Siemens SENTRON 3VA1, Руководство пользователя

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Applications
3.3 Motor protection applications of 3VA molded case circuit breakers
3VA molded case circuit breakers with IEC certificate
152
Manual, 03/2019, A5E03603177010-03
Protection against phase unbalance
All 3VA2 molded case circuit breakers for motor protection have protection against
unbalanced current loading of the motor. This ensures that the motor is reliably protected
against overheating if a phase failure or major fluctuation of a phase current occurs.
The phase unbalance ratio is calculated from the mean value of the three phases L1, L2 and
L3 and the highest possible phase current deviating from this. If the phase unbalance ratio
exceeds the set value, the circuit breaker trips according to tripping time t
unbal
.
With ETU350M, the t
unbal
tripping times are permanently set to 0.7 s for startup and to 4 s for
continuous operation. With ETU550M and ETU860M, the t
unbal
tripping times for startup can
be set from 0.7 s to 60 s and for continuous operation from 4 s to 420 s. The lowest value is
always set as a factory default.
The adjustable tripping time can be used for operation with soft starters, for 2-phase phase-
angle control or DC braking with soft starters.
Calculation formulas:
l
MW
Mean value of the three phase currents
l
i
One phase current
With the ETU350M trip unit version, the unbalance ratio is permanently set to 40%.
With the ETU550M and ETU860M trip unit versions, the unbalance ratio can be set freely
between 5% and 50%. The default setting of 40% is set at the factory.
Trip classes T
C
The trip class T
c
specifies the tripping time T
p
for balanced 3-pole loads, starting from the
cold state, with 7.2 times the set current I
r
according to IEC EN 60947 4-1. Combinations
with class 10 are normally used.
The trip classes with "E" indicate a narrower tolerance band of tripping time T
p
. These trip
classes can also be implemented with the precise electronic trip units of the 3VA2 motor
protection circuit breakers.