Inverters....

[Tiny Spark]

Ok so I had a interesting "conversation" with a contractor today and I have a 2 part question ime hoping you guys can clear up for me.

So the scenario:
3 large cold air handling units each with 2 fans (415v 3 phase) fed through a speed controller, contactor and overloads.
All fans getting extremely hot and burning out.
While testing phase to phase voltages found to be varying some 110v some 300v etc.

Now theres no question in either of our minds that they are burning out because of the fluctuating voltages but we do disagree with reasonings for them.

Now in my mind the inverters will be there to change the frequency of the motors to change the speed and the flow rate of the cold air into the room.

He believes they should never be on a inverter and that its the inverter changing the voltage to change the speed causing the problem.

So 2 questions are:

1) is it possible to change the speed of a 3 phase 415v motor by changing voltage? (because i didnt think it was)

2) should ahu's of this type be fitted with a inverter at all?
(Personally i think its a good thing but the cooling engineer thinks they should be on or off)

Thanks

 

[netblindpaul]

Not really enough info, but, what are you using to measure the voltages, & I mean EXACTLY what.
i.e. make & model of meter.
Some DMM's canot handle the outputs from "inverters" thus, they just display nonsense.
Next question is what "inverters" are they, again exactly.
Next one, is how are the motors wired, i.e. screened cable, etc. cable lengths, csa.
More to follow once you have answered these.

 

[Tiny Spark]

Not really enough info, but, what are you using to measure the voltages, & I mean EXACTLY what.
i.e. make & model of meter.
Some DMM's canot handle the outputs from "inverters" thus, they just display nonsense.
Next question is what "inverters" are they, again exactly.
Next one, is how are the motors wired, i.e. screened cable, etc. cable lengths, csa.
More to follow once you have answered these.

Thanks for the reply.

Afraid I can offer little in answers as I have not looked at the unit myself this is just what the service engineer has told me which I found odd.
But from what I can tell you:

He was using a fluke clamp/meter combo dont know the model.
Dont know what inverters they are but I will have a look tommorow.
The motots are wired in flex inside the units, I would guess at either 1.5mm or 2.5mm flex as they are only rated upto 8A if memory serves right and prob less than 2m run.

 

[Darkwood]

As above - many issues can arise when using inverters ...when you say burning out is that a diagnostic that the winding have melted or you just mean motor failure - if the later exactly what is failing and what are the symptoms etc.
Is this an adapted set-up i.e. an original DOL system that has been converted to Inverter run, you say the motor is supplied via a flex - does this have a screening correctly grounded as required by the Inverter Brand...

Its going to hard for you to solve this if you have no experience in this field but question the Engineer to how speed control is maintained whether it is preset orPOT' control by user ..... i could list you 20+ reasons why the motors may be failing from incorrect motor connection star and delta form to fundamental component currents that can circulate through the bearing sets down the shaft, TBH i feel you may be out of your depth here and yes although we are happy to help we may not get all the info we seek and miss something vital to diagnose correctly and you'll be running round in circle trying to supply the info...

As NBP says the meter used to measure is crucial as true RMS and high sample rate needed with good quality components or your meter just gets confused, the voltage will rise proportionately with the frequency according to the speed chosen its crucial the VSD is set up correctly and underspeed isn't an issue here. If various motors are failing then i suspect its a design issue with the control system, vsd settings or star/delta arrangement set up correctly on the motor termination.

Also check if the inverter is supplying multiple motors or are they dedicated.

 

[netblindpaul]

Tiny,

I doubt VERY much the guy is actually a Registered Engineer.
Also, I very much doubt he has any clue actually what is going on, else he should have come up with some questions or solutions.
This is not an easy situation to solve, if you look at the posts by myself & DW, you must have 20 questions already.
All of which are relevant.
Just another, is this a new install, a modification, or something that has been there a while?
Not sure who would be closest, DW or myself, but as suggested, you "really" need someone who knows this kit as it is a very systematic method required to solve such things, and this would include having all the information including the root causes of the previous motor failures, and if these were bearing related, the bearings themselves for competent dissection after suitably careful removal from the motors.

Just more questions!
No answers, sorry.

 

[Tiny Spark]

As above - many issues can arise when using inverters ...when you say burning out is that a diagnostic that the winding have melted or you just mean motor failure - if the later exactly what is failing and what are the symptoms etc.
Is this an adapted set-up i.e. an original DOL system that has been converted to Inverter run, you say the motor is supplied via a flex - does this have a screening correctly grounded as required by the Inverter Brand...

Its going to hard for you to solve this if you have no experience in this field but question the Engineer to how speed control is maintained whether it is preset orPOT' control by user ..... i could list you 20+ reasons why the motors may be failing from incorrect motor connection star and delta form to fundamental component currents that can circulate through the bearing sets down the shaft, TBH i feel you may be out of your depth here and yes although we are happy to help we may not get all the info we seek and miss something vital to diagnose correctly and you'll be running round in circle trying to supply the info...

