2 gensets on same bus

[Jim Mc]

Actually cujet has it exactly right.  Operating an induction motor as an induction generator does require that it be operated at a speed above synchronous.  In a way, it's the reverse of what happens when it is a motor - then it operates a tad below synchronous speed, with 1750 rpm being common for a 4-pole machine.  If it were being operated as a generator, it would put out its rated current at a speed somewhere around 1850 rpm. 

Induction generators make a lot of sense for some cogeneration schemes where power is sold back to the grid.  Generally, they are not a good solution for stand-alone (back-up or "off-grid") applications since they can deliver power only at a leading power factor.  Without external power factor correction, they can not operate other induction motors, for example, since induction motors operate at a lagging power factor. 

I am a sparky, but didn't learn this stuff in school.  I happen to have an interest in old engineering texts, and this shows up in any good old book on AC machines. 

[Doug]

Ok let me try and explain this:

Introduction to motor generator theory

I assume most of you have looked inside an industion motor before and most are familiar with the rotating poles on a ST alternator so it obvious that the Syncromous machine uses DC to generate a magnetic field and the action of the mganetic field cutting copper wires induces a voltage in the turns blaw blaw blaw. We've touched on this before in amny threads so I won't bore people with it again.
 
Induction generation with a motor is a little different
For our purposes let us assume for a while that when you turn on the AC power to a standard single phase motor it produces a rotating magnetic field in the stator. The rotor has no electrical connection to the stator so energey is transfered to the rotor via the magnetic field to the rotor by trnasformer action. And like a transformer we can consider the motor as having a primary winding ( the stator ) and a secondary winding ( the rotor ). Since the bars in the rotor are shorted together we can assume this is a fully loaded secondary and because of the high current in the rotor a large magnetic field is created that interacts with the magnetic field in the primary stator.
 The effect of this is to produce a powerful forces of atraction and repulsion this creates the torque on the rotor shaft. When an induction motor is unloaded and spining free the speed of the rotor come very close to mtaching the speed of the rotating magentic field produced by the stator and the current induced in the rotor is small because the speed the flux cuts the bars in the rotor is very low. As we load the motor down its speed drops by lets say 5%. More power is consumed in the rotor now because the flux is cutting the rotor bars faster inducing higher current, more rotor flux and electrical energy is being used to create more torque to spin the shaft. And as we all know mechanical power is torque times rotational speed?

Hopefuly this is all clear....

When we spin the rotor above the speed of the rotating field produced by the stator we get a counter torque on the shaft. This is generator action, the power into the rotor shaft now power flow in reverse across the magentic air gap from the rotor to the stator the exactly the same way as before in the motor but mechanical power is being converted to electrical power.

Power factor and reactive power.

Ac is funny stuff, although it you can multiply amps times volts in both ac and dc to figure out how much energy you are producing in both in ac circuts because of the interaction between magnetism and ac power the flow of electrons and the voltage moving them shifts out of phase because of a lag caused by the conversion of power from electron flow and creation of magnetic fields. Capacitors do the opposit and cause electron flow to lead the voltage and a cap builds up a charged of stored electrons. Thus you have reactive power. And this is getting kind of hard to follow for some of you so lets just keep it simple and say capacitors cancel out out inductors ( coils )and the goal when dealing wiith reactive power is to cancel out these forces because they require energy in to produce and they are a load on the prime mover ( engine, wind mill, steam turbine, or mad squirle running in a cage ).
Power factor is the ratio of watts in real power you produce and VARs of reactive power that you are generating but don't actualydo anything for your in the way of real work.

Doug

   

[cujet]

Ok, given that a 6/1 driving a ST will make 4000 watts (for this discussion) and the other 6/1 will drive an induction motor as a syncronous generator. If I couple them together, will the output be 8000 watts? I understand that some power is necessary from the ST to the induction motor to get it to function. So, my question is,,,, 8000W?

Chris

[Jim Mc]

If I slightly twist your question around so that we want to add 4000 VA from the induction generator to 4000 VA from the ST head, then, yes.  It is possible that 4000 + 4000 will equal 8000. 

But that will only happen if your load happens to want to draw power at the same leading power factor as the induction generator wants to supply it.  Not likely to happen.  Most loads operate at a lagging power factor. 

Alternating currents need to be treated as vectors, and when two vectors, each of 4000 are added, the answer could be anything from 8000 to zero.

It’s kind of like driving from your home a mile North, then heading West for a mile.  You’ve traveled 2 miles, but are only 1.414 miles from home.

[Doug]

Yes that sums it up nicely....

Add some capacitors to add a little reactive power to the induction motor and things will get a little better because now the ST head won't have to suply all the VA required by the motor. But one point we haven't touched on is these sort of systems are a little unstable, your voltage will droop as you load this system down more than if you just had a bigger ST and a bigger engine. It may also rise if the system is too lightly loaded. There is also the possibility that the two engines may hunt under some loading conditions.
Juts my gut feeling but I don't think I would want the induction motor to put out any more than 1/3 of tottal peak power required in order to keep the voltage and frequency reasonably stable.

