Siemens SIRIUS 3RT Series System Manual page 725

Load current monitoring (apparent current/active current)
Definition of load current monitoring
The 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 ways of monitoring the load current:
● Apparent current I
● Active currentI
Explanation
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
②
cos φ
I [A]
M [%]
I
p
I
s
Figure 8-5
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
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
apparent current.
Factory setting
Load current monitoring is factory-set to apparent current monitoring I
SIRIUS Innovations
System Manual, 01/2011, A8E56203870002-03
s
p
Strong change in cos φ
Strong change in apparent current
Power factor
Current
Torque as a % of the rated torque
Active current
Apparent current
Load current monitoring diagram
increases along with the torque, in other words as the rated current
s
SIRIUS 3RR2 current monitoring relays
, the product of power factor and
p
.
s
8.8 Operation
725