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Types of Stepper Motors:
It can be classified into several types according
to machine structure and
principle of operation as explained by Kenjo (1984). There are three types:
1. Variable Reluctance Motor (VRM)
2. Permanent Magnet Stepper Motor (PMSM)
3. Hybrid Stepper Motor (HSM)
4.1.1.1
Variable Reluctance Motor:
It consists of a soft iron multi-toothed rotor and a wound stator. When the
stator windings are energized with DC current, the poles become magnetized.
Rotation occurs when the rotor teeth are attracted to the energized stator poles.
Both the stator and rotor materials must have high permeability and be capable
of allowing high magnetic flux to pass through even if a low magneto motive
force is applied. When the rotor teeth are directly lined up with the stator poles,
the rotor is in a position of minimum reluctance to the magnetic flux [12].
A rotor step takes place when one stator phase is DE energized and the
next phase in sequence is energized, thus creating a new position of minimum
reluctance for the rotor as explained by Kenjo (1984). Cross-section of variable
reluctance motor is shown in Figure 4.2.
Figure 4.2 Cross-section of variable reluctance motor
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4.1.1.2
Permanent Magnet Stepper Motor:
A stepper motor using a permanent magnet in the rotor is called a PMSM.
The rotor no longer has teeth as with the VRM. Instead, the rotor is magnetized
with alternating north and south poles situated in a straight line parallel to the
rotor shaft. These magnetized rotor poles provide an increased magnetic flux
intensity and, because of this the PM motor
exhibits improved torque
characteristics when compared with the VRM type [11], [13]. An elementary PM
motor is shown in Figure 4.3, which employs a cylindrical permanent magnet as
the rotor and possesses four poles in its stator. Two overlapping windings are
wound as one winding on poles 1 and 3 and these two windings are separated
from each other at terminals to keep them as independent windings. The same is
true for poles 2 and 4. The terminals marked Ca or Cb denotes connected to the
positive terminal of the power supply as explained by Kenjo (1984).
When the windings are excited in the sequence A - B - A1 - B1 --- the
rotor will be driven in a clockwise direction. The step length is 900 in this
machine. If the number of stator teeth and magnetic poles on the rotor are both
doubled, a two-phase motor with a step length of 45
0
will be realized.
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