Max motor current

I've been working with some motors for the past few days and am getting strange readings with the maximum currents being drawn. It says on the motor that it is supposed to draw "7A continuous". Obviously there is an initial surge, and by my understand this maximum current draw can be calculated by I = V/R where R is simply the resistance of the motor measured with a multimeter.

I would have expected the resistance of the motor to increase as the motor gets hotter (when it has been used for a while), meaning that the max current drawn would decrease. However it seems the opposite is happening with my motors? Measured resistances range from 2.5 ohms to 0.7 ohms. Any thoughts?

First, what kind of motors are they?

Nope, the initial peak surge current is determined by the DC resistance of the windings; Once the motor begins to turn, the current decreases because the motor is generating a "back EMF" which is in opposition to (but smaller than) the applied voltage. The faster the motor runs, the bigger the back EMF, and the lower the current. If you apply a load (torque) to a running motor, the current increases as the motor shaft begins to slow down (because of the reduction in the generated back EMF).

chris414 said:

I would have expected the resistance of the motor to increase as the motor gets hotter (when it has been used for a while), meaning that the max current drawn would decrease. However it seems the opposite is happening with my motors? Measured resistances range from 2.5 ohms to 0.7 ohms. Any thoughts?

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Yes, the resistance of copper increases with temperature so I would expect the dc resistance of a hot motor to be greater than a cold motor.

However, since this increases the motor losses, I would expect the running current for a hot motor to be slightly higher for the same mechanical load.