Features of Stepper Motor:
The Features of Stepper Motor are
- small step angle
- high positioning accuracy
- high torque to inertia ratio
- stepping rate and accuracy
Small Step Angle: The angle by which the rotor of a stepper motor moves when one pulse is applied to the (input) stator is called step angle. This is expressed in degrees. The resolution of positioning of a stepper motor is decided by the step angle. Smaller the step angle the higher is the resolution of positioning of the motor. The step number of a motor is the number of steps it makes in one revolution. The stepper motors are realisable for very small step angle. Some precision motors can make 1000 steps in one revolution with a step angle of 0.36°. A standard motor will have a step angle of 1.8° with 200 steps for revolution. The step angles of 90°, 45°, 15° are not uncommon in simple motors.
High Positioning Accuracy: The quality of a stepper motor is decided by its positioning accuracy. This accuracy of positioning is a significant factor. In any application the stepper motors are expected to rotate by step angle when a pulse is given as input. It should come to rest in a precise position. Care must be exercised in the design and manufacture of a stepper motor as the accuracy at no load depends on the physical accuracy of the rotor and the stator. The positioning accuracy depends only on the machine characteristics and the driving circuit, while other electronic parameters have no effect on positioning accuracy. The stepper motors are designed to have very high restoring torque when the rotor is displaced following load torque.
High Torque-to-inertia Ratio: A stepper motor must move fast response to a pulse or a train of pulses. Fast response is also associated with quick start and quick stop of the motor. The motor must be capable of stopping at a position specified by the last pulse of the pulse train if the train is inhibited when the motor is running uniformly. A stepper motor must have a large ratio of torque to inertia
Stepping Rate and Pulse Frequency: The speed of a stepper motor is indicated by the number of pulses per second. Stepping rate of the motor is therefore used to indicate the speed. As the motor moves by one step for a pulse the speed can also be indicated by pulses per second as pulse frequency. The absolute rotational speed and stepping rate are related by
Where,
S = step number,
f = stepping rate