Faqs - Frequently Asked Questions - Siemens SIRIUS 3RT Series System Manual

SIRIUS 3RF34 solid-state switching devices

3.9 FAQs - Frequently asked questions

3.9 FAQs - Frequently asked questions
Troubleshooting/FAQs
Table 3- 21 Questions/Answers/Solutions
Frequently asked questions
Do solid-state switching devices
galvanically isolate the main current path?
One frequency converter is to be used to
control two motors. At high switching
frequencies, the load switches between
motors 1 and 2.
Are solid-state contactors suitable for this
application?
Can current paths be connected in parallel
to increase current carrying capacity?
Minimum load currents are given in the
technical data for 3RF34 solid-state
contactors.
What happens if these are undershot?
Are there auxiliary switches for the solid-
state contactors?
346
Answers/Solutions
No, solid-state switching devices are not capable of galvanic isolation. When
the supply is disconnected, a low leakage current (approx. 10 mA) still flows.
To carry out maintenance work on the load side, an additional switching
device with isolating features has to be used.
Solid-state contactors are not suitable for switching loads in circuits with
frequency converters or soft-starting devices. The non-sinusoidal voltage
waveform would damage important solid-state contactor components beyond
repair.
Parallel connection of current paths is not permissible. As the contact
resistances of the thyristors vary greatly, the current paths would be put under
significant asymmetrical load. As a result, the current path with the lowest
contact resistance would be subject to thermal overload.
The minimum load current for 3RF34 solid-state switching devices is 0.5 A.
Undershooting the minimum load current can cause the solid-state contactor
to shut down in each half-wave prior to zero crossing of current (current cuts
in and out - similar to generalized phase control). Therefore, the minimum
value should not be undershot.
Proposed solution:
Connect a load (e.g. a resistive load) in parallel to increase the load current
accordingly.
We often hear requests for the switching state to be fed back to the controller.
Since for physical reasons there are no auxiliary switches connected to the
main circuit in the case of solid-state switching devices, it is not possible to
signal the switching state directly.
The state of the switching device or, better, that of the motor, can be signaled
to the controller via the 3RR2 current monitoring module.
The solid-state contactors can be expanded for load monitoring with the
screw-on 3RR2 current monitoring relays. These relays monitor the
connected load, providing a means of feeding back the state of the entire load
feeder to the controller.
SIRIUS Innovations
System Manual, 01/2011, A8E56203870002-03