Rockwell Automation 1395 Digital DC Drive FRN 5-10.10/9.30 - Manual (Page 156)

Chapter 6
Installation
6-32
Tach Velocity – The analog tachometer device generates a DC voltage that
is direction sensitive and proportional to speed. The tach output must be
connected to an analog input channel on the Discrete Adapter Board or
Digital Reference Board. Most industrial tachs have an output greater than
the
±
10V range of the analog inputs. The tach output must be scaled down,
by an external voltage divider network, so that the entire speed range of the
motor can be represented by a
±
9V feedback signal.
CAUTION: Connecting a tach which has an output range
greater than
±
10V directly to the analog input channel can
severely damage the adapter board.
!
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 6.20 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 (R1 should be rated for 0.5W, 1%).
Figure 6.20
Scaling Circuit
R1
10k Ohm
Analog In (+)
Analog In (–)
Resistors 0.5W, 1%
Tach Velocity (+)
TB3–24
TB3–23
(Max Volts Output) x 6666
9V
– 6666 = R1
20k Adapter Board Input Impedance
Tach Velocity (–)
DC
Tach
T
+