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Relay failures
The pictures here show the damage caused to the contacts of a relay used in an industrial setting to control the coil of a hydraulic solenoid.
The relay contacts were rated at 250 VAC / 6A. Contact material was a silver/tin alloy with an overall silver flashing.
The coil was driven by a 24Vdc supply and has a DC resistance of ~8 Ohms. The scientists/engineers will realize that the load is only 3A or 50% of rating. The inductance of the coil was measured to be about 6-7 Henries.
I used a digitizing oscilloscope to determine the magnitude of the undamped voltage spike and found that it was clamping at about 300 to 330 volts. This was due to the breakdown of the air in the relay allowing an arc to strike across the contacts.
Various snubbing means were tried including RC networks MOV's and reverse biased diodes. MOV's were the least effective, clamping the voltage at the MOV breakdown voltage this was better than the undamped condition but still not ideal. An RC snubber comprised of a 0.1uF capacitor in series with a 100 Ohm resistor limited the spike to about 35 to 45 volts. The best protection was obtained with a reverse diode which clamped the voltage at the diode drop of 0.6 or so volts.
The protection diodes were installed resulting in a 3x lifespan for the relays at this time (no failures to date.)
All these pictures were taken with a microscope camera.
The relay cycled 7 times per minute on a 24-7 basis. MTBF was 6 - 9 weeks.
This is a side view of the relay.
Notice the arc tracks and copper oxide buildup on the lower contact.
This is the bottom view of the wiper.
Notice the extreme pitting of the contact. Some of the failures were welded due to the relay closing while the surface was still molten. Also observe the greenish copper oxide deposits where the copper vapor has condensed on the wiper body. The failure mode of this unit was failure to energize the coil due to the high resistance of the copper oxide and slag buildup.
Top of same wiper.
The upper contact was an open circuit and the only marking is mechanical wear. There is a quantity of soot and copper dust which has settled on the upper side of the wiper.
Sorry about the overexposed photo.
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