Schweitzer Engineering Laboratories SEL-701 - Figure E.7 Motor Running Thermal Element

701 Motor Protection Relay Date Code 20010719
Motor Thermal Element
Motor Running Protection
E.10
Figure E.6 shows a graphical representation of the problem and its solution. The
motor normal operating energy is the difference between the ambient and operating
temperature locked rotor times, multiplied by locked rotor current squared. For those
motors that do not publish separate locked rotor times, assume that the locked rotor trip
energy is approximately six times the operating energy in the relation.
Equation E.8
The motor ratings allow the motor to be run continuously at the motor service
factor, thus the service factor, SF, is accounted for in the running thermal element trip
threshold. Figure E.7 shows the final running thermal element.
Figure E.7 Motor Running Thermal Element.
I
L
2
Ta•
I
L
2
Ta To–()•
-
------------------------------------- 6
Ta
To
-------
∴ 1.2 Ta To–()∴ 0.2 T
a
•== =
I
1
2
+ 5 • I
2
2
49T
I
L
2
• (Ta - To) • SF
2
_
+
U
3
Relay
Word
Bit
I
L
2
• (Ta - To)