I am working on an initiative for a new product. The product generates a rotational output force from:

  • Input force of 14.73 NM
  • Input speed 120 rpm
  • Gear ratios from 0.5 - 2

The out rotation is intended to produce electricity so therefore it will need to go into a dynamo, or alternator of some sort to generate the maximum KW output based on the input force and speed.

Can someone help me work out the following.

  • What force does the dynamo/alternator need to rotate?
  • Any advice on which type of Dynamo, alternator, generator will be best
  • What KW output can be expected given the input force and speed?

I look forward to any assistance anyone can offer.

Please excuse me but I am a mechanical person and my knowledge of electronics is very limited.

  • 2
    \$\begingroup\$ power = torque * angular velocity maximum power: 14.73 Nm * 120 rpm * 2 * pi / 60 = 185 W = 0.185 kW \$\endgroup\$ Jun 22, 2022 at 11:14
  • 1
    \$\begingroup\$ 120 rpm is very slow for an alternator. You may need a gear ratio of more than 2, may be 5 or more. Don't expect 100 % efficiency for alternator with gear, may be 50 to 60 %. \$\endgroup\$
    – Uwe
    Jun 22, 2022 at 11:21
  • 1
    \$\begingroup\$ If your product is a radio or similar then have a look at the wind-up radios or torches as design solutions. \$\endgroup\$
    – Solar Mike
    Jun 22, 2022 at 11:39
  • 1
    \$\begingroup\$ Further reading machinedesign.com/motors-drives/article/21168640/… \$\endgroup\$
    – bobflux
    Jun 22, 2022 at 11:45
  • 1
    \$\begingroup\$ Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. \$\endgroup\$
    – Community Bot
    Jun 22, 2022 at 11:57

1 Answer 1


If your product generates a rotational force, then as a mechanical person, you already know much of what you need to know.

Angular speed of 120 rpm = 2 turns/second = 4pi radians/s
Torque of 14.73 Nm (very precise?)
Gives you a mechanical power of 14.73 x 4pi = 185 watts

Most small electrical machines would want to be run far faster than 120 rpm for best efficiency, 1000 to 3000 rpm is quite common at this power level. You may find that a BLDC motor designed for low speed high torque could be a reasonable match for this speed. Power will be lost in any gearing used to increase the speed up to what is ideal.

With 185 watts of input power, you would be lucky to get an output exceeding 100 watts. Small machines are not as efficient as large ones, it's just the way physics scales.


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