# DC Motor/Generator current flow

So I am in the midst of learning circuitry, and cannot for the life of me understand what dictates the direction of current in a DC Motor/Generator. I understand that a charge applied to the negative and positive of a DC motor will be converted to mechanical energy. Conversely, it is also easy to understand that applying mechanical energy to the shaft will result in an electrical charge available via the positive and negative leads. What i don't understand however is if you were to hook a dead battery to the leads and applied mechanical force to the shaft, would it charge the battery? And furthermore, what would prevent any accumulated charge from backflowing and turning the generator into a motor?

Back EMF and internal resistance are the main things that determine the direction and magnitude of the current flow in a DC brushed motor or generator

Back EMF comes from the speed that the motor shaft is turning, spin it faster and the back emf will have more volts.

in normal runiong the motor will reach a speed where the back emf mostly cancels the supply voltage and the difference in voltage divided by the internal resistance will get you the current through the motor (Ohm's law).

If you spin it fast enough that the back EMF exceeds the supply voltage the the motor will act as a generator pushing current into the supply.

A third thing that determines the current is the strength of the magnetic field, this sets an upper limit on the current that a generator can produce, no matter how fast you spin it.

To answer your second question nothing would stop the generator from becoming a motor, but if the spinning speed is fast enough it will be acting as a generator.

You could add a diode to prevent current from flowing the wrong direction.

A DC motor, when run as a generator, would charge a battery.

With the drive taken off, the battery would run the motor in the same direction, were it to be free to run. Otherwise, the battery current would damage it's winding.

This possibility existed in vintage cars having a 'dynamo' (DC generator). It was taken care of by a relay unit known as a 'cutout'.

• The motor would not run in the reverse direction - it would rotate in the same direction but the current flow would reverse. going into the motor rather than out of the motor (when acting as a generator). Commented Jul 10, 2020 at 8:27
• Hi Kevin, Thank you very much for pointing out the blunder. 'Back EMF' totally slipped my mind. Commented Jul 10, 2020 at 11:59