Torque–Displacement Characteristic of a Stepping Motor
When the teeth of one energized phase of stator with the teeth of the rotor are aligned, the output torque is zero. It means that the rotor does not show any resistance to the applied output torque. When the rotor is deflected by a small amount, an attracting pulling force develops between the teeth of the rotor and stator which shows itself as a torque on the rotor. The behavior can be simulated by a sinusoidal output torque. This is shown in Fig. 5.6, when two phases are energized.
The relationship between the output torque and rotor displacement can be ap- proximated as
The second term in Eq. (5.4) is negative because as θ increases over one step of stepping motor, the torque reduces. When θ moves one step, the torque becomes zero. The torque constant ( Kt) and the step angle are given by manufacturers. There- fore, Kθ can easily be evaluated.