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TomEV 02-27-2008 11:22 PM

Series Motor - Speed vs Torque
I came across a statement regarding the correct connection of the field and armature posts on a series DC motor. One thing that I found interesting was this:

"where a series field (S1 and S2) is provided, the polarity of this winding must always be connected for the same polarity of the motor armature"

"Apply + and - to the same number post. + to A1 and S1; - to A2 and S2.
It's A1, A2+S1, S2 for torque, but A1, A2+S2, S1 for speed"

So, from a line drawing perspective, it would look like this:

+ A1::A2>>S1::S2 - for torque (where :: is the motor internal wiring, and >> is either a jumper wire or a Forward/Reverse contactor)

+A1::A2>>S2::S1- for speed.

Turns out that my motor was wired this way -

+S2::S1>>A2::A1- (which would be a torque setting, forward rotation)

In order to test out the 'speed' theory, here was my plan. First step is to change to 'speed'

+S1::S2>>A2::A1- (ends same - speed) But this would reverse the motor rotation, so I reversed the polarity at the ends:

-S1::S2>>A2::A1+ (ends still same - speed, opposite polarity = forward)

Oddly enough, the car went BACKWARDS in this configuration. :confused: I checked it several times to make sure I wired it according to the diagram. I did.

Well, no harm, no foul - I just switched it to 'reverse' and drove home. Same acceleration, same top speed In 'reverse' it swaps S1 and S2, so:

-S2::S1>>A2::A1+ (torque mode... ends opposite.) Essentially the way it was.

Back at home, I changed the polarity on the ends to make it 'go' the other direction -


Again, the car went BACKWARDS :mad:

Anyone out there have a spare series motor on the bench that can tell me if their motor turns the same direction if polarity is swapped in these two cases? I may pull out my motor to see, but if someone has one out already, it would save me some confusion...

Seems no matter what I do, this setup wants to be in torque mode forward, and speed mode reverse (the opposite of what I want to test).

The saga continues!

TomEV 03-01-2008 10:45 PM

Turns out that the direction series motors rotate is not dependent on the polarity of the 'ends'. The only way to change direction of the motor is to change polarity of either the field OR armature (but not both at the same time).

Ryland 03-02-2008 12:21 AM

I'm not sure if I fallowed all of this correctly, are you saying that the citicar motor can be hooked up differntly for higher speeds then is stock? I don't currently have batteries, or a body, or brakes on mine... so I'm a few steps behind you.

TomEV 03-02-2008 12:27 PM

In short, the -theory- is that there may be a difference, but we will not be able to test this theory in the Citicar because in order to make the car move forward, the armature and stator polarities are equal, which (again, in theory) is a torque mode. It can't be changed because our motors are connected directly to the gearbox.

We could flip the differential, but then the speedometer wouldn't work, and the rear brakes wouldn't be as effective.

Good thing about this is that my Citicar will do 0-25 in about six seconds. Bad thing is it takes twice that time to get from zero to 30, and 'tops out' at about 33 (this time of year, with 50f batteries). During the Summer, it goes a few MPH faster because the batteries are warmer.

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