Page 124 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Step angle
The angle through which the motor shaft rotates for each command pulse is called the step angle β.
The step angles can be as small as 0.72º or as large as 90º. But the most common step sizes are 1.8º, 2.5º, 7.5º
and 15º.

N N
β s r 36 0
N .N
s r
360º 360º
β
mN No.of stator phases No.of rotor teeth
r
Rotor and stator poles (teeth) Nrand Ns and number of stator phases (m).

Types of stepper motors
i Variable Reluctance Stepper Motor
ii Permanent Magnet Stepper Motor
iii Hybrid Stepper Motor

1 A stepper motor is a digital electromagnetic device where each pulseinput results in a discrete output i.e. a
definite angle of shaft rotation. It is ideally-suited for open-loop operation because by keeping a count of the
number of input pulses, it is possible to know the exact position of the rotor shaft.

2 In a VR motor, excitation of the stator phases gives rise to a torque in a direction which minimizes the magnetic
circuit reluctance. The reluctance torque depends on the square of the phase current and its direction is
independent of the polarity of the phase current. A VR motor can be asingle-stack or multi-stack motor.
3 A permanent-magnet stepper motor has a permanently-magnetized cylindrical rotor. The direction of the torque
produced depends on the polarity of the stator current.

4 A hybrid motor combines the features of VR and PM stepper motors. The direction of its torque also depends
on the polarity of the stator current. Its step angle b = 360º / mNr.
5 In the 1-phase ON mode of excitation, the rotor moves by one full-step for each change of excitation. In the
2-phase-ON mode, the rotor moves in full steps although it comes to rest at a point midway between the two
adjacent full-step positions.

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CITS : Power - Electrician & Wireman - Lesson 70-75 CITS : Power - Electrician & Wireman - Lesson 70-75