Schweitzer Engineering Laboratories SEL-701 - Motor Starting Protection; Figure E.4 Motor Starting Thermal Element

701 Motor Protection Relay Date Code 20010719
Motor Thermal Element
Motor Starting Protection
E.8
Motor Starting Protection
Figure E.4 Motor Starting Thermal Element.
Figure E.4 shows the thermal element used when the motor is starting. Locked
rotor heating occurs over just a few seconds, so the model assumes that no heat is lost
to the surroundings and the resistor is removed from the thermal circuit. The thermal
trip value is defined by the motor rated locked rotor current, I
L
, squared, times the rated
hot motor locked rotor time, To. The thermal capacitance is selected to match the heat
source heating factor, 3. By setting the capacitance equal to 3, when the motor positive-
sequence current, I
1
, equals locked rotor current, I
L
, the heat estimate, U, reaches the
trip value in exactly locked rotor time, To.
When a successful motor start occurs and positive-sequence current drops below
2.5 times full load current, the relay switches from the starting thermal element to the
running thermal element. The present heat estimate, U, is transferred directly to the
running element, representing the heat build-up that occurred during motor starting.
49T
I
L
2
• To
_
+
3 • (I
1
2
+ I
2
2
)
U
3
Relay
Word
Bit