701 Motor Protection Relay Date Code 20010719
Motor Thermal Element
Interpreting Percent Thermal Element Capacity Values
E.12
Interpreting Percent
Thermal Element Capacity Values
Several of the SEL-701 Relay reporting functions include a % Thermal Capacity
value. At all times, the relay calculates the percent thermal capacity using
Equation E.9.
Equation E.9
By this definition, when the % Thermal Capacity reaches 100%, the heat estimate
equals the trip value and the thermal element trips.
As Example E.1 on page E.11 shows, the thermal trip values for the running and
starting elements are very different. For this reason, it is not generally meaningful to
compare the % Thermal Capacity during a start to the % Thermal Capacity during
running conditions. However, it is quite useful to compare the % Thermal Capacities of
several starts using the relay Motor Start Reports and Motor Start Trend data. Using
this data, you may notice an increasing trend in the Starting % Thermal Capacity, the
final % Thermal Capacity value when the thermal model switches from starting to
running. This could indicate gradually increasing load torque which could eventually
result in an undesirable locked rotor trip and subsequent down-time.
The average and peak running % Thermal Capacity values reported in the motor
operating statistics report also offer a useful basis of comparison, allowing you to
compare the present % Thermal Capacity to normal values.
% Thermal Capacity
Present Heat
Estimate, U
Present Thermal
Trip Value
---------------------------------------
100 %•=
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