701 Motor Protection Relay Date Code 20010719
Settings Calculation
Basic Motor Protection
4.10
EXAMPLE 4.3 Thermal Element Rating Method Setting
A 4160 V, 600 HP motor is to be protected using the SEL-701 Relay
Thermal Element Rating Method. The motor data sheet includes
the following information.
Rated Horsepower (HP) = 600 HP
Rated Voltage (V) = 4160 V
Rated Full Load Current (A) = 76.9 A
Rated Locked Rotor Amps (A) = 462.0 A
Safe Stall Time at 100% Volts
Cold = 21 seconds
Hot = 16 seconds
Service Factor = 1.2
Phase current transformers having 80:5 ratios are selected for the
application. The SEL-701 Relay settings for the application are
calculated as shown below.
Current Transformer Ratio (CTR) = 80/5 = 16
CT Secondary Rating (ITAP) = 5
Full Load Amps (FLA) = 76.9/16 = 4.81 A secondary
Service Factory (SF) = 1.2
Locked Rotor Amps (LRA) = 462.0/16 = 28.9 A secondary
Hot Locked Rotor Time (LRTHOT) = 16 seconds
Cold Locked Rotor Time (LRTCOLD) = 21 seconds
The Locked Rotor Trip Time Dial setting reduces or extends the allowed
accelerating time under locked rotor conditions. You can always safely set this value
equal to 1.00. If you know that the driven load will always accelerate in less than the
rated locked rotor time, you may wish to use a Locked Rotor Trip Time Dial less than
1.00 to provide a faster trip in locked rotor conditions. Do not set the Locked Rotor
Trip Time Dial setting greater than 1.00, except in an emergency to allow a start with a
longer than normal accelerating time.
EXAMPLE 4.4 Locked Rotor Trip Time Dial Setting Calculation
In a particular application, a motor with a 10 second hot locked
rotor time always starts in 5 seconds.
Setting the Locked Rotor Trip Time Dial setting equal to 0.75
causes the relay to trip in 7.5 seconds under locked rotor
conditions. This setting allows ample time for the motor to start,
but does not subject the motor to the full 10 seconds of locked
rotor current if a locked rotor start attempt takes place.
Continue calculating the balance of thermal element settings with Thermal
Capacity Alarm Setting on page 4.21.
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