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Ive had a amusing days fault finding today, Found several faults but have come out of it asking more questions than I have answers.

So Me and a colleague go to this AHU that keeps tripping for unknown reasons, The pannel has been canabalised and the inverter and motors covers are removed but everything appears to be there.

So we started it up and stood there for 20 mins as the thing runs fine, Slightly puzzled we were about to put all the covers back on when it trips out the 63A breaker.
So starting it up again we clamp each phase finding that L1 is pulling no current and the other 2 are pulling 55A, So this firstly puzzles us as again the motor is spinning fine?
So we test voltage from L1 of the panels incomers, through the breaker, contactor & OL, isolator to inverter no problems, Outgoing of inverter also fine.
But still no current being pulled on L1 and the motor still spinning away merrily.

Is this something to do with the inverter??

So going back to the panel and having a poke around I hear some nice buzzing sounds coming from the internal MG board, Followed up by a nice puff of smoke from one of the breakers (but not the one for the supply fan I am concerned about).
So I take the breakers off and find that several of the bus bar connections are broken and burnt.

So after sourceing a replacement I decide to test the motor to check that its all ok before I go any further.
Meter the windings and IR test them to each other and earth all ok, Its at this point I realise another problem.
The motor had been wired in Star but according to the rating plate it is Delta.

Now I replaced the board (allthough not powered up yet) and intend to finish the job tommorow.

But can anyone explain to me how a motor wired wrongly with only 2 phases pulling current but with voltage accross all 3 run fine for 20 mins before tripping the breaker and not taking out the overload?

Ime assuming it must me something to do with the inverter but its got me puzzled.

Its not helped by the fact that everything has been canabalised!

Thanks muchly :)
 
Are the drives 1ph input - 3ph output if so its likely thay are output rated to 230v which on an EU dual voltage motor would be a delta config'.

What are the voltage ratings on the motor in relation to the configeration star/delta.

Where did you clamp the L1 load in relation to the panel supply, drive and motor?

What speed are the drives set to run at when on?
 
Hi DW thanks for the reply.

1) No the inverters are 3 phase in and out (Allthough for some reason between the the inverter and motor there is 4 wires with 2 connected to the same phase both sides? - Possibly a old neutral for a Star motor??)
2) The motor is not set up star delta, I think it was originally as there is a S/D contactor in the pannel however its been mostly disconected and just comes on and off now (Like I said canabalised!!)
3) We only clamped the cables in the pannel as its the only place we could actually get the clamp in (Big clamp!) But I may take my little one in tommorow.
4) No idea what the speeds are set at, They are connected to a BMS so possibly varies.

I Should probably add that this AHU is 20+ years old and nobody knows who or when it was last worked on, Going off some of the other stuff in the others though god knows what has happened to it!!
 
Hi DW thanks for the reply.

1) No the inverters are 3 phase in and out (Allthough for some reason between the the inverter and motor there is 4 wires with 2 connected to the same phase both sides? - Possibly a old neutral for a Star motor??)
Motors don't have neutrals

2) The motor is not set up star delta, I think it was originally as there is a S/D contactor in the pannel however its been mostly disconected and just comes on and off now (Like I said canabalised!!)
I meant what is the voltage on the motor plate for the star config' and the delta config' it may be the case 400v is the delta config'.

3) We only clamped the cables in the pannel as its the only place we could actually get the clamp in (Big clamp!) But I may take my little one in tommorow.
Where did you clamp.... Panel supply in or output of the inverter


4) No idea what the speeds are set at, They are connected to a BMS so possibly varies.

I Should probably add that this AHU is 20+ years old and nobody knows who or when it was last worked on, Going off some of the other stuff in the others though god knows what has happened to it!!


You say it looks like the fans used to be star/delta started if this was the case then they will be 680v/720v star and 380v- 420v delta or there abouts so a delta connection would be correct... carefully read the plates and don't assume they are 400v/230v motors.
 
Sorry just seen this:


2) The motor is not set up star delta, I think it was originally as there is a S/D contactor in the pannel however its been mostly disconected and just comes on and off now (Like I said canabalised!!)
I meant what is the voltage on the motor plate for the star config' and the delta config' it may be the case 400v is the delta config'.tar

There is no Info for star, Only Delta.

