DC Motor Rotation

To change the rotation, you need to reverse the magnetic reaction between the fields and the armature i.e. you need to reverse either the direction of current flow in either the field coils or the armature - but not both. By simply changing the incoming supply polarity you are changing the direction of current flow in both the fields and the armature and the motor will rotate in the same direction as before. This applies whether it is a shunt, series or compound connected DC motor. The only time that reversing the incoming polarity would change the direction of rotation is if the motor had permanent magnets as its field or armature magnetic source, but these are only found in relatively small motors.

Well, I have a small d.c. motor and reversing the input poliity did reverse the direction or rotation.

That works in permanet magnet motors, we removed all our dc drives and motors in the industry I am in, and have great results with AC, VF Drives with quite a cost savings on our energy use.

In response to posts#1 and 2:

You are both right of course. Guest (post #1) explained it all. If simply changing the polarity of the inputs changed the rotation, the motor has permanent magnets rather than series or shunt wiring.

In the course of building an electrically driven dumping trailer for my garden tractor I had a small 12 VDC starter motor with one lead for power and the return path ("ground") was the housing. I disassembled the motor and separated the field wiring from the armature wiring (the wires to the brushes) and ran the two wires for one of them (I forget which but it doesn't matter) outside the housing. This separated the armature and field circuits and allowed me to reverse the direction at will by changing the polarity of those two wires while the original connection remained at a fixed polarity. This is exactly what post #1 means.

Depending on the internal wiring and construction of your motor, it may be easier to separate one circuit or the other. The important thing is that their relative polarity must be able to be changed in order to control rotation direction.

Take care not to damage anything, insulate your connections, pay attention to observe whether the field and armature are connected in series or parallel (compound wiring is a little more complicated because it contains both series and parallel elements but you can figure it out) so that you can maintain this same arrangement with your external switching wiring, use the proper size wire and switch or relays and you have a reversible motor.

The one possible caveat might be that with some motors, it may not run quite as well (premature brush failure, slightly less power) in the opposite direction if the brushes and commutator segments were optimized for the original direction. But for most purposes and with most common motors, this should not be an issue of much consequence.

On a historical note, some large DC motors would have their direction of rotation changed by mechanically rotating the brush holders to a different position in respect to the commutator segments, which had the effect of changing the relative polarities. This method was also used on some diesel-electric submarines to not only change direction of the motor, but to allow it to function as a generator when the diesel engine was running while also allowing for the optimal brush positions for highest efficiency and minimal arcing in all situations.

Greg G has given you just about the most perfect answer that I have ever seen on any CR4 posts. Concide and accurate. Thanks Greg!

That depends on type of Motor:

1.) Permanent Magnet small motors: just by reversing polarity of DC supply to motor.

2.) With Field & Armature wound: by reversing the Field or Armature Supply Polarity.

Note: In Armature in some motors +(ve) & -(ve) brushes differ in material. In that case also interchange the brushes.

3.) In Brush-less DC motors using Hall-effect Sensors [Device] it is a complicated job.

The simplest way is to change the polarity of its connection to the power supply.

If those electrical engineers have confused you,

simply flip the motor over or around to the other side.