Figure 247: Trip time curves
In order to account for the difference between the warm and cold curves the
Weighting factor p is used. This defines the difference, which is based on the warm
and cold locked rotor times defined by the motor manufacturer.
Weighting factor p
can be calculated by the following equation:
ℎ
=
2
∙ 1 −
∙ 100%
(
)
(Equation 53)
k
is the
Overload factor
,
LRT
warm
is the locked rotor time for warm start and
LRT
cold
is the locked rotor time for warm start.
The
Weighting factor p for the rotor is usually lower than for the stator, since the
temperature limit for the rotor is often higher. Therefore, the used thermal capacity,
when running at full load, can be quite low. For example, with warm and cold locked
rotor times being 5 and 6 seconds and
Overload factor being 1.05, the Weighting
factor p could be set to 18.4%.
ℎ
= 1.05
2
∙
1 −
5
6
∙
100% = 18.4%
(
)
(Equation 54)
In addition, the
Weighting factor p is used in the cumulative thermal level
calculation. It is used to properly distinguish between short-time thermal stress
1MRS759142 F
Protection functions
REX640
Technical Manual
431