# Interest group forums > Electrical Contracting Industry Forum > Electrical Load Shedding Forum >  Electric Motor or Generator ?

## daveob

Here's one for the electrical / electronic guru's in the forum ...

We live in a town-house complex, and a noisy generator would not really be allowed or practible. So I'm looking at alternatives like UPS, ( + battery units for extended power ), etc.

After a bit of googling, I've noticed that an electric motor can also be used as a generator.

So instead of providing power to the motor which converts into motion, if you can supply the motion it will convert back into power.

Assumable, this is the basic principle of how a nuclear reactor creates power : steam > turbine > generator ( like a reverse motor ) ?

So, if I have a method of providing the motion ( spinning the motor ) would it be a simple case of : if a 220 volt motor spins at 1000 rpm, and if I can spin it at 1000 prm, it will generate 220 volts ? Or is there some other laws of physics that determine the power output of the motor ?

Then, would a motor be the best solution to convert this freely available motion back into electricity, or are there better / cheaper / more efficient methods ?

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## Dave A

:EEK!:  Where to start?

Short answer - don't do it.

You need to generate electricity in the 220-240V range and at 50Hz. Your motor will produce electricity, but I'd be very worried about its characteristics.

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## daveob

Hi DaveA

Thanks for the reply.

If it is not a good idea, then how does a petrol / diesel powered generator produce electricity ? Is it not the same principle ?

Regards
Dave

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## Dave A

Yes and no. I'm not concerned about you motor producing electricity - I'm concerned about how fit for use that electricity is going to be. Dave, if you put something together, just take a really close look at the output before you hook it up to actually power anything.

I took note of your point around batteries and noise. Now if you are going to use batteries, the way to go is to use an inverter to produce an AC supply or set up equipment that can run off the DC supply that the battery produces. 

My sense is that you might be considering using a DC motor to drive the AC motor. Once you've got together all the control mechanisms to get a reasonable 220V 50Hz supply, it's likely to overrun the cost of going the inverter route, and I'm still a little nervous of what the wave quality will be like.

Go for it by all means - but I think you can expect surprises.

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## daveob

Hi again

Thanks for the reply, Dave_A.

I was seriously considering the following scenerio :

having the 'genny/motor' create 220v and run the non-sensitive equipment ( lights, etc ) directly from that. For the PCs and more sensitive stuff, an inline ups that will receive the rough power and charge the batteries, and the battery power will then pass through an invertor to run the PC equipment which, as it comes from the batteries, would be nice clean PC friendly power.

As far as the drive method is concerned, I have something completely different in mind. I'll not say more until I've tested this ( so I don't look like a twat if it does not work, and don't lose a really strong commercial idea if it does work - and no, it's not wind power ), but suffice to say that I could easily spin a smallish motor at +1000 rpm.

So my original question still remains :

if a 220 volt motor spins at 1000 rpm, and if I can spin it at 1000 prm, it will generate 220 volts ? Or is there some other laws of physics that determine the power output of the motor ?

Then, would a motor be the best solution to convert this freely available motion back into electricity, or are there better / cheaper / more efficient methods ?

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## duncan drennan

Spinning something and producing the correct electrical voltage is quite different from spinning something and also producing the required power.

Fundamental law here: power in x efficiency = power out.

Whatever you are spinning the motor with has to also be able to provide the required amount of power to be useful. Various things can work: wind, steam, water, etc.

Whatever drives it has to turn a a pretty stable speed though, and that is quite important (unless you have a bunch of fancy stuff between it and what you want to power).

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## mikilianis

Depending on the Supply frequency, assuming that it is 50 hz and the number of poles is 4 then yes, your motor will rotate at 1400 R.P.M. 
Mike

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## tec0

Here is the thing, you need 3000 RPM and then only can you scale down. The reason for that is simple; because it comes down to 3 phase then house power 2 phase still it comes down to AMP’s. By now you are thinking I don’t know a damn thing but there is a lot more than just your 230v 50Hz to worry about. 

Your pickup needs to be just right and that is where the extra RPM comes to play. She the higher the demand on your generator the more revolutions it needs to generate and then your rectifier get to play with the raw electricity AKA dirty power and then has to clean it up otherwise everything in your home goes BOOM! 

And you end up buying new stuff. So what do you do??? Well firstly you need to know how much amperes you need and then get the generator to “pickup” at the right moment otherwise pop goes your freezer! 

So what can you do? Well the easiest way is to generate power with something like the sun and then charge big old 12V batteries and then use an inverter to get it up to 220V or better yet get stuff that can work on the 12V directly now don’t worry most inverters comes with build in protection and will cleanly cut the power to your device before something bad happens. Still driving your oven or freezer might be too much to handle so what do you do... 

Well in the old days steam engines can happily generate 3000 RPM without much effort. Now connect your electric motor to a steam engine and then run it at 3000RPM and then let your rectifier and “pickup” deal with the power demand. You will happily get 230V and about 93 AMPs! But because this is a steam engine you get torque a lot of torque and thus you can push it to a hell of a lot more than 93 AMPs  :Yes: 

So what is slowing your motor down when you generate power?????? Well it is called magnetic drag and it happens when the demand for power goes up!!! Thus it needs more power from your “engine” to generate the same amount of power.      :Stick Out Tongue:  but you get more AMPS!!!!!!!!

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## greasemonkey

You also will not be able to use just"any " electric motor.There has to be some form of induction . A standard 3 phase motor won't do that for you.

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## wynn

Not being a lecky and knowing nothing about amps, volts and amperes the system I aspire to is:-

The required number of deep cell batteries outside in a metal lock up cage under the eaves charged by municipal current, with the necessary invertor and safety systems to feed my pc, phone, tv and some emergency lighting.

cooking by gas, fridge/freezer and solar heating boosted by a heat exchanger.

If power cuts are going to be longer than 48 hrs add a generator to keep things together, with PV panels to boost the batteries when the gennie is off.

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## amund

IT will produce electricity but you have to spin it just more than the rpm on the nameplate on the motor to get 50 hz.

I think u might build the same thing as i did, use rare earth magnets they work the best.
and the magnetism will last for about 400 years.

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