Electrical Drives
Selecting Electrical Drives
Some motor suppliers address the need for such special products through field
modification centers located at a distributor site. These centers may offer some
limited variations, usually only for brush-type motors. It is also interesting to note
that the modifications these centers make do not address the overall system or the
controllers. In many cases, other factors can complicate the design process,
even when an off-the-shelf motor seems to provide the right performance. For example,
space limitations may eliminate standard motors from consideration. Numerous applications
today call for a motor built on and around the driven shaft, particularly when the system
must be torsionally stiff (no belts), or have zero lost motion (no gears), or where shaft
runout requirements are in millionths of an inch.
These systems are often typified by a need for high-speed accuracy (&\#177;0.001%) and
are generally regulated with phase-locked servosystems where an optical tachometer
feedback signal is compared to a reference frequency. The comparison generates a system
error proportional to both shaft velocity and position. Typical 0-dB bandwidth is on
the order of 100 to 300 rad/sec. To obtain acceptable system stability in such uses, it
is necessary to minimize effects such as spring rate, or at least to place the response
outside the system bandwidth.
The alternative approach of designing motors and controllers together as a system is
made practical by modern motors that are basically constructed of a few common components.
This allows manufacturers to make motors in a kit form that can be assembled around the
drive spindle during manufacture.
The process of custom tuning all motor parameters for a given application is not as
difficult as it may sound. While this philosophy suggests a generation of specials
without end, there is actually considerable fallout of common parts and subassemblies
when the motor, controller, and feedback devices are considered as a system.
One reason is that the brushless motors often used in high-performance motion-control
applications have quite a simple construction. Motor components can be thought of as
building blocks consisting of laminations and magnets. This allows motion-control
suppliers to inventory a large array of basic building blocks used in developing motors
for special requirements. All other components such as copper wire, bearings, motor
housing parts, shaft and rotors, shaft encoders and readers are either generally available
commodities or can be modified with short lead times.
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