Stepper motors or Step motor are different from all other types of electrical drives in the sense that they operate on discrete control pulses received and rotate in discrete steps. The discrete nature of the operation of a step motor makes it suitable for directly interfacing with a computer and direct computer control. We would discuss about the construction and principle of operations of different types of step motors or stepper motor and elaborate on the drive schemes used.
These motors are widely employed in industrial control, specifically for CNC machines, where open-loop control in discrete steps are acceptable. Electrical a.c and d.c drives are analog in nature and rotate continuously depending on the magnitude and polarity of the control signal received. These motors can also be adapted for continuous rotation.
Step motors are normally of two types
- Permanent magnet
- Variable reluctance type
1. Permanent magnet stepper Motor
The principle of stepper motor can be understood from the basic schematic arrangement of a small permanent magnet step motor is shown in Fig
This type of motor is called a two-phase two-pole permanent magnet stepper motor and the number of windings being two each split into two identical halves and the rotor is a permanent magnet with two poles.
Winding A is split into two halves A1 and A2. They are excited by constant d.c. voltage V and the direction of current through A1 and A2 can be set by switching of four switches Q1 Q2 Q 3 and Q4 . The directions of the currents and the corresponding polarities of the induced magnets are shown.
Let Winding A be energized and the induced magnetic poles denote the switching condition as S1=1. The other winding B is not energized. As a result, the moving permanent magnet will align itself along the axis of the stator poles and in the next step, both the windings A and B are excited simultaneously, and the polarities of the stator poles are as shown in Fig We shall denote S2=1, for this switching arrangement for winding B.
The rotor magnet will now rotate by an angle of 45o and align itself with the resultant magnetic field produced. In the next step, if we now make S1=0 the rotor will rotate further clockwise by 45o and align itself along winding B.In this way if we keep on changing the switching sequence, the rotor will keep on rotating by 45o in each step in the clockwise direction.
The direction of rotation can be reversed by changing the order of the switching sequence. It is also possible to have an excitation arrangement where each phase is excited one at a time and there is no overlapping where both the phases are energised simultaneously, though it is not possible for this configuration.
2. Variable Reluctance Stepper Motor
Variable reluctance stepper motors do not require reversing of current through the coils but at the same time do not have any holding torque as compared to permanent magnet step motors and their step angles are also much smaller.
The rotor is a cylindrical soft iron core with projected teeth. When a particular stator coil is excited, the rotor aligns itself such that one pair of teeth is along the energized stator coil, at the minimum reluctance path. The schematic arrangement of a three-phase virtual reluctance motor with 12 stator poles (teeth) and eight rotor teeth.
When phase-1 is energized, the rotor will align itself and in the next step, if phase-1 is switched off and phase-2 is switched on, the rotor will rotate in a counter-clockwise direction by an angle of 15o
The stator pole pitch
The rotor pole pitch
The full step angle
Switching sequence for rotation in the counter clockwise direction with half stepping would be 1-(1,2)-2-(2,3)-3-(3,1)-1….
Multi-stack stators are also used for achieving a smaller step angle, where there are several stacks of stator windings skewed from each other by a certain angle. It has been already mentioned that the virtual reluctance motors do not have any holding torque. It is natural because, when the stator coils are de-energized there is no magnetic force present and the rotor is free.
Hybrid step motors are improved versions of single stack virtual reluctance stepper motors, where the basic constructions are modified slightly in order to achieve holding torque.