Schweitzer Engineering Laboratories SEL-701, Instruction Manual

Page: 387 / 468

Date Code 20010719
701 Motor Protection Relay
Motor Thermal Element
Motor Running Protection
E.11
EXAMPLE E.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
Example E.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
due to the drop in current-squared and due to the insertion of the resistor into the
model. The decay of U is exponential due to the interaction between the model thermal
capacitance and resistance. Rather than instantly switch 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.