Current Limiting for Stepping Motors
Small stepping motors, such as those used for head positioning on floppy disk drives, are usually driven at a low DC voltage, and the current through the motor windings is usually limited by the internal resistance of the winding.
There is good reason to run a stepping motor at a supply voltage above that needed to push the maximum rated current through the motor windings. Running a motor at higher voltages leads to a faster rise in the current through the windings when they are turned on, and this, in turn, leads to a higher cutoff speed for the motor and higher torques at speeds above the cutoff.
Microstepping, where the control system positions the motor rotor between half steps, also requires external current limiting circuitry. For example, to position the rotor 1/4 of the way from one step to another, it might be necessary to run one motor winding at full current while the other is run at approximately 1/3 of that current.
The remainder of this section discusses various circuits for limiting the current through the windings of a stepping motor, starting with simple resistive limiters and moving up to choppers and other switching regulators. Most of these current limiters are appropriate for many other applications, including limiting the current through conventional DC motors and other inductive loads.
Resistive Current Limiters
The easiest to understand current limiter is a series resistor. Most motor manufacturers recommended this approach in their literature up until the early 1980's, and most motor data sheets still give performance curves for motors driven by such circuits. The typical circuits used to control the current through one winding of a permanent magnet or hybrid motor are shown in Figure 4.1.
in this figure limits the current through the motor winding. Given a rated current of I and a motor winding with a resistance Ohm's law sets the maximum supply voltage as . Given that the inductance of the motor motor winding is , the time constant for the motor winding will be Figure 4.2 illustrates the effect of increasing the resistance and the operating voltage on the rise and fall times of the current through one winding of a stepping motor.
is shown only in the unipolar example in Figure 4.1 because it is particularly useful there. For a bipolar H-bridge drive, when all switches are turned off, current flows from ground to the motor supply through so the current through the motor winding will decay quite quickly.
Note: When the switches in the H-bridge circuit shown in Figure 4.1 are opened, the direction of current flow through will reverse almost instantaneously! If has any inductance, for example, if it is wire-wound, it must either be bypassed with a capacitor to handle the voltage kick caused by this current reversal, or must be added to the H-bridge.
Given the rated maximum current through each winding and the supply voltage, the resistance and wattage of is easy to compute. if it is included, poses more interesting problems. The resistance of depends on the maximum voltage the switches can handle. For example, if the supply voltage is 24 volts, and the switches are rated at 75 volts, the drop across can be as much as 51 volts without harming the transistors.