ABB REM 610 - 6 Application Examples; Setting Calculations; Protected Unit Scaling Factor; Thermal Overload Protection

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6. Application examples
6.1. Setting calculations
6.1.1. Protected unit scaling factor
The FLA secondary scaling factor for phase currents is calculated as follows:
FLA secondary scaling factor =
I
I
I
I
N
NM
NR
N
1
2
×
(5)
I
N1
= rated primary current of the CT
I
N2
= rated secondary current of the CT
I
NM
= rated current of the motor
I
NR
= rated current of the relay
The rated current of the protected unit, I
n
, equals the FLA of the motor provided that
the protected unit scaling factor has been properly set. The rated current of the
protected unit will equal that of the CT when the factor is 1.
Example:
Rated power, P
nm
4500 kW
Rated voltage, U
nm
3300 V
Rated current, I
nm
930 A
CT current ratio, I
N1
/I
N2
1000/5 A
Relay input, I
NR
5:00 AM
The FLA secondary scaling factor is calculated as follows: 1000 A/930 A x 5
A/5 A = 1.075 ≈ 1.08
Example:
Rated power, P
nm
900 kW
Rated voltage, U
nm
380 V
Rated current, I
nm
1650 A
CT current ratio 2000/1 A
Relay input 1:00 AM
The FLA secondary scaling factor is calculated as follows: 2000 A/1650 A x
1 A/1 A = 1.212 ≈ 1.21
6.1.2. Thermal overload protection
The FLA of the motor at an ambient temperature of 40°C determines the highest
permissible continuous load. In this case, an increase of five per cent in the motor
current will eventually cause a trip.
Motor Protection Relay
Technical Reference Manual - ANSI Version
REM 610REM 610
1MRS755537