Motor rotation direction in DC

[Femme_physics]

Kind of a silly question, maybe, but just confirming... If I want the motor at S1 to rotate to one side, and at S2 to rotate to the other side... I just do that?

http://img651.imageshack.us/img651/4167/motorfp.jpg

 

[NascentOxygen]

Yes, that will reverse the direction of rotor current.

 

[Femme_physics]

Ahh...too easy! Thanks.

 

[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.

 

[NascentOxygen]

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.

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?

 

[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.

 

[Ouabache]

Kind of a silly question, maybe, but just confirming... If I want the motor at S1 to rotate to one side, and at S2 to rotate to the other side... I just do that?

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.

 

[NascentOxygen]

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't— 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

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