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I am currently working on a project to harvest electrical energy from a rowing machine. I am trying to make use of gearing ratios to adjust the resistance level offered by the system, and the impact it will have on the generated power. I plan to connect the flywheel of the rowing machine to the shaft of a PMDC motor by chain or belt.

I am looking at an RPM range between approximately 400 and 800 for the flywheel. If the radius of the sprocket attached to my flywheel was lets say 0.3m, and the radius of the sprocket attached to the motor was a smaller size, say 0.1m, I would expect a greater number of RPM in the motor shaft. However, what would I expect in terms of the power generated by it, and the resistance against the mechanical force applied by the rower?

Also, is there a maximum counter-torque I would expect from a given motor, for example if the gearing ratio was adjusted so that more torque was being applied to the motor than it could provide counter-torque? I've noticed that motors are rated by their motor torque. Is the maximum counter torque equal to this value? What would happen if it was exceeded? Would no additional power be generated?

Sorry if the question is a little vague/factually inaccurate i can try to add any additional information needed. This is all very theoretical at the minute, just trying to gain some understanding.

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  • \$\begingroup\$ Usually maximum torque is at its maximum power rating so I don't think you would want to generate more power out of a motor than that. also technically any transmission or anything will be a source of loss. \$\endgroup\$
    – MadHatter
    Apr 18 '20 at 14:58
  • \$\begingroup\$ You don't want the generator to provide as much counter-torque as you have being applied by the rower. If they were equal it would be impossible to row. \$\endgroup\$
    – Finbarr
    Apr 18 '20 at 15:18
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By the principal of conservation of energy, the power output is the power input by the rower minus losses. Mechanically, there are friction losses at each bearing or pivot point, each sliding surface and each area where there is aerodynamic drag. The mechanical power produced by the rower is (force x distance)/time. The mechanical power into the generator is torque x RPM. The electrical power out of the generator is voltage x current. Generator voltage is directly proportional to RPM. Generator current is voltage divided by load resistance. Generator counter torque is proportional to current. Electrical load resistance and losses determine the counter torque felt by the rower.

If the generator torque rating is exceeded, the rated current will be exceed and the electrical losses in the generator will be greater than the generator can dissipate without overheating. The generator can operate at the same current and torque over a wide speed range, but the movement of air inside and through the housing will be reduced at low speed, so it may overheat if there is not a separately powered blower cooling it.

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