Editing Stepper motor (section)

==Driver circuits==
[[File:Stepper Motor 11.jpg|thumb|Stepper motor with [[Adafruit Industries|Adafruit]] Motor Shield drive circuit for use with [[Arduino]]]]
Stepper motor performance is strongly dependent on the [[driver circuit]]. [[Torque curve]]s may be extended to greater speeds if the stator poles can be reversed more quickly, the limiting factor being a combination of the winding inductance. To overcome the inductance and switch the windings quickly, one must increase the drive voltage. This leads further to the necessity of limiting the current that these high voltages may otherwise induce.

An additional limitation, often comparable to the effects of inductance, is the back-EMF of the motor. As the motor's rotor turns, a sinusoidal voltage is generated proportional to the speed (step rate). This AC voltage is subtracted from the voltage waveform available to induce a change in the current.

=== L/R driver circuits ===
L/R driver circuits are also referred to as [[Voltage source|constant voltage]] drives because a constant positive or negative voltage is applied to each winding to set the step positions. However, it is winding current, not voltage that applies torque to the stepper motor shaft. The current I in each winding is related to the applied voltage V by the winding inductance L and the winding resistance R. The resistance R determines the maximum current according to [[Ohm's law]] I=V/R. The inductance L determines the maximum rate of change of the current in the winding according to the formula for an [[inductor]] dI/dt = V/L. The resulting current for a voltage pulse is a quickly increasing current as a function of inductance. This reaches the V/R value and holds for the remainder of the pulse. Thus when controlled by a constant voltage drive, the maximum speed of a stepper motor is limited by its inductance since at some speed, the voltage U will be changing faster than the current I can keep up. In simple terms the rate of change of current is L / R (e.g. a 10&nbsp;mH inductance with 2 ohms resistance will take 5&nbsp;ms to reach approx 2/3 of maximum torque or around 24&nbsp;ms to reach 99% of max torque). To obtain high torque at high speeds requires a large drive voltage with a low resistance and low inductance.

With an L/R drive it is possible to control a low voltage resistive motor with a higher voltage drive simply by adding an external resistor in series with each winding. This will waste power in the resistors, and generate heat. It is therefore considered a low performing option, albeit simple and cheap.

Modern voltage-mode drivers overcome some of these limitations by approximating a sinusoidal voltage waveform to the motor phases. The amplitude of the voltage waveform is set up to increase with step rate. If properly tuned, this compensates the effects of inductance and [[Back EMF|back-EMF]], allowing decent performance relative to current-mode drivers, but at the expense of design effort (tuning procedures) that are simpler for current-mode drivers.

===Chopper drive circuits===
Chopper drive circuits are referred to as controlled current drives because they generate a controlled current in each winding rather than applying a constant voltage. Chopper drive circuits are most often used with two-winding bipolar motors, the two windings being driven independently to provide a specific motor torque CW or CCW. On each winding, a "supply" voltage is applied to the winding as a square wave voltage; example 8&nbsp;kHz. The winding inductance smooths the current which reaches a level according to the square wave [[duty cycle]]. Most often bipolar supply (+ and - ) voltages are supplied to the controller relative to the winding return. So 50% duty cycle results in zero current. 0% results in full V/R current in one direction. 100% results in full current in the opposite direction. This current level is monitored by the controller by measuring the voltage across a small sense resistor in series with the winding. This requires additional electronics to sense winding currents, and control the switching, but it allows stepper motors to be driven with higher torque at higher speeds than L/R drives. It also allows the controller to output predetermined current levels rather than fixed. Integrated electronics for this purpose are widely available.