Stepper motors

Stepper motors

General information on stepper motors

Stepper motors are special versions of the synchronous machine, in which the rotor is a permanent magnet, while the stator consists of a coil package. In contrast to synchronous motors, stepper motors have a large number of pole pairs. Motor operation requires a control unit, which energises the individual motor windings based on a certain pulse sequence. A stepper motor has a tendency to mechanical oscillation. Above its load limit it loses dynamic characteristics and may lose individual steps. Under high load the shaft may even stop. Safe positioning is therefore only guaranteed within the performance limits. If the motor is operated within its load limits, positioning without feedback of the rotor position can be achieved by linking individual steps. This operating mode (open loop control) and the durability of the stepper motor enable it to be used as a positioning drive in price-sensitive applications.

Basic function principles of a stepper motor

Like most electric motors, a stepper motor consists of a stator (fixed external winding) and a rotor (rotating shaft with magnets). The rotation of the motor shaft (rotor) is generated by rapid energising the electromagnetic field of the stator, causing the shaft to turn by the step angle a. In a minimum control configuration, the stepper motor is moved from pole to pole, or from step to step. A full turn of the motor shaft is therefore made up of individual steps. Energising of the motor windings results in a magnetic field in the motor from north to south (or south to north if the power supply has negative polarity and the winding is arranged accordingly). The movable stator with its permanent magnets aligns itself according to the direction of the external magnetic field of the stator.

A special type of synchronous motor which is designed to rotate a specific number of degrees for every o electric pulse received by its control unit. Typical steps are 7.5 or 15 per pulse. It is a motor that can rotate in both directions, move in precise angular increments, sustain a holding torque at zero speed, and be controlled with digital circuits. It moves in accurate angular increments known as steps, in response to the application of digital pulses to the electric drive circuit.

Generally, such motors are manufactured with steps per revolution. Step motors are either bipolar, requiring two power sources or uni polar requiring only one power source.

                                     Θm =2/p*θe 

                                                    wm =2/p*we