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Starting to feel stupid #70049
09/28/06 09:34 PM
09/28/06 09:34 PM
S
Surfinsparky  Offline OP
Member
Joined: Jul 2005
Posts: 73
merritt island fl usa
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??

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Re: Starting to feel stupid #70050
09/28/06 09:42 PM
09/28/06 09:42 PM
L
LK  Offline
Member
Joined: Jan 2003
Posts: 1,429
New Jersey
Is the fuse sized right, and the right type?

Re: Starting to feel stupid #70051
09/28/06 10:18 PM
09/28/06 10:18 PM
B
BigB  Offline
Member
Joined: Mar 2004
Posts: 732
Tucson, AZ USA
Are you using a time delay fuse? Did you check the terminal lugs in the disco? If it is a pullout, does it make solid contact?

Re: Starting to feel stupid #70052
09/28/06 10:29 PM
09/28/06 10:29 PM
Bill Addiss  Offline
Member
Joined: Oct 2000
Posts: 3,902
NY, USA
Has anyone checked the unit out to determine if there might be a mechanical related reason? (ie incorrect freon charge ... etc)

Bill

(edited to clarify)

[This message has been edited by Bill Addiss (edited 09-28-2006).]

Re: Starting to feel stupid #70053
09/28/06 11:11 PM
09/28/06 11:11 PM
J
JoeTestingEngr  Offline
Member
Joined: Nov 2005
Posts: 794
Chicago, Il.
Is it always the same fuse? Have you pulled the power, pushed in on the contactor(s), and measured resistance top to bottom on each pole?
Joe

Re: Starting to feel stupid #70054
09/28/06 11:38 PM
09/28/06 11:38 PM
J
jraef  Offline
Member
Joined: Jul 2004
Posts: 101
San francisco, CA, USA
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.


JRaef
Re: Starting to feel stupid #70055
09/29/06 03:13 AM
09/29/06 03:13 AM
S
skipr  Offline
Member
Joined: Jan 2005
Posts: 39
Huntington Beach, Ca.
I had same problem on a unit ontop of Hunt. beach high school. It's one of the contactors. I forgot which, but like that other dude said it's single phasing the compressor.

Re: Starting to feel stupid #70056
09/29/06 06:15 AM
09/29/06 06:15 AM
S
Surfinsparky  Offline OP
Member
Joined: Jul 2005
Posts: 73
merritt island fl usa
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
09/29/06 12:24 PM
09/29/06 12:24 PM
H
Helectric  Offline
Member
Joined: Jan 2005
Posts: 90
Brandon, SD
This might be a good place to use a circuit analyzer/recorder if you have access to one.

Re: Starting to feel stupid #70058
09/30/06 06:28 PM
09/30/06 06:28 PM
R
Rewired  Offline
Member
Joined: Jan 2006
Posts: 558
Hamilton, Ontario, Canada
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

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