Switching to negative ground question

[Boobtube]

I am going switch my Series IV to negative ground and have looked at the list of things to change and read that there is no change needed to the starter motor. This may be a stupid question but doesn't a DC motor change direction when changing polarity? While I'm relieved that no change is needed for the starter, I have to ask, how is that so?

 

[Barry]

Boobtube,

Permanent magnet DC motors typically change direction of rotation when the electrical polarity is changed because the armature fields are reversed, but the permanent magnet fields stay the same.

Automotive starter motors are series wound DC motors and changing the electrical polarity reverses the magnetic polarity for BOTH the armature fields and the stator fields.

The phasing of the commutator is what determines the direction of rotation for series wound DC motors.

HTH.

 

[Tom H]

If it were a permanent magnet DC motor it would need a change. But starter motors (and wiper motors and blower motors) all use a magnetic winding in the stator (or case) as well as in the rotating armature. So when you change polarity you change BOTH the stator armature and all still goes the same direction. To change direction you need to change the polarity of just ONE of either rotor or stator.

Tom

 

[Boobtube]

Ah, I see now. Thanks for the answer.