Dual Speed Motors: Understanding No-Load Speed of DC Units

[Danger]

While I've worked with an awful lot of motors, including making simple ones and repairing several others, I don't know the technical aspect of what governs the no-load speed of a DC unit. Specifically, I have 3 really nice 12VDC 1/2hp wheelchair drive motors with gear trains. They're extremely handy for lots of things. What sets them apart from any others that I've worked with is that there are 2 sets of field windings, with 6 wires coming out of the case. They're meant to be operated by a joystick that alters the connections between the wires. One connection set-up wires the windings in parallel, and produces a lot of torque. The other wires them in series, which doubles the speed and halves the torque. (The rest are for doing the same things in reverse.)
I don't know the principle behind this effect. Can someone, preferably without invoking math, set me straight?

 

[anorlunda]

ping @jim hardy

 

[jim hardy]

In DC machines, field current sets the magnetic flux in which the armature rotates.

At given armature voltage, probably full battery voltage,
Speed is proportional to inverse of flux
and torque is proportional to product of flux and armature current.
More flux makes the motor run slower.
but gives more torque for the same armature current.

So it's a safe bet that with the fields connected in parallel each field makes its full rated flux and you get not much speed but ample torque. .
When you connect the fields in series, flux will decrease because each field sees only half battery voltage
so speed goes up (it's inversely proportional to flux)
and torque goes down (proportional to flux)

That's a word picture and subject to all the vagaries of word pictures.

You should read a DC motor tutorial and develop vocabulary
https://electrical-engineering-portal.com/basics-of-dc-motors-for-electrical-engineers-beginners

for starters

Counter EMF is the internally generated voltage that results from the armature spinning in the field flux at any given RPM with absolutely zero mechanical load, ie zero armature current .
We measure it by spinning the motor and measuring open circuit voltage with known field
Then we know the value of K_onstant for that particular motor.
In Imperial units

Counter-EMF = K_onstant X Flux X RPM
Torque_ft-lbs = 7.04 X sameK_onstant X Flux X I_{armature current}

in SI units you use Newton-Meters and radians per second and the 7.04 disappearsWhen you grasp those two simple formulas you are well on your way to understanding DC motors.
Armature Reaction is a complicating evil that you deal with later when you get more seriously into the subject.

For permanent magnet motors you have a benefit - flux is constant and there's no field winding.
So it's real easy to find K_onstant - just spin it with your electric drill , measure volts and speed.

old jim

 

[jim hardy]

another tutorial

http://lancet.mit.edu/motors/motors3.html