I get a call about no power to an ac unit.So I see the fused disconnect has a blown fuse.It is a 5 ton dx unit.After inspecting the unit taking voltage and amperage and ohm readings.I chalk it up to a power surge or thunderstorms.Replace fuse and go about my way.A few days later the bldg maint man tells me that the unit has been blowing fuses on a regular basis.So I return to the roof telling myself well I find it this time.After tearing it apart for a few hours and megging the compressor,fan motors etc.Still cannot find a problem.The unit is 1 year old and is made by carrier.On a side note they wire those things pretty crazy.3 phase compressor single phase blower motors.The contactors for the three phase compressor was only switching two legs and a relay was switching the other leg.I am at a loss it is not about money just the fact that this thing is beating me.There are 6 other units on the bldg with no problems.I replaced the fuses today well I should say fuse A phase and baby sat it all day.Ran with no problems for at least 12 hours now.I am tempted to take the maint man suggestion and put some copper tubing in place of the fuse.Just kidding I don't like fire.Anybody got an ideas??
The 3 phase compressor using a 2 pole contactor and a separate 1 pole relay is a cheap and extremely ugly form of "soft start". Essentially, they single phase the motor to get it turning, then close the 3rd pole relay a few seconds later to allow the motor to develop full torque. This does NOT really reduce current, in fact the single phase current is colossally wasteful. But at the 3 phase meter which is averaging all 3 phase currents, it APPEARS to have reduced the starting current because one phase is essentially zero current and pulls the average down! I have seen this done before, I think it is one of the stupidest schemes I have run across. I seriously doubt Carrier did it themselves, I have only seen it done by their dealers or mechanical contractors who are trying to squeeze a few nickels out of a project by not using a soft starter when the HP requires it.
Anyway, time delay or not if you sized the fuses based on the motor FLA you will blow them quite often because the single phasing of the motor, even for just a second or 2, pulls 1.73 times more motor current than it would in a 3 phase starting scenario. The fuses just can't take it.
Re: Starting to feel stupid#70055 09/29/0602:13 AM09/29/0602:13 AM
So maybe it is single phasing it intermittely did I spell that right?I don't know if it the contactor setup was done by the factory or not it appears all original.I am using time delay fuses and everthing is clean and tight.Thanks for all the input this forum has some pretty smart fellars on it.Just not me right know.I am going to check that unit right now.I would love to wash my hands of it but just would'nt feel right about.
Re: Starting to feel stupid#70057 09/29/0611:24 AM09/29/0611:24 AM
I had something like that happen, only difference was it was an older resi unit and had a mercury thermostat controlling it and NO compressor start delay. Homeowner would play with the thermostat while the thing had been running, the unit would turn off, then try to turn on with high head pressure. Compressor is now "stalled" and would end up blowing the time delay fuses before the O/L on the compressor would open... Our old 2-ton lennox would do the same thing if the power flickered, compressor stalled and it would trip a 2-pole 20A FPE breaker within 2 - 2 1/2 seconds!
Perhaps something similar is happening with your unit??? Just a thought... A.D