Question about induction generators

[rcavictim]

Hopefully this will be seen by Doug and others who are more knowledgeable in such matters than I.  In my research of the available literature discussing the AC induction motor used as a generating alternator by adding resonating capacitors in parallel with the motor's stator pole windings I have come to understand that ther resultant 'generator' suffers poor voltage regulation and although acceptable as a power supply for mostly resistive loads like light bulbs (the old fashioned Edison kind) and resistance heaters, perform poorly trying to start motors and operate other inductive loads.

So my question is......
Can the reluctance of the induction generator to start an inductive load be significatly improved by oversizing the induction generator.  For example, I have recently acquired a 100 HP, 3550 RPM, 3-phase, 575 volt induction motor and gravity concentrator.  IF the appropriate resonating capacitors were attached and the unit spun up would it provide acceptable motor starting ability to loads that represented only a small fraction of it's very large nameplate capacity, say up to 10 kW from a 75 kW watt induction generator?

Why?  Well, I have a need for a 10-12 kW, 3-phase AC head for my Changfa 1115 engine and I have this big induction motor that I may not end up converting into a PM alternator for my wind turbine.  I have just discovered that this motor has excellent bearings and is in fine operating condition having spun it up a few days ago in the shop.  To make it into an idea PM alternator I must remove the stator copper and re-wind the thing with more poles to reduce the speed.  I do not possess the oven or otherwise means of baking the stator to burn off the varnish to allow removal of the wire without overheating the laminations and destroying the inter-lamination insulation system.  I have read in motor service books that a temperature controlled oven bake around 350F for several hours (from memory) is necessary for this process.

It would be difficult for me to transport this stator core to a commercial re-wind shop to have this process done.

I am just, as usual struggling to determine the best uses for hardware at hand to solve hardware needs that have without spending money.

[mobile_bob]

my understanding on capacitor excited induction generators is as follows

voltage regulation can be a problem, you can switch in and out additional capacitors to pick up the sagging voltage
under heavier loads, but that requires some relatively sophisticated controls, hysterisis, load sensors etc.

and your assessment that the induction generator is better suited to resistive loads than inductive follows what i have
learned.

from my understanding you figure 10hp of induction generator capacity for 1 hp of induction motor starting

so if you have 100hp induction generator in theory you could manage 10hp of induction motor starting loads.

how big is this wind machine going to be?

that will probably have more effect on your output that any generator issues in my opinion.

your going to need a good wind resource and a quite large mill to harvest it.

interesting project you have there, good luck

bob g

[guyd]

spinning a 100hp genny at 3500 rpm is going to take some serious power - just in gearing losses, windage, etc etc. I wouldnt be surprised at 5hp+ required - which doesnt sound sensible use of power. Theres a 97kw motor in my local scrapyard at the moment - its huge - like 4 feet diameter - is this what you are talking about? I think you are going a bit OTT.

20kw to drive a 10kw would surely be adequate?

[rcavictim]

You fellows may have misunderstood me.  I have no plans to use an induction generator on my VAWT.

I might use the frame from the 100 HP induction motor, rewiring it for more poles and adding P magnets to the armature to make a PM alternator for my VAWT.  Since I need it to also act as a electric brake capable of many more kW than it will make as a wind generator the oversize is not actually OTT.

I was wondering, and asking advise about the possibility of using the 100 HP motor as-is with caps added to work as a replacement for a 3-phase ST type head on my Changfa 1115.  That would save me $$$$$.  I agree that the higher windage losses and small pulley size (3600 RPM) make it less than ideal compared to a real 1800 RPM alternator head.

Thanx.

[mobile_bob]

personally i think it would be a horrible match for an 1115 changfa

bob g

[Doug]

Thats real big Rob.

If you add a little more capacitance to start a heavy load that will help.
Correct the PF of all the motor loads to unity at full load but they will lagg more a little on start up because of the higher current and the extra leakage of flux ( this means they draw more reactive power and have a lower PF thus draw may need a little mor4e capacitor to try and overcome this effect).

