Using an electric motor as a generator

That is a bit short to give any recommendation , please give full data from the motor and you will certainly have good answers.

Hi, if it is a squirrel cage induction motor, you're out of luck. They can't, without major modifications, be used as generators.

guest; you can use a motor as a generator by over speeding the name plate rpm, i have

done this to load down new gas engines for break in & adjustments perry

It must be:

• A "universal" motor (brush type)
• It doesn't work efficiently as a generator.

It is designed to convert electrical energy to mechanical and not vice-versa. This becomes more relevant as the power rate increases.

It is like when you trying to use a larger Transformer for the opposite job that it was originally designed for, it does not work well.

You can debate this as long as you like but remember - you wanna use it in a continuous mode of operation - not in a short bursts.

Let us know what you decide.

Synchronous motors work pretty good as generators. But I'd guess the motor you have is not a synchronous motor (let us know more, OK?). While there is a whole lot I don't know about designing hydro plants, I do know you need to drive the generator at just the right speed if frequency matters (and it usually does). This is accomplished in part by using a generator that has a larger number of poles than you would at a coal or nuke plant. So if frequency matters, this may be an issue for your motor. If you haven't already, you should sanity check your idea by plugging the flow rate and "head" (the distance the water can drop at your location) into the appropriate formula and make sure your stream has enough power to produce 30 kW. Also, there are all kinds of low power hydro kits for sale on the Internet. Some produce 1 kW or less. This might be your best bet.

I hate to say this but if you need to ask on such a forum this question, it means that you haven't a clue yourself.

The fact that you supplied no relevant data with regard to the motor in question only solidifies this thought even more for us.....

It is possible to do, if you are an electro-mechanical Engineer, with a very good workshop and a lot of intimate knowledge of the subject matter......it has been done by others....but you will need quite large volumes of fast running water to make it worth while too.....

The "Clever Clogs" who told you it is easy, should be roped in immediately to guide you further, plus of course the Guys and Dolls from CR4......

One last point, I am such an Engineer, and although I believe I could do it (and have possible mots of the knowledge and the required tools/workshop, I am too lazy to even make an attempt as there is a lot of grief awaiting you at every corner of the build.......and I have far better things to do with what is left of my life that that!!!!

The workshop requirements will be quite extensive and the value of the tools needed, alone will probably be more than buying a finished unit with guarantee, but the fun of doing it for yourself can make this point easily forgotten.....I've still got the T-Shirt!!! I am redesigning a Radar Speed pistol at this time......totally crazy.....

But if you are less that 40 years old, you should still have the energy to carry it through to the end!!!

Best of luck, I wish you well, but do open your eyes to the possible problems before starting!!!

I had a work aquintance (mechanical tech) who did this and uses it to supply power to his greenhouse. It's not as difficult as some want you to think it is. Just requires some considered thought. He's using a squirrel cage, 1-1/2 hp 3ph 480ac motor. He devised some simple mechanical governor to regulate the motor speed and it's all driven by a 6 foot waterwheel. Quite clever and not anywhere as complicated as some here would have you think. Get this though, he's getting 16amps on each of 120v legs. I didn't believe it until I saw it either. Sounds like something outta Stephen Kings Tommyknockers novel. Anyways, here's a schematic he gave me. I'm sure this'll get em decipherin'..........

This is a 'fluke³' http://www.yourdictionary.com/fluke.

Can you explain - how this works? I'm not familiar with the symbols apart from the windings and the fuses they're seems to hard to understand for me.

What did your friend do for excitation? I assume he used a synchronous motor, didn't he?

There are several other issues I can think of but this is a very important one, to say the least.

i will try to explain the schematic

left 3 cores are the 3 phases of the gridconnection

the round with 3 windings are the motor generator in star connected

! please check the voltage of your generator

if there are 6 connections of your motor connection block U2 / V2 / W2

or older motors connected x / y /z must be connected

the litle ~ is the simbol for fuse

the litle round is the simbol for a elelectromagnetic contactor / switch

most to the right are the capacitors here wired in delta

so capacitor voltage must be 480 i believe in your country

! if the capacitor voltage is 260 they must be connected in star not delta ( 480 / 1.73)

aditional to the schematic: if you use the generator in island operation please connect a proper earth connection to the starpoint of the generator

If you are talking about the working voltage of the capacitors needed, it would appear that your theory is maybe OK, but practical experience is = 0 !!

For European systems, with 380 volts phase to phase, an experienced Engineer would specify caps with a working voltage of 800-1000volts. For US systems, 500-600volts is considered to be adequate. The upper value I have given being the safest!!

Specifying less than these values will of course work, for a time, but will never be reliable in the longterm......

