701 Motor Protection Relay Date Code 20010719
Settings Calculation
Basic Motor Protection
4.12
EXAMPLE 4.5 Thermal Element Generic Method Setting
A 4160 V, 800 HP motor is to be protected using the SEL-701 Relay
Thermal Element Generic Curve Method. The motor data sheet
includes the following information.
Rated Horsepower (HP) = 800 HP
Rated Voltage (V) = 4160 V
Rated Full Load Current (A) = 101.0 A
Rated Locked Rotor Amps (A) = 620.4 A
Safe Stall Time, Hot = 30 seconds
Service Factor = 1.15
Each increase in generic curve number increases the hot motor
thermal limit time by 2.1 seconds at six times full load current.
Therefore, we can select the maximum curve number using the
following equation.
Phase current transformers having 150:5 ratios are selected for the
application. The SEL-701 Relay settings for the application are
shown below.
Current Transformer Ratio (CTR) = 150/5 = 30
CT Secondary Rating (ITAP) = 5
Full Load Amps (FLA) = 101/30 = 3.36 A secondary
Service Factor (SF) = 1.15
Curve Number (CURVE) = 14
Curve
Safe Stall Time, Hot (seconds)
2.1 seconds
-------------------------------------------------------------------------------=
Curve 30/2.1 = 14.3; select curve 14 or less=
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