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SE C T IO N 2 .2
Selecting a Motor
When selecting a stepper motor for your application, there are several factors that need to be taken into consideration:
How will the motor be coupled to the load?
How much torque is required to move the load?
How fast does the load need to move or accelerate?
What degree of accuracy is required when positioning the load?
While determining the answers to these and other questions is beyond the scope of this document, they are details that
you must know in order to select a motor that is appropriate for your application. These details will affect everything
from the power supply voltage to the type and wiring configuration of your stepper motor. The current and microstep-
ping settings of your MForce MicroDrive will also be affected.
Types and Construction of Stepping Motors
The stepping motor, while classed as a DC motor, is actually an AC motor that is operated by trains of pulses. Although
it is called a "stepping motor", it is in reality a polyphase synchronous motor. This means it has multiple phases wound
in the stator and the rotor is dragged along in synchronism with the rotating magnetic field. The MForce MicroDrive is
designed to work with the following types of stepping motors:
1) Permanent Magnet (PM)
2) Hybrid Stepping Motors
Hybrid stepping motors combine the features of the PM stepping motors with the features of another type of stepping
motor called a variable reluctance motor (VR). VR motors are low torque and load capacity motors which are typically
used in instrumentation. The MForce MicroDrive cannot be used with VR motors as they have no permanent magnet.
On hybrid motors, the phases are wound on toothed segments of the stator assembly. The rotor consists of a permanent
magnet with a toothed outer surface which allows precision motion accurate to within ± 3 percent. Hybrid stepping mo-
tors are available with step angles varying from 0.45° to 15° with 1.8° being the most commonly used. Torque capacity
in hybrid steppers ranges from 5 - 8000 ounce-inches. Because of their smaller step angles, hybrid motors have a higher
degree of suitability in applications where precise load positioning and smooth motion is required.
Sizing a Motor for Your System
The MForce MicroDrive is a bipolar driver which works equally well with both bipolar and unipolar motors (i.e. 8 and
4 lead motors, and 6 lead center tapped motors).
To maintain a given set motor current, the MForce MicroDrive chops the voltage using a variable chopping frequency
and a varying duty cycle. Duty cycles that exceed 50% can cause unstable chopping. This characteristic is directly
related to the motor's winding inductance. In order to avoid this situation, it is necessary to choose a motor with a low
winding inductance. The lower the winding inductance, the higher the step rate possible.
Winding Inductance
Since the MForce MicroDrive is a constant current source, it is not necessary to use a motor that is rated at the same
voltage as the supply voltage. What is important is that the MForce MicroDrive is set to the motor's rated current.
The higher the voltage used the faster the current can flow through the motor windings. This in turn means a higher
step rate, or motor speed. Care should be taken not to exceed the maximum voltage of the driver. Therefore, in choos-
ing a motor for a system design, the best performance for a specified torque is a motor with the lowest possible winding
inductance used in conjunction with highest possible driver voltage.
The winding inductance will determine the motor type and wiring configuration best suited for your system. While the
equation used to size a motor for your system is quite simple, several factors fall into play at this point.
The winding inductance of a motor is rated in milliHenrys (mH) per Phase. The amount of inductance
the wiring configuration of the motor.
The per phase winding inductance specified may be different than the per phase inductance seen by your MForce Micro-
Drive driver depending on the wiring configuration used. Your calculations must allow for the actual inductance that the
driver will see based upon the wiring configuration.
Part 2: Connections and Interface
Motor Sizing and Selection
will depend on
2-7
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