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Old September 26th, 2013, 03:28 PM   #1

Skyba is offline
Join Date: Sep 2013
Location: Lima, Peru
Posts: 7
How many VFDs to share a load ?

Hi everyone,
I have a project in which I need to lift a cylindrical tank of 20 tons with a dozen 0.5 HP motors over 15 minutes. It's important that the load distributes evenly over every motor.
What is better :
1) 1 VFD for 15 motors, control in V/f mode
2) 3 VFDs with 5 motors each, control in V/f mode
3) 15 VFDs with 1 motor each, and interconnect the VFDs for Torque/speed share.

In the first two options, i would protect every motor with a thermal overload. Option 2) has two sub-options, see Do you know which is best for a good load share though "slipping" of the motor ?
Option 3) is pricier and more complex. Only in last resort.

Thank you very much for your help !
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Old September 26th, 2013, 06:08 PM   #2
The Plc Kid
United States

The Plc Kid is offline
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Join Date: Feb 2009
Location: Macon, Georgia
Posts: 3,233
Option 3 is going to be the best and most reliable and easiest to maintain.

Vendor selection would be critical though.
I would only consider Rockwell,Siemens or ABB likely Rockwell or ABB. With Rockwell I would use 755 series with CIP Sync and CIP Motion.

Just my 2 cents.

Yes, training is expensive - but ignorance is where the REAL money is .Ron Beaufort

Some jobs start out best with a Sawzall and a Dumpster on casters. OkiePC
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Old September 26th, 2013, 07:26 PM   #3
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DickDV is offline
Join Date: May 2002
Location: Kalamazoo MI
Posts: 2,321
With multiple motors on a single VFD, you cannot manage individual motor torques. The variations in mechanical friction and especially variations in the motor slip/torque relationship will make torque management impossible.

If you want to be sure to load each motor equally, individual VFD's with torque follower control signals will be required.

My choice would be ABB ACS800 drives due to the precision of the torque management and the response speed if something gets unbalanced. Also, you should be able to do this job without encoders on the motors.
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