Asymmetry (phase unbalance) and phase failure, Table 3.k, Protection against thermal overload – Rockwell Automation 825 Smart Motor Manager User Manual User Manual
Page 55
Functions 3-22
Publication 825-UM001B-EN-P January 2001
Table 3.K Protection Against Thermal Overload
➊ Thermal utilization %
➋ If auxiliary relays #2 and #3 are assigned to the communication (refer to page
) they cannot be selected
here.
Asymmetry (Phase Unbalance) and Phase Failure
Asymmetrical phase voltages usually occur when the leads closest to the motor are too long.
The resulting current asymmetry in the motor windings may then be 6…10 times the voltage
asymmetry. The Smart Motor Manager takes into account the additional temperature rise and
thus prevents the life of the motor from being reduced. Higher asymmetry or the failure of a
phase can be caused by defective contacts in circuit breakers or contactors, loose terminals,
blown fuses, and faults in the motor itself. Rapid detection and interruption of these factors
help to prevent damage caused by overheating in such equipment. The stress on the
installation and the motor bearings is reduced. The Smart Motor Manager measures the phase
currents and calculates the total copper losses according to the definition of voltage
asymmetry per IEC and NEMA.
Definition of voltage asymmetry per IEC and NEMA:
Warning
Trip
Function
Factory setting
Off
On
Response Level ➊
Setting range
55…99%
—
Factory setting
75%
100%
Setting increments
1%
—
Output Relay ➋
Selection
AL, #1…#5
MR, No output relay
Factory setting
AL
MR
P
Cu
,
2
M
k
,
2
G
+
(
)
≈
∆
U %
( )
Max. deviation from the average of the phase voltages
100
×
Average of the phase voltages
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