You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

---------->>>>>Get FREE PLC Programming Tips

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

PLC training tools sale

Reply
 
Thread Tools Display Modes
Old September 3rd, 2013, 08:56 AM   #1
g.mccormick
Member
United States

g.mccormick is offline
 
Join Date: Jul 2012
Location: IN
Posts: 457
VFD to 2 motors. Alternating run time

We have a used piece of water filtration equipment. Long story short, 2 motors/pumps that are currently on contactors. Pump1 needs to be sent out for repair. Pump2 runs good, but building water supply cannot keep up at 60hz. We are going to put a vfd on and control pressure.

I plan to use the vfd through the 2 contactors to alternate the pumps run time. I will have to figure out interlocking the contactors so that only 1 can be on at a time, and 1 has to be on for the vfd to come on.
Since I will only be running 1 at a time, I don't think I need to use overloads on the contactors correct? Any thing I should be looking out for on this?

Thanks
  Reply With Quote
Old September 3rd, 2013, 09:44 AM   #2
Highland Controls
Member
United States

Highland Controls is offline
 
Join Date: Oct 2012
Location: Richfield, Ohio
Posts: 393
I think you need 2 drives. Are you planning on stopping the first motor, disabling the drive, switching contactors, then starting the drive? Can your filtration system handle the no-flow condition for a short time? You do not want to throw contactors on and off at the output of the drive(while on).

You say that one pump running at 60Hz cannot supply enough water. Are you planning on running one pump at more than 60Hz?
  Reply With Quote
Old September 3rd, 2013, 09:51 AM   #3
balash
Member
Croatia

balash is offline
 
Join Date: Dec 2012
Location: Koprivnica, HR
Posts: 412
overloads protect motor not vfd. vfd should have it so it is not necessary but as i heard regulations in North America require it.
as for alternation some vfd's have cascade controller which is supposed to do exactly what you want and more. you check it
I have one danfoss with cascade control doing what you want with alternation after 100h runtime controlling 2 pressure pumps on city water supply.
  Reply With Quote
Old September 3rd, 2013, 09:57 AM   #4
g.mccormick
Member
United States

g.mccormick is offline
 
Join Date: Jul 2012
Location: IN
Posts: 457
The water system is on the make up water to our cold well. We will run it in a way that there will be long off times (hours, days, week? depending on how much we lose to evaporation). We will NOT be switching motors at run time.
When we decide we need to top off the cold well, we will decide which motor to turn on and energize that contactor. That motor will run for the complete fill cycle (and however many fill cycles/ total time).
The building supply cannot keep up with a good pump running at 60hz. We will be slowing down the motor so that the building supply keeps up. The contactors will be interlocked with the drive "run enable" DI so that if no contactor is on, drive will not run.
  Reply With Quote
Old September 3rd, 2013, 10:03 AM   #5
James Mcquade
Member
United States

James Mcquade is offline
 
Join Date: Oct 2007
Location: Tennessee
Posts: 2,117
you cannot have the vfd energized when you turn on a motor. you will burn out the electronics.

also, you will need an inverter rated motor to keep it from overheating and possibly burning out.
regards,
james
  Reply With Quote
Old September 3rd, 2013, 10:06 AM   #6
g.mccormick
Member
United States

g.mccormick is offline
 
Join Date: Jul 2012
Location: IN
Posts: 457
Quote:
Originally Posted by James Mcquade View Post
you cannot have the vfd energized when you turn on a motor. you will burn out the electronics.

also, you will need an inverter rated motor to keep it from overheating and possibly burning out.
regards,
james
The vfd run enable will be interlocked with the contactors. If a contactor is not on, the vfd will not go into run mode. as far as the interlocking is concerned, it will be no different than an auxilliary contact on a motor disconnect.

