Schweitzer Engineering Laboratories SEL-749M, Instruction Manual

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F.9
Date Code 20080918 Instruction Manual SEL-749M Relay
Motor Thermal Element
Interpreting Percent Thermal Capacity Values
Interpreting Percent Thermal Capacity Values
Several of the SEL-749M reporting functions include a % Thermal Capacity
value. At all times, the relay uses
Equation F.9 to calculate the percent thermal
capacity.
Equation F.9
By this definition, when the % Thermal Capacity reaches 100%, the heat
estimate equals the trip value and the thermal element trips.
As Example F.1 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 may indicate
gradually increasing load torque, which could eventually result in an
unwanted locked rotor trip and subsequent downtime.
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.
The SEL-749M helps to ensure that a motor start is not attempted while the
motor is still too hot to be started safely. The TCSTART setting allows you to
define a fixed value of thermal capacity used above which the relay asserts the
Thermal Lockout until the motor is cool. See
Equation 4.3 , Equation 4.4 , and
the associated description for setting criteria.
Thermal Element Trip-Time Equations
Figure 4.1 and Figure 4.2 in Section 4: Protection and Logic Functions show
trip-time curves for selected settings of the thermal element. Following are
equations for calculating the trip times or curves for any settings.
As stated earlier, the motor model consists of distinct starting and running
thermal elements (see
Figure F.4 and Figure F.7 ). Equations for each element
appear in the following text. For simplicity, all motor currents are assumed to
be balanced three phase and in per unit of full load current (e.g., motor
current I = motor current/full load current).
% Thermal Capacity
Present Heat
Estimate, U
⎝ ⎠
⎛ ⎞
Present Thermal
Trip Value
⎝ ⎠
⎛ ⎞
--------------------------------------------
100% • =
Courtesy of NationalSwitchgear.com