As load increases the output from the generator will drop in voltage and frequency so you have to slighly increase speed to compensate. This is where the load regualtion problem comes in, most engine slow slightly with load you need to actualy spin faster with load on an induction generator.

Hope this helps

[rcavictim]

Thats real big Rob.

If you add a little more capacitance to start a heavy load that will help.
Correct the PF of all the motor loads to unity at full load but they will lagg more a little on start up because of the higher current and the extra leakage of flux ( this means they draw more reactive power and have a lower PF thus draw may need a little mor4e capacitor to try and overcome this effect).

As load increases the output from the generator will drop in voltage and frequency so you have to slighly increase speed to compensate. This is where the load regualtion problem comes in, most engine slow slightly with load you need to actualy spin faster with load on an induction generator.

Hope this helps

Doug,
Yes your comments here were helpful and I understand what you said in other ways that the slip will increase with loading necessitating an increase in prime mover RPM to maintain 60 Hz, dynamically based on load conditions.  With a synchronous alternator it is only necessary to hold prime mover RPM constant with increasing or decreasing loads.

If I can dig up the capacitance necessary I'll likely give this a try.  I have the Changfa bolted to a pair of heavy I-beams right now and the exhaust run outdoors through a hole in the wall.  I have already load tested two really large generating heads with the setup in the past week.

Bob,
Totally agree that this is far from ideal.  Gee, being flat broke makes everything far from ideal but a fellow must make the best of the situation at hand with the materials available to him, or take the easy route and say "screw it".  I choose to make the effort.  Looking at the bright side, using a 100 HP induction generator on a home genset makes it virtually theft proof. 

[mobile_bob]

RCA:

don't let me rain on your parade, if you got the engine, the motor already, belts and pulleys, what can it hurt to give
it a try?

heck it might be wildly successful,,

ok, maybe even if if is moderately successful?

marginally?

works at all?  yes even if it just worked at all it would be useful from a variety of viewpoints
if for no other reason, lessons leaned

i vote go for it!

if after your done you end up needing more hp, you can always get an old car engine to couple it to
and give that a try, that might be the answer for you anyway.
iirc you were looking for power for you homeshop, that usually can be intermittent so maybe an auto engine
might be useful even if it is a bit less efficient?

anyway don't let my lack of vision and ambition throw a wet towel on your project

bob g

[rcavictim]

RCA:

don't let me rain on your parade, bob g

Not at all Bob.  I did not consider you throwing a wet project on my towel.      The last point you made about the exercise being valuable if only as a learning tool is not to be underrated IMO.  If everybody just got off their hands and tried things without fear because there was a chance of learning something useful, knowing full well that the chances of commercial or other physical success were probably a longshot, imagine what a better world we might find ourselves in.

[Stan]

Also don't worry if you screw up and things break, that happens.     Everything eventually survives and continues on, maybe a little bit later than planned due to waiting for the refinancing of the repairs etc.  My big worry is losing Whitworth nuts! 
Stan

[Doug]

I make nuts.
Does that make me nuts?

[Stan]

Only you can tell that for sure, and then only if you want to first.  If it helps Doug, I was born and raised on a nut farm.  I guess this makes a lot of sense to the guys down on the global warming thread. 
Stan

[Tom]

I'm from the land of fruits and nuts. 

[mobile_bob]

awe comeon guys step into the 20th century already!

whats wrong with computer control?

i know you guys are just dieing for a full blown computer controlled listeroid!
egr, variable timeing, emission controlled, common rail electronic injection set to run at 23k psi ,electronic governor
accurate to .000000001 rpm, fault codes, 47 miles of wiring and weather pack connectors, 37 redundant sensors, etc etc.

and the requisite 50k dollars of diagnostic equipment and a service diagnostic manual 3 inches thick!

might even make it to tier 2?

geesh,, why not?

you guys are so stoneage, so last century (1800's)




bob g

[mobile_bob]

oops, this last post was supposed to be in reply to Tom on  the global warming thread



bob g