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Vfd Output Chokes


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#1 Mohankumar.L

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Posted 12 November 2008 - 10:30 AM

Hi I am facing a problem while using the single 11 Kw VFD for 65 Nos of 0.3Hp motors. In this the motors are burning very frequently. Anybody is having any solution for this. Kindly help me.

#2 mariomaggi

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Posted 12 November 2008 - 03:48 PM

Dear friend,
if the motors and cables are properly selected, motors will never burns.

It is possible that such motors are not enouugh insulated to run with VFD. Do you find an electric burning in the first winding of each phase winding?

What is the cable lenght? Do you connected all 65 motor using 65 cables, or only one cable as daisy chain?

What kind of damage you find in such motors? Are they protected by bimetallic thermal switch in their windings?

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Mario

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#3 jraef

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Posted 13 November 2008 - 06:33 PM

QUOTE (mariomaggi @ Nov 12 2008, 07:48 AM) <{POST_SNAPBACK}>
Dear friend,
if the motors and cables are properly selected, motors will never burns.

It is possible that such motors are not enouugh insulated to run with VFD. Do you find an electric burning in the first winding of each phase winding?

What is the cable lenght? Do you connected all 65 motor using 65 cables, or only one cable as daisy chain?

What kind of damage you find in such motors? Are they protected by bimetallic thermal switch in their windings?

Best regards
Mario

All very excellent questions. To get the best help on the motor issues, please respond to all of Mario's questions.

From another angle however, I think your VFD is seriously under sized. 0.3HP x 65 motors is 19.5HP, which is 14.55kW. It is a good rule of thumb to over size a VFD for multiple motors by 10% of the aggregate total, so I would have recommended at least a 16kW rated VFD, but you are using an 11kW drive. Chances are, your VFD is going into current limit in order to protect itself, which means it will limit the speed as well. If someone noticed that and didn't understand what was happening, then tried to adjust the output frequency to compensate, they would have set up the incorrect V/Hz ratio for your motors and they will over heat.

It is also a very very good idea to have output reactors on multiple motor installations like this, because you can end up with issues of cable capacitance and resonance from all of those separate motor circuits, causing sever voltage anomalies at the motors and VFD.
"He's not dead, he's just pinin' for the fjords!"




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