F.3
Date Code 20080918
Instruction Manual
SEL-749M Relay
Motor Thermal Element
The Basic Thermal Element
Figure F.1
Motor Thermal Limit Characteristic Plotted With Motor Starting Current
The Basic Thermal Element
shows a simple electrical analog for a thermal system. The thermal
element includes:
➤
A heat source, modeled as a current source.
➤
A thermal capacitance, modeled as a capacitor.
➤
A thermal impedance to ambient, modeled as a resistor.
➤
A comparator to compare the present heat estimate, U, to the
thermal trip value.
Figure F.2
Electrical Analog of a Thermal System
0
2
4
6
8
10
1
10
100
Current in Multiples of Full Load Amps
Time in S
ec
onds
1000
Overload
Running
Current
Locked Rotor
Starting
Current
Thermal Trip
Thermal Trip Value
+
—
Heat
Source
U
C
R
Summary of Contents for SEL-749M
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