Using a Motor as a Generator

All motors can not be as it is used as a generator. In fact most of them can not be without major rewiring.

Better take a car alternator, that is a generator.

DC Motor- yes.

Induction (most likely what you have)- not easy

Synchronous motor- yes.

Single phase motors- require rewiring.

pretty sure only D.C. motors could be used with out going through alot of trouble.

Like the other person said,Just use a car alt.

All machines can be used either as motors, when absorbing electrical power and putting out mechanical power or as generators, with mechanical power input and electrical power output. The motor/generator situation is basically established by the difference between mechanical and electrical power. If mechanical power exceeds the electrical power ( + all losses in the machine), it operates as a generator, if not, it operates as a motor.

When operating as a generator, there are 2 situations:

1. Connected to an (electrical) power grid, when output parameters are imposed: voltage, frequency (in case of AC), sequence of phases (AC), etc. Any machine, when connected to a grid, can switch from motor to generator operation and vice-versa, by varying the input mechanical power parameters (usu speed), provided that equipment installed to avoid this is off.

2. Indepenedently operated (stand-alone) when those parameters have to be set by the generator (via excitation, driving motor speed, etc).

Definitely some types are more easily to operate as stand-alone machines ( synchronous and independently/parallel excited DC machines), but I do not think the others need major rewiring, as long as the inputs are supplied with those values for which the machine has been designed.

Sorry, but you appear to be living in another world with other laws of physics.....Nice laws, but not the ones that work here - sadly!!

Andy, Number 7 is correct, and induction motor when connected to a system Will generate power beck into the system when powered from and external source at about 20 % overspeed, We do it all the time on windmill generators. Makes no difference between single phase or 3 phase. The excitation comes from the capacitive reactance component of the system.

If the induction motor is not connected to a system, then (particularly on a 3 phase motor) it will likely NOT GENERATE unless a capacitor is connected across one winding to supply the needed reactive excitation currents.

Many co-generation systems use induction motors as generators in this manner.

Originally the co-generation people fought the power companies requirement for a disconnect device to prevent power from back feeding into the system during a power outage, stating there would be no source of excitation.

As part of an EPRI project I have personally tested over 30 different induction motors of various designs (10 to 75 Hp) and they all self excited when connected to the system.

These systems are ideal where you cannot maintain synchronous speed as in a Synchronous motor, such as a windmill. Note, the current wave forms may be rich in harmonics. but it's still power.

We published several IEEE papers on this subject.

You may need to reconsider your comment and possibly you owe some one an apology.

Don Jeerings

I think not!

I took a different stance on the post, generating using an induction motor is eminently possible, but requires detailed knowledge that I do believe was missing here......the reason for my negative comment.

But thanks anyway for your involvement.....have a great day.

Generator was the term applied to the older DC generating "generators". When automotive mfrs changed to AC generators they were called alternators however they were still generators. You might want to look onto a GE or Siemmens website. Ge advertises their 100 years of experience in manufacturing "generators". Having been instrumental in the purchase of a 30,000 KW steam turbine driven "generator" (AC-4600 volts I believe) many years ago from GE I agree with #17 in that you do owe someone an apology and the term "guru' may not be as well earned as you think it is.

Sorry,

The laws of physics are the same everywhere and for everybody, irrespective if they are aware of them or not.

All machines can be used either as motors, when absorbing electrical power and putting out mechanical power or as generators, with mechanical power input and electrical power output.

You are technically right, but then as you yourself have mentioned,

2. Indepenedently operated (stand-alone) when those parameters have to be set by the generator (via excitation, driving motor speed, etc).

Of course I need not mention here the requirement of the magnetic field for the conversion of the mechanical to electrical energies.

For the grid connected motors, of course the field is established by the grid and the transfer of energy takes place smoothly.

But what do you do when the motor is an island ? Can you give an excitation to an Induction motor (any type) without rewiring to create the total excitation field and along with the excitation controls?

Unfortunately most of the motors we come across in regular usage are the AC-Induction motors- single phase or poly phase.

And as you said the Laws of physics are same everywhere but it says that an electric current is generated in a conductor moving in a magnetic field - or rather the Electric field generated is due to the change in flux linkages (maxwells equations). The movement is given, where is the magnetic field?

