The Electrical Contractor Network

ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Books, Tools and Test Equipment for Electrical and Construction Trades

Register Now!

Register Now!

We want your input!

Featured:
   

2017 NEC and Related
2017 NEC
Now Available!

   
Recent Posts
Sprinklered equipment 26-008
by bigpapa
12/02/16 04:24 PM
On Delay Relay with Auto Reset
by Potseal
12/01/16 09:59 AM
Wow, that was close!
by jraef
11/28/16 07:06 PM
Earthquake in New Zeeland
by RODALCO
11/27/16 11:25 PM
Calling all Non-US members!! (Non-US only)
by Tjia1981
11/27/16 06:33 AM
New in the Gallery:
12.5A through 0.75mm˛ flex (just out of curiosity)
Shout Box

Top Posters (30 Days)
gfretwell 13
HotLine1 9
Texas_Ranger 8
Trumpy 8
sparkyinak 7
Who's Online
1 registered (sparkyinak), 220 Guests and 4 Spiders online.
Key: Admin, Global Mod, Mod
Page 1 of 2 1 2 >
Topic Options
Rate This Topic
#70049 - 09/28/06 06:34 PM Starting to feel stupid
Surfinsparky Offline
Member

Registered: 07/29/05
Posts: 77
Loc: 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??

Top
Work Gear for Electricians and the Trades

Work Gear for Electricians and the Trades
Work Gear for Electricians and the Trades
Arc Flash Clothing, Gloves, KneePads, Tool Belts, Pouches, Tool Carriers, etc. etc....

#70050 - 09/28/06 06:42 PM Re: Starting to feel stupid
LK Offline

Member

Registered: 01/21/03
Posts: 1721
Loc: New Jersey
Is the fuse sized right, and the right type?

Top
#70051 - 09/28/06 07:18 PM Re: Starting to feel stupid
BigB Offline
Member

Registered: 03/31/04
Posts: 727
Loc: 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?

Top
#70052 - 09/28/06 07:29 PM Re: Starting to feel stupid
Bill Addiss Offline
Member

Registered: 10/07/00
Posts: 4196
Loc: 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).]

Top
#70053 - 09/28/06 08:11 PM Re: Starting to feel stupid
JoeTestingEngr Offline
Member

Registered: 11/04/05
Posts: 786
Loc: 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

Top
#70054 - 09/28/06 08:38 PM Re: Starting to feel stupid
jraef Offline
Member

Registered: 07/21/04
Posts: 91
Loc: 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

Top
#70055 - 09/29/06 12:13 AM Re: Starting to feel stupid
skipr Offline
Member

Registered: 01/03/05
Posts: 41
Loc: 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.

Top
#70056 - 09/29/06 03:15 AM Re: Starting to feel stupid
Surfinsparky Offline
Member

Registered: 07/29/05
Posts: 77
Loc: 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.

Top
#70057 - 09/29/06 09:24 AM Re: Starting to feel stupid
Helectric Offline
Member

Registered: 01/07/05
Posts: 91
Loc: Brandon, SD
This might be a good place to use a circuit analyzer/recorder if you have access to one.

Top
#70058 - 09/30/06 03:28 PM Re: Starting to feel stupid
Rewired Offline
Member

Registered: 01/01/06
Posts: 567
Loc: 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

Top
Page 1 of 2 1 2 >



ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals