Rockwell Automation Publication 440R-RM003A-EN-P - July 2021 11
Chapter 2
CU3 Control Unit
In late 2019, Rockwell Automation launched the MSR55P safety relay to provide
additional features for monitoring the speed of a motor as the motor spins
down to a stopped condition by sensing the back EMF of the motor. The
MSR55P safety relay has been well accepted, and the CU3 control unit is now
being obsoleted.
Feature Comparison Table 1 compares the key feature differences between the CU3 control unit and
the MSR55P safety relay. The MSR55P safety relay has several features that the
CU3 control unit does not.
Power Supply and
Thresholds
The power supplies for the CU3 control unit and the MSR55P safety relay are
similar, however, 24V AC cannot power the MSR55P safety relay. If the CU3
control unit is powered by 24V AC, you must provide an AC to DC converter to
use the MSR55P safety relay.
Table 2 on page 12
lists the CU3 control unit catalog numbers, threshold
voltage range, and typical applications. Table 3 on page 12
lists the MSR55P
safety relay catalog numbers, threshold voltage range, and typical applications.
Table 1 - Key Feature Comparison
Feature CU3 MSR55P
Panel space 45 mm (1.77 in.) 45 mm (1.77 in.)
Single-phase motor monitoring Yes Yes
Three-phase motor monitoring No Yes
Output monitoring Yes Yes
Fault output reporting No Yes
Monitors induction motors Yes Yes
Monitors servo motors Not recommended Yes
Adjustable threshold Yes Wider range
Delayed output capability No Yes
Safety rating Cat 1 Cat4 PLe, SIL 3
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