It is true that in order to operate an induction motor as a generator ( in a stand-alone situation), it needs a capacitor connected in parallel to the output. The rating of that capacitor battery has to be determined to provide the magnetization field required. Definitely, when rotating the machine at the rated speed (rpm) to have a certain output voltage & frequency, first in a no-load situation and later starting to load it up, a speed regulator for the driving engine (Diesel motor, wind turbine or other similar device) shall maintain the speed at that value. But since the output voltage, even at constant speed falls with increasing load, is falling at the same capacity, to compensate that voltage drop, additional capacitors have to be connected.

It is true that the quantity of capacitors that have to be connected varies based on how the magnetic circuit of the machine has been designed ( more or less saturated). Usually standard (general use) induction machines are optimally designed to work as motors, with a minimum imposed power factor, efficiency, etc, but in order to work as generators no changes to the windings have to be done.

Definitely, the "start-from-scratch" design of an induction generator will lead to probably a slightly different winding, cross-section, length, etc, because an optimization process is used, to make it best for the operation as a generator.

So a general use induction machine (motor) can work as a generator, but at non-optimal parameters as compared to a machine designed specifically to work as a generator.

It's certainly possible; what are you going to turn your motor with? What kind of motor do you want to use?

Anyone starting with your "apparent" level of knowledge has a REALLY steep learning curve in front of them....

May I suggest that you start learning/studying all you can about various motor/generator types before you invest one red cent in your hobby......it will save you a lot of money in the end.....buying a generator will probably be the cheapest way....and certainly the quickest.

Have a great day.

Don't Know why you would want to build something that consumes more than it puts out, but here goes:

If you have a synchronous motor, rated at 1200 rpm, and a motor rated at 3600 rpm coupled to it,the slower motor will become a generator, producing approximately it's rated voltage and amps.The load will have to be applied before starting the "generator, to produce eddy currents to generate the voltage.Alternatively, you could use a capacitor to kick start the process.

This is very inefficient process.The driver will always consume more KWH than the generator can produce.

Do you simply want to change voltages? A transformer would be better.

Are you trying to build a rotary phase converter, to run a three phase motor from single phase supply?

Please elaborate on your intention.More info= more help.

EZStreet

I don't know how you referenced it when you Googled but the proper term is "Induction Generators". I just did and there was a vast amount of information given under that reference. Most induction generators "float" on a main system, that is they are controlled as far as cycles by the utility system they are connected to and hence the utility system acts as a speed governor. Wind generators are a form of induction generators that are quite common. One major thing that you have to protect against is maximum output. If the main system goes down then your induction generator is going to try and put out more amps than it is designed for since it no longer is being governed by the utility and will blow out as it overspeeds and produces excessive amperage.

GA

You really understand the problems well.....

JRaef,

That's one of the best & most concise answers I've ever seen for such a complex question. GA!

Well done. Great short course if there ever was one. GA!

Buy Fairbanks Morse (Electrical Machinery Catechism) Bulletin E100Q This contains a lot of information you need. Generally Wound Rotor or Synchronous motors.

This has been an interesting thread. So how would a synchronous motor work as a rotary phase converter? I looked up rotary phase converters, and everyone does what I do; induction motor fed with single phase and kick started. Is it because synchronous motors are generally large?

No, I think you are referring to an Asynchronous (induction) motor being used as a phase converter. That's another long post, but with relation to the above, it is like a grid-connected alternator. The 1 phase power you feed into it excites the unused winding of a 3 phase motor and it adds-in a 3rd phase for OTHER connected smaller loads.

I'm looking for a sourse of generator for 50 watts to 1 kW, to be powered by a steam turbine. I guess I can't use a power tool because they're all cage motors, likely. I don't think a car alternator puts out 120v though, needed to run a fridge. Other wise I would get any motor and use a bridge rectifier to convert it to DC and feed it to an inverter. Maybe a battery before the inverter?

The cheapest Marathon generator is $2000.

May I point out that steam turbines are not the easiest things to build at home (I assume you do not want to buy something generally in the MW range)......the gearbox alone to get the revs of a small turbine down to manageable values for your generator will usually cost a lot too.....

For small steam systems, usually a steam (piston) engine is more manageable, or a Stirling motor maybe? (with no steam....!)

I hope this helps....

http://www.metalwebnews.com/howto/ph-conv/ph-conv.html

It seems to me.... its not just enough to know know the technical configuration (as experiments) on how machines-motors operates...upside-down discussion of electrical-mechanical energy transformation...but theorytically "Law of Physics still applies".