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# How much power is generated by a motor when we rotate it? Answered

For a special project, I need a dynamo but I can not found it,because I belong to rural area.So I want to use dc motors instead of a dynamo,but how much efficient is this I don not know.Please tell me how much power is generated by a 500 watt motor.How to find it?

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## Discussions

1 horse is 746 watts or 550 footpounds per second.

So a 500 watt motor is 0.67 horse or 368.5 footpounds per second.

Definition of 746 watts is equal to 1 HP (Horse Power).

Somebody measured an English horse to come up with that comparison.

Maybe if you improve the efficiency you could get a better number ;-o

It would be nice if he was using a normal generator, then you could just look up the efficiency.

He want to know the efficiency of a 500 watt motor when used as a dynamo.

Impossible to tell without doing the experiments.

Little hobby motors can produce a little bit of power, maybe a few watts when you rotate the shaft. How much power is generated depends on a number of things.

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1) To add to what Steve mentioned, if the motor/generator is not connected to a load, or if the windings are shorted, then you are not going to generate any power.

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2) Like others mentioned, If the 500W motor is a nominal efficiency of 80%, then the mechanical output power under an optimal load is going to be about 400W. If you input 400W of mechanical energy, then the output will be about 320W.

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3) The coils inside the generator will generate a voltage that is proportional to how fast you are turning the rotor. The faster you rotate the rotor, the more EMF is generated. However because of the internal resistance, this voltage will be less due to ohmic losses.

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4) With no load on the generator, the rotor should turn relatively freely. The EMF is still generated but it is supplying no current. Once you attach a load to the generator expect it to get harder to rotate the rotor, as more torque is required to maintain speed. If the windings on the motor are dead shorted, it should become very stiff and hard to turn the shaft. This is the basis for electric regenerative braking in cars. Except that we use energy storing devices to load the generator down so that we can turn some of that kinetic energy of a moving car back into electricity stored in something like a capacitor or battery.

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Power is always going to be derived either from the product of voltage and current, or from torque and rotational speed.

No power is generated by a motor JUST by turning it, it has to be connected to a load. Dynamos are not as efficient as alternators. Cars are all fitted with alternators.

The motor needs 500W to run with nominal load at full speed.
If the motor is suited to be used as a generator and you would reach the nominal speed you would get about 400W on power out that you can use - if you keep the speed up.
A dynamo is usually for voltages between 6 and 12V and provides just enough to power some standard light bulbs on your bike.
So you might want to use a smaller motor ;)

It depends how fast you can turn it.