Armature Voltage Feedback; Quick Stops - Control Techniques Focus 3R User Manual

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.
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).
tachometers output a linear voltage proportional to their RPM and can provide the Focus
3 with accurate motor speed feedback information. With good DC tachometers up to
0.5% speed accuracy can be achieved.
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
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

The DB option provides a rather quick stopping action and provides motor turning
resistance when the drive is not in the RUN condition.
with a Dynamic Braking resistor should this requirement be desired.
Regenerative drives are capable of stopping the motor more quickly than dynamic
braking by rapidly reducing the speed command to zero and then disabling the drive.
This requires the drive to be active and some external means of disabling the drive after
the motor reaches zero speed.
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.
should this event occur
Note: DB Resistors require Contactor Option - See
Armature Voltage Feedback (or
.
The Focus 3R can be outfitted
Options
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DC