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Saturday, 24 March 2018 15:45

Basics of Stepper Motors

  THE Most Common Types of Stepper Motors

Motor Selection - Unipolar vs Bipolar
Stepper motors in common use can be divided into general classes, Unipolar driven motors and Bipolar driven motors. In the past unipolar motors were common and preferred for their simple drive configurations. However, with the advent of cost effective integrated drivers, bipolar motors are now more common. These bipolar motors typically produce a higher torque in a given form factor.

  RUNNING Torque

Steppers can’t deliver as much running torque as standard DC motors of the same size and weight. A typical 12-volt, medium-sized stepper motor may have a running torque of only 25 oz-in. The same 12-volt, medium-sized standard DC motor may have a running torque that is three or four times more. However, steppers are at their best when they are turning slowly. With the typical stepper, the slower the motor revolves, the higher the torque. The reverse is usually true of continuous DC motors. Fig. 19.5 shows a graph of the running torque of a medium-duty, unipolar 12-volt stepper. This unit has a top running speed of 550 pulses per second. Since the motor has a step angle of 1.8°, that results in a top speed of 2.75 revolutions per second (165 rpm).
torque stepper motor FIGURE 1.0 With a stepper motor, torque increases as the speed of the motor is reduced.

 BRAKING Effect

Actuating one of the windings in a stepper motor advances the shaft. If you continue to apply current to the winding the motor won’t turn any more. In fact, the shaft will be locked, as if you’ve applied brakes. As a result of this interesting locking effect, you never need to add a braking circuit to a stepper motor because it has its own brakes built in. The amount of braking power a stepper motor has is expressed as holding torque. Small stepper motors have a holding torque of a few oz-in. Larger, heavier-duty models have holding torque exceeding 400 oz-in

  MICROSTEP Technology

Microstep Technology Microstep drive technology is used to divide the basic step angle (0.72 ̊ ) of the 0.72 ̊  stepper motor into smaller steps (up to a maximum of 250 divisions) without the use of a speed reduction mechanism. Features The stepper motor moves and stops in increments of the step angle determined by the rotor and stator’s salient pole structure, easily achieving a high degree of precision in positioning. The stepper motor, on the other hand, causes the rotor speed to vary because the motor rotates in step angle increments, resulting in resonance or greater vibration at a given speed.

Microstepping is a technology that achieves low resonance, low noise operation at extremely low speeds by controlling the flow of electric current fed to the motor coil and thereby dividing the motor’s basic step angle into smaller steps. The motor’s basic step angle (0.72 ̊/full step) can be divided into smaller steps ranging from 1/1 to 1/250. Microstepping thus ensures smooth operation. With the technology for smoothly varying the motor drive current, motor vibration can be minimized for low noise operation. Up to 250 Microsteps based on Basic Step Angle Thanks to the microstep driver, different step angles (16 steps up to 250 divisions) can be set to two step angle setting switches. By controlling the input signal for step angle switching via an external source, it is possible to switch the step angle between the levels set for the respective switches.

  STEPPER Motor Advantages and Disadvantages

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 repeatabilityof movement since good stepper motors have an accuracy of 3 – 5% of a step and this error is non cumulative from one step to the next.
  • Excellent response to starting/stopping/reversing.
  • Very reliable since there are no contact brushes in the motor.Therefore the life of the motor is simply dependant on the life of the bearing.
  • The motors response to digital input pulses provides open-loop control, making the motor simpler and less costly to control.
  • It is possible to achieve very low speed synchronous rota tion with a load that is directly coupled to the shaft.
  • A wide range of rotational speeds can be realized as the speed is proportional to the frequency of the input pulses.

Disadvantages:

  • Resonances can occur if not properly controlled.
  • Not easy to operate at extremely high speeds

 STEPPER Motor NEMA Size

 NEMA Motor Dimensions (mm)
Dimension Part NEMA 8 NEMA 11 NEMA 14 NEMA 17 NEMA 23 NEMA 34 NEMA 42
[DD] Motor Size (DD) ~0.8 (20.32) ~1.1 (27.94) ~1.4 (35.56) ~1.7 (43.18) ~2.3 (58.42) ~3.4 (86.36) ~4.2 (106.68)
[ a ] Bolt Hole Distance 0.630 (16) 0.905 (23) 1.024 (26) 1.220 (31) 1.854 (47.14) 2.744 (69.7) 3.500 (88.9)
[ b ] Motor Shaft Diameter 0.157 (4) 0.197 (5) 0.197 (5) 0.197 (5) 0.250 (6.35) 0.375 (9.5) 0.625 (16)
[ c ] Motor Shaft max Length       0.945 (24) 0.810 (20.57) 1.250 (31.75) 1.380 (35.05)
[ d ] Pilot max Diameter 0.590 (15) 0.866 (22) 0.866 (22) 0.866 (22) 1.500 (38.1) 2.875 (73) 2.186 (55.52)
[ e ] Pilot Depth (max) 0.059 (1.5) 0.079 (2) 0.079 (2) 0.079 (2) 0.062 (1.6) 0.062 (1.6) 0.062 (1.6)
[ f  ] Bolt Hole Circle Diameter       1.725 (43.82) 2.625 (66.67) 3.875 (98.42) 4.950 (125.73)

nema size

 

 

Read 311 times Last modified on Tuesday, 10 April 2018 03:02
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