Danfoss MCD3000 Series Manual page 51

Appendix
Typical Causes of SCR Damage
Overview
The reliability of the modern SCRs as used in the
MCD3000 Series soft starters is such that failures
attributable to faulty manufacture are almost nil. SCR
damage is almost always caused by external
influences . Often these influences can be identified but
in other cases the identification may prove difficult or
impossible because the damaging event was
temporary in nature.
Typical causes of SCR damage
SCR damage is generally caused by one of three
mechanisms, overcurrent, overvoltage or
overtemperature. Before replacing damaged SCRs it is
important to identify the cause of damage if at all
possible. The fo llowing list details some of the
common problems.
Overcurrent
•
Cable fault on output of soft starter.
• Motor fault.
•
Start current and/or start time exceeds the soft
starter ratings.
Overvoltage
•
Power supply transient or surge.
•
Lightening strike 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.
•
Faulty cooling fans.
•
Inadequate ventilation.
•
Excessive ambient temperatures.
MG.15.B2.02 – VLT is a registered Danfoss trademark
MCD3000 Series
Protecting SCRs
The SCRs used in modern soft starters are rugged
and provide reliable operation in most industrial
environments without the need for additional
protection. However the potential for SCR damage
can be reduced by use of semiconductor fuses and/or
line contactors.
Semiconductor fuses: Use of semiconductor fuses
reduces the potential for SCR damage caused by
short circuits on the output of the starter. Note that
protection systems such as circuit breakers or HRC
fuses do not operate quickly enough to protect SCRs
from short circuits.
Line contactors: SCRs are most vulnerable to damage
caused by overvoltage when they are in the off state
and have voltage applied to their input terminal. In this
condition the SCR is blocking the full line voltage. Use
of a line contactor to remove voltage from the SCR
input when the starter is in the off state eliminates the
chance of SCR damage due to overvoltage.
Note that when the soft starter is operating and the
SCRs are fully conducting the SCR is not blocking line
voltage and is thus immune to damage caused by
voltage fluctuations.
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