# Selecting the right PMDC motor and battery

I'm looking at generating electrical energy from a mechanical input being used to rotate the shaft of a Permanent magnet DC motor. However, i'm a bit stuck with selecting the right motor and battery to store the energy in(and everything else needed between)

My average mechanical input to the motor will have a power of approximately 200W, and the shaft RPM would be in the range of 500-1500. However, the RPM will be prone to fluctuation within this range (It's an ergometer so RPM and Torque will vary throughout each individual stroke)

I plan to store the produced electrical power in a battery. However, i'm not very clued up at all with the electronics side of things.

My first question is: Is it best to use a Permanent magnet DC motor as a generator, or an Alternator? I'm not quite clear which is best in this situation and why. Since I will be storing the energy in a battery, i have been going on the assumption that a DC motor is best?

Secondly: What other components are needed in the circuit between the motor and the battery? And what kind of efficiency can i expect from each component (Very approximate?)? IE if i put 200W in, how much can i expect to actually reach the battery?

Finally: Is it possible to charge a battery with an unstable voltage? or would this present problems and some method of controlling the voltage would be needed?

Thanks!

My first question is: Is it best to use a Permanent magnet DC motor as a generator, or an Alternator?

An automotive alternator is probably more power fuel than you need and more complex to use that you would like.

A permanent-magnet DC motor with a commutator would probably be ok. You may also want to consider a 3-phase brushless DC motor. That is essentially the same as a permanent-magnet synchronous motor. It would generate AC, but it would be quite efficient and easy to rectify to DC.

What other components are needed in the circuit between the motor and the battery?

You need a voltage converter to convert the variable DC from the generator to the battery charging voltage. You also should have a charge controller that could be combined with the voltage converter.

And what kind of efficiency can i expect from each component

Generator: 75 - 90+%

Rectifier, converter, charge controller: 80 - 90+% combined

Mechanical system: 85 - 97%

The system efficiency curve vs. speed starts at zero speed zero efficiency, then rises, rapidly at first, to maximum efficiency at or near maximum speed.

Is it possible to charge a battery with an unstable voltage?

Some variation is ok, but you will need a voltage converter and charge controller to get the voltage into acceptable limits and control the current.