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# What force is required to rotate a dynamo or alternator

Question – What force is required to rotate a dynamo or alternator – Asked today Modified today Viewed 61 times
https://electronics.stackexchange.com/questions/624580/what-force-is-required-to-rotate-a-dynamo-or-alternator

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.

power = torque * angular velocity maximum power: 14.73 Nm * 120 rpm * 2 * pi / 60 = 185 W = 0.185 kW –
Roland Mieslinger
15 hours ago

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 %. –
Uwe
15 hours ago

If your product is a radio or similar then have a look at the wind-up radios or torches as design solutions. –
Solar Mike
15 hours ago

bobflux
14 hours ago

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. –
Community Bot
14 hours ago

Sorted by:

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.

Neil_UK