Date Code 20080918
Instruction Manual
SEL-749M Relay
Appendix F
Motor Thermal Element
Instructi on Manual
Overview
The SEL-749M Relay uses a patented protection algorithm to provide
effective motor thermal protection. The relay offers two convenient methods
to set the thermal element. They are:
➤
Motor ratings method
➤
Thermal limit curve method
These setting methods are described in detail in
. The two methods accommodate differences in protected
motors and in the amount and type of motor information available. They also
offer the majority of relay users at least one familiar setting method.
While the implementation details of each setting method vary, the
fundamental thermal element is the same for both, so this generalized
discussion applies to both methods equally. Regardless of the setting method
used, the thermal element provides motor protection for the following
potentially damaging conditions:
➤
Locked Rotor Starts
➤
Running Overload
➤
Operation Under Unbalanced Currents
➤
Too Frequent or Prolonged Starting
In order to provide integrated protection for all these conditions, the thermal
element:
➤
Continuously maintains a numeric estimate analogous to the
heat energy in the motor.
➤
Adjusts the heat estimate based on the measured positive-
sequence and negative-sequence current flowing in the motor.
➤
Weights the heating effect of negative-sequence current as five
times the heating effect of positive-sequence current when the
motor is running.
➤
Weights the heating effects of positive- and negative-sequence
current equally when the motor is starting.
➤
Models the heat lost to the surroundings when the motor is
running.
➤
Compares the present heat estimate to an adaptive starting trip
threshold or a running trip threshold, depending on the state of
the motor.
Содержание SEL-749M
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