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How to / DIY => Generators => Topic started by: scott p on December 26, 2019, 05:29:50 PM

Title: A hypothetical generator application
Post by: scott p on December 26, 2019, 05:29:50 PM
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. 
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??
Title: Re: A hypothetical generator application
Post by: glort on December 26, 2019, 08:31:25 PM


Only the output of the generators is rectified to DC. From the description given it is safe to assume they are still both AV induction type generators rather than permanant Magnet so increasing and lowering the speed May have little effect on VOLTAGE output depending on other factors Not specified.

If the output is through Diodes the Ripple will be way too fast for the 2 output waves to allow the outputs to sync. On an AC generator that is going to be 50-60Hz. Through a diode that is going to be Khz.

Diodes generally have a forward current/ Voltage. Exceed that and they will backflow. this may cause the larger genny to push back to the smaller on and the magic smoke escape. Possibly in a spectacular Manner.

The better ( lesser of 2 Evils Practice would be to have the bigger generator as the master and the smaller one cut in after the load was applied.  This way the full output of the larger unit is absorbed and it would be trying to push anything bar the DC motor. The smaller gen can then make up the shortfall.
It is possible to run 3 AC gennerator together or a generator and a solar inverter PROVIDED, the full output of the generator is being used before the Inverter kicks in. A generator will always want to push where an inverter is flexible and will provide whatever demand required and no more.

The governors will probably hunt trying to find their happy place. May be better to just set the larger generator Flat out and just have the smaller one on the gov.

Overall I think this idea is fraught with danger and complication and has a lot of potential bad practices involved.
main generator should be able to provide all the power on it's own anyway. .
Title: Re: A hypothetical generator application
Post by: scott p on January 20, 2020, 07:28:44 PM

Yes it is an unusual proposition but doable if you donít 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.

Title: Re: A hypothetical generator application
Post by: Hugh Conway on January 21, 2020, 09:13:23 PM
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'
No concrete there!
Title: Re: A hypothetical generator application
Post by: EdDee on January 22, 2020, 07:54:16 AM
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....
Title: Re: A hypothetical generator application
Post by: AdeV on January 22, 2020, 09:40:09 PM
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.
Title: Re: A hypothetical generator application
Post by: scott p on January 25, 2020, 05:17:49 AM
Glorks cautious approach

Glorks cautious approach to this hypothetical proposition is well taken. It could easily backfire into a smoking Extravaganza.

     The diodes I have in mind have a back-flow of 1600 volts but I wouldnít trust even that. Fuses (not breakers at this time) too protect the components.
     This application is a sawmill powered by a diesel. Since it is only under a load about half the time because I am  messing around getting ready for the next cut I idle it down. The hydraulic system therefore idles at 300 PSI and is variable under load depending on the log and condition of the saw and can go as high as 1500 PSI. Much beyond that and the relief valve opens.
  The numbers are based on the 30 HP DC motor running under full load. Crude calculations indicate the sawmill needs about twenty HP max, I empathize crude but probably close. I based that on a formula of hp/hydraulic pressure. So realistically the DC motor might never reach full load though I would like to have a good safety margin.
That link is a nice easy to understand explanation of what an accumulator is and does. Basically a hydraulic flywheel. Looks like a properly designed system would be better than a mechanical flywheel. Unfortunately this simple hydraulic system is about the limit of my knowledge concerning hydraulics. What to expect beyond that I donít have a clue.
A couple things I have considered is a flywheel mounted on the saw shaft to carry the hydraulic motor through that initial sudden load when the saw hits the log but it would have to tuned to sync with the system.   
I thought about rigging up say a five HP electric motor with an electric clutch that would be connected through belts to the saw shaft. As I pull up the diesel rpm the clutch activates and away I go. That might work.   
I am much more comfortable with electricity. I thought about the two generators (or twenty 12 volt batteries) long before I decided to go hydraulic but the concept seemed a bit too out side the box compared to a simple hydraulic system.

If I were rich I would most certainly go battery.

Not sure what you mean by (allowing it to sync and freewheel with the mains when it is out of use.) EdDee.

A generator connected to the bus can be brought up to just the bus voltage but no more and it will float. That is it will not add any current to the bus until the governor is tweaked a little like EdDee said.

Would the big engine running as a float be more efficient than running it at a idle?

I can get around 8 HR of sawing for around four gallons of fuel give or take, havenít seriously checked that. I just remember looking at the amount of lumber sawed and the fuel used and being impressed.

Taking this to a serious level canít ignore what has been posted. Forget idling, forget two engines, much more practical rig up the larger engine and go from there at a constant 1800 RPM. If the big engine canít do it then add the smaller one.

Title: Re: A hypothetical generator application
Post by: scott p on January 25, 2020, 08:27:41 AM
By the way thanks all for the input.
Title: Re: A hypothetical generator application
Post by: scott p on February 04, 2020, 07:10:40 AM
Well darned if I didnít find the problem with my hydraulic system. I had a flow control valve in the system I used as a switch to shut off the saw in case I had to work around the saw for some reason. I took it out and now the system does not hunt/oscillate like before.

I hated to see it go on account there have been many occasions that called for quick shut off.

I will be doing some research on a better way of having an emergency shut down with out running over to shut down the prime mover, which is quite some distant away. What I be looking for is another type of valve that will do the same thing but with out causing the hunting effect.
Title: Re: A hypothetical generator application
Post by: mike90045 on February 04, 2020, 07:45:52 AM
have a valve that simply bypasses the hydraulic motor
(in parallel with the motor, and valve normally closed to force all flow thru the motor)