Section 4
1MAC050144-MB C
Protection functions
208
615 series ANSI
Technical Manual
When the voltages supplied to an operating motor become unbalanced, the
positive-sequence current remains substantially unchanged, but the negative-sequence
current flows due to the unbalance. For example, if the unbalance is caused by an open
circuit in any phase, a negative-sequence current flows and it is equal and opposite to the
previous load current in a healthy phase. The combination of positive and
negative-sequence currents produces phase currents approximately 1.7 times the previous
load in each healthy phase and zero current in the open phase.
The negative-sequence currents flow through the stator windings inducing
negative-sequence voltage in the rotor windings. This can result in a high rotor current that
damages the rotor winding. The frequency of the induced current is approximately twice
the supply frequency. Due to skin effect, the induced current with a frequency double the
supply frequency encounters high rotor resistance which leads to excessive heating even
with phase currents with value less than the rated current of the motor.
The negative-sequence impedance of induction or a synchronous motor is approximately
equal to the locked rotor impedance, which is approximately one-sixth of the normal motor
impedance, considering that the motor has a locked-rotor current of six times the rated
current. Therefore, even a three percent voltage unbalance can lead to 18 percent stator
negative sequence current in windings. The severity of this is indicated by a 30-40 percent
increase in the motor temperature due to the extra current.
4.1.9.7
Signals
Table 193: 46M Input signals
Table 194: 46M Output signals
4.1.9.8
Settings
Table 195: 46M Group settings
Name
Type
Default
Description
I2
SIGNAL
0
Negative phase sequence current
BLOCK
BOOLEAN
0=False
Block signal for activating the blocking mode
Name
Type
Description
TRIP
BOOLEAN
Trip
PICKUP
BOOLEAN
Pickup
BLK_RESTART
BOOLEAN
Overheated machine reconnection blocking
Parameter
Values (Range)
Unit
Step
Default
Description
Pickup value
0.01...0.50
xIn
0.01
0.20
Pickup value
Operating curve type
5=ANSI DT
15=IEC DT
17=Inv. Curve A
18=Inv. Curve B
5=ANSI DT
Selection of time delay curve type
Machine time Mult
5.0...100.0
0.1
5.0
Machine related time constant
Trip delay time
100...120000
ms
10
1000
Trip delay time
Содержание 615 Series ANSI
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Страница 389: ...Section 4 1MAC050144 MB C Protection functions 382 615 series ANSI Technical Manual ...
Страница 485: ...Section 6 1MAC050144 MB C Supervision functions 478 615 series ANSI Technical Manual ...
Страница 635: ...Section 10 1MAC050144 MB C Other functions 628 615 series ANSI Technical Manual ...
Страница 707: ...Section 15 1MAC050144 MB C Applicable standards and regulations 700 615 series ANSI Technical Manual ...
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