Author Topic: induction motor as generator fail  (Read 4882 times)

gusbratz

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induction motor as generator fail
« on: April 05, 2017, 11:39:57 PM »
I have heard the urban legend of an induction motor as generator for years, I have a little pamphlet from lindseys bookstore entitled "alternator secrets" I bought in the 90's. I also used info from this link http://www.redrok.com/cimtext.pdf. well I set up a nice 3 phase 5hp 1725rpm motor coupled to the little 4hp lister LR1 I bought over the winter. made a nice base and used the doghouse from my ST head that I removed. wired it up in Star and it did nothing. waited for caps to come in the mail and added (3) 25uF caps in delta on the 3 legs and still nothing. Flashed the field and nothing. I wanted to use Star with the center connection as the neutral.  Just for kicks I rewired the motor in delta and hooked up the caps. started it up and the light on the doghouse lit right up. I thought that was cool so without a second thought I throttled her up to 1800 rpm and boom boom out go the lights followed by one of the caps being on fire. shut it down and  then in put the 20uf caps I bought as backups on and tried again. this time just idling It maks about 150 volts and if you give it any rpms the voltage goes way over the scale of the meter.  what is the deal? you supposed to add load and caps a little at a time as you load the motor as a generator? any body got experience with this? pics to follow if anybody is interested.

mike90045

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Re: induction motor as generator fail
« Reply #1 on: April 06, 2017, 03:30:08 AM »
I think they are required to be hooked up as a MOTOR, and thenn driven overspeed to backfeed the grid

glort

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Re: induction motor as generator fail
« Reply #2 on: April 06, 2017, 05:17:00 AM »

Been posting about IMAG's on other forums trying to see if anyone has come up with a controller for the things so they can be used as stand alone gennys with varying loads. Thought it might be relatively easy to do something with an arduino or cobble together some Chinese pre made boards but no one has put anything forward so far.

I have been playing with these things for years and set them up in every configuration possible.  Here's some early play time setups.

https://www.youtube.com/watch?v=HUE1RQRAXlg

https://www.youtube.com/watch?v=tqy4nJ4cZ5A&t=13s


First thing I pick up on from what you have written is that you say you put caps on the 3 legs.  You should only put caps on TWO legs unless you are going from each leg and neutral, IE, 3 seperate outputs. Is that star?
I know the difference but stuffed if I can remember which is which!  If you have put them on 3 legs, Delta?, then as they say in the classics  THERE"S your problem! "  :0)

The caps in that configuration of bringing the 3 phases back to a single phase ( 2 legs) act like an electronic delay between the phases which are 120o apart and pulled back into line. The  first phase is the "base" and needs no cap. The 2nd phase has one cap ( or value)  as it needs to retard the phase 120o and the 3rd phase needs twice as much delay and therefore has 2 caps or double the value of the first as it has to hold the phase back 240o to line up with the base.  You have 3 caps connected but not one on each leg ( in delta, single phase output) you have one on one leg, 2 on the next leg and none on the 3rd.

If you have a cap on each leg, you can either pull from each leg individually and neutral or across any 2 legs for 3x 1/3rd rated outputs  ( EG, 3 x 3,3 KW outputs instead of one 10KW output).
If you go the None, 1, 2 method  Which is called C2C, then you get one single phase 10 KW output. Just to clarify, You can use one 20 and one 40 Uf caps for 2 caps instead of 3 Physical units.  I have found here that caps up to about 25 UF are much cheaper than Buying larger sizes so it's cheaper to buy 2 smaller caps than one double the value. IF you have one double the value, then good and well.

The second thing with your account is speed.
When the motor is lightly or low loaded, it takes little speed increase to make a big difference in output.  This also depends on how your gearing between the engine and motor is.  Obviously if you have a 2:1, any speed increase on the engine is doubled.  I like to over drive myself to keep the engine speed and noise down and have a larger motor working with a decent load at lower revs rather than a smaller one having to rev harder to get the desired output.

If you are getting 150V at idle, then either your gearing is way high, ( which having an output at that speed would suggest) or you are running too much capacitance.  For a 5HP motor, what you have sounds right to me but if the caps are wired wrong, that's a whole different ball game.  Never done that so can't speculate on what the outcome may be. To me that would create 2 phases, a double output and a single that were 120o out of sync and fighting with one another. That I can see causing a problem with the caps.  I'd also think you were lucky they just caught fire. Caps that size go off with a hell of a bang. Never done one and I take care not to but I have seen vids of it and it's not something I would want to be around.

If you want to persue this, Maybe investing $10 in an optical tachometer and a voltage/ frequency meter would help.  That way you can set the thing up to be at the right frequency ( If it matters, doesn't with resistive loads like lights and heaters) and you can see how hard you are driving your 3 phase.

You should only have to go about 10% over nominal speed to get excitation BUT, if you are over capped, that will happen much lower and your output will be very touchy. When you do go over that sync speed, the voltage will go North exponentially unless you have a decent load to hold it down.  My induction motor is 5.5 HP and I can pull 2 KW off it which is about the practical load. Go over than and the thing will create so much drag it will make a 12HP diesel smoke and only gain less than a  KW in output no matter what size caps you put it on.  Run it no load and I have seen the thing go to 1200V before the multi meter literally blew itself off the ground and I killed the motor before the caps turned to grenades . There is definitely a sweet spot with these things.

