- A stepper motor consists of a slotted stator having multi-pole, multi-phase winding and a rotor structure carrying no winding. They typically use three and four phase windings, the number of poles depends upon the required angular change per input pulse.
- The rotors may be of the permanent magnet or variable reluctance type,
- Stepper motors operate with an external drive logic circuit. When a train of pulse is applied to the input of the drive circuit, the circuit supplies currents to the stator windings of the motor to make the axis of the air-gap field around in coincidence with the input pulses.
The rotor follows the axis of air-gap magnetic field by virtue of the permanent magnet torque and/or the reluctance torque, depending upon the pulse rate and load torque (including inertia effects).
- Permanent-magnet stepper motor:
- Fig. 28 shows the phases or stacks of a 2-phase, 4-pole permanent-magnet stepper motor.
- The rotor is made of ferrite or rare-earth material which is permanently magnetised.
- The stator stack of phase II is staggered from that of phase ‘I’ by an angle of 90°.
- When the phase’I’ is excited, the rotor is aligned as shown in Fig. 28 (i). If now the phase ‘II’ is also excited, the effective stator poles shift anti-clockwise by 22.5° [Fig, 28 (ii)] causing the rotor to move accordingly. Now, keeping the phase ‘II’ still energised, if the phase ‘I’ is now de-energised, the rotor will move another step of 22,5°. The reversal of phase ‘I’ winding current will produce a further forward movement of 22.5°, and so on. It can be a easily observed/visualised as to how the direction of movement can be reversed.
- Each phase is provided with double coils to simplify the switching arrangement (which is electronically accomplished).
2. Variable-reluctance stepper motor:
- A variable-reluctance stepper motor has no permanent magnet on the rotor and the rotor employed is a ferro-magnetic multi-toothed one.
- The large differences in magnetic reluctances that exist between the direct and quadrature axes develop the torque. The stationary field developed by the direct current in some stator coils tends to develop a torque which causes the rotor to move to the position where the reluctance of the flux path is minimum.
Stepping angle, irrespective of the type of stepper motor is given as :
3. Hybrid stepper motor:
- This is infact in permanent-magnet stepper motor with constructional features of toothed and stacked rotor adopted from the variable-reluctance motor.
- The stator has only one set of winding-excited poles which interact with the two rotor stacks.
- The permanent magnet is placed axially along the rotor in the form of an annular cylinder over the motor shaft.
- The stacks at each end of the rotor are toothed. So all the teeth on the stack at one end of the rotor acquire the same polarity while the teeth of the stack at the other end of the rotor acquire the opposite polarity. The two sets of the teeth are displaced from each other by one half of the tooth pitch (also called pole pitch).
- The primary advantage of the hybrid motor is that if stator excitation is removed, the rotor continues to remain locked into the same position, as before removal of excitation.
This is due to the reason that the rotor is prevented to move in either direction by torque because of the permanent magnet excitation.
- Typical step angles for stepper motors are 15°, 7.5°, 2° and 0.72°. The choice of the angle depends upon the angular resolution required for application.