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The formulae for a DC motor with permanent magnet or constant current field , consistent with my names for them are:
Vback = k1 * w (in, say, rpm)
Torque = k2 * Iarmature
This is somewhat bogus, because the k's are not magic, but something you can (must) figure.
If the motor is running free, with little current entering, Vback = Vsupply (= 12V, say).
thus k1 = 12V / free speed rpms.
If the armature, aka rotor, is locked/stalled, then the (stall) torque is spec'd, at locked rotor amps (you could allow for field current in parallel), so k2 = stall torque / locked rotor current.
Essentially, the back voltage (usually called back emf) is linear with speed, and torque is linear with armature current, for a given field current in a given motor.
Bear in mind that this is first approximation stuff, but we probably need not go further with it for now.
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