Schweitzer Engineering Laboratories SEL-749M - Page 282

F.8
SEL-749M Relay Instruction Manual Date Code 20080918
Motor Thermal Element
Motor Running Protection
Equation F.8
The motor ratings allow the motor to be run continuously at the motor service
factor. The service factor, SF, is accounted for in the running thermal element
trip threshold. Figure F.7 shows the final running thermal element.
Figure F.7 Motor Running Thermal Element
NOTE:
The Motor Starting Trip
Threshold calculated in Example F.1 is
the limiting value. The actual
threshold ramps up during the
starting mode and will not reach the
limit for a successful motor start.
Example F.1 illustrates the difference between the trip thresholds of the
starting and running thermal elements. When the relay switches from the
starting to the running thermal element, it maintains the present heat
estimate, U, which begins to decrease because of the drop in current squared
and because of the insertion of the resistor into the model. The decay of U is
exponential as a result of the interaction between the model thermal
capacitance and resistance. Rather than instantly switching from the high
starting trip threshold to the low running trip threshold, the relay allows the
trip threshold to decay exponentially, using the same time constant as the
thermal element RC circuit from the initial starting value to the final running
value.
EXAMPLE F.1 Starting and Running Trip Level Calculations
Given a motor with the following characteristics, calculate the
starting and running thermal model trip thresholds.
Service Factor, SF = 1.15
Locked Rotor Current, I
L
= 6 per unit of full load amps
Locked Rotor Time From Operating Temperature, To = 12 seconds
Locked Rotor Time From Ambient Temperature, Ta = 14.4 seconds
Motor Starting Trip Threshold
= I
L
2
• To
= 36 • 12
= 432
Motor Running Trip Threshold
= I
L
2
• (Ta – To) • SF
2
= 36 • 2.4 • 1.323
= 114.3
I
L
2
Ta•
I
L
2
Ta To–()•
--------------------------------------6=
Ta
To
-------1.2=∴
Ta To–()0.2 To• =∴
_
+
3
I
1
2
+ 5 • I
2
2
I
L
2
•(Ta –To)
I
L
2
•(Ta –To) • SF
2
U
49T
Relay
Word
Bit
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