Multifunction Protection and Switchgear Control unit REF542plus
Manual: Motor Protection with ATEX-Certification
Where:
I
average (1-pole)
Root-mean-square value of the current causing the temperature
rise in a single-pole functional test
I
L1
Current in conductor L1,
Therefore in a single-phase test, the current
3 )(3pol(1pol) II = (19.)
shall be increased by factor √3 in order to obtain a temperature rise that is compara-
ble to the one in a three-pole functional test.
4.5 Unbalanced load protection
The unbalanced load protection is intended to provide protection and monitoring of
electrical equipment against/for asymmetrical loading. Unbalance load protection is
mostly applied for protecting motors or generators.
The unbalanced load is calculated from the negative-phase-sequence component of
the three-phase conductor currents and has to be generated — in accordance with
the definitions given in the applicable regulations — on the basis of the relationship
between the current of the negative-phase-sequence component and the rated cur-
rent of the equipment that is to be protected. Since an unbalanced load leads to im-
permissible temperature rises in the laminated core of the rotor, it is necessary to
generate a tripping signal which is square-dependent on the unbalanced load in case
the permissible values are being exceeded. By means of the square-dependency, it is
possible to replica a temperature rise without losses (adiabatic curves). The tripping
time can be derived as follows:
22
2 s
II
K
t
−
=
(20.)
Where:
t Tripping time derived from the above temperature rise constant
K Temperature rise constant depending on the type of equipment
I
2
Unbalanced load related to the rated current
I
s
Response value for monitoring impermissible temperature rises
When a tripping took place, it is usually advisable not to restart the motor immediately
after the trip. The unbalanced load protection in the REF542plus has therefore been
provided with the option to block motor reconnection by means of an output signal.
Within the blocking time, the memory contents is subjected to linear clearing for the
tripping time. If the component to be protected is reconnected without waiting for a
complete cool-off, it is possible that another trip will take place - if the unbalanced
load limit is exceeded again - much faster than would be expected theoretically. In
addition, it is possible to reduce the duration of the blocking time, if necessary, in per-
centages.
1VTA100114-Rev. 04 en PTMV, 19.01.05 Motorprotection 27 / 66
Valid from Version V4D02
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