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 July 14th, 2008, 10:00 AM   #1
nathanht
Member
United States

nathanht is offline
 
Join Date: Aug 2006
Location: Hobart, Indiana
Posts: 5
VFD to Motor Starter

Hi;
I have a project I am designing to run multiple 3 phase induction motors from 1 vfd. I have two motor starters , one is connected from the output of the vfd
To the motor, the other is connect from line to the motor. The vfd has flying start which gives it the ability to catch a motor while it is still spinning. I monitor the speed of the vfd from an analog output of the vfd to a PLC. When the VFD reaches 60 hz the PLC disables the vfd and switches the motor to the line contactor.
When the speed drops below 60 hz the PLC switches the motor back to the vfd, re-enables the vfd and sends a start command.. All this works nicely except when the motor goes from the vfd to to line most of the time I get a thump which could damage couplings. Some people have told me I can eliminate this by zero cross detection, can I use a PLC for this does anyone have any suggestions?
  Reply With Quote
Old July 14th, 2008, 10:34 AM   #2
milldrone
Member
United States

milldrone is offline
 
milldrone's Avatar
 
Join Date: Mar 2005
Location: in the dog house
Posts: 1,294
Quote:
Originally Posted by nathanht
Hi;
I have a project I am designing to run multiple 3 phase induction motors from 1 vfd.
I'm having trouble with your description. Where are the multiple motors described?

Quote:
Originally Posted by nathanht
I have two motor starters , one is connected from the output of the vfd
To the motor, the other is connect from line to the motor.
Sounds like a typical bypass arrangement.


Quote:
Originally Posted by nathanht
All this works nicely except when the motor goes from the vfd to to line most of the time I get a thump which could damage couplings. Some people have told me I can eliminate this by zero cross detection,
This is done all the time with generators (syncing with the line). But it is done with equipment that is designed for the job not a plc. My guess is this equipment will cost at least 10 times of the price of a dedicated VFD (1 FVD to a motor).
__________________
Vaughn

If you can read this, thank a teacher.
  Reply With Quote
Old July 14th, 2008, 10:43 AM   #3
nathanht
Member
United States

nathanht is offline
 
Join Date: Aug 2006
Location: Hobart, Indiana
Posts: 5
VFD to Motor Starter

At this point there is only one motor for testing. There will be another set of contacts for a second and a third motor. I am using reversing starters(set not to reverse), this way I only need one overload and they are interlocked so as not to energize at the same time. One starter feeds from the vfd to the motor, the other starter feeds from line to the motor with the same overload. The idea for the single motor test was to assure that the VFD would catch the motor while spinning with no issues, which it does. The issue is going bak to the line starter which causes a jerk.
Thanks
  Reply With Quote
Old July 14th, 2008, 11:49 AM   #4
tragically1969
Member
United Kingdom

tragically1969 is offline
 
Join Date: Jun 2007
Location: uk
Posts: 1,313
I sense a big bang and a smell of smoke about to hit the air

Can you provide a schematic or block diagram of how all this is connected ?
  Reply With Quote
Old July 14th, 2008, 12:23 PM   #5
OkiePC
Lifetime Supporting Member
United States

OkiePC is offline
 
OkiePC's Avatar
 
Join Date: Mar 2005
Location: ENE of Nowhere Oklahoma
Posts: 9,983
Quote:
Originally Posted by nathanht
...I monitor the speed of the vfd from an analog output of the vfd to a PLC. When the VFD reaches 60 hz the PLC disables the vfd and switches the motor to the line contactor.
When the speed drops below 60 hz the PLC switches the motor back to the vfd, re-enables the vfd and sends a start command...
So while the line contactor is engaged, you are still able to monitor the speed via the VFD? External Encoder?

And, what kind of load are you operating that causes the motor speed to droop?

I don't know of any way to ensure a bumpless transfer back to the contactor other than really fine tuning the VFD control to the motor and load. But even then, when the contactor reverses, the motor may experience a momentary break in current flow...I don't think an overlapping contactor would work either unless you like the smell of exploding IGBTs...

Paul
__________________
It's not all the variables I am most concerned with, it's the undiscovered constants.
  Reply With Quote
Old July 14th, 2008, 12:26 PM   #6
milldrone
Member
United States

milldrone is offline
 
milldrone's Avatar
 
Join Date: Mar 2005
Location: in the dog house
Posts: 1,294
Quote:
Originally Posted by nathanht
I am using reversing starters(set not to reverse), this way I only need one overload and they are interlocked so as not to energize at the same time. One starter feeds from the vfd to the motor, the other starter feeds from line to the motor with the same overload.
I'm curious, how does the cost of the extra equipment compare against the price of one VFD to a motor? My experence has been just the opposite of what you are are attempting. As I mentioned before the technology exists for your problem but I'm sure it's way more than the price of one VFD to a motor.

PS. Welcome to the forum.
__________________
Vaughn

If you can read this, thank a teacher.
  Reply With Quote
Old July 14th, 2008, 12:31 PM   #7
nathanht
Member
United States

nathanht is offline
 
Join Date: Aug 2006
Location: Hobart, Indiana
Posts: 5
Hi Paul

Once the motor is on the line contactor the vfd is disabled, the reason for this is the vfd cannot catch a spinning motor while it is enabled, it will generate a hardware fault because of the back emf of the spinning motor. When the motor is returned to the vfd control the vfd is re-enabled. There is no problem with any of this, the vfd catches the motor ,determines the speed to adjust to and runs fine. The problem is going from vfd to line. The load is a pump.
  Reply With Quote
Old July 14th, 2008, 12:33 PM   #8
robertmee
Member
United States

robertmee is offline
 
robertmee's Avatar
 
Join Date: Feb 2008
Location: NC
Posts: 1,428
This sounds like a very ugly application. You mention sharing a common overload for multiple motors. I would not suggest this as even though you are isolating one motor at a time, OL's are time/temperature dependent and have memory. Motor A running through the OL will heat the element (assuming you are using thermal type) to some degree. Switching to Motor B now gives a false history to the OL of Motor B's operation. Probably not an issue, but why risk it for the minimal savings. I agree with everyone else here. VFDs are so cheap nowadays, you can nearly buy them for the cost of one NEMA style reversing contactor.
  Reply With Quote
Old July 14th, 2008, 12:42 PM   #9
OkiePC
Lifetime Supporting Member
United States

