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Slick1122AC vs DC voltageWe have a heater that operates on either AC or DC current at 90W to 120W. DC current flows in one direction. AC current flows in two directions. During operation the heaters work perfectly using AC or DC voltage. We know that a heater must be in contact with another surface to transfer energy or it will eventually burn itself out. Our operators occasionally will have to disengage the heater and do not always turn them off immediately. When running on AC voltage the heater may fail during this production stoppage, however when we wire the heaters in DC voltage we rarely or never see a heater failure during this same production stoppage. Can someone help explain why the difference? Wednesday, March 9, 2011
7:12 AM

Comments (2)
JeffHAC vs DC voltageAC causes a mechanical vibration in the element. If you listen carefully you can hear the heating elements in a toaster hum at 60 cycle when it’s on. If the element is already brittle from over-temperature that 60 cycle vibration could be enough to cause failure. The DC would also cause some vibration, depending on how it’s rectified. If you use a full-wave rectifier on the AC power of the machine you’ll get pulses at a rate of 120/second. The mechanical vibration from this would probably be less than from straight AC power though. Monday, March 14, 2011
9:13 PM
P_StephensAC vs. DC operation of a heating elementA resistance heating element should operate the same on AC or DC voltage as long as it’s the rated voltage of the element. You’re describing a situation where the heating element is de-coupled from the material that it would normally transfer its heat to. This then causes it to exceed its rated operating temperature and fail – like disconnecting the radiator from a running car engine. I don’t know of any reason why alternating current would make it more likely for the element to fail under this condition. If you rewire to DC after a failure has occurred it could be that the operator has also become more aware, and therefore more likely to shut off the heater upon disengagement. It could also be that your observations of element failure are too few to yield reliable statistical information. It may just appear that failure is more often with AC, however if you analyzed 1,000 such failures you might find a 50/50 probability between AC and DC. It could also be that the rapidly reversing current of AC has some negative effect on the element when it comes under heat stress. I’m unaware of such an effect but I suppose it’s possible. Wednesday, March 9, 2011
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