1) Definition: A stepper motor is an electromechanical device which convert the electric pulse into discrete mechanical movement.
2) Types of stepper motor:
A) Variable Reluctance (VR)
-most easier to understand
-consist of a soft iron multi-toothed rotor and wound stator.
-when stator windings are energized with DC current the poles become magnetized.
-rotation occurs when the rotor teeth are attracted to the energized stator poles.
B) Permanent Magnet (PM)
-tin can/ canstock
-low cost and low resolution step (typical step angles 7.5-15 degree or 48-24 steps)
-does not have teeth.
-the rotor is magnetized with alternating north and south poles situated in a straight line parallel to the rotor shaft.
-provide an increased magnetic flux intensity and hence exhibits improved torque characteristics when compared to VR.
C) Hybrid (HB)
-more expensive than PM.
-better step resolution, torque and speed.
-typical step angle: 3.6 to 0.9 degree ( 100-400 step).
-combines the best features of both VR and PM.
-multi-toothed like VR and contains an axially magnetized concentric magnet around its shaft.The teeth guide the magnetic flux to preferred locations in the airgap.
-increase the dentent, holding and dynamic torque
D) Disc Magnet Motor (special type)
-designed as a disc with rare earth magnet.
-low inertia and optimized magnetic flow path with no coupling between the two stator windings.
3) Most commonly used stepper motor: PM and HB
-if a designer is not sure which type best fit, he should first evaluate the PM type as i it is normally several time less expensive.
4) Size and Power:
-in addition to being classified by their step angle, stepper motor are also classified according to frame sizes (diameter of the body of the motor)
-size 11 (body diameter 1.1 inches)
-size 23 (body diameter 2.3 inches = 58mm)
-torque output from a motor of a particular frame size will increase with increased body length.
-power levels for IC driven stepper motors typically range for very small motors (10W) up to larger motors (20W).
-maximum power dissipation P = VI.
-eg: a size 23 stepper motor rated at 6V and 1A per phase. Therefore with two phases energized the motor has a rated power dissipated of 12W.
5) Application: A stepper motor can be a good choice whenever controlled movement is required or control rotation angle, speed, position and synchronism.
- eg: printer, plotters, highend office equipment, hard disk drivers, medical equipment, fax machine, automotive.
6) Advantages:
-the rotation angle of the motor is proportional to the input pulse.
-the motor has full torque at stand-still ( if the windings are energized.)
-precise positioning and repeatability of movement since good stepper motors have an accuracy of 3-5% of a step and this error is non cumulative from one to next.
-excellent response to start/ stop/ reverse.
-very reliable since there are no contact brushes in the motor. Therefore the life of the motor is simply dependent on the life of the bearing.
-response to digital input pulses and provides open-loop control, making the motor simplet and less costly to control.
-low speed synchoronous rotation with a load that is directly coupled to the shaft.
-wide range of rotational speeds. ( v proportional to f)
7) Disadvantages:
-resonances can occur if not properly controlled.
-not easy to operate at extremely high speeds.
8) Operation of steeper motor:
-open load control.No feedback information about position is needed.
-eliminates the need for expensive sensing and feedback devices such as optical encoders.
-position simply known by keeping track of the input step pulses.