I will be enabling/disabling the vfd seperately also.
  Reply With Quote
Old September 3rd, 2013, 10:07 AM   #7
Highland Controls
Member
United States

Highland Controls is offline
 
Join Date: Oct 2012
Location: Richfield, Ohio
Posts: 393
OK, that makes more sense. As long as you are sure the drive if off when switching contactors(open or closed), then is should work fine. Drive provides the overload protection.
  Reply With Quote
Old September 3rd, 2013, 10:11 AM   #8
g.mccormick
Member
United States

g.mccormick is offline
 
Join Date: Jul 2012
Location: IN
Posts: 457
Quote:
Originally Posted by Highland Controls View Post
OK, that makes more sense. As long as you are sure the drive if off when switching contactors(open or closed), then is should work fine. Drive provides the overload protection.

That is what I was thinking. I am going to go astep farther and use a reversing kit for the two contactors so that only 1 can physically be on at a time (though the motors will not be wired for reversing). Since only 1 motor at a time will run, I was pretty sure that the VFD overload protection was sufficient and did not need motor overloads.

Thanks
  Reply With Quote
Old September 3rd, 2013, 01:33 PM   #9
TConnolly
Lifetime Supporting Member
United States

TConnolly is offline
 
TConnolly's Avatar
 
Join Date: Apr 2005
Location: Salt Lake City
Posts: 6,147
OK, if I understand you correctly,
  • You have a cold well that you are supplying make up water too.
  • You do not have to provide make up water often.
  • You have two make up water pumps, but only one runs at a time.
  • Your pump feed water flow is not sufficient at the pump suction, so you want to decrease the rate at which the pumps move water.


Its just me, but I wouldn't approach this as a controls problem. I'd put a trimmed impeller in the pumps and leave the controls alone. Its maybe 5% of the cost of a drive, will take substantially less time, and will be far more reliable. Resizing the impeller is one of those rare cases where you really can have all three: cheap, good, and fast.

Your pump repair people can help you resize the impeller. Chances are the impeller can be chucked on a lathe and trimmed to the right size for the application, if not most pump manufacturers provide trimmed impellers. The pump volute, motor, plumbing, and controls remain unchanged.

Trimming will decrease your pump efficiency, but it will also decrease the load on the motor, which means a lower current draw, so despite the lower efficiency its a win.

Here is some light reading about impeller trimming as a better solution to your problem:
http://www1.eere.energy.gov/manufact...impellers7.pdf
http://www.pump-zone.com/topics/pump...-impeller-trim
http://saraswathula.tripod.com/trimming.htm
http://fluidfuture.ksb.com/en/fluidf...ller-trimming/
http://www.waterworld.com/articles/p...dify-them.html
__________________
True craftsmanship is only one more power tool away.

That's the beauty of processors, they don't have emotions they just run code - The PLC Kid.

Last edited by TConnolly; September 3rd, 2013 at 01:56 PM.
  Reply With Quote
Old September 3rd, 2013, 04:56 PM   #10
DickDV
Member
United States

DickDV is offline
 
Join Date: May 2002
Location: Kalamazoo MI
Posts: 2,307
Unless the two motors have identical nameplate data, you will still need two overloads, one in each set of motor leads.
  Reply With Quote
Old September 3rd, 2013, 05:06 PM   #11
g.mccormick
Member
United States

g.mccormick is offline
 
Join Date: Jul 2012
Location: IN
Posts: 457
Quote:
Originally Posted by TConnolly View Post
OK, if I understand you correctly,
  • You have a cold well that you are supplying make up water too.
  • You do not have to provide make up water often.
  • You have two make up water pumps, but only one runs at a time.
  • Your pump feed water flow is not sufficient at the pump suction, so you want to decrease the rate at which the pumps move water.


Its just me, but I wouldn't approach this as a controls problem. I'd put a trimmed impeller in the pumps and leave the controls alone. Its maybe 5% of the cost of a drive, will take substantially less time, and will be far more reliable. Resizing the impeller is one of those rare cases where you really can have all three: cheap, good, and fast.

Your pump repair people can help you resize the impeller. Chances are the impeller can be chucked on a lathe and trimmed to the right size for the application, if not most pump manufacturers provide trimmed impellers. The pump volute, motor, plumbing, and controls remain unchanged.

Trimming will decrease your pump efficiency, but it will also decrease the load on the motor, which means a lower current draw, so despite the lower efficiency its a win.

