Avoiding Damage; Scrs; Mg.17.L4.02 - Vlt; Typical Causes Of Scr Damage - Danfoss MCD500 Service Manual

Avoiding Damage

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 semicon-
ductor 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.
MCD 500 Service Manual

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 three programmable output
relays. These relays are often used to control external
contactors.
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.
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MG.17.L4.02 - VLT

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