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CHAPTER 6: SETPOINTS
339 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
MOTOR
STANDARD CURVE MULTIPLIERS
CURRE
× 1
× 2
× 3
× 4
NT
(× FLA)
10.00
1.39 2.78 4.16 5.55 6.94 8.33 9.71 11.1
15.00
1.39 2.78 4.16 5.55 6.94 8.33 9.71 11.1
20.00
1.39 2.78 4.16 5.55 6.94 8.33 9.71 11.1
Table 6-2: Conversion Between NEMA Curves and 339 Curve Multiplier
NEMA Curve
Class 10
339 Curve Multiplier
4
UNBALANCE BIASING
Unbalanced phase currents, that is to say negative sequence currents, cause rotor heating
in addition to the normal heating caused by positive sequence currents. When the motor is
running, the rotor rotates in the direction of the positive-sequence Magnetomotive Force
(MMF) wave at near synchronous speed. The induced rotor currents are at a frequency
determined by the difference between synchronous speed and rotor speed, typically 2 to
4 Hertz. At these low frequencies the current flows equally in all parts of the rotor bars,
right down to the inside portion of the bars at the bottom of the slots. Negative-sequence
stator current on the other hand causes an MMF wave with a rotation opposite to rotor
rotation, which induces rotor current with a frequency approximately 2 times the line
frequency: 100 Hz for a 50 Hz system or 120 Hz for a 60 Hz system. The skin effect at this
frequency restricts the rotor current to the outside portion of the bars at the top of the
slots, causing a significant increase in rotor resistance and therefore significant additional
rotor heating. 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 balanced motor construction.
To account for this additional heating, the relay allows for the thermal overload curve to be
biased with negative sequence current. This biasing is accomplished by using an
equivalent motor heating current rather than the simple motor terminal current (I
equivalent current is calculated according to the equation:
where:
I
= equivalent motor heating current in per-unit on an FLA base
eq
I
= average of each motor terminal's RMS current in per-unit on an FLA base I
avg
negative sequence to positive sequence current ratio k = value of the Unbalance K Factor
setpoint, which is used to adjust the degree of unbalance biasing.
k may be estimated as:
where I
is the locked rotor current in per-unit on an FLA base.
LR
If a k value of 0 is entered, the unbalance biasing is defeated and the overload curve will
time out against the average per-unit motor current.
× 5
× 6
× 7
× 8
× 9
12.4
0
9
12.4
0
9
12.4
0
9
Class 15
6
S3 PROTECTION
× 10 × 11 × 12 × 13 × 14 × 15
13.8
15.2
16.6
18.0
19.4
8
7
5
4
3
13.8
15.2
16.6
18.0
19.4
8
7
5
4
3
13.8
15.2
16.6
18.0
19.4
8
7
5
4
3
Class 20
Class 30
8
12
). This
avg
/ I
2
1
20.8
2
20.8
2
20.8
2
Eq. 7
=
Eq. 8
6–39
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