DC Motor as generator

[jason8598]

I own a scale model jet turbine engine. To start this engine up I have to use a hair dryer to spin the engine up to 3000rpm's, then I can ignite the gas. I have seen small DC motors used for this step, connected directly to the shaft. I have heard that the motor can also act as a generator when the engine is at running speeds, from 35,000 to 90,000 rpm's. Is this true and if so what size motor would handle these speeds?

 

[DickDV]

i would recommend that you check out the DC spinup motor setup a bit closer. I suspect that you will find an overrunning clutch or some other means for disconnecting the DC motor from the turbine shaft.

I know of no DC motor that will tolerate these speeds. There may be some but I certainly have not seen one.

And, yes, as long as a DC motor is driven at safe speeds and has its field energized (it might be permanent magnet), the motor is essentially a generator and can be used as one.

 

[aolalde]

For high speed, the smaller rotor diameter is best. The more power is required the larger the diameter. It will be a compromise between enduring very high speed and handling the power requirement without destruction of the rotor armature due to the high speed (90,000 rpm?)

 

[OperaHouse]

There are some small electric motors for light duty high speed light machining that will go 35,000 rpm. They run about $6,000. They also wouldn't have enough torque to start the turbine. The problem is the windings in the aramture will fly apart and they are not balanced for that speed. The brushes would also lift off the commutator on an air film. The high speed motors are brushless. The answer to having a starter and generator that wound make your model actually usefull for something is no.

 

[jason8598]

Keep in mind that this is a small turbine engine. Currently I use a hair dryer to spin the turbine up to speed so it does not take much force to spin the engine. In fact I can blow through the engine and get it up to 1,000 rpm. Also keep in mind that these are being used by at home hobbyist using the engines in R/C planes.

 

[sreid]

For small DC brush type motors check out Micromo.

http://www.micromo.com

 

[Barry1961]

A Dremel tool runs at to 35,000 rpm. There may be some other brands that run faster. I do not know if there is a speed up gear set on it.
You may need to run this like a turbo prop to regulate the speed down.
Sounds like fun with the potential for profit.

Barry1961

 

[jason8598]

Yeh they are alot of fun. The only problem is that I can only run the engine for about 3 minutes before the battery pack running the control unit runs out of power. Would love to have a way to sustain the batteries longer or atleast till I run the fuel tank dry.

 

[Barry1961]

You should be able to use some of the air to run a small turbine generator. Maybe a small spring loaded flapper to maintain a fairly constant pressure/speed.

Have fun!!

Barry1961

 

[jason8598]

Barry, where could I find a small turbine generator?

 

[aolalde]

Use a step down gear-box around 50/1 and a standard DC machine 1800-2000 rpm to generate electricity.

 

[sreid]

Is it possible that the controller is pulling too much current or thst the batteries are old Nicads that have lost capacity?

 

[jason8598]

Batteries are fine. The battery pack is a one of a kind type and is designed to only last about 3 minutes. I can always have a battery with a longer capacity made, but I just like the idea of having a recharging system integrated into the entire system. But thats just me.

 

[jason8598]

How would I determine the amount of electricity the motor will produce at a certain RPM?

 

[Clyde38]

I suggest that you look into a switched reluctance motor. They, as well as brushless dc motors are commonly used for exactly the purpose you describe (larger scale). By the way, where do you get a model "jet engine"? Sounds like something I'd be interested in.

 

[jason8598]

Clyde38,
Check out

http://www.artesjet.com

or

http://www.behotec.com

they make great engines. Can also just do a search for model jet engines and find many sites. Also the Gas Turbine Builders Association is a great site with several pictures

http://www.gtba.co.uk