# Motor Resistance as Generator

Discussion in 'Sensors and Actuators' started by MadMechanic, Sep 11, 2012.

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Aug 28, 2012
I have another question about motors as generators.

If I measure the resistance across the leads of a motor and they read 10 ohm, and the motor is labeled 9v...this would mean the motor at 9V = 0.9A Draw @ 8.1W.

If I am using the same motor as a generator to output the same in reverse, supplying a circuit of 6.8 ohm resistance 5.1V. 750Ma at 3.82W, The generator should be able to supply this circuit without burning out correct?

Is measuring the resistance across the leads of a motor an accurate way of calculating this?

and also is it a general rule of thumb that the circuit does not exceed the resistance rating of the motor? Thanks

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
The DC resistance of a motor will determine the stall current (i.e. the maximum current at a particular voltage).

When the motor is turning, it will always draw less current, maybe a little, usually a lot less. As you load the motor, the current rises (and the speed reduces) until you get to the stall current (and the motor is not turning)

Does that help?

3. ### duke37

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Jan 9, 2011
If it is a motor with carbon brushes,the brush/commutator resistance is not constant and is higher when stationary. This effect will depend on brush composition which is sometimes loaded with metals.

The heating in the motor will depend on the current and the speed (which will cool it). Do not put a high current through a stationary motor, the commutator can be damaged.