Sealed Enclosures; Primary Resistance Starters - Danfoss VLT MCD 500 Pocket Manual

Sealed enclosures: Can soft starters be installed
in sealed enclosures?
Soft starters can be installed in sealed enclosures, provided the
ambient temperature within the enclosure will not exceed the soft
starter's rated temperature.
All heat generated within the enclosure must be dissipated, either by
ventilation or through the enclosure's walls. This includes heat not
only from the soft starter but also from other components such as
fuses, cabling and switchgear. Heating from the soft starter can be
minimised by installing the starter in a bypassed configuration. To
minimise external heating, protect the enclosure from direct sunlight.
WinStart includes a function to help design enclosure ventilation.
Primary Resistance Starters: How does soft start
compare to primary resistance starting?
Soft starters are more flexible and reliable than primary resistance
starters.
Primary resistance starters cannot accommodate varying load
conditions (e.g. loaded or unloaded starts) and the start torque cannot
be fine-tuned to match motor and load characteristics. Performance
may vary with multiple starts in close succession, because the start
profile changes as the resistance heats up. Damaging torque and
current transients still occur at the steps between voltages, and
primary resistance starters are not capable of providing soft stop.
Primary resistance starters are large and expensive, and liquid
resistance starters require frequent maintenance.
32 | Danfoss Drives · DKDD.PB.551.A1.02