A "blonde" moment

[phil d]

Well they do say it happens to the best of us, and this tale proves it, despite many years in the industry I found myself dealing with a schoolboy error on a job yesterday, the annoying thing was I had created the problem!

On the farm we have large fans in the end of the building to provide ventilation, we had a problem and it was found we needed more airflow, the solution was more fans. Not a problem said I, when I designed the system I installed bigger cables than needed for the fans, partly in case there were any changes, and partly, because of cost. I was using vast amounts of the larger cable and was getting it very cheaply, so it made sense to run the fans in the larger cable, anyway here's the story.

2 more fans were being installed, I  checked my calcs and could indeed run 2 fans off one feed, the problem was the overload in the main panel, it was designed for 1 fan, with a loading of 3.5 Amps, so I came up with a plan, make a small enclosure containing 4 overloads, remove the one in the main panel, take the 2 fan feeds into this box, and split them across 4 overload units, each cable feeding 2 overloads, each overload feeding 1 fan, simple.

I got the parts and set to work, built the unit in a nice enclosure with a clear front panel, so as to enable easy viewing of the trips, installed it all on Sunday and away we went, it worked fine, then one of the motors tripped, I reset it and after about 30 minutes, it tripped again.

Now previous to this I had never had an issue with this motor, it had run fine for 4 years, I reset it and it ran for about 8 hours, then tripped again!

I rang technical to see if they could shed any light on this, they asked what my overloads were set at, 3.5 Amps, rated full load current, they said they recommend setting at 115% of FLC.

I went back yesterday to make the adjustments, then realised something else, I had made a really neat job of the panel, 4 overload units, side by side, and a bank of Weidmuller connectors next to them to connect the cables to, it looked lovely, really neat, then it hit me, all the overloads were butted up next to each other, no airgap between them, well that wouldn't help.

I then realised why the unit hadn't tripped for 7 hours, it had gone cold in the late afternoon!

To think that only a couple of months ago, there was I advising someone who was doing an install using large breakers to not place all the high loads next to each other to avoid nuisance tripping caused by heat build up.

What a silly beggar I am. lol :facepalm:

 

[Rob.]

So you've still got a problem then. 

Modern overloads are designed to be zero spaced. If they're getting hot enough to influence each other you've a real problem. 

Which manufacturer of overload did you use? Never once been advised to use 115% FLC. (This is a hangover from motors that were protected with dashpots). 

 

[phil d]

So you've still got a problem then. 

Modern overloads are designed to be zero spaced. If they're getting hot enough to influence each other you've a real problem. 

Which manufacturer of overload did you use? Never once been advised to use 115% FLC. (This is a hangover from motors that were protected with dashpots). 

I used Telemechanique overloads, the enclosure is only small, and has no ventilation as it's outdoors, tbh I thought the 115% was a bit weird, but there you go. TBH everything about the farm is turning nightmarish, they are making a lot of what are in my opinion unnecessary changes at the moment, and some which are not thought out.

There is a drying plant that runs from a PLC, recently they wanted to modify the system and add another grain elevator, to divert it to another location when not using the cleaner. rather than alter the programme and run a couple of new cables in, they wanted a changeover switch (manual) to shunt between 2 drive motors, they thought this was a great idea, until I pointed out that if someone left it in the wrong position then they could potentially feed all the product into the waste bin.

It used to be ok but now the owner seems more interested in spending time away from the business and he just wants everything done on a quick fix minimum outlay.

I'm only assuming the heat to be a factor, since the motor ran longest when the temperature was down.

 

[Rob.]

You need to clamp the motors running current. 

I presume you've checked that you are receiving 400V, I know some farms run lower due to transformer distance and loading. 

 

[phil d]

You need to clamp the motors running current. 

I presume you've checked that you are receiving 400V, I know some farms run lower due to transformer distance and loading. 

Motor pulling 3.92 amps running, volts at terminal without motor running are 430, dropping to 420 when all motors running, so well within for volt drop.Our TX is pole mount and just outside the gates

 

[Rob.]

If the motor name plate is saying 3.5A and you're getting 3.92 there's an issue.

Are all fans mechanically free, are they running in the correct direction, any duct work that could be too restrictive/not restrictive enough. 

An it may seem obvious but if they are centrifugal fans they will still pull air running in both directions. Just running in the wrong direction will be a lot more inefficient. 

I've got the same issue on a 315KW extraction system, pulling too high a current. Input pressure is looking likely to be the issue. 

 

[roys]

Must admit I never liked butting contractors and overloads next to each other and have had problems in the past when they have been, usually due to a combination of factors, high ambient, high percentage of contractors energised and motors running near FLC.

i use to put a terminal rail end block between the contractors so that everything was still tight on the rail but things had room to "breathe".

Also as per Robs comment setting at 115%FLC  a thing of the past.

Just seen your last post Phil 3.92A certainaly going to cause probs with a motor FLC of 3.5 A

 

[phil d]

Must admit I never liked butting contractors and overloads next to each other and have had problems in the past when they have been, usually due to a combination of factors, high ambient, high percentage of contractors energised and motors running near FLC.

i use to put a terminal rail end block between the contractors so that everything was still tight on the rail but things had room to "breathe".

Also as per Robs comment setting at 115%FLC  a thing of the past.

Just seen your last post Phil 3.92A certainaly going to cause probs with a motor FLC of 3.5 A

Yes I had to replace them, with bigger ones, the original motors had a lower wattage, it would appear that they've had some kind of issue and changed motor suppliers, the old motors pulled just over 3amps and the new ones just under 4 amps. to be honest I'm glad it it's all sorted now, I suspect they are actually not that accurate on motor wattage, maybe they're "approximately" the stated wattage and it's their way of getting around tripping problems. I didn't like doing it the way I had to but that's life, as I said they used to be ok but now they're playing silly beggars with everything, I put a supply into one area for a load of 20 amps, I made it big enough to  carry 32A, just in case, later on they were asking if we could squeeze 50 amps out of it!

When the place was built they were told the maximum power available, they went over! fortunately the DNO was able to give them a bit more, then they were on about expanding again, I told them there was no spare capacity to expand as big as they wanted and they'd need a new TX, they lost interest after that.

 

[Rob.]

What does it say on the motor nameplate  for flc Phil?

 

[roys]

Why has auto correct changed contactor to contractor, in my posts Dam you autocorrect 

 

[Sharpend]

Why has auto correct changed contactor to contractor, in my posts Dam you autocorrect 

I just thought you had something against contractors? 

 

[phil d]

What does it say on the motor nameplate  for flc Phil?

can't remember and I'm not there at the minute, will have a look when I go up tomorrow. but on the spec sheet it said something like 3.5A however when I clamped it it was almost 4A.