3) We only clamped the cables in the pannel as its the only place we could actually get the clamp in (Big clamp!) But I may take my little one in tommorow.
Where did you clamp.... Panel supply in or output of the inverter

Pannel - Supply to the inverter

 
I take it then the way we are talking the motor has 6 terminals and at present is Star set up... just be careful because not everything about motors is always as you see them... are they dual speed motors that have been put into high speed for a drive to be fitted, ref' the plate and model on line if unsure, as the panel has been butchered this could be any kind of old set-up that may not be what it seems , the Star Delta you see in the panel could have been an old 2 speed set-up which looks very similar in arrangement, I cannot see how the motor hasn't burnt out or the drive tripped through basic SQ/Root calcs of the motor demand against speed if it was a 680v Star with a 400v full frequency connection... Just tread carefully here, its hard to say without seeing it.

If in doubt take pics of motor, connection block and plate and post on here.
 
Cheers DW, I to am completely stumped on this.

From what I understand from the Older guys years ago they all used to be star delta, Then at some point they were modified to be Inverter controlled.
Just to really throw a spanner in the works no unit has been done exactly the same, theres no up to date drawings and we are just trying to make do with whats there.
I have no idea if it has ever worked or if it did when it last worked as it should.

There is no evidence of it being a 2 speed motor and no control to switch speeds so I dont think that is the case.

How the motor has not burnt out or become damaged in anyway is what has really foxed me and tbh I just cant explain how I can see a voltage but no current on L1.

Just for the sakes of interest though here is the board I replaced today.
Never seen inside one of the MG slider boards so I was interested in looking for my own sakes and was rather suprised by the somewhat basic nature of the sliders.

A strange fault and a interesting days fault finding.... {filename} | ElectriciansForums.net

A strange fault and a interesting days fault finding.... {filename} | ElectriciansForums.net
 
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That looks like the screws were never tightened in the first place to the busbar pins ...

Sorry never answered the 2 phase load issue .... Basically you had lost a phase (high resitive joint under load) but it still give enough off load to show a reading, although you had contact voltage it couldn't supply the ampage due to the burnt out board upstream. How can the motor work then you ask??? --- as is in the case of some drive models they will run on 2 phase if one is lost thus supplying 3ph out as normal, the only difference here is the demand on the drive input with increase 3 to 4 fold to compensate and this would be what seems to have happened (Not all drives can do this)...

Ask all your experienced workmates to answer this with your new found knowledge and see if they can figure it out ;)
 
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Thanks DW that goes some to explaing this head scratcher.
I figured it had to be something to do with the drive but my knowledge of them is somewhat limited.
The clamp meter reading however was taken after the breaker which confused me more. I figured I could have a broken connection with a hot point giving me the voltage but I would of expected the current to be higher rather than non existant :s

As for the screws they were not loose, Not up to my standard of tightness but they were clamped down on the pins.
I think a few of the breakers had been knocked about tbh breaking the outgoing pin.
The board is mounted as you see it in the panel and there is no protection around the breakers etc so it would be very easy to whack them about once the panel door is opened.
 
Thanks DW that goes some to explaing this head scratcher.
I figured it had to be something to do with the drive but my knowledge of them is somewhat limited.
The clamp meter reading however was taken after the breaker which confused me more. I figured I could have a broken connection with a hot point giving me the voltage but I would of expected the current to be higher rather than non existant :s

As for the screws they were not loose, Not up to my standard of tightness but they were clamped down on the pins.
I think a few of the breakers had been knocked about tbh breaking the outgoing pin.
The board is mounted as you see it in the panel and there is no protection around the breakers etc so it would be very easy to whack them about once the panel door is opened.


Think about it, if you have a very hot point maybe even glowing red hot it will act as a massive resistor so what will happen to the current if you fit a resistor in line with a load??? In your case it got so bad it stopped any loading of that phase.

