13-10-2012, 03:00 PM
More a second opinion required on this one. Item is a contactor on a Class 73 locomotive that runs the cooling fan in the radiator compartment. The fan is controlled by a device called a Teddington switch which basically comes on when temp in the cooling system reaches a preset level. 110 volt DC power runs from a supply rail into the teddington, then out to the contactor coil and from the contactor coil to earth. The coil is rated 330R at 110vdc, a nice easy setup or so we thought.
What has been happening is that the silver contacts in the teddington are being spark eroded whenever it opens to break supply to the contactor, and with a decent spark I hasten to add, the coil is producing a decent back emf and slowing eating the contacts away. First impression is that the contacts in the switch are not man enough for the job and I would agree with this. In order to reduce the sparking down to a managable level it has been proposed to fit a 0.1uf 630v self healing capacitor across the coil to reduce the back emf kick, but then thinking back I remember that a diode was used on relays driven by semiconductors to protect them so at this moment it's recommendation time. Should we got for our first option of capacitor only, or fit a diode, or fit both in parallel?
Thanks.
What has been happening is that the silver contacts in the teddington are being spark eroded whenever it opens to break supply to the contactor, and with a decent spark I hasten to add, the coil is producing a decent back emf and slowing eating the contacts away. First impression is that the contacts in the switch are not man enough for the job and I would agree with this. In order to reduce the sparking down to a managable level it has been proposed to fit a 0.1uf 630v self healing capacitor across the coil to reduce the back emf kick, but then thinking back I remember that a diode was used on relays driven by semiconductors to protect them so at this moment it's recommendation time. Should we got for our first option of capacitor only, or fit a diode, or fit both in parallel?
Thanks.