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Old September 25th, 2007, 03:11 AM   #1
leitmotif
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VFD -- Reverse only one of two motors

We have mixer where two motor gearboxes drive two screw type mixer blades. Blades are six foot long 14" dia and are horizontal. When mixing they counter rotate to pump this liquid "slurry food stuff" around the bathtub shaped mixer. To empty one motor reverses and both now rotate in same direction. Boss wants to drive these with VFD and use one VFD for two identical motors - each 7.5 HP 480 3 phase.

I know we can use one VFD (15 HP or slightly larger) to drive both.
I also know we should not use relays on VFD output.
VFD can reverse two motors OK but not just one
OR is there a VFD that has dual outputs and is able to reverse just one?
The only other way is to put reversing contactors on VFD output for just one motor.
Sequence would be something like this
Operator shifts select switch from mix to empty.
VFD would have to come to a stop
VFD would then allow the reversing contactor to reverse only one motor and then restart both.

I personally would prefer to use two VFDs and drive each motor independently.

Anybody got any better ideas??

Dan Bentler
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Old September 25th, 2007, 04:32 AM   #2
Andybr
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Dan.

You COULD do this by fitting reversing contactors for one drive in the VFD output and ensuring that the drive is stopped before you operate them. I have seen this done with two speed motors and there have been no problems. The question is whether it is actually worth the effort. A 7.5HP drive is not very expensive and you could easily spend just as much installing contactors and interlocks (and PLC I/O) trying to use a single drive. I would go for two drives every time.

Andybr
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Old September 25th, 2007, 05:19 AM   #3
plcsoftvsd
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I would not suggest one VSd and two motors especially in this application where there is reversal.

Two 10 hp VSd ( I always size the VSD one notch above the motor size to allow for temperature deration).... is the way to go.

Good Luck !!
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Old September 25th, 2007, 10:57 AM   #4
TConnolly
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Cost of the extra (7.5hp) drive: < 800 bucks.

If you have two motors on one drive, each motor needs its own OL. Then you have the reversing contactor. If you use NEMA overloads and contactors, you're already over the cost of the extra drive. If you use cheaper IEC components, you're into it for at least half the cost of the drive. Now add in the margin for the larger drive, and you're over the cost of using a second drive.

Get a quote on what it will cost for a 15HP drive with two sets of overloads and the reversing contactor and put it side by side on a spreadsheet showing two 7.5HP drives. Show how much panel space is required for wiring the reversing contactors and the extra overloads compared to the panel space for the drives.
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Old September 25th, 2007, 11:12 AM   #5
Jimmie_Ohio
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Quote:
Originally Posted by Alaric
Cost of the extra (7.5hp) drive: < 800 bucks.

If you have two motors on one drive, each motor needs its own OL. Then you have the reversing contactor. If you use NEMA overloads and contactors, you're already over the cost of the extra drive. If you use cheaper IEC components, you're into it for at least half the cost of the drive. Now add in the margin for the larger drive, and you're over the cost of using a second drive.

Get a quote on what it will cost for a 15HP drive with two sets of overloads and the reversing contactor and put it side by side on a spreadsheet showing two 7.5HP drives. Show how much panel space is required for wiring the reversing contactors and the extra overloads compared to the panel space for the drives.
This is an elegant solution to his problem, but the REAL ISSUE is the boss.

No matter what you tell him, he wants to do it his own way. If you design a building and tell Mr. Boss it will take a million bricks, he'll want to do it with 999,999 and spend thousands of dollars in additional design time to do it his way. Then when it's built, you have to come in during the night to figure out why the "duct tape" solution (his) doesn't work...
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Old September 25th, 2007, 11:25 AM   #6
Big John T
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Quote:
Originally Posted by Jimmie_Ohio
This is an elegant solution to his problem, but the REAL ISSUE is the boss.

No matter what you tell him, he wants to do it his own way. If you design a building and tell Mr. Boss it will take a million bricks, he'll want to do it with 999,999 and spend thousands of dollars in additional design time to do it his way. Then when it's built, you have to come in during the night to figure out why the "duct tape" solution (his) doesn't work...
Depends on the BOSS! I'm the boss 1/2 the time in my shop and we always do it my way 1/2 of the time...

John
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Old September 25th, 2007, 01:10 PM   #7
leitmotif
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Thank you gentlemen for the help.

Alaric I looked in Grainger cost of two 7.5 drives about that of 15.

John -- so you get your way 25 percent of time - of those how many worked out well??

What I think is going to happen is that we are always doing things RUSH RUSH and take as many shortcuts as possible. I think when boss realizes the wiring changes required by using single VFD he will go with two.

Now for those who see problems well this is the way it is --
you don't want to install or maintain your equipment correctly
well I cannot allow myself to care
-- I just get agitated and frustrated
-- besides it is just more job security for me.

HOWEVER I will NOT take chances with electrocuting people.
I do not like conduit grounding and will always pull a ground wire preferably bare. I DO take the time to install a ground buss in cabinets and hook up all them little green wires boss has no time to bother with and lets em dangle.

Dan Bentler
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Old September 25th, 2007, 01:41 PM   #8
rsdoran
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Dan, I usually keep Grainger on speed dial I use it so much BUT in this case may want to look around a little.

AD offers 7.5 HP drives for $435 US, worth looking at http://web3.automationdirect.com/adc...trol)/GS2-27P5

If you think you need sensorless vector they are $575
http://web3.automationdirect.com/adc...trol)/GS3-47P5

May want to check with your vendors, they may match (or lower) those prices if you deal with them on a regular basis.
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