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MCD 500 Service Manual
5 Avoiding Damage
5.1 SCRs
5.1.1 Typical Causes of SCR Damage
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)
5.1.2 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.
5.1.3 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.
5.1.4 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.
5.2 Output Relays
MCD 500 soft starters have four programmable output relays. These relays are often used to control the main contactor.
The electronic contactor coils used in many contactors have a high initial inrush current, which can damage the soft starter's internal relays if the contactor
coil is switched directly.
MG.17.L1.02 - VLT
®
is a registered Danfoss trademark
5 Avoiding Damage
5
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