Not sure what voltage your motor is because to be blunt, from my POV the US electrical system is as screwed up as the way you write the date but I would always use a minimum rating of 400V caps.
I would prefer 600V but can't seem to get them at an affordable price here.  Wether you are aiming for 110V or 220, it's best to under drive the caps. Also look at the rting of your motor. If it's 240V and you are only trying to get 110-120 out of it, it won't need much speed or capacitance to get there.

This is far from an urban ledgend and I find its extremely easy to make power this way. Controlling that power is however a very different thing. Piss easy to power lights or a heater if you aren't switching anything on or off  but once you start doing that the required capacitance changes, the load on the motor goes up and the field crashes although sometimes in slow motion.
hat I am trying to do is find a cheap and cheerful way of stabilisng that output.

Seems like it would be an easy to do thing given what people are doing with arduino and the like but practice is proving reality to be a very different thing. One thing I have heard of is wiring caps across the loads to be switched so the extra capacitance makes up for the increased load but there can be issues with that too and it would work in some circumstances and not others.

If you want to run a string of lights or a heater as a constant load, it's pretty much just a matter of getting a multi meter, connecting the motor up however you want it and setting your engine speed to give you the voltage you want and it will stay at that in perpetuity till the light's blow or the fuel runs out.  :0)

EdDee

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Re: induction motor as generator fail
« Reply #3 on: April 07, 2017, 10:01:46 AM »
Hi Gus,

+1 to what Glort said....

Run the unit as an assister assistor "helper" generator to a smaller unit, ie a smaller prime mover plus the inductance gen to make up and boost the current - keep the revs to within about +5% of the frequency required and watch the output. The prime will excite the inductor and govern its output frequency too. The +5% rpm/frequency of the inductive gen is to give it a little slip so it can generate and backfeed the prime.

I have a setup where a "prime" genhead is hard belted in ratio to an inductive head and can run them hooked either individually or simultaneously into my house feed. A bit twitchy to set up as you have to custom machine pulleys to get the right ratio, but worth it in the long run for ease of use later....

I was planning to initially use a inductive gen only as a prime, but with slight load variances in current and PF, the cap ratios change requirement and things are not easy to sync and keep the smoke in... Gave up on that eventually.....

Cheers
Ed
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buickanddeere

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Re: induction motor as generator fail
« Reply #4 on: April 07, 2017, 01:15:54 PM »
As previously stated the only practical way to operate an induction generator is via a grid tie system for excitation and frequency control. I have ran a stand alone induction generator in the test lab with capacitors to supply the excitation current. The unit had to operate at rated load current and approx 5% over synchronous  rpm for proper voltage and frequency.

gusbratz

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Re: induction motor as generator fail
« Reply #5 on: April 07, 2017, 02:25:14 PM »
thanks for the responses, i have it coupled direct so idle is around 800 rpm and full governed rpm is around 1800 with my laser tach. caps are hooked together and then attached to L1 L2 L3. i will try hooking it up with each cap hooked to the neutral attached to 6-5-4 leads  wired in star next time i am out there. i want to run a 3 phase motor i have with it and it would be nice to have as a backup generator.  i bought this engine on a whim and can't come up with a use for it. too heavy for a lawn mower, too weak for my splitter ect.ect. thought it may be handy to drag around and use as portable power to run the block heater on the backhoe or dozer when they are out of reach or something. i built the skid out of material i had laying around, an old 3 phase motor out of the dumpster at work, and the doghouse i took off my other liseroid project. don't really have anything into it but time and the cost of the engine.


BruceM

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Re: induction motor as generator fail
« Reply #6 on: April 07, 2017, 03:26:15 PM »
Those look like motor start capacitors, not run capacitors.  They will not survive in this application.  Motor run capacitors are typically in a metal can, with much smaller ratings per size.  They must dissipate a significant amount of heat.


gusbratz

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Re: induction motor as generator fail
« Reply #7 on: April 07, 2017, 04:08:56 PM »
Those look like motor start capacitors, not run capacitors.  They will not survive in this application.  Motor run capacitors are typically in a metal can, with much smaller ratings per size.  They must dissipate a significant amount of heat.



i was wondering the same thing, they did say run in the description . i will have to do a little more research.
http://www.ebay.com/itm/292044102917?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

glort

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Re: induction motor as generator fail
« Reply #8 on: April 08, 2017, 05:36:57 AM »
I bought some New caps for this very application about 6 Months ago and they were plastic cased although had spade terminals  rather than leads. Definitely run caps. The place I bought them specialises in AC and they only do run caps now because they can be used as starts as well and it avoids any confusion or using the wrong cap for the wrong application.  I get a nail and scratch the value in the bottom of the things because playing around with them like this it's easy to get Fuel or oil on them which takes the printing off and then you don't know what the hell they are less you bugger around metering them and even then you can't be sure with teh tolerance of some and whether they are 16 or 20 UF. Not a big difference on their own but when you are using them in series/ parallel with others it's best to keep them identical.

