F.6
SEL-749M Relay
Instruction Manual
Date Code 20080918
Motor Thermal Element
Motor Starting Protection
current. When the summed current is greater than 2.5 times the motor rated
full load current setting, the relay uses the starting model. When the summed
current drops below 2.4 times the rated full load current, the relay uses the
running model.
Motor Starting Protection
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. This is true for the motor initially at or below
the normal operating temperature and the rating method is used. If a less
conservative curve method is used, the locked rotor trip time will adapt to the
actual initial temperature (heat estimate at the beginning of the motor start).
For example, the locked rotor trip time will be 120 percent of To for the motor
at an ambient temperature.
Figure F.4
Motor Starting Thermal Element
NOTE:
The trip value shown in the
is the limiting value.
Actually, the trip value ramps up
during the starting mode and will not
reach the limit for a successful motor
start.
When a successful motor start occurs and positive-sequence current drops
below 2.4 times full load current, the relay switches from the starting thermal
element to the running thermal element. The present heat estimate, U, and the
present trip value are transferred directly to the running element, representing
the heat build-up that occurred during motor starting.
_
+
3
3 • (I
1
2
+ I
2
2
)
I
L
2
• To
U
49T
Relay
Word
Bit
Summary of Contents for SEL-749M
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