How to / DIY > Generators
A hypothetical generator application
scott p:
Two AC generators, one 25000 watt 104 amp at 220 volts and one 6500 watt 30 amp at 220 volt, diesel powered with mechanical governors.
Both generators altered to DC through appropriate heavy-duty high PIV diodes.
One 30 hp DC motor, 220 volt, separately excited, 100 amp draw fully loaded. Draw at no load unknown.
The generators are paralleled to produce 134 amps at 220 volt DC.
1. The major generator is variable speed so as to idle in no load situations.
2. The minor generator is set to constant speed.
The theory
The minor generator is set to 220 volts thereabouts (perhaps slightly over) via the governor. The major generator is set to 220 volt via the governor when pulled up to full speed.
When a full load is coming on the major generator is brought up to speed. When the load hits, the minor generator will droop and fall back to 220 volts thereby allowing the major generator to share the load.
When paralleled, because of the PIV of the diodes the generators cannot catastrophically interfere with each other in the usual ways as when paralleling straight DC or straight AC generators.
QUESTIONS
Under normal paralleling operations the two generators will lock up and become one as long as both are producing. Will these two hypothetical generators have a tendency to lockup in spite of the diodes??
Will a 100-amp normally AC circuit breaker work under these DC conditions??
Two mechanical governors might be a problem??
scott p:
unusual
Yes it is an unusual proposition but doable if you don’t mind the inefficiency involved. Needs more research to be sure, fuses before the diodes and after to protect the units.
I have all the components required including high power diodes. It would be nice to have a stand-alone system where all electrics involved were run by the two engines.
The noisy engines could be off somewhere and boxed in. I do think trying to get the two governors to work together would likely be a problem. That would require a single governor probably electronic to bring both machines together.
The primary reason I would even consider a system like that is because my present system is not working out. I elected to use hydraulics to power the saw, which works fine except for a persistent tendency to hunt or oscillate when sawing. It can get quite violent, affecting even the prime mover. So I am forced to take evasive measures to keep the hunting down toned down, which is a pain in the butt and takes the fun out of it.
From what I have read since hydraulic fluid is non compressible any change in velocity or direction or reduction sends a shock wave of some form through the system. A friend who is a career navy veteran told me a hydraulic system practically needs to be set in concrete to keep it from jumping around. That plus I am using a flexible coupling to connect the hydraulic motor to the saw shaft and another one off the prime mover to the pump.
He indicated a large flywheel might solve the problem. Any way live and learn as they say.
Hugh Conway:
My knowledge on the generator side is miniscule. As for the hydraulics, a suitably sized accumulator may be helpful in smoothing out the variables. Accumulators work well and are extensively used in aircraft hydraulic systems to buffer loads'
https://www.aircraftsystemstech.com/p/withnitrogen-or-air.html
No concrete there!
Cheers
Hugh
EdDee:
An accumulator in a hydraulic motor environment can be a PITA.... As the bulk of hydraulic motors off the shelf are rev/flow coupled, an increase in speed might be a problem as the accumulator unloads through the motor, particularly if motor torque varies, such as used on a power saw. This would require a somewhat more upmarket motor with a built in "governor" or a flow control valve directly before the motor....
An easier solution might be to have two generators, the smaller one being a prime, the second, larger being an induction generator permanently coupled to the network. The induction could be on a slip/one way drive, allowing it to sync and freewheel with the mains when it is out of use... As net load increases, the second generator can come up to speed with only a small bit of current sensing circuitry to up the governor, thereby assisting the prime mover....
My .0000001c worth....
AdeV:
Is the efficiency loss of running the 25kW unit for all loads going to be offset by having the big engine running constantly (mostly at idle, one assumes) and the smaller unit running harder more of the time? Diesels are efficient, yes, but not when they're idling.
Are those numbers based on what you have, load wise, or what you're expecting? e.g. would it be worth looking into having a BFO battery bank to provide the peak loads, running just the smaller engine to re-charge and assist the battery bank when load exceeds capacity? A battery bank could also be supplemented by solar/wind generation to help defray the costs of diesel (although whether it would pay back in the lifetime of the system, who knows).
If the 25kW unit is way oversized for your peak load (but your 6.5kW unit isn't big enough), could you gear up the 25kW head so you're running the big diesel slower? e.g. drop it's max output to 12.5kW at 60% of RPMs, for example.
Just wet-finger-in-the-air thinking there.
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