Brochure

Circuit Breaker/MSP 3RV1
SIRIUS System Manual
GWA 4NEB 430 0999-02 DS 01
2-73
2.8 Application notes for the use of 3RV1 downstream from frequency
converters/ inverter with pulsing voltage
The use of thermal motor protection devices downstream from frequency
converters / inverters with pulsing voltage results in influences on the
switching devices that could lead to the nuisance tripping of those devices.
In the following examples, practical guidelines are provided for such applica-
tions.
2.8.1 Influences of high frequency currents upon the thermal overload release
The thermal overload release on the circuit breakers/MSPs and overload
relays generally consists of a bimetal strip and a heating winding, that heat
up when motor current runs through them. The excessive bending of the
bimetal strip (such as with high motor current) results in the breaking of the
motor circuit.
This style of thermal overload release is set for 50 Hz AC current. So that the
trip point is also only for currents which have the same thermal effect (r.m.s.
value) or are similar to the set current in the required range of the standard.
This would be the case for AC current from 0 to 400 Hz and for DC current.
With high frequency currents, as occurs downstream of converters, the
bimetal strip becomes increasingly hot. This can be attributed to eddy cur-
rent loses induced by harmonics and to the Skin effect in the heater wind-
ings. Both lead to the tripping of the overload release even with lower cur-
rents (nuisance tripping!).
The influences are dependent on the frequency of the current. The higher
the frequency of the converter and the lower the adjustable range/rated cur-
rent, the higher the reduction of the trip current.
In order for the trip limits to once again fall into the standard range, the over-
load release setting needs to be corrected. The following table shows the
correction factors for the various setting ranges depending on the pulse fre-
quency of the converters.
Setting range /
Rated current
Pulse frequency [kHz]
0 2 4 6 8 10 12 14 16
3.2 - 50 A 1.00 1.07 1.12 1.16 1.18 1.19 1.21 1.22 1.23
0.5 - 2.5 A 1.00 1.08 1.13 1.17 1.21 1.24 1.26 1.28 1.29
0.32 - 0.4 A 1.00 1.09 1.15 1.21 1.25 1.29 1.33 1.35 1.37
0.16 - 0.25 A 1.00 1.10 1.17 1.24 1.28 1.33 1.38 1.42 1.46
Table 2-20: Correction factors for the various setting ranges