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#58110 10/30/05 09:47 AM
Joined: Mar 2003
Posts: 97
Was called out to a cooling tower main breaker tripped. Found that the motor contactors for the 480V 30 HP fan were arc welded closed. This is actually a two speed one winding type fan with interlocked contactors. It appeared that the low speed welded shut then the high speed pulled in and welded shut. Found the overload relays set on auto reset.

These are IEC style contactors and overloads, it appears that the overloads did in fact trip and then reset until the main breaker tripped. Some control wires that have nothing to do with the motor circuit run right next to the overload relay are browned from heat.

I have megged out the motor and found no problems.... yet... need to hook up new contactors then stand back.

I have ordered new parts and will set all overloads on manual reset. Just wondering if anyone here sets any sort of overload relay to auto reset.

Many thanks

#58111 10/30/05 10:27 AM
Joined: Jan 2003
Posts: 4,391
Drillman I have never set OLs to Auto and I doubt that the ones you are working on should be.

430.43 Automatic Restarting.
A motor overload device that can restart a motor automatically after overload tripping shall not be installed unless approved for use with the motor it protects. A motor overload device that can restart a motor automatically after overload tripping shall not be installed if automatic restarting of the motor can result in injury to persons.

The only auto resetting motor overload application I see regularly are the OLs contained in hermetic refrigerant motor-compressors.

Bob Badger
Construction & Maintenance Electrician
#58112 10/30/05 02:54 PM
Joined: Oct 2004
Posts: 806
I think a cooling tower fan (which based on the HP rating is in a very large tower) would not generally qualify as having the possibility of injury to persons, as defined in that reference. That would tend to apply to conveyors, mixers, process machinery and the like, where sudden restarting could injure someone. It is reasonably assumed (yeah, we know what happens when one assumes) that when the tower is being worked on, LOTO would be applied.

Since this is a dual-speed application (maybe- I really think it's a wye-delta start arrangement) though, a manual reset overload would probably have prevented this breakdown.

UL has a standard for overload protection for fan motors (I think it's #508) which specifies that auto-reset overloads shall not be used where a fan is unattended (like a tower) or where the operator may not detect a locked-rotor or failure to start condition.

So, the upshot here is that the overloads should have been manual-reset to begin with. Drillman's doing the right thing.

Im my field, the only overloads set for auto reset are for lamp exhaust and cooling fans. Projector and transport motors have fuse/breaker or manual reset protection.

Drillman megger'd the motor..are you sure it's not locked up? Bad bearings, etc.?

In any event, be very careful when testing. 480 and a motor of that size can bite pretty hard!

Stupid should be painful.
#58113 10/30/05 07:56 PM
Joined: Jun 2004
Posts: 1,273
One would presume that the fan motors are lugging under too much drag. This will reduce their back-emf to the point that the heaters are toasty.

It is high time to convert this system over to intelligent, variable speed cooling fan motors. The economics are overwhelmingly favorable.

It doen't matter if you can reuse the motors or anything else. In economic terms they're worthless; kind of like a 12 year old personal computer.

Don't repair it. Replace it.

#58114 11/07/05 08:24 PM
Joined: Mar 2003
Posts: 97
An update.

I want to make it clear that this thing is pretty big. There is only one fan and it is least 10 or 12 feet in diameter. and the blades are shaped like an airplane wing. The tower is about 2 stories high.

First I climbed up there and manually turned the fan to make sure that the bearings were not locked up.

Got all the contactors and overloads and installed. Fired it up and the fan did not move. Finally figured out that there was at one time dampers on the top of the tower that were taken down and the damper motors left on the tower. There is an aux switch in the motors to lock out the fan if the damper is closed. Jumped it out and the fan worked.

I wonder if the aux switch caused the contactors to chatter which caused the damage?

The motor is in fact a two speed one winding. First one I have seen. The contactor and overload setup is very similar to a wye delta type starter.

As for replacing the motor and controls, there is talk to replace the entire tower in a few years. As I work in house for a government agency part of my job is to keep worn out equipment operating untill they get around to replacing it.

Set all overloads to manual reset.

Thanks for all your help.

#58115 11/08/05 07:44 AM
Joined: Nov 2001
Posts: 75
It was standard practice at a GM facility where I worked to automatically reset the OL's on their IEC starters. The production line was fenced in and each gate had switches on them to E-Stop things if opened at the wrong time.

If an overload tripped it created an error message so you would know if there was a persistent problem with one motor.

Engineering & production supervisors loved these starters for this auto-reset feature, saying it eliminated downtime. Personally, I believe most of these IEC starters and pushbuttons are crap.......but they are smaller and cheaper than NEMA so guess who wins everytime.

#58116 11/08/05 03:43 PM
Joined: Feb 2003
Posts: 939
myself i will not even think about leaving the O/L relay on auto resetting at all because of safety reason.

as Iwire [ bob] stated about leave it on manual reset because it is easier to spot the trouble and the only place i useally leave on auto resetting is hemispear sealed refrigent commpressor [ they useally have some kind of thermal proctetion on it allready { some size not all } ]

i did work on one machine shop the owner insteted useing automatic reseting i say no way due the sisuation the machine have to run.

i did conveted the owner just leave it on manual resetting and he found out why because few time machine did jammed up the O/L kicked out and got it unjammed compared to automatic reset it can start unexpected at all unless the power is off completly

speaking of IEC vs NEMA starter i rather use the NEMA starter hand down due stronger construction

Merci, Marc

Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

#58117 11/08/05 06:56 PM
Joined: Jul 2004
Posts: 101
Auto reset is a valid consideration in a lot of applications, but should not be used indiscriminately. For instance, if it is a 2 hour drive on a dirt road to get to a remote pump station, automatic reset is a good idea since it will attemp to reset if there is a clog or stuck valve that may clear up. That said, I always recommend a short cycle timer to go with that so that you don't exceed the starts-per-hour rating of the motor.

Also, the OL relay must have a reset time that does not allow reset until the motor is sufficiently cool. Cheap-o ones unfortunately do not necessarilly provide that feature, so it comes back to "you get what you pay for". Thats why I try to use newer electronic overloads if the customer wants auto-reset now. The ones I use have starts-per-hour and minimum time between starts lockouts as well.

Given that an auto-reset OL, even a cheap-o, will still take 10 to 15 minutes minimum to reset, I seriously doubt that was the cause of your welded contacts. I would believe the interlock contact as a more likely scenario.


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