Thank you for your concern, but both my theory and working experience are quite up to par (many references available on demand ). voltage rating of the system is only one parameter for a capacitor. what about Capacitance? or, in other words, how many Farad (or perhaps an answer in kVAR is possible, since as you so quickly concluded, the Voltage is a given) to get the circuit running.

I still feel you are lacking in practical experience or you would not have tried to deflect my comments so quickly (and so obviously), with talking about the value of the capacitors in Farads. That has simply nothing to do with the problems I was trying to make you more aware of!!!! and you should know and understand that!!!

So either you do not accept that there are such problems or you do know and are trying to cover up. Make your choice!!

The maximum working voltage of any component must be far above the expected working voltage as AC systems tend to have occasional very large spikes in terms of voltage. They can be produced from equipment being fed for example at switch on and switch off times.....especially equipment with a large values of inductance like motors and transformers etc...These tend to damage components over a fairly short time frame and they eventually fail.

I spent many years working for various US companies on their European computer products, over 30 years and hardly a product ever left the US shores that was adequately built for the European voltages. I could easily bore you too death with lists of the corrections that had to be made.....and with 4 different companies......so its a common problem with US design Engineers.....so please do not feel either isolated or picked upon, you are one of many!!!

please accept my most humble apologies for my ignorance.

my reference to capacitors was to a drawing posted by someone early on in this thread, showing a squrirrel cage motor to be used as a generator with capacitors added to the circuit, I assume to get the phase shift.

My comment was regarding the parameters for the capacitor.

I really would like to know how to calculate the necessary capacitor needed in order to get the squirrel cage motor to act as a generator, even though I was not the original poster.

...by the way, I am on your side of the Atlantic, and my engineering license reflects European standards.

I have found the only similarity between the electrical distribution systems in North America and in Europe to be the free electrons in the copper! Even the copper is not the same (awg vs mm), never mind standard consumer voltage levels, or worse, the difference in distribution and grounding practices (3 wire split phase vs grounded star single phase, for example)

It is impossible to know where you are based as you do not tell us (as many do not on CR4, which makes answering their questions sometimes harder as the location affects the answer in many cases).

Please could you and the many others, please add some little tip as to your location like USA or UK or Europe.....? It would really help a lot of us out. You can add this in your personal profile quite easily!!

You have no need to apologize, that was not what I was looking for! But many thanks for your courteousness in this matter

The "Simple" diagram you mentioned (from another poster), was far too "simple" to be of any assistance to anyone at all as someone who knows the method would not need it in the first place and anyone who did need such help, was not helped in the least!!! I also feel we sort of agree on that point!!(but please feel free completely free to disagree with me on that or any other point, its your right!)

I must admit that I would like to "know" as well, but more from an Engineers "nosiness" than a personal need to build something...... I just like to know things....sad really!! I would go about it in a completely different (not necessarily better) way.....but thats just me all over!!

I really liked your comment:-

I have found the only similarity between the electrical distribution systems in North America and in Europe to be the free electrons in the copper!

...which really fits the bill well, many thanks for that!!

Have a great day.

Does it mean that not only me but many of you guys do not quite understand the diagram placed on by the the person who raised this issue on the first place?

I am still struggling to interpret it you know.

I believe I can interpret it for you to some simple degree, but to use it to make something practical that also works I cannot!!

You need a little residual magnetism, or a 3 phase power source.

Assuming a 3 phase source, you run the Motor up to speed.

Start the prime mover (add fuel and ignition as it is already turning at whatever the base frequency is of 50 or 60 Hz).

When motor is running, remove mains power.

The prime mover keeps the motor (now generator) turning and the caps store a charge that allows a voltage to be developed in each phase, which recharges the caps again - if you are very lucky!!!

There are some web sites around with a better (and probably more accurate!) description!!

I would never attempt this, but thats just me!!!

It sounds pretty dangerous and usually I know my way round mains voltages......but I would not like to get this up and running!!!! Just call me chicken!!!

Thanks Andy,

By having the Caps in place then ideally I suppose there's no need for residual magnetism, is there?

However, the resonance of this LC circuit should it not match the rev of the rotor within close tolerance?

Only if you use the mains to get the motor running, if you want to do it without mains, you need some residual magnetism in the poles/fields, to get things started.

Here is a link to someone who has done this often.

Thanx for the link Andy.

Yeah, finally I've got the mystery understand.

It is as I suspected all along that this motor works on the principle of a LC resonant circuit formed by the Cap/s (C) and by the Stator (L) of the motor.

As the guy mentions in his blog, you can only load it up-to 10% of its nominal ratings; which is normal with a resonant-circuit.

Basically, you just take a sample from it otherwise you'd need to buffer it to make more use of it, but that would beat the purpose of things since our friend, who started this discussion, wants to create a Power and not a Signal source.

I well and truly would not recommend this to be ever connected to the GRID as it would never operate reliably, especially frequency wise, within specs.

