I am working on a project to harvest electrical power from a rowing machine by connecting a permanent magnet DC motor to the flywheel of the rower. However, my knowledge of electrical motors, and especially using them as generators to convert mechanical energy to electrical energy, is limited at best, non-existent at worst.
I'm trying to figure out how I will be able to vary the level of resistance felt by the user. My current plan is to attach a couple of different sized sprockets to the flywheel of the rowing machine and the shaft of the motor and make use ratios between their diameters, similar to what you'd find on a bicycle with gears. I am aware that by doing this, the shaft of the motor will be spun at different speeds for a given force applied to the rowing machine.
My question is this: By spinning the motor at different speeds as planned, what will the affect be on the efficiency of the motor? For example, by operating below the rated wattage, or very close to it, would I expect a lower/higher efficiency, i.e., less electrical energy out per unit mechanical energy that goes in? If there are any resources i could be pointed to that could explain this to me id be MASSIVELY grateful.
Also, any advice on suitable rating for the motor in question would be a huge help. I'm looking at an average sustained mechanical power input to the rowing machine of approximately 80 watts, with maximum as high as 700 watts (only for very short instants during the beginning of each rowing cycle)
Apologies this is very long and painfully under/mis-informed, very new to this.