Posted by David Sherman on Thursday, January 29, 2004 at 4:55PM :
In Reply to: reversing 12V DC elec. motor posted by David N. Lundstrom on Thursday, January 29, 2004 at 4:26PM :
There are basically two electrical parts in a normal DC motor, the "field" or "stator" and the "rotor" or "armature". Normally, the "field coil" is mounted to the frame of the motor and the rotor is on the spinning part and connects to the non-spinning world via the brushes. The direction the motor turns depends on the polarity of the magnetic field of both parts. Some small motors have a "permanent magnet" field rather than one wound with a coil of wire. In that case the "field" is always the direction it was magnetized in and you can reverse the motor simply by reversing the external electrical connections to it. Fan motors and wiper motors are often (but not always) this type. Heftier motors need a "wound field". There are different ways to connect the field and the rotor windings (series, parallel, and compound) but as far as the direction of rotation goes, you need to swap the terminals of either the field or the rotor (brushes) but not both in order to reverse the direction. If you just reverse the normal external connections, you've reversed both coils and the motor still goes the same way it did before. Winch motors are pretty much like starter motors. They are probably "series wound" in order to have the torque go up as the speed goes down so that they'll run fast when lightly loaded but will really "get down and work" when they need to. I have not tried to reverse one of these motors, but it should be relatively simple to take it apart and bring out both the field and rotor leads separately so you can connect a reversing switch. The switch will need to handle a lot of current, so you'll probably want relays mounted close to the motor, or something really beefy like an old-fashioned knife switch.
Follow Ups: