Question:
For which utilisation category (AC1, AC3) should the mains contactors for drive controllers be dimensioned?
Answer:
The different utilisation categories for contactors are described in the IEC 60947-4-1 standard.
The utilisation categories which may be used for the mains input of drive controllers are listed below:
- AC1: Non-inductive or slightly inductive loads, resistive furnaces.
- AC3: Squirrel-cage motors: starting, switches off motors during running time.
Lenze recommends to dimension the mains contactors for Lenze controllers according to
utilisation category AC3 to ensure long-term reliable system operation.
Practical experience has shown that problems with 'sticky' or 'burnt' contactor contacts can thus be avoided.
Utilisation category AC3 also covers short-term high overcurrents during switch-on and switch-off typically occurring when connecting and disconnecting motors directly on the mains (inductive load). Though starting currents are not that high when motors are operated on controllers and the inductive load is disconnected from the mains via the inverter, short-term overcurrents may also occur during operation and load the mains contactors (overcurrent capability of the controllers). Furthermore, the mains contactors will be exposed to high current loads in error situations when the mains fuses respond.
The following must be considered if, in special cases, mains contactors according to utilisation category 'AC1' are to be used:
- The rated contactor current specified for AC1 must not be exceeded even not during switch-on and switch-off.
- The charging current for the DC bus occurring during switch-on must be considered. For Lenze controllers with charging current limitation (i700, 9400, 8400) consisting of charging resistor and bypass relay, the initial charging current is usually lower than the rated controller current. I. e. dimensioning may be based on the rated device currents.
- The controllers must be inhibited when the mains contactor is switched off to ensure that there will not flow any current when the contactor contacts are opened.
- After triggering of the mains fuses, the mains contactors must be checked for proper functioning and replaced, if necessary.
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