But as a stand alone generator it would be ok with a little stand by surge-power to fire it up if it fails due to lack of sufficient magnetic residue in the rotor-core, more so than fire it up from the mains first. (In that case what's the point?)

Your last sentence was perfect!!! The one in ()....

I got a good laugh at the "simple" diagram.

Only one "minor" detail left out- how do you calculate the capacitors?

Add the complete list of calculations for all capacitor parameters, and yes, it may very well be simple.

Thats a normal schematic, also needed of course, (although no excitation circuit is shown, which is needed for MOST, but not all applications). I feel that the schematic is possibly/probably missing some important & vital circuitry still......

But I feel that the requester will have more problems of a mechanical/electrical safety type due to his inexperience and seemingly lack of knowledge.....I hope I am wrong of course and he achieves his goal......but developing high voltages carries with it a heavy risk to the uninitiated.....

Dear sir

the fig is on black BG & contrast is not good to view the diagram

Regards

First off, let's assume you intend on doing this off-grid. You can do this with a grid-connected motor without all the excitation issues, but the connection to the grid is highly highly complex and can be deadly for people such as utility workers, so it is NOT something to be undertaken by an amateur.

So in that light (off-grid), then Yes, it can be done, but requires a couple of very important things:

1. A good working knowledge of electrical engineering principals, especially in the area of safety and circuit protection, To attempt this without that knowledge would be akin to driving a car because you can reach the gas pedal.
2. A lot of luck. You need a motor with really good residual magnetism. The circuit in the above example is used in motors that have a fairly good amount of residual magnetism in the core, so that when it is run over synchronous speed, the capacitors get enough of a charge that they can build up and supply the excitation energy to the windings. Without that residual magnetism, the motor will just be a spinning weight on your water wheel. Unfortunately, 90% of all basic induction motors do NOT have enough residual magnetism to accomplish this task. So that is where the luck comes in.
3. A lot of water. 30kW is quite a bit of energy. Read this wiki article in its entirety and go to the referenced articles to get an idea of how complex the water supply issue really is. http://en.wikipedia.org/wiki/Small_hydro
   
4. A good handle on mechanics and civil engineering. You will need gearboxes, a dam, water channels, flow control devices, mounting structures, protective enclosures, the whole lot. Just having engine and wheels of a car does not make it driveable.

Good luck with that, and be safe above all else.

a small trick if the remanent magnetism is not enoug in island operation

give a small burst ( 100msec) of a 12V batery to the compensation capacitors that will initiate the magnetism the capacitors will do the rest

jbz

I assume a 30kW motor is a 3 phase type, 90% change it is a squeral cage motor

all squeral cage motors can easy used as generators

just let them drive the oposite rotation as the grid would do

so hook it up to the grid with a motor switch (best with thermal protection), give a short burst with the switch and make sure he is going oposite to the flows direction

if it is going in to the wrong direction change 2 of the 3 phases

then open the water valve and he will do as generator

ok thats as far as basics

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to improve the gridquality you can mound a capacitor which size is depending on your motor , for general count on 30uF / kW ( for cosinus phi = 1 ) betweeen voltage and current

the danisch type where not different than this just powerd by wind not by water

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to the water basics / mechanics if you nod did already

make your water rad. Let it run careful by opening the water and see what rpm you can do with 25% of the available water

now you can calculate for the speed adaption wheel to motor/generator

so if you reached 100 rpm you will need 2 wheels (as an example with a gear ratio of 100 / 1850)

best is tooted belts because in case of the V type it could burn down if the belt slips

again check the generator / water rad combination, unloaded connected to the grid

it should run in the oposite direction

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now some automation

if the water will slow down it will go automatic as motor again

to prevent that, mount a rpm counter / contactor

again let the motor run as motor unloaded and measure the rpm

now let the rpm switch decide when to connect the generator to the grid , if the rpm is higher as so called synchronius speed for me: mostly above above 750 / 1500 / 3000 rpm

if at your place 60Hz it will be 900 / 1800 / 3600 rpm

mostly you can read the motor speed of the nameplate where the 30kW is indicated

the diference to this synchronius speed is caled the slip

so the unloaded motor is rotating 1800 and its indicated 1750 as rpm as motor then it has to run 1850 for full loaded generator

you will have the lowest inrush currents just above synchronius speed so as example switch it on at 1815rpm and switch it off at 1805

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2 warnings

electricity is dangerous be carefull if your not an electrical engineet

switch off and defuse befor you do anything on the motor

your grid connection must be capable to handle the inrush current

if you can connect the generator as motor unloaded without blowed fused thats ok

goor luck and be carefull

I feel it is irresponsible to suggest connection to the grid by someone who is not familiar with all of the issues involved. A utility worker could easily be killed.

in stead of killing his ideas

help him how to do it safe

i think negative people are more than wanted on the internet

together we can help him

no I

not you

together we can improve the world

Im sorry for this emotion

I will now help him to stay alive:-

To the original poster of this Blog:-

DO NOT TRY AND DO THIS YOURSELF, EITHER GET A "HANDS ON EXPERT" OR JUST GIVE UP. IT IS SIMPLY TOO DANGEROUS.

