Posted 20 September 2010 - 02:39 PM
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?
Posted 20 September 2010 - 03:06 PM
Posted 21 September 2010 - 09:06 PM
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 by Michael E Brown, 21 September 2010 - 09:07 PM.
Posted 22 September 2010 - 06:43 PM
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.
Posted 23 September 2010 - 12:31 PM