induction motor as generator fail

[gusbratz]

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]

I think they are required to be hooked up as a MOTOR, and thenn driven overspeed to backfeed the grid

[EdDee]

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

[buickanddeere]

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]

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]

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]

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

[BruceM]

Thanks, I stand corrected.  They seem to be selling run caps in plastic cases now.

[oldgoat]

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]

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]

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...

[oldgoat]

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]

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. 


 

[dieselgman]

Kohler made this design for many Listers marketed in the US... now out of favor and production. It required a DC commutator and lots of additional windings plus brushes to make into a starter motor.

dieselgman

[BruceM]

For using the ST-3/5 as motor, power transistors have advanced to the point that it is now more practical to do commutation via electronics, thus avoiding a major mechanical modification.  I think rotational position feedback will be required. (added to the shaft or pulley; maybe a just a couple magnets for two hall sensors)  The field coils can be alternated as well as the stator, if need be.  

I'm busy on an low EMI, 120V input inverter project now. I've got a modified square wave version with a single controlled rise/fall H-bridge and a toroidal step up transformer, and a stepped sine presently using 2 H-bridges and two toroidal transformers. I'm only doing 3 steps in each direction plus zero, at present.  The goal is to make motors happy; some get hot and growl on MSW.  This is a low frequency, transformer isolated design based on the venerable Trace SW design. Running 500 watts of heat lamps, the output AC can only be heard on the lowest end of the AM radio within a few inches of the power cord.  The DC side is totally silent.  My THD with my on-hand transformers is 18%; the wave is too broad at the top to get to 120VRMS.  A typical MSW output is 24%.  I should be able to get down to generator class which is 9-15% THD, or better.