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VSD underated


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#1 bob

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Posted 04 October 2005 - 05:41 AM

Hi,

We had previously one switched reluctance motor rated 45 k W driving a conveyor. The latter was starting the conveyor fully loaded without any problem. Due to scarcity of spare parts , the SR drive has been repalaced by a VSD of 55 k W . However, for the time being we have only at hand a 75 k w motor which has been installed for this application. Off load, the vsd drove the conveyor easily but when the conveyor is loaded it is almost impossible to start the load without overloading the drive.
The vsd is set for sensorless vector mode. I am planning to install a choke betwen the drive and the motor. Is it worth trying ? Is there any other solution ? I understand that the obvious solution would be to use a 55 k W motor instead .
Thanks for any comment.

Bob

#2 marke

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Posted 04 October 2005 - 06:17 AM

Hello Bob

Loaded materials conveyors commonly require over 160% torque to achieve breakaway. This at zero load speed.
Achieveing 160% Breakaway torque can be difficult for a drive, even an openloop vector drive.
If the flux is not high enough at zero load speed, the motor just will not produce enough torque. The slip wil increase as the drive accelerates and the motor current will rapidly climb and the drive will trip on overload.

Depending on the drive you are using, you may be able to provide a short term flux boost at zero speed to gain the extra torque.

DTC (direct torque control) drives claim up to 400% torque at standstill.
Some open loop drives can produce a high breakaway torque, others need to be in closed loop mode.
I believe that you may need to look harder at the drive rather than the motor, may be a setting, maybe going closed loop (add an encoder), or maybe a different drive.

I do not believe that chokes between the drive and motor, I expect that this will actually make things worse.

See what comments Jeff has!!

Best regards,

#3 jraef

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Posted 04 October 2005 - 11:46 PM

Assuming that the 55kW motor worked in the past when trying to start under full load, the problem as I see it is that the VFD is 55kW, and the motor is 75kW. I would venture to say that you cannot enter motor parameters into the VFD for anything larger than it's rating, so it "thinks" it has a 55kW motor connected to it. When you try to start it loaded, the vector control algorithm boosts the voltage with frequency to the levels expected to provide maximum power from a 55kW motor, but as it raises the flux voltage, the current is rising too fast becuase it is actually a 75kW motor. So it is probably going into current limit, which would lower the flux in the 75kW motor, which is probably reducing the output torque too much. Just a guess though, because you are sailing in relatively uncharted waters to apply a VFD that is too small for the motor in the first place. I do that for tests all the time, but never with a load.
"He's not dead, he's just pinin' for the fjords!"

#4 bob

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Posted 05 October 2005 - 04:04 AM

Thanks Marke and jraef,

jraef is right. I would like to know how to foul the vsd because honestly I believe that the vsd would have pull the load if the motor was rated 55 k W.

Bob

#5 jraef

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Posted 05 October 2005 - 06:22 AM

That is probably something you will need to discuss with your VFD supplier. The VFD capabilities and programming from from one mfgr to another is very different. Explain to them what you are doing, they may have suggestions. Some allow complete manual tuning of the vector algorithm, some do not. You will need to be able to do that on your drive if it is to work.

For instance the PDL drives that I am familiar with allow adjustment of what is called the Field Weakening Point in the Open Loop vector algorithm. This allows the VFD to reach maximum flux at a lower voltage, effectively creating a lower powered motor out of a larger one, so that there is additional flux available to boost torque temporarily. This is actually exactly what your application appears to need, but I have no way of knowing if your drive allows you to do that.
"He's not dead, he's just pinin' for the fjords!"




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