Here is some light reading about impeller trimming as a better solution to your problem:
http://www1.eere.energy.gov/manufact...impellers7.pdf
http://www.pump-zone.com/topics/pump...-impeller-trim
http://saraswathula.tripod.com/trimming.htm
http://fluidfuture.ksb.com/en/fluidf...ller-trimming/
http://www.waterworld.com/articles/p...dify-them.html


I don't want to modify the pumps because at some point in the future (hopefully within the next 6 months) we should have water supply updated and be able to use full 60hz.



Quote:
Originally Posted by DickDV View Post
Unless the two motors have identical nameplate data, you will still need two overloads, one in each set of motor leads.
Identical motors and pumps. The reason for wanting to alternate is just to keep the two pumps about even on run time.
  Reply With Quote
Old September 3rd, 2013, 06:27 PM   #12
mendonsy
Member
United States

mendonsy is offline
 
mendonsy's Avatar
 
Join Date: Feb 2009
Location: Western NY
Posts: 716
Why not just get two VFDs now and get it over with. They are not all that much more expensive than contactors and give you a lot more options.
  Reply With Quote
Old September 3rd, 2013, 06:42 PM   #13
Paully's5.0
Lifetime Supporting Member
United States

Paully's5.0 is offline
 
Join Date: Jan 2006
Location: WI
Posts: 1,991
Quote:
Originally Posted by g.mccormick View Post
I don't want to modify the pumps because at some point in the future (hopefully within the next 6 months) we should have water supply updated and be able to use full 60hz.
I have to agree with TConnelly on this one. Get a second impeller, have it trimmed and just swap it out when you get better water supply.

I personally hate trying to use "controls" as a "fix" for process issues, just because you can bandaid it doesn't mean it's something I want to take ownership of!
  Reply With Quote
Old September 3rd, 2013, 10:20 PM   #14
g.mccormick
Member
United States

g.mccormick is offline
 
Join Date: Jul 2012
Location: IN
Posts: 457
Quote:
Originally Posted by mendonsy View Post
Why not just get two VFDs now and get it over with. They are not all that much more expensive than contactors and give you a lot more options.

I already have the contactors in place. Only need to run 1 pump/motor at a time. Where are contactors the same price as VFDs?



Quote:
Originally Posted by Paully's5.0 View Post
I have to agree with TConnelly on this one. Get a second impeller, have it trimmed and just swap it out when you get better water supply.

I personally hate trying to use "controls" as a "fix" for process issues, just because you can bandaid it doesn't mean it's something I want to take ownership of!
If we modify the pumps, once we have sufficient supply flow, the pumps are undersized. If I put on a vfd, I can vary the flow now to get around the supply bottle neck, but run up to 60hz later once the supply is fixed. An Automation Direct GS3 30hp drive is less than $2k
  Reply With Quote
Old September 3rd, 2013, 10:35 PM   #15
TheWaterboy
Lifetime Supporting Member + Moderator
United States

TheWaterboy is offline
 
TheWaterboy's Avatar
 
Join Date: May 2006
Location: State of Denial
Posts: 717
We did just what you propose and it worked well enough, to prevent the multiple modes that would equal disaster it was a complicated wiring mess that we later replaced with 2 VFD's and are much happier.
But . . . it did work. For years.
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
alternating a drive on multiple motors DRThorne LIVE PLC Questions And Answers 29 May 17th, 2017 01:53 PM
Elapsed time calculation in s7 400 Ultimate99 LIVE PLC Questions And Answers 3 November 2nd, 2009 11:18 AM
Run time Problem in Wincc 5.2 nehe bhimaji LIVE PLC Questions And Answers 0 October 1st, 2008 08:56 AM
PIPE DREAM Electric car using VFD and 3 phase motors leitmotif LIVE PLC Questions And Answers 10 April 19th, 2005 07:36 AM
Can a VFD do magic with motors Goody LIVE PLC Questions And Answers 37 February 25th, 2004 02:01 PM


All times are GMT -5. The time now is 10:12 AM.


.