# Speed of a Cumulative Compound DC Motor

I came across an equation for the Speed ω in Radians/seconds of a Cumulative Compound DC motor.

ω= K(Vt-IA[RA+RF)/($\Phi$P+$\Phi$S)

• How would we calculate K in this equation?
• Shouldn't the Terminal Voltage Vt be replaced by Armature Voltage Ea?
• I came across a lot of varying torque speed curves. WIll the curve be exponentially decaying or linear?