The stepper motor is a special type of synchronous motor which is designed to rotate through a specific angle (called a step) for each electrical pulse received by its control unit. Typical step sizes are 7.5°, 15° or larger. The stepper motor is used in digitally controlled position control systems in open loop mode. The input command is in the form of a train of pulses to turn a shaft through a specified angle.
There are two advantages in using stepper motors. One is their compatibility with digital systems and secondly no sensors are needed for position and speed sensing as these are directly obtained by counting input pulses and periodic counting if speed information is needed. Stepper motors have a wide range of applications; paper feed motors in typewriters and printers, positioning of print heads, pens in XY-plotters, recording heads in computer disk drives and in positioning of work tables and tools in numerically controlled machining equipment. The range of applications of these motors is increasing as these motors are becoming available in larger power ratings and with reducing cost.
Elementary operation of a four-phase stepper motor with a two-pole rotor can be illustrated through the diagram of Fig. 10.32. Let us assume that the rotor is permanent magnet excited.
Such a rotor aligns with the axis of the stator field with torque being proportional to the sin θ, θ being the angle of displacement between the rotor axis and stator field axis. The torque-angle characteristics is drawn in Fig. 10.33(a) with phase a excited and also with phase b excited. It is easily observed that the stable position of the rotor corresponds to that angle at which the torque is zero and is positive for smaller angles and negative for larger angles. Thus with phase a excited, the stable (or locked) position is θ = 0° but not θ = 180° (unstable) and the torque has a maximum positive value at θ = — 90°. It is therefore easily concluded that each excitation pattern of phases corresponds to a unique position of the rotor. Therefore the excitation sequence a, b, c, d, a … causes the rotor to move in positive sequence in steps of 90°.
With rotor in position θ = 0 and a and b both excited the rotor will move to 45°, which is a stable position (net torque due to a and b zero and torque-angle slope negative). So excitation sequence a, a + b, b, b + c, c … make the rotor to move forward in steps of 45°. Patterns for phase winding excitations can be easily visualized for steps of 22.5°, 11.25°, and smaller per pulse input to the circuit. Another feature of a PM stepper motor is that, when excited, it seeks a preferred position which offers advantage in certain applications.
Consider now that the rotor (projecting pole) is made of just ferromagnetic material (no permanent magnet). The device now behaves as a variable-reluctance motor. The ferromagnetic rotor seeks the position which presents minimum reluctance to the stator field, i.e. the rotor axis aligns itself to the stator field axis. In Fig. 10.32 with phase a excited, this happens at θ= 0° as well as θ = 180° in which positions the torque on the rotor is zero. At θ = 90° the rotor presents infinite reluctance to the phase ‘a’ axis and so the torque has also a zero there. Thus the rotor torque is a function of sin 2θ as drawn in Fig. 10.33(b). For a reductance stepper motor there are two possible stable positions for a given excitation or set of excitations.
Having illustrated the operation of an elementary stepper motor and its two types, we shall now consider some further details.
Variable-reluctance Stepper Motors
A variable-reluctance stepper motor consists of a single or several stacks of stators and rotors-stators have a common frame and rotors have a common shaft as shown in the longitudinal cross-sectional view of Fig. 10.34 for a 3-stack motor. Both stators and rotors have toothed structure as shown in the end view of Fig. 10.35.
The stator and rotor teeth are of same number and size and therefore can be aligned as shown in this figure. The stators are pulse excited, while the rotors are unexcited.
Consider a particular stator and rotor set shown in the developed diagram of Fig. 10.36. As the stator is excited, the rotor is pulled into the nearest minimum reluctance position; the position where stator and rotor teeth are aligned. The static torque acting on the rotor is a function of the angular misalignment θ. There are two positions of zero torque: θ = 0, rotor and stator teeth aligned and θ= 360°/(2 x 7) = 180°/T (T= number of rotor teeth), rotor teeth aligned with stator slots. The shape of the static torque-angle curve of one stack of a stepper motor is shown in Fig. 10.37. It is nearly sinusoidal. Teeth aligned position (θ = 0) is a stable position, i.e., slight disturbance from this position in either direction brings the rotor back to it. Tooth-slot aligned position (θ= 180°/7) is unstable i.e, slight disturbance from this position in either direction makes the rotor move away from it. The rotor thus locks into stator in position θ = 0 (or multiple of 360°/7). The dynamic torque-speed characteristic will differ from this due to speed emf induced in stator coils.
While the teeth on all the rotors are perfectly aligned, stator teeth of various stacks differ by an angular displacement of
where n = number of stacks.
Figure 10.38 shows the developed diagram of a 3-stack stepper motor with rotor in such a position that stack c rotor teeth are aligned with its stator. Here
In a multiple stack rotor, number of phases equals number of stacks. If phase ‘a’ stator is pulse excited, the rotor will move by 10° in the direction indicated. If instead phase b is excited, the rotor will move by 10° opposite to the direction indicated. Pulse train with sequence abcab will make the rotor go through incremental motion in indicated direction, while sequence bacba will make it move to opposite direction. Directional control is only possible with three or more phases.