Thanks darkwood.

Well in terms of the motors they are getting EXTREMELY hot, He isolated them before this did any damage but we have recently replaced 2x motors in these units recently which had seized up totally (probably due to this reason).

I am not sure if the speed control was a retro installation or not as the inverters are situated in a seemingly odd place and have been there longer than me so I really cant answer that one.
But I can tell you that the units are controlled by a BMS system.
As for the flex I dont think it has a screening but I cannot confirm this atm, Again I can have a look tomorrow.

I dont have a huge amount of experience with air con units which is why I asked wether they are normally speed controlled or not.

But there are around 30 of these units on site in total, All pretty much the same so I really dont think the problem lies with the fact they have a inverter however ime happy to be told ime wrong on this one LOL

However I have just had a thought, Now this may be entirely circumstantial but:
The 3 units that have this suposed inverter problem are all on the same gantry separate from all other units, But also on this gantry is a large heating AHU with 2 large inverter controlled motors.
One of these inverters seems to of failed but we have not investigated this fault yet.
So is it possible there could of been some kind of spike or something which could of damaged the inverters in this area?

I dont believe in circumstance so I doubt it but thought it may be worth mentioning.

 

[Tiny Spark]

Oh and just to add no this is a old installation, been in for many many years.

 

[netblindpaul]

OK, but, have the inverters, is the BMS new?
Are this air distribution fan units or chiller fans?

 

[Darkwood]

Just to add if you can get a prognosis from the motor repair company as to why the motor failed in his view it can lead you to a good starting point even when motors fail their is a wealth of info as to why when you take them to bits by the kind of damage and where... some older motors have issues with been inverter driven as they can suffer specific winding damage due to the original winding insulation isn't designed for the additional stresses that a VSD can put on the insulation..... im hoping you are really seeing the hole here you may dig yourself, ive been working with drives over 10yrs now and some issues still have me flummoxed to diagnose id hate to be walking into this situe with no experience and would personally advice against it.

PS NETBP if the OP requires our help on site ill leave this one to you its a tad out of my catchment area and im a tad busy to boot.

 

[Tiny Spark]

Paul the BMS and inverters are both existing also, Literally nothing has been changed recently. (other than the 2 motors previously mentioned.)

As for the units, Basically they are large cold air blowers.
They have a cold water feed into them which is pumped through a heat exchanger system and the fans draw the air through and blow it out basically.

 

[netblindpaul]

OK, are the new motors "inverter duty", or cheapest you could get?
Any other changes on the system?

 

[Tiny Spark]

Well thankfully its not up to me to fix them as they are under service contract and it has to be said drive systems are not my strong point!
But I know my boss is going to quiz me tomorrow for a explanation and at the moment I cant give him one LOL.

My plan for tomorrow atm is basically to have a look at these units for myself and see what readings I get from them and from a good unit.
I was not with the guy while he was carrying out the work but my apprentice had a few concerns about him which he relayed to me afterwards which made me raise a eyebrow.

I was not aware that the voltage could appear to change from a inverter so I will keep that in mind but what the engineer told me before he left about the inverter changing the voltages rang alarm bells with me as I was always under the impresion they just changed the frequency to change the speed of the motor and the voltage would stay constant?
But from my limited understanding it is possible to change voltages with inverters.

 

[netblindpaul]

It is normally the frequency that changes, however depending on the meter this could appear as a changing voltage, it depends on the meter spec.
The d.c. bus voltage is normally fixed, it is the on time of the output gates that changes normally.
Again different "inverters" can be set up in different ways.

Too many variables.

DW, I work country wide TBH, but I'm not cheap, and also a bit busy!!!

 

[Tiny Spark]

OK, are the new motors "inverter duty", or cheapest you could get?
Any other changes on the system?

They were direct replacements for the original ones, And I think (not 100%) that the inverter controls both motors in the unit but again ile be looking at that one tomorrow.
I shall take some pics to so you can see

And no other changes that I am aware of.

 

[Darkwood]

This shows the proportional gain of voltage to frequency as the motor starts from 0-speed up to base speed (motor running speed) then also show what happens to the torque as you overspeed the motor which you can do within limits if you don't require full torque.

 

[netblindpaul]

I'm confused by your last post DW

 

[Tiny Spark]

No problem,

Thanks for the help thus far guys.
I will get some more info and pics tomorrow which may help.
Appreciate it

 

[stef]

Reading through this, not by any chance the motors are wired in delta whilst the inverters are set up for star connection? The output voltage is then too high. This you can't measure easily as the output is not a pure sine, too many harmonics are on it. Don't measure to earth, you will get rubbish. Just a thought...

 

[Darkwood]

I'm confused by your last post DW

Edit ..... remove as we got our wires crossed

 

[netblindpaul]

Because that was not the post I was confused by!
You blighter you've changed it!