You are DEFIANTLY wiring those caps wrong for a single phase output.
 You need one cap across 2 legs and 2 caps across another 2 legs.
I believe what you have there is actually  25Uf across one leg and 12.5 UF across the other. This would make the thing under capped and very finick to any applied loads.
One terminal should have one wire, another should have 2 wires and the last should have 3 wires joined at the terminal from the caps. Gives you one cap and a pair of caps in parallel. Connecting to 3 terminals on the motor will give you caps across 2 legs on the motor in a 1,2,3 layout. The 3rd ( uncapped) leg of the motor being terminals 1 & 3.


You are going to need a single phase output to run a block heater because I think the motor may be struggling at full output on those loads  let along just using one phase.
Check the rating of your block heaters. I suspect they are going to be significant.  You'll be pushing to get more than 2Kw out of this setup without going too high on the voltage.
the good thing about IMAG setups is you can't overload them or really case damage with shorts etc. The field in the motor will just collapse and the thing will stop making power. That will also happen if you drop a big load on the things without revving the engine to take the load.  The heater will pull down the motor which will slow the engine which will drop under the synchronous speed and the field will collapse and no power.
What I do is speed the engine up before I drop the load on and then be ready to catch it and re set the engine speed to carry the load.
Doesn't matter if the voltage goes 400 or so briefly, it will pull down as soon as you apply the load.

IF you cannot get the motor to hold the block heater load because it is too much current, You could get a cheap little AC PWM controller from ebay. These are often called motor speed controllers  and will allow you to run a large load at reduce power from what would be an undersized source.  I use one to boil a 2200W kettle from an inverter that only does 900W on my solar setup. Because the load is rapidly pulsed, it appears as a smaller current to the source.  IF I put the kettle directly on the inverter it would just trip or it would fry. The other advantage of a PWM on your setup would be could could feed the load in slowly instead of a dump when you flick the power switch.  Bringing the load up gradualy will mean the motor has a better chance to keep up and you can ajust the revs if need be if you see the voltage not keeping up.
Just look for Motor speed controller on fleabay.  they come in 200 and 4000w capacitys which is more than what you need. Much cheaper to buy the module only and wire it in to your control box than a unit with leads ans a case.  

One other thing to note, you cannot start the motor generating with a load attached. It won't let the field come up. You have to start the engine, get the voltage to come up ( Blipping the throttle works for me to save flashing)  and once it is generating spped it up a bit again to take the surge current of the load.  If you can't get it to hold, you might look at adding more capacitance then.
I have run  20-25UF on a 5.5 HP motor and also run 50 and 10UF.  Works with them all just gives the motor output different characteristics.

Smaller caps will mean you have to run the motor faster which can allow it to get up into it's power curve better and hold the load rather than running it lower down where it will "sag" more easily.  
Larger caps will mean the power starts earlier and the voltage will climb easier but it will be more tolerant of the loads applied and not crash as easy.  Both will mean the frequency will be way off but that's the last thing you have to worry about with a "dumb" load like a heater.  Anything resistive just doesn't care at all.  

I like your set up. Nice and neat and well made.  It should do what you want quite well once you figure it out and tune it a bit.
« Last Edit: April 08, 2017, 05:39:10 AM by glort »

BruceM

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Re: induction motor as generator fail
« Reply #9 on: April 09, 2017, 03:23:39 AM »
Thanks, I stand corrected.  They seem to be selling run caps in plastic cases now.


oldgoat

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Re: induction motor as generator fail
« Reply #10 on: April 09, 2017, 04:04:26 PM »
Google "Motors as generators by N. Smith available as a PDF and goes into a lot more theory and will help you calculate the starting size for your capacitor. Originally developed for hydro in Sri Lanka but applicable to your project

buickanddeere

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Re: induction motor as generator fail
« Reply #11 on: April 09, 2017, 10:46:17 PM »
It is the stand alone aspect without a grid tie to control frequency and voltage which dooms a induction generator to wide variations in frequency and voltage.

Thob

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Re: induction motor as generator fail
« Reply #12 on: April 10, 2017, 12:01:36 AM »
I keep asking myself "why bother?".  Why not use the solution that is known to work?  After you blow up a meter, a capacitor, and who knows how many electric loads, how much have you saved?  Why not just go buy a generator with an AVR, and be done with it?  Why mess with a touchy system that drops dead one minute, and goes wild the next?  Inquiring minds want to know...
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oldgoat

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Re: induction motor as generator fail
« Reply #13 on: April 10, 2017, 01:22:21 PM »
The answer is a lot of us have time to experiment The sense of achievement after we have got it working is great. This system will work well on steady loads such as lights and heaters but for fluctuating loads the alternator with an AVR is the answer.

BruceM

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Re: induction motor as generator fail
« Reply #14 on: April 10, 2017, 07:12:30 PM »
The project I'd like to see is the inverse of this one; turn the ST-3 or ST-5 into a starter motor, which will then got back to being an alternator.