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ganeshakce-electrical
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Join Date: Jun 2008
Location: Tamilnadu,india
Posts: 64

VFD application

04/05/2011 7:55 AM

dear all,

i am working in heavy industry. in that we are using more number of fans. For energy efficient we are controlling all the higher rating fans by VFD. we have different rating of motors like 90KW,110KW,160KW. for standization purpose we changed 90 KW motor to 110 KW.My question is fixed loss only vary according to the motor KW or not & what about variable loss if i am using VFD.

regards,

N.Ganesan

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electricalexpert65
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#1

Re: VFD application

04/05/2011 10:28 PM

Please understand that the energy saving with VFDs in case of Fan Loads, follows the Affinity Laws (i.e.) flow is proportional to speed, pressur or head is proportional to square of speed and the power is proportioal to cube of speed. Thus, supposing that your flow requirement is only 50%, you run the motor at 50% of its rated speed (with a VFD), but the power requirement woul only be 12.5% of the rated power (0.5 x 0.5 x 0.5); thu, you acheive a tremendou power saving of 82.5%, just for a 50% reduction in flow. This was not possible in mechanical means of flow control like dampers. Considering this level of power savings, the savings in losses (either fixed or variable) is immaterial.

But, to answer your question straight, fixed losses would definitely vary according to kW (frame size, to be more precise) but this is negligible. coming to variable losses, if use with a VFD, the losses would increase, in fact, as the motor efficiency at part loded conditions is poor (unless, of course, you use an energy efficient motor).
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ganeshakce-electrical
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Location: Tamilnadu,india
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#2
In reply to #1

Re: VFD application

04/06/2011 12:20 AM

thanks for good answer.

my required shaft power is 85 KW appox.so i selected 90 KW motor.for the standarization i choosen 110 KW(remanining motors 110KW). in this case loss will increase or not. we are using energy efficient motors only. advise me

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electricalexpert65
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#3
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Re: VFD application

04/06/2011 5:44 AM

If you operate your 110kW motor at 85kW (assuming the connected equipment draws full load) then your motor is loaded to about 77%. Pl. check from the motor manufacturer's data sheet, what is the efficiency of the motor at this % load? Similar excercise can be done for various partial loadings while on a VFD. