What should the resistance of a motor or generator commutator be to ground with the brushes raised?

When testing the resistance of a motor or generator commutator to ground with the brushes raised, the correct expectation is for the resistance to be infinite. This is due to the fact that in a properly functioning electrical machine, the commutator should be electrically isolated from the ground when the brushes are lifted.

An infinite resistance indicates that there is no electrical path between the commutator and the ground, which is the desired condition to prevent any leakage currents that could lead to electrical faults or operational issues. It also ensures that the machine is safe for maintenance and testing, as there should be no potential for accidental shock or damage to the apparatus.

In practice, if the resistance were to measure as zero, low, or even high, it could suggest a fault such as insulation failure, which poses serious risks to safety and functionality. Thus, achieving an infinite resistance reading confirms that the electrical integrity is intact and that there is an appropriate isolation from ground potential.