Rockwell Automation 1771-QC , D17716.5.25 SER/B SERVO POS - Manual (Page 65)

Installing the Assembly
Chapter 6
6Ć15
Provide one transformer for the master-control relay (CRM) circuit, the
loop-contactor relay (LCR) circuit, the dc power supplies, and any ac I/O
chassis. Provide a separate transformer for the servo drives to provide
noise immunity.
Use normally-open LCR contacts to switch power from the servo drive to
the servo motor. Also, use normally closed LCR contacts to switch in the
dynamic braking resistor across the servo motor whenever power is
removed from the servo motor. Check with the servo drive and servo
motor manufacturer for the resistance and power rating for the dynamic
braking resistor.
WARNING:Without a dynamic braking resistor, removing
servo motor power while the axis is in motion allows
momentum to keep the axis in motion. In an emergency
situation, this could be dangerous. A dynamic braking resistor
can help stop the servo motor by quickly dissipating the energy
of momentum. Even with dynamic braking, a vertical axis may
also require an electric brake or counter balance.
An extreme overtravel limit switch or an emergency stop switch can
de-energize the LCR, thereby turning off servo motor power. However,
abruptly stopping an axis in this way stresses the servo motor and the
mechanical linkage. Therefore, use the LCR to stop a moving axis only in
an emergency. To stop an axis in a non-emergency situation, use the
slide-stop bit in the command block thru the ladder diagram program. A
slide stop decelerates the axis feedrate before stopping it. After a slide
stop you can use an emergency stop switch if you want to remove power.
Connect a set of normally-open CRM contacts in series with servo
transformer overload, servo drive fault, and servo motor overload
contacts. Connect this series of contacts between the hardware stop input
terminal and common. The opening of any of these contacts indicates that
power to the servo motor is interrupted. When any of these sets of
contacts open the hardware stop circuit, the following occur:
1. When this circuit opens, the 1771-ES expander immediately sets the
velocity command output to zero and disables the serve drive by
turning off the drive disable circuit.