Motors versus VFD issue

[SparkyJunior]

Hi guys. I am working on a project where our client has a 150HP motor and a 150HP VFD driving it. They want to replace the motor with a 200HP motor and also the corresponding drive. However, they want to do this one at a time. Either replace the drive first and then the motor or the motor and then the drive. I have been told that you should replace the VFD first and then set the electronic overloads in the 200HP VFD to trip a 150HP motor.

Is there any scenario where you can replace the motor first and rive it with a smaller VFD? Any input would be appreciated.

Thanks

 

[dV8r]

Probably not, especially if the 150 Hp is alrady too small.

 

[ozmosis]

it is current that determines overloads in your VFD, not hp. Check the current rating of your VFD and see how this looks against the current rating of the motor you plan to fit.
However, as you have invested the time and effort into a project whereby you have first determined that you need more power and then decided that you will invest in new equipment; why are you thinking about doing it piecemeal? It seems to be a waste of time to me.
If you are going to do it, do it all. It is not just the VFD and motor you will need to change, you may need to alter cabling, switchgear, breakers/fusing etc.

 

[aolalde]

To me sounds logic to replace the VFD for 200 HP first and the motor later. You will be able to setup the larger drive to handle the 150 HP motor. Certainly this is only one opinion.

 

[jraef]

I love engineering by budget managers. It keeps me employed in reworking their failures.

You could theoretically replace the motor first, then set the current limit of the VFD to not allow the VFD to deliver more current than it is rated for. The effect on the motor will be to limit HP, which in many cases must involve lowering the speed. So if you can live with the motor running slower, then you can replace the motor first.

But as others have pointed out, you already have determined that the 150HP motor is not sufficient, so why bother either way? Do the whole thing or do nothing and live with what you have. sed2developer brought up a very valid issue, that of cable size, protective devices etc. Per the latest code rules you cannot change any component without changing all of that too.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[macmil]

As a mechanical guy, I would suggest that you obtain information on the load requirements before changing out drive components. It might well be that this has been done but you should be made aware of the reasons for the proposed change. Is it because of starting difficulty ? or normal operation ? or overload conditions ? Increasing motor sizes without looking at the whole picture can give rise to other problems~ e.g. is there a speed reducer / gearbox in the drive train~ is it rated for the imcreased motor size ? couplings ? etc. I know, not the the answer to the question you asked, but.... mac

 

[Senselessticker]

Do you know for certain that the existing VFD needs replacing? Get the info on the existing VFD and determine for certain that it needs replacing.

If so, do it all at the same time. Oh...and don't forget as others have mentioned: feeder size changes, breaker size changes, coordination changes, changes to short circuit current and ratings for breakers, etc...

 

[dcasto]

It would be interesting to know what piece of equipment the motor is driving. As stated above, the amps set the limit on the VFD. If the equipment being driven can do its job with the amp load of the 150 HP, then the motor can be changed first, then the VFD. Is the equipment being driven being replace too?

 

[Skogsgurra]

jraef,

I do not love engineering by bean counters. Yes, it will keep you occupied. But my guts do not like that kind of "occupation". I had a recent case with an old DC drive. Two perfectly identical motors where load sharing was terrible. No apparent reason - until I opened the motors to have a look at commutator and brushes. Instead of three rows - there was just one row. Bean counting had decided not to buy brushes until they needed so many that they could get a better price. OK - I know, not subject of this thread, but perhaps you can learn something from it. Do the whole job. But plan the whole job first. Macmil has some good points - we need clankeys, too. And involve them in the planning!

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

Gunnar,
I was being sarcastic...

 

[Skogsgurra]

I did understand that. Know your preferences. But, given the possibility to tell a bean counter horror story, I couldn't resist.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[jraef]

Tangent Alert!
I just had one last year involving a pump and a VFD. They had a 250HP pump on a soft starter, they wanted to save energy. I suggested they put in a VFD and modulate speed when full flow was not necessary. They (the bean counters) listened to a pump salesman who convinced them that for less money, they could use his new 100HP pump and put it on a VFD. I didn't get a call back until they had already done it and "SURPRISE!", for some reason couldn't get it to work under anything but the lowest flow conditions and the VFD was always at full speed. They thought that by turning up the speed of the pump with the VFD they could pump more water, i.e. as much as they had with the 250HP pump! Needless to say, this was a financial disaster in the long run, which was especially satisfying for me as I got paid by the hour to inform them (tactfully of course) that they were idiots!

Now I am getting paid again this year to commission the 250HP VFD that I told them they should have used in the first place, then again to recommission the 100HP VFD on a different application.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[itsmoked]

Ha! Sweet. jraef.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[macmil]

Skogs~~~~~ Clankeys ????????????????
but thanks for the kind ref~~ yes I have found in my somewhat lengthy life passage that it is always worth while to review the whole project with all disciplines ~~ sometimes it's redundant, and sometimes it's time really well spent. mac

 

[itsmoked]

Yes, Clankeys in lue of zahzahzahzahzzzzzsssss<sizzle>.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

clank 1. noun - sound made by clankeys (which see) when hitting metal object with heavy tool. 2. verb - to clank, to hit metal objects with heavy tool. First appeared in early 1940 comic strips - illustrating sound generated M. O. was hit with H. T.

clankey 1. noun - person thats clanks (which see). 2. noun - in a wider sense, professionals that once clanked, but now do it on computer screens - aka mechanical engineers. Both nouns derived in same vein as sparky (which see).

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[DickDV]

Hmmm! I've been in Sweden seven times and not once did I see a body of water called "Witch Sea"!

But then, I barely came close to pronouncing "sjo" anyway!
(Sju wasn't any easier!)

 

[macmil]

Skogs~ for the record, my " clankeys ?? " was not a true question but perhaps suggesting that I was inferring that it was a disparaging remark, made in good humour and fun and accepted that way !! no offence taken, or intended. As a mechanical engineer ( clankey ? ) I appreciate the enlightenment you bring to this forum. And like to remind you that many of the things that get driven by your motors are mechanical, hence the need for multi discipline co-operation.

 

[jraef]

I've never heard "clankeys" before, but that is definitely going into my lexicon! I used to resent being refered to as a "sparky" because I had always heard that used to refer to electricians, not EEs. I would prefer "smokey", because when I make a mistake, it ususally involves more than mere sparks!

<tip-o-the-hat to Keith, aka member itsmoked >

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[Skogsgurra]

macmil,
You are so right. I will actually be part of a conference this May where I shall give my view on co-operation between disciplines like mechanical, electric, process and so on. The theme of this conference is "Mech is Mech and 'tric is 'tric - will they ever meet?" This conference is to make them meet and understand each other a little better.

Dick,
There are lots of lakes (seas) as you probably noticed. Around 100 000 of them - I am sure there must be one named Haxsjon (Witch Sea). Now, I have to find that lake.

And, Jeff,
I used to work with an ABB guy that is known all over the world as "Smokey" after having set a paper mill on fire. His comment: "It just burned down". Are you in the same division, Jeff?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...