Intelligent Motion Systems MForce PowerDrive Plus MForce Series Microstepping PowerDrive, Operating Instructions Manual

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11
Part 2: Interfacing and Configuring
SECTION 2.3
Motor Selection and Interface
Selecting a Motor
When selecting a stepper motor for your application, there are several factors that need to be taken into consider-
ation:

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 microstepping settings of your Microstepping MForce PowerDrive 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 PowerDrive 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 PowerDrive 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 motors are available with step angles varying from 0.45° to 15° with 1.8° being the most com-
monly 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 PowerDrive 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 PowerDrive chops the voltage using a variable chopping fre-
quency 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 PowerDrive 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 PowerDrive 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. There-
fore, in choosing 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 will
depend on the wiring configuration of the motor.