Troubleshooting
ASTAT XL Service Manual
3.4 SCRs
SCR damage is generally caused by overcurrent, overvoltage or overtemperature. To prevent future
damage, check that the soft starter has been installed properly. Common causes of SCR problems
include:
Overcurrent:
• cable fault on soft starter output
• motor fault
• start current and/or start time exceeds the soft starter's rating
• starts per hour exceed the soft starter rating
Overvoltage:
• power supply transient or surge
• lightning strike (direct or indirect) on power supply
• motor fault
• loose connection in power circuit, before or after the starter
• power factor correction connected to the output of the soft starter
• over-corrected bulk power factor correction on a lightly loaded system causing severe ringing
voltages
Overtemperature:
• blocked heatsinks or restricted ventilation
• inadequate ventilation
• excessive ambient temperatures
• bypass relay fails to close during running (internally bypassed starters only)
3.4.2 Protecting SCRs
Modern SCRs are generally rugged and reliable. However, the risk of SCR damage can be reduced by
using semiconductor fuses and/or a main contactor.
3.4.2.1 Semiconductor Fuses
Semiconductor fuses reduce the potential for SCR damage caused by short circuits on the output of the
starter.
Protection systems such as circuit breakers or HRC fuses do not operate quickly enough to protect
SCRs from short circuits.
3.4.2.2 Main Contactors
SCRs are most vulnerable to overvoltage damage when voltage is applied to their input terminal while
they are off. In this condition the SCR is blocking the full line voltage. Using a main contactor to remove
voltage from the SCR input when the starter is off eliminates the risk of SCR damage due to
overvoltage.
DET-813 13
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