<P> A step motor can be viewed as a synchronous AC motor with the number of poles (on both rotor and stator) increased, taking care that they have no common denominator . Additionally, soft magnetic material with many teeth on the rotor and stator cheaply multiplies the number of poles (reluctance motor). Modern steppers are of hybrid design, having both permanent magnets and soft iron cores . </P> <P> To achieve full rated torque, the coils in a stepper motor must reach their full rated current during each step . Winding inductance and counter-EMF generated by a moving rotor tend to resist changes in drive current, so that as the motor speeds up, less and less time is spent at full current--thus reducing motor torque . As speeds further increase, the current will not reach the rated value, and eventually the motor will cease to produce torque . </P> <P> This is the measure of the torque produced by a stepper motor when it is operated without an acceleration state . At low speeds the stepper motor can synchronize itself with an applied step frequency, and this pull - in torque must overcome friction and inertia . It is important to make sure that the load on the motor is frictional rather than inertial as the friction reduces any unwanted oscillations . The pull - in curve defines an area called the start / stop region . Into this region, the motor can be started / stopped instantaneously with a load applied and without loss of synchronism . </P> <P> The stepper motor pull - out torque is measured by accelerating the motor to the desired speed and then increasing the torque loading until the motor stalls or misses steps . This measurement is taken across a wide range of speeds and the results are used to generate the stepper motor's dynamic performance curve . As noted below this curve is affected by drive voltage, drive current and current switching techniques . A designer may include a safety factor between the rated torque and the estimated full load torque required for the application>> </P>

Three step modes which use to operate stepper motor