# Homework Help: Motor rotation direction in DC

1. Apr 24, 2012

### Femme_physics

Last edited by a moderator: May 5, 2017
2. Apr 24, 2012

### Staff: Mentor

Yes, that will reverse the direction of rotor current.

3. Apr 27, 2012

### Femme_physics

Ahh...too easy! Thanks.

4. Apr 27, 2012

### OldEngr63

DC machines have some really nice characteristics, this being one of them. Easy speed control is another. They have some no-so-nice characteristics, like carbon brushes that wear and commutators that spark, but it is all a trade off.

5. Apr 27, 2012

### Staff: Mentor

The one I like most of all is the BIG starting torque!

OldEngr63, you might be able to answer something that I glossed over in my brief answer here. Notice I said reverse the rotor current. With the typical DC motor having field and rotor in series, I'm wondering if we reverse the polarity of both together will the rotor spin not reverse?

6. Apr 27, 2012

### OldEngr63

To change the direction of rotation, we need to change the direction of the rotor current while holding the field direction constant. If the rotor and field are in series, then reversing the external terminals should have no effect on the direction of rotation.

7. Apr 27, 2012

### Ouabache

Practical example: I have a 1/2"-drive, electric drill. The reversing switch recently broke. I wired an external DPDT switch in a small box, that accomplishes exactly what you have indicated.
Yay it works again. Wooo hoo.

8. Apr 28, 2012

### Staff: Mentor

On my first read of this question I wondered whether it originated with a trick question. We are all told how reversing the voltage to a DC motor reverses its rotation. Except it doesn'tâ€” not unless the motor has permanent magnets for the field. Otherwise, one must reverse the rotor current, or the field current, but not both together. https://www.physicsforums.com/images/icons/icon6.gif [Broken]

Pondering this further: I guess it would be feasible, especially with the lower current of a shunt field, to include a bridge rectifier inside the motor to maintain the direction of the field current even when the motor's terminal voltage is reversed. So then we really would have a DC motor whose rotation reversed when the terminal voltage was reversed! https://www.physicsforums.com/images/icons/icon10.gif [Broken]

Last edited by a moderator: May 5, 2017