Emerson Control Techniques Focus 3N, User Manual, Page: 13 / 82

Page 7
Speed Control
Armature Voltage Feedback
The Focus 3 can vary the speed of the motors mentioned above as a function of the
Speed potentiometer setting (or external speed command signal) by simply varying the
output Armature voltage (field excitation if used typically remains constant). A great
many motor applications do not require ultra precise speed control. Because of this, the
Focus 3 is factory set for Armature Voltage Feedback. Armature Voltage Feedback (or
simply Armature Feedback) does not require any special motor mounted speed
feedback device and is therefore inherently quite reliable and is capable of providing up
to 1% speed regulation.
Speed Feedback- Tachometer
Should more precise speed control be required, the Focus 3 can accommodate those
motors equipped with DC Tachometers (AC tachometers require an option kit). DC
tachometers output a linear voltage proportional to their RPM and can provide the Focus
3 with exact motor speed feedback information. With good DC tachometers up to 0.5%
speed accuracy can be achieved. AC tach’s are not as accurate as DC tach’s but DC
tach’s require brush maintenance whereas the AC tach has no brushes.
Tach failure will typically result in motor speed runaway. One who designs
systems with Speed Feedback devices such as DC tachometers should be
cognizant of this fault condition and must take machine design precautions
should this event occur
.
We would recommend that all DC Drives be initially run using Armature Voltage
Feedback to verify operation even if Tach Feedback is the ultimate goal.
Quick Stops
A non-regenerative drive cannot stop a motor faster than the motors normal coasting
rate unless a Dynamic Braking resistor is employed. The DB option provides a rather
quick stopping action and provides motor turning resistance when the drive is not in the
RUN condition. The Focus 3N can be outfitted with a Dynamic Braking resistor should
this requirement be desired.
Note: DB Resistors require Contactor Option - See Options
Power Outages
Shunt Wound Motors
Should a power outage occur, the drive would turn off and the motor would coast to rest.
If a Dynamic Braking resistor were employed there would typically be enough decaying
field strength to enable some faster stopping action.
Permanent Magnet Motors
Should a power outage occur, the drive would turn off and the motor would coast to rest.
If a Dynamic Braking resistor were employed, full Dynamic Braking force would be
exerted because the field is maintained by the motors internal permanent magnets.
Therefore, there would be motor turning resistance during power outages as well.