Ps you have to ignore how tight the screws felt, they have got so hot they probably got stuck solid in their captive threads even though loose contact on pins... the busbar pins are very solid and robust, I doubt they had snapped internally unless this is what you found.
 
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Think about it, if you have a very hot point maybe even glowing red hot it will act as a massive resistor so what will happen to the current if you fit a resistor in line with a load??? In your case it got so bad it stopped any loading of that phase.

Ps you have to ignore how tight the screws felt, they have got so hot they probably got stuck solid in their captive threads even though loose contact on pins... the busbar pins are very solid and robust, I doubt they had snapped internally unless this is what you found.

Yes sorry, Its been a long day LOL I was thinking backwards :p

And yeah once I removed the breakers the pins were wobbleing around, Its a bit hard to see but if you look at the 2nd from bottom pins its not quite straight.
All 3 of the bottom pins were wobbly and 2 of the top ones were also.
I couldnt think of any other logical explanation than them being hit but as with most of this stuff who knows!

Thanks once again for your time DW, I learn a little more about this crazy stuff every time I come accross them.

I will give you a update tommorow once I have everything connected back up.

Hopfully fully working without any tripping :D
 
You maybe right about the pins but Schneider (Merlin) even this generation are well built boards... do what I would and strip it down to learn from your findings ;)

PS change the mcb for new... the heat damage could have damaged the internals of the MCB as well as the properties of the metal contacts so ensure you don't promote the same situation again.


Edit sorry just seen the pics... yep a failure on the pins but where it originated from is anyones guess from a loose mcb clamp to the actual pins themselve been as you said broken away from a good whack!! Bet the board was dropped before installing IMHO.
 
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I destroyed it for my own curiosity more than anything :p

The sliders seemed to operate fine and the main bus bars were spotless so I cant conclude anything there.
One of the sliders did fall out of its location before I had actually took it apart, No idea how that actually happened and couldnt investigate it as it typically fell down a hole as sods law ususally dictates!
The screws on the breakers rotated fine and were not seized, Allthough like I said they were not what I would class as tight. Thats not to say of course that they were loose and burnt before and someone tightened them up before I got there.

Who knows...

But I have worked on these type of boards all my carreer and it is the first time I have ever come accross wobbly bus bar points.
Seen them burnt and bent but never loose and wobbly!
 
Well I got back on it again today, First I went and looked in the unit next door which is basically the same as the unit I am currently in and has the same motor, inverter etc. Only real difference is it actually works!
I had a look how the motor was wired in that unit just out of interest and sure enough its wired up in delta.
So I Re-wired the motor into Delta as per the rating plate and started it all up, Worked well for a few seconds and then for some reason it accelerated continually until the inverter decided to stop it allthough I was about to stop it myself as the thing looked like it was going to take off!
So after chatting to my boss he said put it back into Star as it would prevent it accelerating fully, I understand his logic behind this but disagree with the idea as its just going to kill the motor surely? Even though its through a inverter??

Anyways put it back into star as requested to try it out and the motor runs up fine, Well so I thought....
Bearing in mind the problems from yesterday I decided to clamp the phases to check the loading and lo and behold they are still out, Even more so than they were before I replaced the board!

New readings being L1 - 0A, L2 - 23A, L3 - 45A.
Needless to say it was then stopped again, Also worked out why the O/L was never tripped. Turns out its been bypassed!!

So I have now come to the conclusion that the Inverter is cream crackered so ime going hunting tommorow to hopefully find a un-used one to change it over as I cant find anything else wrong with the system well except for the O/L but thats a easy fix assuming that still works and hasnt been bypassed becasue thats ****ed to.

As for my further investigation of the original board melting issues I stripped it down fully this morning and also had a look at the breaker I took off.
I believe this may of been a 2 part problem.
First possibly a loosely connected breaker as talked about yesterday as the burning on the breaker did not match the burning on the board, The majority of the damage to the breaker was on the bottom terminal but most of the board damage was around the middle bar.
A strange fault and a interesting days fault finding.... {filename} | ElectriciansForums.net

Also the plastic around the middle pin has drooped inwards which would suggest to me its got very hot.
A strange fault and a interesting days fault finding.... {filename} | ElectriciansForums.net

So with this I would conclude that the chances are the breaker I removed was fitted after some of this damage was done.

