Motor as generator

I’m prototyping a turbine. Looking for a high speed generator. I can’t find one so I thought of using a dc motor as the generator. The one I’m thinking is rated 300 watts and 25000 rpm so it’s not quick enough but it’ll ok to prove the concept. Really need more wattage so I thought I may be able to join 3 together and draw current to simulate load if the shafts don’t break. Will this motor output 300 watts when used as a generator JOHNSON RS-775 Electric Motor DC 12V 18500RPM High Speed High Power Torque 300W | eBay.

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That’s a very complex question, with way too many unknowns. For something this complicated you’re probably best to get in touch with an engineering firm or a University - they’re the only ones who are going to have the knowledge, skills, and resources to give you an answer.

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What’s complicated about it. Will a dc motor output the same current when used as a generator

Maybe, as Oliver said there’s a huge number of variables here. The actual components of the motor should be able to handle that much current running through them, but the actual current that flows through a circuit is actually only a relationship between the resistance and voltage being produced (this is also known as EMF when no current is flowing in physics and is based on the rotation speed, the strength of the permanent magnets if any in the motor, and the number of turns of wire that are turning at a given angle through the magnetic field). If you have any other questions please let us know!

Basically It will produce current according to the conditions outlined by Bryce. Given these conditions are right It should go close.
One other thing. Using a motor as a generator may be OK for a short time to carry out an experiment but should not be considered as any sort of permanent solution. In a generator the relationship between armature position and brush/commutator contact is not “dead centre”. In other words the brush/commutator contact leads or lags (I don’t remember which) the armature position and in practise is set or adjusted to cause minimal arcing at the brush contact while under load. A DC generator with fixed brushes should only be rotated one direction. One with adjustable brush position can be rotated either direction subject to brush adjustment. Failure to heed this would result in abnormal wear and burning of the brushes and pitting/burning and possible irreparable damage to the commutator.
In your situation just to “prove concept” I don’t think it would be running long enough to cause much damage but I just thought I would throw this in as something to think about.
That motor in the pic on E-Bay looks pretty small for 300W. I was thinking about a golf buggy motor which has a “stall” current of 23-25A and they are a whole lot bigger than that. Maybe the measuring technique could be optimistic (marketing???).
I think what Oliver may have been getting at is the mechanical side of things. Thinking of the disastrous damage that could happen at your proposed speed if the whole thing came apart due to unbalance or misalignment. As you are carrying out this experiment I assume you know what you are doing along these lines. Me?? I would prefer to be outside a solid brick wall watching on Television. A believer in the old saying, act like a small chicken now and grow up to be a big rooster later.
Good luck and I wish you success.
Cheers Bob


Might be a combination of optimistic measuring, and the very high RPM needing bugger all torque to get 300W of power out.

Did you ever have a go at reverse engineering one of those little DC water turbines?

Hi Oliver

Never had a need to. Probably never will. Could be a fun bit of kit to experiment with though. I could not imagine using it to power anything, the water cost and waste could outweigh any benefits.
What I could imagine however is using it as a sensing device and use the 5V output to trigger something else like light’s or whatever. up to the old imagination.
Don’t anyone dare suggest using this to power a pump to drive the turbine to power a pump etc. One golden rule worth remembering. In this cruel world you get nothing for nothing and people have being trying for perpetual motion for centuries.
Looks like a great little gizmo though.
Cheers Bob

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Ah, I actually meant Rodney - I suggested it to him in his previous topic:

Yeah, definitely not a high power thing, much more useful as an educational tool, but if you’ve ever got a situation where there’s a regular supply of water (eg. in agriculture or gardening) and there’s a little bit of energy to spare from the flow it’d be handy for running a data logger of some kind.

Re the perpetual motion gismo:

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Ideal in that situation where the water is being used as it was meant in the first place.
However, we are digressing a bit. The subject of another post maybe?
Cheers Bob


Thanks. So as a proof of concept if the load is 300 watts ,the motor being used a generator should go close to producing the 300 watts although it may have a short lifespan. Didn’t have a go at reverse engineering the turbine. The prototype I have will be idling at 20,000 rpm so just need to show it produces power efficiently

Hi Rodney
We all await the results.
As a matter of interest, how are you spinning the turbine??
Cheers Bob

Water vapour. At 512 metres per second

Hi Rod.
By water vapour I assume you mean steam.
That is a pretty fair velocity. 30km / minute. That would blow most weather type anemometers off the roof.
I would take from this that you have far better than home/back yard facilities available to be able to generate this sort of velocity, steam or otherwise, and you are able to measure this sort of parameter.
You know more about what you are doing than I, or I think most of us on this side of the conversation do so please keep us posted re the outcome as I think all are interested.

As a thought have you researched a jet aircraft APU. I am pretty sure this is turbine driven to supply power while not flying and you used to be able to hear it back in the days when you walked in the open to board your flight. I think the turbine in the old Focker Friendship turbo prop aircraft did something like 30k RPM but I don’t know what speed the actual generators did.
Cheers Bob

Using a vacuum it is not that hard to generate that velocity. Machine shop is doing the build of components. I have a homemade jet that does 90,000 rpm and generates 56 lb of thrust using a turbo as the jet. Currently getting some university students to double check my calculations but I envisage a full speed of 1/2 the velocity. Turbines are different in that full torque is at stall and they have no torque at full rpm.

Gearing generators is not that hard but adds to the cost. It’s quite easy to break the speed of sound with a belt or a gear and they don’t like it lol

Thanks for that bit of interesting trivia. I didn’t realise that although I have not made a study of this either. I usually get by by knowing they spin very fast and get me from A to B pretty quick in an aircraft.

No they don’t do they. It mostly is a very uncertain point of operation. An everyday example is a helicopter rotor blade tip. Exceeding the speed of sound may cause the rotor to delaminate with disastrous results. This also limits forward speed as at one point this has to be added to the leading rotor spinning speed.

Thanks for that info anyway. interesting.
Cheers Bob