Electric motor used as Generator?

[cgaday]

theoretically I believe this is possible:

basically I am trying to decide if I could use an electric motor as a generator to charge a battery source, and when needed use it as a motor as well.

I think the only issue is the rpm's which it is operating at, to get the most efficient electrical generation.

Insights would be appreciated.

 

[nucleus]

There are several things that have starter generators, but usually requires special wiring.
http://www.simpletractors.com/service/electrics/starter-generator.htm

 

[Topher925]

Yes, you can certainly do this but good motors tend to make terrible generators. What kind of electric machine were you planning on using?

 

[cgaday]

If good motors make terrible generators, can good generators make good motors.

I haven't decided on the electric motor, just trying to figure out some good ideas. and what I need to do.

I am trying to develop a direct drive ( or other type of gear box etc, depending on what is needed) to a generator from an IC engine, to charge the batteries via the generator. And when needed use the generator as a motor when the IC engine is not needed.

The problem I will most likely encounter is that the IC engine will be fluctuating while it is in use, for example driving a car. Don't know if this will harm the motor, or just not provide sufficient rpm.

 

[sophiecentaur]

In principle, the answer to the op is yes and spinning many motors will produce some volts- to prove a point. But:
A brush motor, which you find in the majority of domestic devices and in many low voltage applications, has a commutator and brushes in it. These keep the motor rotating in the same direction all the time and they will work with AC or DC supplies quite happily. If you turn a motor (especially one with a permanent field magnet in it) it will generate. However,because of the constant switching action of the commutator, the current produced is constantly interrupted, although all in one direction. Also, the carbon brushes bridge between the commutator segments; good for a motor as it stops sparking but wasting power from a generator. Not good.
It's much better to generate AC, using an alternator (which has no commutator) and then rectify if you want DC out. Since decent rectifier diodes were invented, about 50yrs ago, cars all use alternators because the old DC 'Dynamos' were really rubbish - especially at low speed.
However, having said that, automotive motor generators are now used which do both functions well. They are very high tech and involve clever control circuitry - V.expensive.

 

[cgaday]

Is this possible with high power output approx 80hp. and what kind of cost do you mean by V.expensive? Do you know of a company which can provide these generators with the advanced circuitry.

 

[sophiecentaur]

Have a word with Toyota, perhaps?
Or Citroen in their 'stop start' C something use a motor alternator with quite high power.

 

[Topher925]

Is this possible with high power output approx 80hp. and what kind of cost do you mean by V.expensive? Do you know of a company which can provide these generators with the advanced circuitry.

You may want to look into the aerospace industry for a solution. The electrical machines in aircraft turbine engines function as both motors (for starting) and generators and do so with a very high power density. Machines with these kinds of characteristics are almost always permanent magnet synchronous machines or asynchronous induction. Although, I have seen a few switched reluctance motors with some very high power densities.

 

[agtee]

So for mechanism having rpm around 1000, what is better to use alternator or generator?

P.S.: i am considering generating power only. As for the alternator we need to give some starting voltage by connecting it to a battery. While for a generator it doesn't need starting voltage. And again is it better to use motor as a generator at this rpm?

 

[sophiecentaur]

Is it fatuous to say that, if you want to generate, then you should use a generator? The 1000 revs speed is a bit low for anything, though, but you would normally use a belt or gears to get the optimum speed in any case. If money is an issue then you are more likely to get a cheap motor plus a cheap generator than a single unit to do their combined jobs as satisfactorily.

 

[sophiecentaur]

You do not need an exciter voltage for all alternators but automotive devices ( the cheapest) would require a battery and would give you regulation, too.

 

[agtee]

Hi there,
Money is not an issue, I tried to find out generator which works at around 100rpm and 12-14 volts output but couldn't get it. So the remaining option is to have an assembly.

 

[agtee]

And also does alternator only give AC output?

 

[sophiecentaur]

An alternator basically produces AC but automotive ones have diodes built in and give you DC. But if you look at what comes off a 'DC' dynamo you will probably see a pretty raggedy waveform.
I don't see the problem with revs. Is there a problem with arranging suitable gearing?

I think you need to define a bit more accurately what you actually want. If money is not a problem then go for the 'right thing', properly engineered. This involves coming up with a proper specification.

 

[agtee]

Hi,
I want to generate electricity at 1000rpm aprox. and than to charge a 12.4 volt battery. So that i can use battery whenever i want. What could be suitable way?

 

[sophiecentaur]

"electricity"? How much?
"12.4V battery"? How 'big' what charging current do you want?
"1000rpm"? What's so special about 1000 rpm? With gears or a belt an pulleys you can suit the drive to the generator.
You have been much too vague about your requirements for any sensible suggestions to be made. You need to be more helpful if you want help.

 

[agtee]

Basically I want to generate electricity by bicycle pedaling. So far what I am aware of is human can pedal with 50-60 rpm, so in my design we have this conversion ratio of 1:30, accounting efficiency decrement with number of transmission system, Around 1500 rpm can be produce at max with my design. So for this purpose i want a generator that can give me electricity. Furthermore i want to store this to battery which has terminal voltage of 12.4 Volts. Wattage is around 150 from generator that can be generated without much of a pain and considering we need to produce around 14 volts to charge the battery.

 

[sophiecentaur]

Have you looked at existing designs?
I have a feeling that you will need a rather fancy design of generator if it really has to work at this low rev speed. An auto alternator will start to deliver current with the engine doing about 1000rpm but there is an up-gearing of about 1:2 (from memory) tonto the alternator shaft.

 

[agtee]

Yup I have looked for the existing designs. Basically we have tried to run it with an alternator but its rather hard to pedal while generating electricity with alternator as compared to generator (we have one small generator but we got it as a scrap so no idea how to manufacture another one). Could you throw some light on this?

 

[sophiecentaur]

I think you are mixing things up here. If you find it hard to pedal against the alternator and easy to pedal agains your generator it will be more likely that the generator is just not producing as much electrical power. Did you compare current and volts during each condition?
If you can sustain 100W pedalling, that should correspond to about 8A at 12V (but there will be a few percent loss, of course). I would take a bet that your generator wasn't producing that much. An alternator, on the other hand, could well be producing as much as that.
Also, the commutator which needs to be part of a DC 'dynamo' makes for very inefficient operation at slow speeds - because of the wasted times during switching from winding to winding.

And you still haven't made it clear what electrical power you require- just some random observations about things you have tried. That's not real Engineering.

 

[lqxpl]

Recently did something similar to this for a class project.
Major difference being that our motor (a very nice hobby motor) was being driven by a prop, with the prop's revolutions being geared to the motor at 1:3.5 (7,500 from prop turned into 26,250 on the motor).
Motor was three-phase. Needed a three-phase rectifier.
All said-n-done got to keep about 80% of the expected power. Approx 18V and a little under 4 amps (~72 watts)

In short: yes, it is doable, but as you are discovering, you need very high revolutions to your motor to produce anything usable.

 

[sophiecentaur]

Having read all this talk of motors as generators, I am gobsmacked that anyone bothers to design and use purpose built generators.
If this project is supposed to be anything to do with Engineering then why not treat the problem seriously and go for a proper alternator? If you want to generate power efficiently with a low shaft speed then it calls for a specially designed multipole generator. Such things have been available for decades (for very low powers) in the form of the 'Dynohub' alternator which mounts on the hub of a bicycle wheel and functions at shaft speeds of 100rpm and upwards. (The rate of a cycle wheel at moderate cycling speed).

@Lqxpl: what was the mechanical input power to your motor / generator? How did you measure efficiency?