Low power stepper motors, such as those used in floppy disk drives, are usually powered at low dc voltages, and the value of the motor windings current is usually restricted by the internal resistance of the winding. Very low resistance windings are usually used for building high torque motors; when powered by any suitable supply voltage, these motors typically require external current limiting circuitry. The requirements for stepper motor drive circuits have changed at a very rapid rate and hence digital integrated circuits have been developed to avoid any complexity and provide facilities to be used in association with microcontrollers. The reduction of discrete circuit components has enhanced the reliability and permitted the use of more sophisticated drive techniques at a reasonable cost [1]. The rotor of a stepper motor usually aligns itself with the stator magnetic field generated by a dc current applied to the stator coils. When the rotor is driven by an external force, a restoring torque is developed. The torque becomes maximum when the rotor is turned by one step angle on either direction. This maximum torque is called the holding torque and has the unit of ounce-inches (oz-inches). It may be good to say that the running torque or the pull-out torque should be less than the holding torque; otherwise the rotor will not turn. The pull-out torque is the true indication of the torque output capability of the motor. This torque varies with the stepping rate or rotor speed [1-2]. Thus the running torque of stepper motor may be considered as the peak load torque that can be subjected to the motor without affecting the rotor equilibrium position while the appropriate stator windings are energized. The dynamic characteristics and efficiency of the stepper motor drive may be improved if a steady dc current switched mode power supply is used. This paper discusses two types of power electronic circuits for limiting the current through the windings of the stepper motor. These two current limiters are suitable for many other industrial applications, including limiting the current rise and decrease through the dc motor windings and other highly inductive loads. This paper also covers the basic principles of stepper motors and stepper motor control systems. It focuses on a Bipolar permanent magnet, from the elementary circuitry needed to control its speed, to the methods used for improving its time constant and stepper rate [1-5].
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