Parameters
3UG4 / 3RR2 monitoring relays
370
Manual, 05/2016, NEB927043002000/RS-AC/004
Current monitoring principle
The monitoring relays monitor an AC load current which flows through the device terminals
depending on whether monitoring has been set to check for overshoot or undershoot, or
window monitoring.
There are two current monitoring principles:
●
Apparent current I
s
●
Active currentI
p
Whereas apparent current monitoring is primarily used in the rated torque range or for
overload, active current monitoring can be used to observe and evaluate the degree of
loading across a motor's entire torque range.
①
Strong change in cos φ
②
Strong change in apparent current
cos φ
Power factor
I [A]
Current
M [%]
Torque as a % of the rated torque
I
p
Active current
I
s
Apparent current
Image B-2
Load current monitoring diagram (shown using the example of a real motor)
The diagram shows that it only makes sense to use the power factor cos φ to monitor the
rated current up to approx. 70%. At a higher rated current the change in the power factor is
too slight to produce measured values of any significance. In contrast, the monitoring quality
of the apparent current I
s
increases along with the torque, in other words as the rated current
rises. It is for this reason that monitoring of the apparent current is primarily used if a load
needs to be protected against overload. Integrated monitoring from no-load operation up to
overload is only possible by monitoring the active current I
p
, the product of power factor and
apparent current.
Note
Determining the thresholds in practice
You can determine the thresholds by operating the motor at the load limits, for example. You
can then read the thresholds as the up-to-date measured values off the display of the
monitoring relay.
