michael Collier · September 20, 2010

On a union set, whose ultimate responsibility is it to maintain a balanced generator? How does that actually work? I've heard the genny op call out high and low legs before, but let's say current at the plant reads 13A, 103A, and 11A. What's more, 90% of the load is non-PFC kinos. In my theoretical understanding that would be out of range on a 1200A generator (I always heard balance should be weighted no more than 20% over average for the 3 phases. Eg: 127A draw, 42.3A average, no leg should be more than 50.75 A)

Is my understanding correct, or is balance not as vital when the plant is dramatically under-loaded. Under that theoretical, 79A should be flowing through the neutral, well within the ampacity of the cabling used. Am I just overthinking things? Who do I turn to to ensure proper balance? Or just listen for low legs and hope that I have a lunchbox on that leg handy?. At what point does balance become critical? What do I listen for to ensure I'm not making the problem worse?

John Sprung · September 20, 2010

Officially it's the Best Boy's responsibility to balance the load. The operator's responsibilty is to inform him of the ammeter readings.

-- J.S.

Michael E Brown · September 22, 2010

Is my understanding correct, or is balance not as vital when the plant is dramatically under-loaded.

There are two big issues with improper load balancing:

1) Gensets are designed with an engine large enough to turn the actual generator to rated load at xxxx-RPM. If you severely unbalance the load, you end up with a motor that is larger than required (for one leg) and it's theoretically possible to overload that leg. The engine will keep up with the increasing load on one leg since it's 2-3x overpowered, but that one winding will overheat and burn out, catch on fire, spew fireworks, etc. Now, your over current protection (and all those fancy electronics) should prevent this from happening but there are all sorts of variables here also - adjustable breakers, shady single/three phase switch overs with common breaker, etc, etc.

2) You could theoretically snap the drive shaft from serious vibration by severely unbalancing the load. This would be more common in a 2 pole generator vs a 3 pole generator but it could happen either way. You would need to be doing something stupid probably like running 800a on one leg with zero on the other(s). But this is something manufactures warn against so it's worth noting.

Your example is pretty unbalanced and really you would have to plug most of the Kinos into one leg. That would just be bad planning on dropping boxes. Say there are 3 units here, 3 units there, and 25 units over there - some people's first instinct would be to run a lunchbox to each world from each leg. That would give you a situation similar to your example. A better plan would be to run a box to each of the trio groups that came back to a splitter on one leg (6 units total) and two runs from each of the other legs over to the 25 unit bank (12 and 13 units each). That's about as good as you are gonna get without silly cables going everywhere and would give you approx 24a, 48a, 52a.

That said - 13a, 103a, 11a on a 400a/leg genny is probably not going to bother it at all - may not even throttle it up much. It's still a bad practice - if you wire it correct now, you are more likely to wire it correct when you are pushing the limits of the genny. Tripping main breakers or blowing fuses makes you look dumb, esp when the schedule is tight.

The fact the Kinos are non-PFC doesn't affect this situation any more than the fact that they will pull more current than a PFC unit.

Edited September 22, 2010 by Michael E Brown

Michael Collier · September 22, 2010

Thanks for all the replys. Of course I have no reason to doubt the (theoretical) best boy in this (theoretical) situation. Actually when I think back to the theoretical, I had mistated it was 90% kino (implying several small unit that could have been balanced) I forgot entirely the T12 that was burning. That would account for 100A load on a single phase, and the kinos were actually well balanced across the other two legs. It really couldn't have been balanced any better without two dummy loads placed on the low legs.

So my follow up question is how often are dummy loads utilized? It seems like if 90A imbalance is acceptable (on a genny of that size), then I can't think of a situation where a dummy load would be necisarry, unless the genny was smaller.

JD Hartman · September 23, 2010

Another theoretical question. How likely is it that a production would have four 5k's (or two 10k's) on standby to use a dummy loads for the other legs? Striking them to balance the generator would certainly garner some odd looks and questions.