Date Code 20010719 701 Motor Protection Relay
Motor Thermal Element
Motor Starting Thermal Capacity
E.13
Motor Starting Thermal Capacity
A normal motor start is expected to use a significant percentage of the available
starting thermal capacity. After a motor start, it is generally necessary for the motor to
cool for a time before another start is permitted. The cooling can usually take place
while the motor is stopped or running.
If a motor usually requires 70% of the available thermal capacity to start and if a
start is attempted while the present % Thermal Capacity exceeds 30%, a locked rotor
trip is likely because the motor is still too hot.
The SEL-701 Relay provides two facilities to help ensure that a motor start is not
attempted while the motor is still too hot to be started safely. The Thermal Capacity
Used to Start (TCSTART) setting allows you to define a fixed value of thermal capacity
that you expect the motor to use on a start. The relay adds a 10% safety margin to this
setting, thus if you set Thermal Capacity Used to Start equal to 65%, the relay requires
the % Thermal Capacity to fall below 25% (100% – 65% – 10% = 25%) when the
motor is stopped before another start is permitted. The relay asserts the Thermal
Lockout until the motor is cool.
In addition, the relay learns a thermal capacity used to start by recording the
thermal capacity used during each of the past five successful motor starts. This learned
parameter is reported in the motor operating statistics report. When you set Use
Learned Starting Thermal Capacity (TCLRNEN) equal to Y, the relay uses the largest
of the last five starting percentages in place of the TCSTART value to determine when
to release the thermal lockout.
NOTE: Since the Thermal Capacity Used to Start percentage is
based on the starting thermal element trip threshold, the relay
automatically switches to that threshold for % Thermal
Capacity calculation as soon as the motor stops. You may
observe this as a large drop in the % Thermal Capacity value
when you stop the motor. This is normal operation for the
SEL-701 Relay.
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