Induction motor as a generator

[halfcrazy]

Ok I know that I can spin an induction motor a little faster then its nameplate and it will back feed the grid. My real questions are Where can one find a good deal on an induction motor and what HP would you need to look for to dump say 3kw back. Also is there anything special one needs? I am under the impression you just spin the thing up and when it is at speed throw the switch and then put the power to it? I talked to my local Power Co and he was ok with it as part of my grid tie system as long as I could exhibit that it will stop flowing power when the grid is disconnected.

I understand that I will not save money and this isn't the goal. just an experiment with an odd purpose

[Flux]

Yes if this is 3 phase, it's pretty simple. In theory you need a 4hp motor to produce 3 kW but say 6 to allow for efficiency.

Single phase is a very tricky case and it doesn't always work , in fact it never works really well but if you stick to 2 or 4 pole it may be ok. Efficiency will be much lower.

To be absolutely sure it doesn't self excite you must make sure there are no capacitors in circuit. It is very unlikely but long power leads connected to it could have significant capacitance but at such low power there is almost certain to be some load connected somewhere if the grid is isolated. Some remote pf correction capacitors somewhere on the disconnected line could be a worry but it is unlikely to excite. As a back up you could use a frequency switch to knock it off if the frequency went above nominal grid frequency. You could back this up with a voltage sensitive switch as well because it would  most likely self excite above nominal volts.

Flux

[halfcrazy]

Well I have a typical single phase circuit could I use a 3 phase motor and only use 1 phase? Do you have a recommendation for a motor?

[Flux]

Bad luck, single phase is not good.

You might as well use a 3 phase motor, you will have far more choices and the start circuit of a single phase one is wasted anyway.

You will only get 50% of the 3 phase rating at best so choose a big one 10hp plus and ideally a 2 pole but 4 pole will probably work. The modern ones seem to be better suited than the older cast iron frame ones.

It would probably work better with a capacitor across the un used phase but you can't do that for fear of self exciting into capacitive open circuit grid lines.

Flux

[Warrior]

Hello,

Glad someone posted this cause I have been toying around with the same idea. I live in a third world country where batteries, magnets & grid tie inverters are prohibitively expensive. So the simplest solution for me is a grid tied induction turbine similar to the Enertech designs. They made 1.5 kw, 1.8 kw & 25 Kw models, all of them single phase.

I know single phase is the worst way to go, but as a first project it may be simpler that going three phase.

I've tried to grid tie a couple of motors and it seems like no success yet. I bought a mechanical rotating disc type kwh meter to see if speeding up the motor made it run backwards. So far these are the results:

First try with a single phase 1/6 Hp motor, geared 15:1 to a speed increaser and pushed by hand with a handle on the input shaft, I couldn't get it above synchronous speed. Humans are way to weak

Second try with a small 100 watt fan motor driven by variable speed electric drill. I managed to make the meter almost stop turning but never backwards. Increasing the speed above a certain point drew more power rather than send back. I don't have a clue why the meter never turned backwards. Guessing that the motor is so inefficient that it uses more power than what it can produce. Anyone have an idea why this behavior?

Thanks,

 

[Flux]

Yes I think you are right, the fan motor will be too inefficient to generate. Once you drive it above pull out speed it will effectively stop generating and just draw losses and magnetising current.

I don't think you will do any good with any high slip low efficiency motor especially on single phase.

Flux

[dbcollen]

The Hydro that I built is an induction unit, but it is Cap excited for battery charging. It is a 4 pole 3HP 3 phase washdown motor. I have 6uF in delta across the motor output with the motor wired for 440V. My unloaded rpm is just over 4000rpm, and it goes to full speed for about half a second before the caps excite it and pull it down to around 1700, I don't know what the slip is when running as a generator, But it is running a 57Hz @ 370Vac into 3 transformers and a 3 phase bridge, charging my 48v batteries.

Dutin

[PeterAVT]

I would keep everyone's comments in mind, above. Being in the US, I favor www.surpluscenter.com for motors and such: look under the "electrical" section. Most of it should apply to Canada also. They have all kinds, and bare caps for starting/running, etc. You can choose based upon mounting style, single or 3-phase, etc.

[hydrosun]

How efficient is your induction motor hydro system?  A Homepower article a number of years ago had a report on 5 induction systems in central america. They had transformers and then into battery banks. The efficiency was under 20% for all those systems. In comparison the one dc direct harris system was over 50% efficient. There was some discussion that poor matching of frequency through the transformers was the problem.  It worked but much of the power was lost, so more water was needed if available.  I've looked at the idea of induction hydro for long distance transmission but am put off if most of the power is lost.

Chris Soler

[fabricator]

Realizing 20% power production out of a hypothetical 100% is 20% more than nothing out of a hypothetical 100%.

[dbcollen]

There is 2.756KW in 68 meters of dynamic head at 65.5 gpm. I am getting 820 watts presently, which puts me at just over 30% efficient overall. I have a pelton wheel on the machine right now, with poor geometry, and is too large in diameter for the right RPM. My homemade turgo runner was actually closer to 35% efficient overall, and it was far less than ideal.

I have a really nice pelton runner on the way from Italy right now that is the proper diameter and has really good geometry on the cups. I am hoping to see 45%+ with the new runner. My homemade PM alternator conversion with an old Harris runner did over 60% overall through an MX-60.

I have over 1000 ft I have to send the power, so I am using the high voltage induction setup. I can run the motor at almost any voltage I want because the 3 torriod transformers each have 2 primary windings and 6 secondary windings. one primary at 120 vac puts 28vac out each secondary, so between all the combinations of series, parallel, and star and delta I can choose pretty much whatever ratio I want.

Dustin

[bob g]

there is a flaw in how most everyone tries to use a single phase motor as an induction

generator, the issue is one needs to reverse the start winding connections from how they are connected for motor use.

i would have to look it up, but Bill Rogers book explains how this is the problem with very poor efficiency typical of single phase motors

Bill's book, is available at utterpower.com

not much useful for windpower so to speak, but lots of good solid stuff on induction generation.

bob g

[Flux]

You can improve the single phase results by energising the start winding via a capacitor but this is not advisable if you rely on the thing going dead when it looses the incoming line.

The start winding effectively makes it approximate to a 2 phase motor. It must be a start winding designed for continuous use. Those machines that only use the start winding momentarily and switch it out often have thin wire in the start winding to cut capacitor size and this winding will not be continuous rated.

With a 3 phase motor running on single phase there are again options you can use with capacitors in various connections to improve things but again you may sacrifice the fail safe loose volts if you loose the line property.

Flux