When a phase winding of a stepper motor is switched on, ideally the current in the winding should instantly reach the full value (hence providing the full magnetic field instantly). Similarly, when a phase is switched off, its current should become zero immediately. It follows that the ideal shape of phase current history is a rectangular pulse sequence, as shown in Figure P7.5. In actual motors, however, the current curves deviate from the ideal rectangular shape, primarily because of the magnetic induction in the phase windings. Using sketches, indicate how the phase current waveform would deviate from this ideal shape under the following conditions:
(a) Very slow stepping
(b) Very fast stepping at a constant stepping rate
(c) Very fast stepping at a variable (transient) stepping rate
A stepper motor has a phase inductance of 10 mH and a phase resistance of 5 Ω. What is the electrical time constant of each-phase in a stepper motor? Estimate the stepping rate below which magnetic induction effects can be neglected so that the phase current waveform is almost a rectangular pulse sequence
Figure P7.5
