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ADVANCED MICRO SYSTEMS, INC. ADDENDUM
61
About Step Motor Current
There is much confusion regarding the operation of step motors. Depending on your application, the step
motor offers several advantages over servo motor designs, including lower cost and simplicity. The step (or
stepper, or stepping) motor is a digital “synchronous” motor with a pre-designed number of “steps” per
revolution. The most common motor has 200 full steps per revolution. Simple driver electronics can
subdivide these steps into ½ step or more complex “microsteps.”
Step Motor Characteristics
• The positional repeatability of each full or half step is very close to exact.
• While microsteps are repeatable, they tend to be somewhat non-linear.
• The torque is maximum at zero speed.
• The motor shaft RPM exactly correlates with the steps-per-second.
• Torque decreases with speed, eventually to zero or a “stall” condition.
• Resonance at certain speeds can cause undesired stalls or erratic operation.
There is little difference between today’s step motor and the first generation of 60+ years ago. The magnetic
materials and torque have been improved, yet it remains a simple, reliable workhorse of industrial motion
control. Over time most improvements have been made to the drive and control electronics, i.e., microstep,
solid-state components with higher voltage, current and switching speeds.
One insatiable hunger of a step motor is torque output at higher speeds. Winding inductance is the villain
that limits speed. As the windings are switched on, the magnetic flux must be built up from current flow in
the windings, producing mechanical torque. Higher step rates reduce the time available for flux to buildup
and average current flow is reduced.
Note: The DCB-274 is rated for 40 volts only.
This reduced current results in reduced torque. The rate of current change depends on the voltage applied
across it. High voltage applied across the coil will shorten the time constant.
Today’s systems strive for low inductance motors and high voltage supplies. The above curves show the
increased speed that might be obtained with higher supply voltages, up to 160Vdc. At standstill the average
motor voltage is regulated to approximately 3Vdc.
0
20
40
60
80
100
120
140
160
180
200
o
z
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10 100
1000
10000 100000
half steps / sec
AM23_210_3 4A RMS MOTOR
40V
80V
160V