Figure 6.7-1 - Simple Stepper Motor Driver; Figure 6.7-2 - Current Build-Up In Phase; Figure 6.7-3 - Effect Of A Short On Time On Current - Newport ESP6000 User Manual

Figure 6.7-1 — Simple Stepper Motor Driver
This driver works fine for simple, low-performance applications. But if high
speeds are required, having to switch the current fast in inductive loads
becomes a problem. When voltage is applied to a winding, the current (and
thus the torque) approaches its nominal value exponentially (Figure 6.7-2).
Current
Figure 6.7-2 — Current Build-up in Phase
When the pulse rate is fast, the current does not have time to reach the
desired value before it is turned off and the total torque generated is only a
fraction of the nominal torque (Figure 6.7-3).
Phase ON
nominal current
Figure 6.7-3 — Effect of a Short ON Time on Current
How fast the current reaches its nominal value depends on three factors:
the winding's inductance, resistance, and the voltage applied to it.
The inductance cannot be reduced. But the voltage can be temporarily
increased to bring the current to its desired level faster. The most widely
used technique is a high voltage chopper.
If, for instance, a stepper motor requiring only 3V to reach the nominal
current is connected momentarily to 30V, it will reach the same current in
1
only / of the time (Figure 6.7-4).
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