So then I ripped it to bits and had a good look at the sliders themselfs.
The bottom right middle one was rather black and burnt....
A strange fault and a interesting days fault finding.... {filename} | ElectriciansForums.net

A strange fault and a interesting days fault finding.... {filename} | ElectriciansForums.net

I also found that in comparisson to the better sliders that it was a loose fit on the bus bar pin the breaker screws onto.
So again potential for a hot point.
Obviously I am no fire investigator and only have my ideas to go off but ime intrested in other peoples ideas :p
No real way of knowing how this occured other than my original theory of the breakers being knocked about or just general wear and tear.
 
Its possible the slider hadn't been pulled across fully and only just making contact.

The O/L will be by-passed because thats what your inverter is designed to do unless it covers multiple motors.

As for the motor connections, if you are right is it possible the drive parameter have been set for overspeed because the previous connections may has run it slow and they tried to fudge the speed using the drive.. get a parameter list and check max speed is set at 50hz ... if what you say as all correct it does sound like someone has been fiddly without understanding - it is also possible one of your drive inputs has broken down forcing the drive to slave off 2 phase but this shouldn't effect the output in such a manor as to overspeed it... what make of drive and model is it? - there are simple ways to check the drive with a few command changes.

Your drive will more than likely have external command cues for speed, run stop etc ensure you are aware of all the inputs/outputs and how they work before finding a replacement... speed control can be done with various methods and usually 0-10v or 4-20mA source so check you understand it and whether the control is set as PNP or NPN. Even an identical replacement of the drive may be useless if you cannot configure it and set it up.


If I was you I would do a full disconnect test of the motor without the shorting out termial links in place for IR and winding resistance to ensure it tests out ok, if the motor is small enough you could rig it up DOL to confirm it works without the drive and clamp the phases to check for balance.

I would get the motor plate details and confirm the motor is what you believe it to be (Star/delta) motor... trust me when I say they can be very confusing if you don't know what to look for on the plate other than the basics and even worse when plates are missing, can you get pic of the plate?
 
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Thanks DW.
Cant run her up DOL as its a pretty hefty motor.
I took a pic of the plate but afraid its not great, very dark inside!!

A strange fault and a interesting days fault finding.... {filename} | ElectriciansForums.net

I did a full test of the motor yesterday which was fine.
Hopefully still is!

I did wonder if there was some parameters wrong on the thing causing it to go into over drive.
But as said before i know naff all about drives.
Not to mention I have no paperwork for them.

This is the drive details if its of any interest to you:

A strange fault and a interesting days fault finding.... {filename} | ElectriciansForums.net
 
Sorry not read through all of this but a motor driving a fan is liable to “windmill” while idle due to natural draft. Apply 2 phases to a rotating field and the motor will create its own artificial 3[SUP]rd[/SUP] phase and accelerate to more or less normal speed. The O/L’s should have detected this so you’ve several problems.

TS you know where to look for motor testing.
 
Well I am rather happy to say that today we managed to sort this piece o crap out.
So we original went to this thing with the POV that it hadnt run in a while due to some fault nobody had fixed.
As it turns out there is no way in hell this thing had ever run properly in the condition it was in, Which is very probably when it was modified to be inverter driven.

The final list of faults (and I use that term loosely) is rather long, So we came away with the POV the guy who mod'd it was a flaming idiot who had no idea what he was doing!

Highlights of the problems include:

Burnt out Dis Board
Burnt out breaker
Incorrectly programmed or U/S inverter (not really sure whats up with it)
Incorrect wiring between the Inverter and the Motor
Incorrect wiring of the Motor windings

So thanks again for your time DW, Was extremely helpfull and as usual I learned much from this episode.

Tony, Thanks for the reply, There was no chance of it windmilling but I do know what you mean, Used to have that problem with the air con units at my old site.
 
If all your peers at work are old gits then I bet one of them has a lot to answer for but I dare say they will be keeping stum about it all .... :38:
 

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