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]

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!

 

[Tiny Spark]

Why? A standard VFD for an Induction motor will ramp up both Frequency and voltage so as to limit inrush currents there are other drives and methods but your basic off the shelf model will ramp voltage up to full which will then be at plated base speed in line with frequency ...if you were to extend the ramp up from instant to slow say 10seconds and put you meter across the phases you will see the voltage rise to full over the 10seconds not full voltage straight away with frequency rise over the ramp but both together.

That makes sence (and thanks for the graph) But the voltages that were measured were fairly stable.

 

[Darkwood]

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

Was struggling getting that pic over as a copy and paste so had to save and upload but had already posted before i realised i couldn't see pic lol ... i get you now it did seem strange you put that lol felt i was teaching you to suck eggs

 

[Tiny Spark]

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...

I think they are wired in star but cant remember.
Been a while since I had one open LOL

Either way it wont of been changed.

 

[Darkwood]

That makes sence (and thanks for the graph) But the voltages that were measured were fairly stable.

If your getting a steady 300v and the meter isn't a cheap and nasty then the motor is probably not running at full speed if the users have a habit of running the motors at a slow speed the air passing over them may not be enough to cool the motor down with a standard induction motor where the fan on the opposite end is used to cool the motor while running everything is fine at base speed but if you run same motor at 25% of base speed then the fan also runs at 25% speed and won't be able to cool motor and the motor can burn out without been detected by drive until it fails.... in such cases where slow speed are required for long durations the a forced fed fan needs fitting that cools at full speed regardless of motor speed. As there are so many applications and set-up as well as different motors you need some background as i said before to know when things are an issue or whether its not a concern.

In your situation the fans may have been given a user speed control outside the motors limts to run long term that may be why it only affects one area as the may just want a slow speed in said area you need to do alot of digging including ringing the manufacturers of motor asking what suggested minimum speed it can maintain... as we have both said too many variables its hard to give advice without misguiding you as our suggestions may not apply to the set-up you have its all shots in the dark.

If the system is designed for variable speed function then we are to assume the motors are good for low speed operations but if the system has been adapted from a full speed set-up it could be a consequence that the motors are overheating due to low speed and air cooling .... which as mentioned may only happen to a certain section because Ethel in reception get a draft at higher speeds.

 

[Tiny Spark]

Thanks darkwood.

Like I said I will do some digging and take some piki's tomorrow, These things can be hard to explain over the tinternet LOL
As for the speeds they can not be changed willy nilly so to speak as they are controlled from the BMS system, Obviously not sure how exactly they are operated as I know nothing about BMS programing etc but I would imagine that they would have built in parameters to stop that kind of thing?
Like I said before these units have been operating with no issues for many years which is why this whole thing is so puzzling to me.

If the inverter just went bang (as they have in the past) or the motor had just died of natural causes as they do I could understand it but they way this is panning out ime just struggling to get my head round it to be honest.

 

[Tiny Spark]

Good day gents.

Well I have been and investigated these things for myself, well attempted to anyways...
Now unfortunately I have been unable to carry out any live testing of my own as the state the engineer has left the units in does not allow for it.
I have confirmed a few more motors have seized (including one of the ones that was replaced 2 weeks ago) and a few are sounding/feeling rough to turn.

The flex that feeds the motors is a 4mm screened cable.
These are taken off the back of 2 contactor/ol's and they are fed via a single inverter which is a eurotherm 584sv.

Oh I have also confirmed that the motors are wired in delta and are rated at 4kw.

 

[Tiny Spark]

Update 2:

Just went to test a few good units.
Tested one unit that had a different inverter inside (SSO 650) using my Metrel mx26 i got a nice steady 430v between phases.

I then found a unit with the same inverter as the u/s unit and measured between phases again.
On start up the voltage climbed steadily over a period of about 5 seconds.
It started at around 100v and finished holding steady at 379v.

Now the motors operating voltages are between 380-440v so this is obviously a little low (assuming my meter is being truthful)

So with this information can I assume that either these inverters are just not up to the job or that they have been programmed wrong?

Thanks for the continued advice btw.

Think ive learnt more about inverters in the last 24 hours than i ever have lol

 

[Darkwood]

Update 2:

Just went to test a few good units.
Tested one unit that had a different inverter inside (SSO 650) using my Metrel mx26 i got a nice steady 430v between phases.

I then found a unit with the same inverter as the u/s unit and measured between phases again.
On start up the voltage climbed steadily over a period of about 5 seconds.
It started at around 100v and finished holding steady at 379v.

Now the motors operating voltages are between 380-440v so this is obviously a little low (assuming my meter is being truthful)

So with this information can I assume that either these inverters are just not up to the job or that they have been programmed wrong?

Thanks for the continued advice btw.

Think ive learnt more about inverters in the last 24 hours than i ever have lol

Don't be misguided by these plated values your inverter changes the frequency and voltage the plated value you see is at 50hz

 

[Tiny Spark]

Anyone have any ideas then?

 

[Darkwood]

As expressed yesterday this field has so many variables and so many questions that need asking that its not one your going to solve easily on a forum .... you say the other motors don't sound too good ..is that turning free hand does it spin and sound or feel coarse, the real info we need is an Motor Engineers break-down of what exactly happened to the failed motors... if the bearing are repeatedly going and seizing up then they need to split open the bearing and see if they have any electrical pitting if this is the case then you will require at least one insulated bearing on the motor pref' both .... this is getting very advanced for you experience and liaisons with both supplier of the motors, the inverter company and the company who fit and maintain the system.... simply replacing motors when they burn out as has been done before isn't the way forward.. but you knowledge and understanding of this field is many many yrs away from what is required and your going to end up in a big hole as you will be questioned on many key things you won't be able to give answers too.

If this is a repeat issue with various motors failing then their is possibly an incompatibility issue these may show more so in motors set at certain speeds as oppose to other settings ...the list goes on its not something we can guide over a forum too many questions every answer will generate another set of questions and so on.

I personally if the cause is very elusive would be spending a good 3-4hrs just testing all the basics checking all the drive settings trying to get info on the failed motors etc etc then id be calling a meeting of all required parties to bounce a few ideas off each other ...that is if i can't figure the cause which in some cases is due to lack of info.