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

 

[Darkwood]

So just to clarify the original motors were 400/690 and correctly wired in delta, then the replacements were 230/400 and also wired in delta which due to the different winding ratings will have seen up to 400v depending on speed set across a 230v set up hence burn-out express!

 

[Tiny Spark]

No the originals were rated at 400 star and 700 delta.
The replacements were rated at 400 delta and 700 star.

So basically they have not seen enough voltage.

 

[Darkwood]

No the originals were rated at 400 star and 700 delta.
The replacements were rated at 400 delta and 700 star.

So basically they have not seen enough voltage.

I would be checking things again i think you may be mistaken .... if you have a duel voltage motor say 230/400 the winding itself is rated at 230v...so in delta the 230v exists across 1 winding but when connected in star you have 2 winding in series with the supply so you are able to connect a high voltage.

The Star connection of a standard duel voltage motor is the higher voltage rating of the 2 voltages and the Delta is the lower

230 delta / 400 star
400 delta / 690 star

 

[Tiny Spark]

Well, I never took a piki of the old motors plate but this is the new motors plate sorry about crap quality its dark in them thar units LOL):

http://i676.photobucket.com/albums/...8D34F053-524-0000007BB0ECC553_zps27622a2b.jpg

These were wired in Star.

 

[Darkwood]

Well, I never took a piki of the old motors plate but this is the new motors plate sorry about crap quality its dark in them thar units LOL):

http://i676.photobucket.com/albums/...8D34F053-524-0000007BB0ECC553_zps27622a2b.jpg

These were wired in Star.

Well this motor should be connected in Delta 2nd row down 400v 50hz is your supply.

 

[Tiny Spark]

Well this motor should be connected in Delta 2nd row down 400v 50hz is your supply.

Yup.

I can only hazard a guess that these were swapped "like for like" and only on the basis of finding a really old motor in one of the old units that was rated at 400v star (possibly it was lower voltage delta but i thought it was higher, But wasnt really paying attention to anything other than the 400v bit lol)
The original motors were scrapped a few weeks back so I have no way of being able to confirm or deny this theory.

But at the end of the day atleast I know what the problem was now, Or atleast part of it!

 

[Darkwood]

Yup.

I can only hazard a guess that these were swapped "like for like" and only on the basis of finding a really old motor in one of the old units that was rated at 400v star (possibly it was lower voltage delta but i thought it was higher, But wasnt really paying attention to anything other than the 400v bit lol)
The original motors were scrapped a few weeks back so I have no way of being able to confirm or deny this theory.

But at the end of the day atleast I know what the problem was now, Or atleast part of it!

Crucial part of electrical engineering is paying attention it saves time money and confusion it my be the case the star and delta symbols are at either side of the 400v but one relates to the other voltage ...its crucial you read and understand the plates .... read up and star delta connections and voltage rating also read up on theory of star delta starting this should give you insight to why you are connecting the motors the way you are .... Parrot fashion work ethics is not for the Engineering scene you need to understand why you are connecting the way you are and the consequences of incorrect connections you can't learn anything following advice and reading a motor plate you need to be in a position to understand why its the way it is.

 

[Tiny Spark]

Crucial part of electrical engineering is paying attention it saves time money and confusion it my be the case the star and delta symbols are at either side of the 400v but one relates to the other voltage ...its crucial you read and understand the plates .... read up and star delta connections and voltage rating also read up on theory of star delta starting this should give you insight to why you are connecting the motors the way you are .... Parrot fashion work ethics is not for the Engineering scene you need to understand why you are connecting the way you are and the consequences of incorrect connections you can't learn anything following advice and reading a motor plate you need to be in a position to understand why its the way it is.

Well although i agree with you that its important to pay attention (As this wrongly wired motor proves) some times its easier said than done.
At the end of the day me investigating this issue was not a high priority (as I said up to contractors to sort out) And it was something I wanted to do for my own peace of mind more than anything so I didnt want to get bogged down with what voltages a motor which is running fine would be expecting if it was wired wrongly, Only that it was wired correctly and running
In the case of the motor in question it was wired in star and the rating plate said 400v star.

As I said before I can only makes guesses as to what the original motor was based on the other motors knocking around the site as I was not present when they were changed nor do I have access to them.

 

[Darkwood]

That said then you've had an insight to the basics of motors and trust me after 20yrs im still learning as i discover old technology and learn new... its never ending but its something to think about if your interested in the area of Electrical work.... there is a mass of old retiring Electrical Engineers out there with no-one to fill their shoes as their is a black hole in the market due to dumbing down of the industry and everyone only able to do domestic out of college...

I just be head hunted from Ireland to a firm to help them design a control .... i don't advertise in any form its all word of mouth and unless the project appeals to my interest i usually turn these down as i try stick within 50mile radius but im getting more and more call further afield and its clear from conversations that many are retiring

 

[Tiny Spark]

Yup, Every day is a school day.

Unfortunately for me I went from being the learner to the learned fairly fast.
Well I say fairly, More like very!
Recession has much to answer for including the redundancy of my mentors which left me holding the reigns so to speak (not sure how that happened exactly)
So instead of me having people to ask questions to I now have people asking me which is all well and good until I or they come across something which I know nothing about or have a limited knowledge of!
Sadly this happens more often than I would like as its a large factory with a very wide range of plant and services. Obviously its not possible for me to learn and know everything but I do my best with what I have lol.
That being said I have only ever known industrial work and would feel uncomfortable and out of place in a domestic situation!

But when I get stuck I am thankful for experienced people like yourself and friends to give me a shove in the right direction LOL