GE 469 Instruction Manual page 197

CHAPTER 5: SETTINGS
469 MOTOR MANAGEMENT RELAY – INSTRUCTION MANUAL
This extra heating is not accounted for in the thermal limit curves supplied by the motor
manufacturer, as these curves assume only positive-sequence currents from a perfectly
balanced supply and motor design.
The 469 measures the ratio of negative to positive-sequence current. 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 accomplished by creating an equivalent
motor heating current rather than simply using average current (I
current is calculated as shown below.
I
eq
where: I = equivalent motor heating current
eq
I = per unit current based on FLA
per_unit
I = negative sequence current, I
2
k = constant
The figure below shows recommended motor derating as a function of voltage unbalance
recommended by NEMA (the National Electrical Manufacturers Association). Assuming a
typical induction motor with an inrush of 6 x FLA and a negative sequence impedance of
0.167, voltage unbalances of 1, 2, 3, 4, and 5% equal current unbalances of 6, 12, 18, 24,
and 30% respectively. Based on this assumption, the GE Multilin curve illustrates the motor
derating for different values of k entered for the
that the curve created when k = 8 is almost identical to the NEMA derating curve.
1.05
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0 1 2
PERCENT VOLTAGE UNBALANCE
NEMA
FIGURE 5–14: Medium Motor Derating Factor due to Unbalanced Voltage
If a k value of 0 is entered, the unbalance biasing is defeated and the overload curve will
time out against the measured per unit motor current. k may be calculated as:
175
k (typical estimate); k
-------- -
=
2
I
LR
where I is the per-unit locked rotor current.
LR
Motor Cooling
The thermal capacity used value decreases exponentially when the motor current is less
than the
OVERLOAD PICKUP
cooling time constants should be entered for both stopped and running cases. Since
cooling is exponential, the time constants are one-fifth of the total time from 100%
thermal capacity used to 0%. A stopped motor normally cools significantly slower than a
running motor. Motor cooling is calculated as:
I
⎛ ⎛ ⎞
2
2
I ⋅ 1 k ⋅ --- -
= +
per_unit
⎝ ⎝ ⎠
I
1
= positive sequence current
1
UNBALANCE BIAS K FACTOR
1.05
1.00
0.95
0.90
0.85
0.80
0.75
0.70
3 4 5
230
(conservative estimate)
-------- -
=
2
I
LR
settings. This reduction simulates motor cooling. The motor
). This equivalent
per_unit
2
⎞
⎠
settings. Note
0 1 2 3 4
PERCENT VOLTAGE UNBALANCE
GE MULTILIN
(EQ 5.3)
k=2
k=4
k=6
k=8
k=10
5
808728A1.CDR
(EQ 5.4)
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