Special machines:Multi-stack VR Stepper Motor

Multi-stack VR Stepper Motor

So, far, we have discussed single-stack VR motors though multi-stack motors are also available which provide smaller step angles. The multi-stack motor is divided along its axial length into a number of magnetically-isolated sections or stacks which can be excited by a separate winding or phase. Both stator and rotor have the same number of poles. The stators have a common frame while rotors have a common shaft as shown in Fig. 39.5 (a) which represents a three-stack VR motor. The teeth of all the rotors are perfectly aligned with respect to themselves but the stator teeth of various stacks have a progressive angular displacement as shown in the developed diagram of Fig. 39.5 (b) for phase excitation.

Three-stack motors are most common although motors with upto seven stacks and phases are available. They have step angles in the range of 2º to 15º. For example, in a six-stack VR motor having 20 rotor teeth, the step angle b = 360º / 6 ´ 20 = 3º.

Permanent-Magnet Stepping Motor

(a) Construction. Its stator construction is similar to that of the single-stack VR motor discussed above but the rotor is made of a permanent-magnet material like magnetically ‘hard’ ferrite. As shown in the Fig. 39.6 (a), the stator has projecting poles but the rotor is cylindrical and hasradially magnetized permanent magnets. The operating principle of such a motor can be understood with the help of Fig. 39.6 (a) where the rotor has two poles and the stator has four poles. Since two stator poles are energized by one winding, the motor has two windings or phases marked A and B. The step angle of this motor b = 360º / mNr = 360º / 2 ´ 2 = 90º or b = (4 - 2) ´ 360º / 2 ´ 4 = 90º.

(b) Working. When a particular stator phase is energized, the rotor magnetic poles move into alignment with the excited stator poles. The stator windings A and B can be excited with either polarity current (A + refers to positive current I in the phase A and A – to negative current i ).

Fig.39.6 (a) shows the condition when phase A is excited with positive current iA+. Here, q = 0º.

If excitation is now switched to phase B as in Fig. 39.6 (b), the rotor rotates by a full step of 90º in the clockwise direction. Next, when phase A is excited with negative current iA -, the rotor turns through another 90º in CW direction as shown in Fig. 39.6 (c). Similarly, excitation of phase B with iB- further turns the rotor through another 90º in the same direction as shown in Fig. 39.6 (d). After this, excitation of phase A with iA+ makes the rotor turn through one complete revolution of 360º.

Truth tables for three possible current sequences for producing clockwise rotation are given in Fig. 39.7. Table No.1 applies when only one phase is energized at a time in 1-phase-ON mode giving step size of 90º. Table No.2 represents 2-phase-ON mode when two phases are energised simultaneously. The resulting steps are of the same size but the effective rotor pole positions are midway between the two adjacent full-step positions. Table No.3 represents half-stepping when 1-phase-ON and 2-phase- ON modes are used alternately. In this case, the step size becomes half of the normal step or one- fourth of the pole-pitch (i.e. 90º / 2 = 45º or 180º / 4 = 45º). Microstepping can also be employed which will give further reduced step sizes thereby increasing the resolution.

(c) Advantages and Disadvantages. Since the permanent magnets of the motor do not require external exciting current, it has a low power requirement but possesses a high detent torque as compared to a VR stepper motor. This motor has higher inertia and hence slower acceleration. However, it produces more torque per ampere stator current than a VR motor. Since it is difficult to manufacture a small permanent-magnet rotor with large number of poles, the step size in such motors is relatively large ranging from 30º to 90º. However, recently disc rotors have been manufactured which are magnetized axially to give a small step size and low inertia.

Example 39.3. A single-stack, 3-phase VR motor has a step angle of 15º. Find the number of its rotor and stator poles.