Siemens SIMPRO-100 Instruction Manual page 212

E
Motor Thermal Element
Example: Starting and Running Trip Level
Calculations
Given a motor with the following
characteristics, calculate the starting and
running thermal model trip thresholds.
Service Factor,
SF = 1.15
Locked Rotor Current,
I = 6 per unit of full load amps
L
Locked Rotor Time From Operating
Temperature,
To = 12 seconds
Locked Rotor Time From Ambient
Temperature,
Ta = 14.4 seconds
Motor Starting Trip Threshold
2
= I • To
L
= 36 • 12
= 432
Motor Running Trip Threshold
2
= I • (Ta – To) • SF
L
= 36 • 2.4 • 1.323
= 114.3
The example "Starting and Running Trip Level
Calculations" illustrates the difference between
the trip thresholds of the starting and running
thermal elements. When the relay switches from
the starting to the running thermal element, it
maintains the present heat estimate U, which
begins to decrease due to the drop in
current-squared and due to the insertion of the
resistor into the model. The decay of U is
exponential due to the interaction between the
model thermal capacitance and resistance.
Rather than instantly switch from the high starting
trip threshold to the low running trip threshold, the
relay allows the trip threshold to decay
exponentially using the same time-constant as
the thermal element RC circuit from the initial
starting value to the final running value.
210
E.6
Several of the SIMPRO-100 Relay reporting
functions include a % Thermal Capacity value. At
all times, the relay calculates the percent thermal
capacity using 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 the example "Starting and Running Trip Level
Calculations" 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
2
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.
Interpreting Percent
Thermal Element
Capacity Values
SIMPRO-100
Equation E.9
PRIM-2400C