Doug

[Jim Mc]

Turning back to the original post about two ST heads on the same bus, driven from two Listeroids:  That was a 'pet' project of mine a few years back.  I spent considerable time working through the design.  In the end, I concluded that for me, there was too much technical risk.  The main concern I had was the circulating, or synchronizing current that flows between the two ST heads.  Ideally it is close to zero.  But in the real world, this current flows when the two ST heads are not in perfect sync.  And they are always falling out of perfect sync (maybe by only a few degrees, but nonetheless significant) as the Listeroid speed varies throughout its 4-stroke cycle.  It runs a tad fast at the start of the power stroke, and slows down through exhaust, intake, and most dramatically, during compression. 

I wasn't able to convince myself that this synchronizing current was under control, and could be maintained at a low value. 

This is a problem at its worst with single-cylinder prime movers - made worse in the case of a Diesel because of its high compression ration, and made still worse by the low speed of the engine, and the large ratio of the belt-drive.  I concluded parallel operation was a tough nut to crack and went ahead with a single, larger engine.

So, I'd like to hear from anybody that has put two listeroid-driven generators in parallel. 

[cujet]

Certainly, 2 engines can be used to drive one gen head. Tis easy to pop a belt off for single engine use, no tools needed with the common serpentine belt.

In fact, this may be the all around best way to do this type of thing.

I sure would love to have such a setup, as my needs are mostly for lower power.

Chris

[mobile_bob]

cujet:

makes the most sense to me, coupling two engines to one genhead.

perhaps compling by means of a cogged belt, one could phase the two engines 180 apart, to help smooth up the power pulses.might even make for balancing the racks easier.

bob g

[firefly]

I was looking to do the same thing but wanted to avoid the hassles of two gensets. So the simplest method I came up with was to drive one large head off of two smaller engines and use a one way bearing on each engine.
To expand on this a little for clarity in the design I drew up you would have one long base with an engine at each end and a generator head in the middle. The generator would be driven by a lay shaft so the pulley on the generator would connect 1 to 1 with a pulley on the lay shaft. On the same shaft there would be two additional pulleys (the same diameter as on the generator sized for 1800 rpm from your flywheels) each mounted on one-way bearings.
This would allow one engine to be started and run the generator with no load from the other engine or both engines would be started to allow for full output of the generator.
By using one way bearings if one engine suffered a failure or had to be shut down it could stop without affecting the rest of the system (as long as you were not using both at full capacity) The governers would have to be close but it would not be nearly as critical as with two generators. Besides the lister twins are essentially two engines attached by one crank and that seems to work fine.

[pigseye]

Pardon my ignorance, but what are one way bearings?

Thanks

[Andre Blanchard]

Pardon my ignorance, but what are one way bearings?

Thanks

Turn in one direction and they are a bearing turn in the other direction and they lock up.
http://www.zxz-bearings.com/onewaybrg.htm

China web page but near as I can find there are no ball bearings being made in the states.
Should make WWIII with China interesting.

[emerald]

The Sprag clutch is a common item and manufactured widely.

The guy over at Oldstyle Listers used one of those on his twin 6HP engines driving a common gen head. I dont have any links to show because the site went under rather unusual circumstances after a few months of rather unusual site posts.

The net effect of all that was, rather unusual.

/end political correctness/


Emerald
 

[pigseye]

Hi Andre,
So it was as simple as it sounded.  Thanks for the explanation and the link.

Steve

[bitsnpieces1]

  Along Andres line, since Listeroid governors are a little shaky anyway, go with an external governor controlling both engines through a common linkage.  Have a large gen head powered by one engine and the other engine ready to clutch in as needed for extra hp.  Run all three cylinders at first and get them adjusted for even loading to each cylinder.  That way you could run the 2 cy. most of the time, clutch in the 1 cy. when needed and have it pick up its' part of the load.  Or, could run the 1 cy. with the twin clutched out. 
  Isn't automatic, but, only takes a half-minute to switchover, just engage or disengage clutchs.  Don't even need to crank second engine, let the one running do it.  Also could use two identical engines and accomplish same thing, just make sure that governor is driven from a spot that won't allow for runaways, perhaps a connection directly to the crankshaft itself. 

 

  Found some drawings from Lister for a clutch that I was talking about.  It's from the Internal Fire site, copy of a manual for the LD or SL models. 
Can't get it to show up here,  you'll have to look on Coppermine. 

[creighto]

   a clutching arrangement would do the job much easier, and might require a few seconds of brownout while you clutch one engine
   out and the other in. this would require much less cost and be far safer in my books. this is for two engines driving one head of
   course.


I was thinking of doing something like this using two 'override' clutches, like you find on large tractors.  The genhead would sit in the middle and either or both engines could be ran to drive one genhead.  If either engine slows down; by force or failure, than the override clutch would just ratchet as the other engine drove the shaft within it.  I would use a PLC as a two headed electronic governor to keep the loading even, but I think it would be much safer and require less attention than 'syncing' by any method.  Perhaps much cheaper than the safer options as well.