Baldor 23H Series Installation & Operating Manual page 124

Application Notes
Vector drives are used in high-performance adjustable-speed applications, machine tool spindles,
and industrial test stands just to mention a few applications. Line regen units are used in winders,
hoist/crane, presses, HVAC, and other applications. For positioning applications, vector drives are
used with existing programmable position controllers.
DC Servo Drives
In the world of servos, there are DC servos and brushless servos. The brushless servos are termed,
by manufacturers, as either DC or AC.
The DC servo package takes AC power in, and converts it to DC. The amount of DC output applied
to the motor is directly proportional to the desired operating motor speed. The performance
provided by DC servos may be considered as "traditional", and comparable to vector technology
when used with a positioning package.
Advantages of DC servos include proven reliability and well known technology (they have been
around forever). In positioning applications, and in comparison to vectors, DC servos come in a
smaller package size and provide lower inertia, which translates to faster acceleration (gets into
position faster). When used within the designed capability range, the DC servo motor, with its
brush design, provides a long adequate life for many applications.
DC servos are used in machine tool, factory automation, packaging, woodworking, and many
other applications.
Brushless Servo Drives
As indicated, brushless servos are available as either DC brushless or AC brushless. The feedback
device determines whether it is considered DC or AC, since this dictates the control scheme. The
feedback device may be either Hall sensors, encoders, or resolvers.
With Hall sensor feedback, the three-phase brushless motor is powered by energizing two of the
three motor windings at a time. There are six different commutation sections for one mechanical
revolution, and within each commutation section, a DC level of power is applied. The amount of
DC applied is directly proportional to the desired operating motor speed. Thus the term "DC
brushless".
Encoder feedback is used when position data is required in the application. Some encoders are
available with Hall outputs. Again, the Hall signals are used for commutation of the brushless
motor.
With resolver feedback, a sinusoidal waveform is applied on the motor windings. Thus the term
"AC brushless". The advantage of this technology is that, for the same torque (compared to "DC
brushless"), the "AC brushless" will require less current. Therefore a smaller drive may often be
used in the application. This becomes possible since the motor has a three phase sinusoidal
winding being powered by a three phase sinusoidal current waveform.
Advantages of brushless technology include higher speed capability, higher torques in a smaller
package, much lower inertia (thus much faster acceleration capability), and of course, long reliable
maintenance free life in the application. These drives provide good low speed operation down to
zero speed. Brushless controls are available with auto-tuning capability.
Brushless drives are used in robotics, packaging, electronic assembly, semiconductor equipment,
textile, and any cutting, printing or labeling which may even be performed on a moving web.
Many other applications of course exist.
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