Appendix; Typical Causes Of Scr Damage - Danfoss MCD 200 Series Service Manual

Appendix

•

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 following 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.
•
Starts per hour exceed the soft starter rating.
Overvoltage
•
Power supply transient or surge.
•
Lightening 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.
In most cases, failure caused by Overvoltage will be
indicated by physical damage to the MOVs (2). These
are located on the underside of the Interface PCB.
Overtemperature
•
Blocked heatsinks or restricted ventilation.
•
Inadequate ventilation.
•
Excessive ambient temperatures.
If the internal bypass relay does not operate, SCRs will
eventually fail due to thermal stress.
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
30
MCD 200 Series
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 in Run
mode, the SCRs are bypassed by the internal bypass
relay. In this situation, the soft starter is immune to
damage caused by voltage fluctuations.
MG.17.D3.02 – VLT is a registered Danfoss trademark