PM DC brushless generator sizing

[shawnk]

I want to build a small generator using a permanent magnet DC brushless motor. The output capacity need to be about 30VDC @ 10A. I don't think I'll have a problem making a rectifier, but I would like to find out how to go about determining the size motor that I will need. Does anyone know what calculations are involved, or where I can go to get this information?

TIA

 

[jbartos]

Suggestion: Please, would you clarify the generator/motor output capacity of 10A and 30VDC. It makes a difference if this is a continuous load, e.g. heater; or motor with frequent starting/plugging/jogging, or some pulse load.

 

[shawnk]

It would be a continuous load.

 

[electricpete]

I've never been involved with design of a motor-generator like you mention, but I can make some guesses.

As far as the horsepower rating of the motor, simple reasoning would suggest that you should take the output power of your generator, increase it to account for losses generated within the generator, and select a motor with the corresponding shaft horsepower output.

I.E. 30a*1-vdc=300watts power output of generator. ASSUME 75% efficiency of your generator => need approx 400 watts power output of the motor which corresponds to slightly more than 1/2 horsepower (why not round up to 3/4hp for conservatism).

That was the easy part. Now you have to pick a speed (and corresponding torque) compatible with your generator. I would suppose that the way to measure that would be to hook up a 3-ohm load and somehow raise the speed of the generator until you meausred the desired 30volt 10 amp output. Record that speed (with a tach?) and select your motor accordingly.