Question

How often is current reversed in the loops of a simple DC electric motor?

Answer 1

Never

Explanation:

As long as the split ring commutator is in contact with the metal brush, current will always flow the same way as shown in the diagram. When the contact is broken, no current flows. The loop however still rotates and the contact is established yet again with current flowing in the same direction as before .

In this case, current always flows "into the page" on the right side of the loop. The batteries indicate the motor to be a DC motor hence there is no reverse flow of current at any given time.

If you want a reversing current to work the motor, just switch the batteries with an AC source and get rid of the split ring .

I believe the frequency of the AC supply must match the strength of the magnets. Ideally you wan the current to reverse at every half cycle of the rotating wire loop. This requires careful engineering and calculations.

Cheers.