Special Issues Relating To Motor-Side Contactors And Circuit Breakers - Siemens SINAMICS G130 Engineering Manual

1.9.2

Special issues relating to motor-side contactors and circuit breakers

General
Motor-side contactors and circuit breakers are not required for the majority of applications. In special cases they may
be needed, however, for example if
·
a bypass circuit is provided for the converter,
·
a means of disconnecting the converter from the motor must be provided for safety reasons,
·
one converter is provided for multiple motors and one motor at a time is connected to the
converter.
·
motors in group drives need to be individually protected against overload.
Contactors
Motor-side contactors are normally designed according to utilization category AC-3 (starting of squirrel-cage motors)
depending on the rated voltage and current ratings of the motor. For the range of power ratings of the converters and
inverters described in this engineering manual, it is not generally necessary to overdimension the contactors to
handle the capacitive charge/discharge currents associated with long motor cables.
However, switching at low output frequencies of less than around 5 Hz, which is possible in theory at the converter
output, is an issue of critical importance. Because the lower the output frequency, the longer it takes until the arc at
the contacts is interrupted by the voltage zero passage. As a result, the contacts can wear after just a few switching
operations. Switching at low output frequencies should therefore be avoided wherever possible. In applications which
do not require the contactor to operate during operation, the contactor should not be opened during operation, i.e. the
converter sequence control should always issue the pulse disable command for the inverter before the motor-side
contactor is opened.
Circuit breakers
Motor-side circuit breakers are normally designed according to the voltage and current ratings of the motor and can
be used at frequencies of up to 400 Hz. However, the following points need to be taken into account:
The response value of the instantaneous short-circuit release changes as a function of frequency. Typical reference
values are given below:
·
5 Hz: standard value according to data sheet for 50 Hz
·
50 Hz standard value according to data sheet for 50 Hz
·
100 Hz standard value according to data sheet for 50 Hz
·
200 Hz standard value according to data sheet for 50 Hz
·
300 Hz standard value according to data sheet for 50 Hz
·
400 Hz standard value according to data sheet for 50 Hz
These changes are of only secondary importance from a practical design viewpoint, however, since the standard
response value according to the data sheet for 50 Hz corresponds to more than 10 times the rated current value.
The response value of the thermally delayed overload release can be reduced by a significant amount from the value
stated in the data sheet owing to the current harmonics associated with the pulse frequency and pulse pattern and
the capacitive charge/discharge currents which typically occur with long motor cables. This is because the thermal
overload release of circuit breakers generally consists of a bimetal strip and a heater coil which are heated as the
motor current flows through them. When the bimetal strip is deflected beyond a certain limit the circuit breaker trips.
Releases of this kind are calibrated with an alternating current of 50 Hz. The release point is thus calibrated only for
currents within the required standard range which have an rms, i.e. a thermal effect, which is identical or similar to the
calibration current. This applies to alternating currents in the 0 to 400 Hz range. The relatively high-frequency,
capacitive charge/discharge currents associated with long motor cables cause increased heating of the bimetal strip.
This is attributable in part to the induction of eddy currents and in part to the skin effect in the heater coil. Both effects
cause the thermal overload release to trip prematurely.
In consequence, motor-side circuit breakers should be selected such that the motor rated current is at the lower end
of the setting scale of the thermal overload release. This means that the circuit breaker does not need to be replaced
when corrections are necessary at the drive commissioning stage. The lower the motor output and the longer the
motor cable, the larger the setting margin will need to be.
Circuit breakers with thermal overload releases need to be overdimensioned when they are installed in group drive
systems in which a large number of low-output motors are supplied by a single high-output converter and the motors
needed to be individually protected by circuit breakers with thermal overload releases. If the motor rated currents are
within the single-digit ampere range, the circuit breakers should be sized such that the setting scale of the thermal
overload release extends to a value which equals approximately 2 to 3 times the motor rated current.
Fundamental Principles and System Description
SINAMICS Engineering Manual – November 2015
Engineering Information
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