TOO MANY NICE PEOPLE HAVE DIED IN THIS WORLD TRYING TO WORK WITH HIGH VOLTAGE WITH NO PROPER KNOWLEDGE, SO I WILL NOT ENCOURAGE YOU FURTHER IN YOUR POSSIBLE BID TO BE MENTIONED FOR A "DARWIN AWARD".

Please click on the following link for a better explanation:-

Darwin Awards

I do not recommend connecting to the grid unless you know what you are doing.Someone could be killed.Possibly you, or a utility worker miles away.

If so, your local utility must be notified, and approve of the installation, which will include phase and frequency balancing circuitry and isolation circuitry to prevent back feeding of power to the grid when it is switched off by the utility company for repair, such as storm damage, routine maintenance, etc.Improper(mismatched phase,voltage, etc) and unexpected voltage on the grid can cause a catastrophic shutdown of an entire section of grid, and could result in loss of power to hundreds of homes and facilties, followed by a massive lawsuit.

A simple means of phase matching is accomplished with 3 clear lens lamps, one in each phase, connected from grid to generator.When the filment is completely cold, and not glowing, the phases are matched.At least close enough for the balancing circuitry to take over without a big bang.

Good luck, you will need it. And if you don't know what you are doing, make sure your life insurance is paid up to date, as well as a huge liability policy.

I completely agree with you, because as somebody has already pointed it out before, the operation of this motor, as a generator, will be at the mercy of several factors and one of them is the residue magnetism in the core of the rotor; which ideally should be NIL.

Whilst the residue of magnetism will not be perfectly zero, but will be very low, and that in itself will cause for a concern during start-up. (Just remember even a car-generator does require, as an added assurance, a little excitation current through the charge indicator lamp on the dashboard to start up reliably.)

I could not work out how the above diagram supposed to sort out the excitation during operation other than the caps and the windings form a resonant circuit at a frequency that might not even be the desired one that you need.

Frankly, I do not believe that an insurance policy would give an adequate protection because no insurance firm in its sober mind would insure a design like this without a legitimate technical approval.

I might be wrong but I only believe what I see, nothing less.

To work easily, normally a little residual magnetism is required.....not having that would involve some special electronics to ensue that the generator actually generates something.

A traditional AC generator (depending upon physical size) has either permanent magnets or a little DC generator to excite the main rotor....

Andy, I agree with the permanent magnet or a little DC generator exciter concept and they are easy to understand and control both the frequency and the amplitude.

Most larger Alternators have on their spindle a smaller Generator (as a DC power source) to supply the exciting current required and this is what I would call a normal method.

But to understand the excitation and more so to perfectly match the freq on this squirrel cage 3phase motor, without any mods, I find it hard to comprehend both in operation and reliability.

I never heard of this suggestion before, but then again there's always something new to learn about I suppose.

A better method is to cross couple two of the three lamps so that when the 3 phases are lined up you have one lamp off, two lamps on. This is a far better indication of the correct time to connect.

Also, it has the further advantage that the "off" lamp rotates, showing (when properly connected) as to whether the oncoming Generator is leading or lagging the mains. We used a clockwise rotation to show leading and anticlockwise to show lagging....it depends on exactly how the connections are made as to which is which.....

Leading means that power will be taken immediately and the generator will stay online, lagging means that if the connection is made, reverse power relays will immediately trip and take the generator off line again....

A small but important point!!

Im not agree with this idea

3 lamps will work for a synchhonius generator which will have a voltage running in island operation

however the SQR generator will get its reactive energy from the grid

before it will generate voltage

the trick we do in windturbines is energize it with a industrial softstarter just before synchronius speed and after 200msec bypass it by a contactor

if you want to run it in island operation

you dont need to synchronize anyway

It depends on what motor u have, like DC or AC. If it is DC then Series / shunt/Compound?if It is AC Cage induction or slip ring type (Wound Rotor)or Synchronous.Since DC , ac Synchr. and wound rotor are not so frequently employed we assume it is a simple 3 Phase induction motor with a cage rotor.In that case motor has to connected to source of supply and when the turbine rotates the rotor above synchronous speed it will star generating.In your term slip becomes possitive.Of course suitable control and other circuits will have to be incorporated .This is a simple explanation.

See this article....

http://www.reuk.co.uk/Ametek-PM-Motors.htm