Stepper Motor and its Types

STEPPER MOTOR

These motors, also called stepping motors or step motors, because they rotate through a fixed angular step in response to each input current pulse from its controller. The stepper motors are able to provide very precise position and (or speed control) without needing any feedback loop.

A stepper motor has salient poles with field coils. The rotor is usually salient pole but has no exciting coil. The unique feature of a stepper motor is that its shaft rotates is a series of discrete steps in response to command pulses received from an electronic control circuit. The only moving port in a stepper motor is its rotor which has no winding. Hence, it needs no commutator and brushes. This feature makes the stepper motor quite robust and reliable.

Types of Stepper Motor

Based on their construction and working, the stepper motors can be divided into the following three basic categories.

1. Variable reluctance (V_R) stepper motors

2. Permanent magnet (P_m) stepper motors

3. Hybrid stepper motors.

1. Variable reluctance (V) stepper motor

It has wound stator poles but the rotor poles are made of a ferromagnetic material. It is known as variable reluctance motor because the reluctance of the magnetic circuit formed by the stator and rotor poles varies with the angular position of the rotor.

Construction

Fig. 4.24 shows the stepper motor has six stator poles and four rotor poles. The rotor pole width is same as the stator pole width. The coils on diametrically opposite stator pole are connected in series to form one circuit or phase such that on emerging it, one pole become N‒ pole and the other one is S‒ pole. There are three phases A, B and C. Each of these phases can be energized by a direct current pulse from a drive controller circuit.

A simple full step operation of a VR stepper motor is shown in Fig. 4.25. When none of the coils is energised, the rotor is completely free to rotate. On energizing one or more coils, the rotor assumes a position thats forms a path of least reluctance with the magnetized stator poles.

Operations

The operation of one phase on mode, full step operation VR stepper motor is explained below.

The switch S₁ has been closed for energise the phase A. The nearest rotor poles are attracted by N and S poles of the stator so that the rotor takes the position as shown in Fig. 4.24. Let us mark by a star symbol the rotor pole just under the N‒ pole of the stator.

Now let us open the switch S₁ and close the switch S₂ to energies the next phase B. The rotor poles of unmarked pair are now nearest to the new position of alignment. Therefore the rotor rotates by 30° anticlockwise to align along the path of least reluctance. Again open the switch S₂ and close switch S₃ to energies the phase C. Once again, the nearest rotor poles to align are from the marked pair. These poles align themselves by rotating a further 30° anticlock wise direction. This process will be repeated by excitry the coil in sequence A‒B‒C‒A‒B..., the rotor rotates anticlock wise in step of 30°.

Now, if the phases are energised in the reverse sequence, A‒C‒B‒A‒C..., the rotor will rotate clockwise in steps of 30°. Since only one phase is energised at a time, this mode of operation is called one‒phase ON operation. The stator phase switching truth table for this operation is given in Fig. 4.25(b).