Brochure

Circuit Breaker/MSP 3RV1
SIRIUS System Manual
2-74 GWA 4NEB 430 0999-02 DS 01
Example Circuit breaker/MSP with the setting range of 1.1 - 1.6 A behind a frequency
converter with a pulse frequency of 8 kHz and a r.m.s. value of the motor
current at rated load: 1.2 A.
Set to: 1.2 A x 1.21 = 1.45 A
This compensates for the influences of the high frequency current. The trip
current lays within the standard range.
Attention
Harmonics can cause the r.m.s. value of the motor current to be higher than
the rated motor current. In this case nuisance tripping can still occur despite
the use of the correction factors.
In order to remedy this, the r.m.s. value of the motor current at rated load
needs to be ascertained and used as the basis current for the above
described correction procedure. Only measuring instruments suitable to
ascertain the values are those that can measure the true r.m.s. value up to
the frequencies that appear and can also reproduce them. Devices well suit-
able for this would be hot-wire instruments for example. Moving-iron mea-
suring elements are in fact r.m.s. measuring instruments, but can only be
used for frequencies up to 1 kHz and therefore can’t be used in most of the
above described cases. Common instruments such as a multimeter or a clip-
on ammeter are not suitable for measuring in the above described cases.
2.8.2 Other possible influences
a) Capacitive leakage currents
Despite adjusting the setting, nuisance tripping can still occur in individual
installations. Extensive investigations have shown that installations with
pulsing voltage can also lead to other effects that lower the trip current of
the overload release, such as an increase of current flowing through the trip
element.
A practical example:
Consider an installation that is fed from an inverter with 3 kHz pulse fre-
quency and has motors connected with a 80 m long cable. An analysis of the
actual flowing current shows a ripple amplitude of the motor current with
very high frequency currents (up to 150 kHz) and a peak value of 1.5 A. The
influence on the thermal overload release is still significantly higher than
described in example 1 at these frequencies. Moreover, capacitive leakage
currents appear in this installation due to the length of the cable and the
high frequency. These increase the current that flows through the trip ele-
ment and can lead to nuisance tripping.
In cases where high frequency currents of well over 16 kHz appear and the
procedure described in example 1 no longer leads to success, then you can
proceed as follows. In an overload free operation of the motor the overload
release needs to be set so high that the unit will not trip. After that the
motor needs to run for about 1.5 h at full load. Then the overload release
needs to be reduced to the trip limit and then set about 10 % higher than