OkiePC is offline
 
OkiePC's Avatar
 
Join Date: Mar 2005
Location: ENE of Nowhere Oklahoma
Posts: 9,983
I have seen a single VFD used to run multiple motors and it was functional, but the downtime incurred in troubleshooting the first failure probably exceeded the cost to use multiple drives in the first place.

I am still not understanding how you know when the motor begins to slip excessively while connected across the line, and why is it necessary to go back and forth from VFD to line contactor.

What are you going to do when you have say four motors running on contactors, and two of them suddenly need the "VFD boost" at the same time?

As Robert stated, each motor must have its own overload relay, and certain types of those will not play well with VFD carrier frequencies.

And, again, I think when going from VFD to line contactor you will always see a bump in the motor.

What kind of pumps are they? How many horsepower?
__________________
It's not all the variables I am most concerned with, it's the undiscovered constants.
  Reply With Quote
Old July 14th, 2008, 12:58 PM   #10
nathanht
Member
United States

nathanht is offline
 
Join Date: Aug 2006
Location: Hobart, Indiana
Posts: 5
Hi;

This is not a new idea, there are many of these systems on the market, vacuum pumps, well drilling rigs etc... I'm just trying to figure out how they overcame these problems.

The overload is on one motor, not multiple motors, the same motor just switches from vfd to line. Right now I'm using a 25 HP 480V three phase induction motor.
Thanks
  Reply With Quote
Old July 14th, 2008, 02:05 PM   #11
leitmotif
Member
United States

leitmotif is offline
 
Join Date: Nov 2004
Location: Seattle Wa. USA
Posts: 3,680
I can sorta see what he is trying to do (uhhhh I think). Basically run 1 to 3 pumps at full load and run one at partial load on VFD.
Pump A starts gets to 60 Hz and switches to line while pump B starts on VFD etc and repeats until Pmp C is at 60 connd to line and D is on VFD.

All well and good while climbing the ladder
EXCEPT
In short term switch from VFD to line the motor is in generator mode (especially if no discharge check on pump - it may even run backward if left long enough)
BUT in the short term the motor is a generator how does he ensure motor and lines are matched ie paralleled to prevent burning of contacts or worse yet paralleling motor 180 out of phase (goodby contacts and maybe motor)??

Dont think I would do it. Tell the beancounter to write bigger check for more VFDs.

There is a good example of this
switch a two speed 3 phase from hi to low quickly. You will see the motor instantaneously slow to get generated in sync with line. Real hard on couplings and loads. The motor will rock around a little.

On alternators this kind of thing (parallel out of phase) can rock the alternator & prime mover out of the mounts or break the crank on a diesel.

ONE set of overloads per motor. Be cheap at the front end and repent for years in maintenance costs.

Dan Bentler
  Reply With Quote
Old July 14th, 2008, 02:39 PM   #12
Vic
Member
Canada

Vic is offline
 
Join Date: Jun 2002
Posts: 236
nathanht

You are right when you say this is commonly done. That doesn't mean it is a good solution.
There are problems if you switch the motor to the across the line starter too fast or too slow. It you switch the motor too fast, the EMF generated by the motor has not decayed and when the across the line power is applied, it will probably be out of phase with the motor EMF and will cause a large current and torque spike. This can damage the motor, coupling and/or pump. If you switch too slow, the motor, being under full load, will rapidly slow down when the power is removed. When the power is reapplied, the motor will again see a large current and torque spike, although probably not as bad as the other case. There is probably an optimum switching time but it may be hard to find.

This topic was discussed extensively in This Thread, particularly in reply #28 by jraef.
  Reply With Quote
Old July 14th, 2008, 02:59 PM   #13
milldrone
Member
United States

milldrone is offline
 
milldrone's Avatar
 
Join Date: Mar 2005
Location: in the dog house
Posts: 1,294
nathanht,

Any possibilty this is the system your looking for?

Pay attention to the comments from DickDV

http://www.plctalk.net/qanda/showthread.php?t=30957
__________________
Vaughn

If you can read this, thank a teacher.
  Reply With Quote
Old July 14th, 2008, 03:08 PM   #14
nathanht
Member
United States

nathanht is offline
 
Join Date: Aug 2006
Location: Hobart, Indiana
Posts: 5
Thanks;
This is very similar to what I am looking for, thanks to all for the great tips and suggestions.
  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
VFD stop mode options on a pallet hoist? bikerdude LIVE PLC Questions And Answers 27 August 30th, 2015 10:27 PM
VFD to control heaters ?? leitmotif LIVE PLC Questions And Answers 24 March 30th, 2014 05:14 PM
VFD -- Reverse only one of two motors leitmotif LIVE PLC Questions And Answers 7 September 25th, 2007 01:41 PM
VFD switching for one vfd to either motor mordred LIVE PLC Questions And Answers 16 January 24th, 2005 11:19 AM
VFD Standard labingtone LIVE PLC Questions And Answers 11 January 31st, 2003 07:28 PM


All times are GMT -5. The time now is 11:12 PM.


.