Danfoss MCD500 Service Manual

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Danfoss MCD500 Service Manual Manual pdf 34 page image

5 Avoiding Damage5.1 SCRs5.1.1 Typical Causes of SCR DamageSCR damage is generally caused by overcurrent, overvoltageor overtemperature. To prevent future damage, check thatthe soft starter has been installed properly. Common causesof SCR problems include:Overcurrent:• cable fault on soft starter output• motor fault• start current and/or start time exceeds the softstarter's rating• starts per hour exceed the soft starter ratingOvervoltage:• power supply transient or surge• lightning strike (direct or indirect) on power supply• motor fault• loose connection in power circuit, before or afterthe starter• power factor correction connected to the output ofthe soft starter• over-corrected bulk power factor correction on alightly loaded system causing severe ringingvoltagesOvertemperature:• blocked heatsinks or restricted ventilation• inadequate ventilation• excessive ambient temperatures• bypass relay fails to close during running (internallybypassed starters only)5.1.2 Protecting SCRsModern 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 FusesSemiconductor fuses reduce the potential for SCR damagecaused by short circuits on the output of the starter.Protection systems such as circuit breakers or HRC fuses donot operate quickly enough to protect SCRs from shortcircuits.5.1.4 Main ContactorsSCRs are most vulnerable to overvoltage damage whenvoltage is applied to their input terminal while they are off. Inthis condition the SCR is blocking the full line voltage. Usinga main contactor to remove voltage from the SCR inputwhen the starter is off eliminates the risk of SCR damage dueto overvoltage.5.2 Output RelaysMCD 500 soft starters have three programmable outputrelays. These relays are often used to control externalcontactors.The electronic contactor coils used in many contactors havea high initial inrush current, which can damage the softstarter's internal relays if the contactor coil is switcheddirectly.Avoiding Damage MCD 500 Service ManualMG.17.L4.02 - VLT® is a registered Danfoss trademark 335 5