ABB 615 Series ANSI, Техническое руководство, Страница: 324 / 713

1MAC050144-MB C
Section 4
Protection functions
615 series ANSI 317
Technical Manual
Setting the overload factor
The value of Overload factor defines the highest permissible continuous load. The
recommended value is 1.05.
Setting the negative sequence factor
During the unbalance condition, the symmetry of the stator currents is disturbed and a
counter-rotating negative sequence component current is set up. An increased stator
current causes additional heating in the stator and the negative sequence component
current excessive heating in the rotor. Also mechanical problems like rotor vibration can
occur.
The most common cause of unbalance for three-phase motors is the loss of phase resulting
in an open fuse, connector or conductor. Often mechanical problems can be more severe
than the heating effects and therefore a separate unbalance protection is used.
Unbalances in other connected loads in the same busbar can also affect the motor. A
voltage unbalance typically produces 5 to 7 times higher current unbalance. Because the
thermal overload protection is based on the highest TRMS value of the phase current, the
additional heating in stator winding is automatically taken into account. For more accurate
thermal modeling, the
Negative Seq factor setting is used for taking account of the rotor
heating effect.
(Equation 23)
R
R2
rotor negative sequence resistance
R
R1
rotor positive sequence resistance
A conservative estimate for the setting can be calculated:
I
LR
locked rotor current (multiple of set Rated current). The same as the startup current at the beginning of
the motor startup.
For example, if the rated current of a motor is 230 A, startup current is 5.7 x I
r
,
Setting the thermal restart level
The restart disable level can be calculated as follows:
(Equation 24)
Negative Seq factor
R
R
R
R
=
2
1
Negative Seq factor
I
LR
=
175
2
Negative Seq factor = =
175
5 7
5 4
2
.
.

i
startup time of the motor
operatetime when no prior load
= − ×
100 10 % 00%
+
⎛
⎝
⎜
⎞
⎠
⎟
margin