Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 62)

M
3~
~ ~ ~
~ ~ ~
=
=
Fig. 2.4-5
The basic design of a circuit with rectifier, intermediate circuit and converter of the inverter. Frequently
filters are provided on the input side (whether internally or externally) to reduce supply interference.
As the reactive current of the motor is provided by the intermediate circuit capacitance, the
supply current is smaller than the motor current and its power factor cos
φ is nearly 1. Because
of the harmonic content of the current, the thermal overcurrent release of the circuit breaker
should be set to approx. to 1.2 times the motor rated current, but no more than the permissible
current carrying capacity of the connecting cable.
Short-circuit protection
Inverters usually protect themselves against short-circuits on the output side. Short-circuits on
the supply side between pole conductors or pole conductors and ground are switched off by the
upstream short-circuit protective device (circuit breaker or fuse).
Circuit breakers, motor protection relays or contactors in the output circuit of inverters
If low-voltage switchgear is installed in the output circuits of inverters, this must be compatible
with the high switching frequency of the output signal of several thousand Hz. The harmonic
content of the current can result in overheating of the devices. Especially in circuit breakers with
plunger-operated magnetic releases, the sensitivity to current harmonics increases with
decreasing rated current. As a guideline can serve: Due to the large number of windings on
plunger coils, excessive heating should be expected with versions with < 10 A rated current.
The manufacturers’ specifications should be observed.
With long screened lines between the output of the frequency converter and the motor, switch
contacts located between the devices can be stressed by high peak currents caused by the
capacitance of the line that may even result in welding of the contacts. The high charging
currents can under certain circumstances also result in undesired releasing. Filters can have
similar effects. The switching mode of inverters generating steep voltage slopes can lead to
additional voltage stress on long lines by traveling wave effects. Suitable filter measures may be
required as a remedy.
Generally it should be ensured by the control circuit that loadside contactors are switching
without load, i.e. that the frequency converters are switching on after the contactor and switch-
ing off before it.
Overload protection on the output side
LVSAM-WP001A-EN-P - April 2009 2-34
Overload protective devices with bimetal strips (bimetal relays and circuit breakers with bimetal
tripping mechanism) are designed for 50/60 Hz. As their mode of operation is based on the
heating of the bimetal strips by the motor current, the release values relate to heating by
50/60 Hz currents. Depending on the design of the device, the switching frequencies of
frequency converters extend from several kHz to the ultrasonic range and generate harmonic
currents in the output that result in additional temperature rise in the bimetal strips. Long
shielded lines to the motor can cause additional increases of the harmonic components
because of the line capacitance.