Stepper motors operate by sequentially energizing coils located in the stator. Depending upon the stepper

　　motor’s type; variable reluctance， permanent magnet， or hybrid， the rotor contains a toothed soft-iron core，

　　permanent magnets， or both. What sets a stepper motor apart from other motor types is the ability to step

　?。╫r turn） its rotor in small， precise increments and lock it in place

　　Three configurations are typically found for wiring a stepper motor’s windings; variable reluctance，

　　unipolar， and bipolar. Each configuration requires different drive circuitry and its associated method of

　　control. This application report focuses on controlling stepper motors with bipolar windings. Bipolar stepper

　　motors are driven using an H-bridge circuit since current flow is bi-directional through the windings.

　　Based on its internal winding resistance， stepper motors are rated for maximum current at a particular

　　voltage. Maximum torque is developed when maximum current is flowing through the windings. Operating

　　at this voltage， however， is inefficient due to the ramp-up time it takes for current flow to overcome the

　　winding‘s inherent inductance. Applying a voltage much higher than it’s rated for to a stepper motor

　　reduces this ramp-up time. To do this without overheating and destroying the motor， however， the winding

　　current must still be limited to its maximum rated value. This is done by using a chopper circuit which

　　controls the ratio of winding current on time to off time once the maximum current level is reached.