GE M60 UR Series, Instruction Manual

Page: 278 / 662

5-150 M60 Motor Protection System GE Multilin
5.6 GROUPED ELEMENTS 5 SETTINGS
5
substantial current in the rotor. This current will have a frequency that is approximately twice the line frequency: 100 Hz
for a 50 Hz system or 120 Hz for a 60 Hz system. Skin effect in the rotor bars at this frequency will cause a significant
increase in rotor resistance and therefore, a significant increase in rotor heating. This extra heating is not accounted for
in the thermal limit curves supplied by the motor manufacturer as these curves assume positive sequence currents
from a perfectly balanced supply voltage and motor design.
The thermal model may be biased to reflect the additional heating that is caused by negative sequence current when
the motor is running. This biasing is done by creating an equivalent motor heating current rather than simply using
average three phase RMS. This equivalent current is calculated using the equation shown below.
I
eq
= equivalent motor heating current
I
per_unit
= per unit current based on FLA
I_1 = positive sequence current
I_2 = negative sequence current
k = constant
(EQ 5.12)
The motor derating as a function of voltage unbalance as recommended by NEMA (National Electrical Manufacturers
Association) is shown below. Assuming a typical induction motor with an inrush of 6

FLA and a negative sequence
impedance of 0.167, voltage unbalances of 1, 2, 3, 4, and 5% equals current unbalances of 6, 12, 18, 24, and 30%
respectively. Based on this assumption, the amount of motor derating for different values of k entered for setting
UNBALANCE BIAS K FACTOR
is also shown below. Note that the curve created when k = 8 is almost identical to the
NEMA derating curve.
Figure 5–58: MEDIUM MOTOR DERATING FACTOR DUE TO UNBALANCED VOLTAGE
If a value of k = 0 is entered, unbalance biasing is defeated and the overload curve will time out against the measured
per unit motor positive sequence current. The following equations can be used to calculate k.
(EQ 5.13)
• COOL TIME CONSTANT RUNNING / STOPPED: The thermal capacity used value is reduced in an exponential man-
ner when the motor current is below the full load amps

service factor settings to simulate motor cooling. The motor
cooling time constants should be entered for both the stopped and running cases. A stopped motor will normally cool
significantly slower than a running motor.
Motor cooling is calculated as follows:
(EQ 5.14)
(EQ 5.15)
where:
– TCU = thermal capacity used.
– TCU
start
= TCU value caused by overload condition.
I
eq
I
2
per_unit
1 k
I_2
I_1
--------
 
 
2

+
 
 

where :

=
808728A2.CDR
0.70
0.75
0.80
0.85
0.90
0.95
1.00
1.05
0 1 2 3 4 5
PERCENT VOLTAGE UNBALANCE
DER
A
T
IN
G
F
A
C
T
O
R
NEMA
k=2
k=4
k=6
k=8
k=10
0.70
0.75
0.80
0.85
0.90
0.95
1.00
1.05
0 1 2 3 4 5
PERCENT VOLTAGE UNBALANCE
DERA
T
ING
F
A
CT
O
R
GE Multilin
k
175
I
LR
2
----------
=
(typical estimate); k
230
I
LR
2
----------
=
(conservative estimate), where I
LR
is the per unit locked rotor current
TCU TCU
start
TCU
end
–
 
e
t
 
–
 
TCU
end
+ =
TCU
end
I
eq
overload_pickup
-------------------------------------------
 
 
1
hot
cold
-----------
–
 
 
100%

=