Rockwell Automation 1395 Digital DC Drive FRN 5-10.10/9.30 - Manual (Page 161)

Chapter 6
Installation
6-37
The tach signal then must be scaled in the adapter board to determine the
proper relationship of output voltage/ motor velocity to base speed in Drive
Units. This scaled configuration data must then be linked to Parameter 156
“Tach Velocity.”
Many problems relate to the scaling of the tach signals. Below is a
procedure for checking the scaling of the analog tach feedback for proper
drive operation.
1. Determine the Volts/RPM rating of the tach (refer to tach name plate).
Multiply this rating times the absolute maximum speed the motor will
be commanded to accelerate to. This value should also be programmed
in Parameter 607 “Rev Speed Lim” and 608 “Fwd Speed Lim” to
assure that the velocity command will be properly clamped.
Volts/RPM Rating x Max Speed = Max Volts Output
2. The Max Volts output must then be scaled to a level within the
±
10V
analog input channel range. This can be accomplished by using a
voltage divider network external to the drive. The voltage divider will
take the Max Volts output and scale it to a maximum 9V input. This
allows for protection against 10% overshoot.
Figure 6.25 uses a l0k ohm resistor across the input channel. Rl represents
the dropping resistor for the scaling network. To determine the value of Rl
use the equation that follows:
Figure 6.25
Scaling Circuit
R1
10k Ohm
Analog In (+)
Analog In (–)
Resistors 0.5W, 1%
Tach Velocity (+)
TB3–33
TB3–34
(Max Volts Output) x 6666
9V
– 6666 = R1
20k Adapter Board Input Impedance
Tach Velocity (–)
DC
Tach
T
+
3. The analog input channel on the adapter board must now be scaled to
represent an accurate velocity feedback signal. First determine the
analog input signal for base speed. Parameter numbers are given in ( )
where applicable.
Base Motor Speed (606) x 9V
Max Speed
= Base Speed Input