Why I have to turn the rotor slowly to find the resistance value?

Question

in a dc brushed motor, I have to turn the rotor slowly in order to find the lowest resistance value? why is that?

anyway, I find the resistence value by connecting the motor positive and negative terminal to the digital multimeter

Answer 1

A dc brushed motor does not have a rotor. You can do a resistance check of the armature by measuring opposite segments with the brushes withdrawn and turning the armature to the next set. All the readings should be similar. showing that no windings are short or open.

Spinning the armature with brushes connected and the meter attached may damage your meter as the motor becomes a generator and serves no useful reading.

Answer 2

turning the rotor slowly insures that the brushes are making good contact. Slowly so that any DC voltage generated will be low and not change the reading.

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Answer 3

Likely, you need to find average resistance in case it varies with rotation.  That might work better with a needle-type voltmeter that has inertia than with a digital meter that gives instantaneous values.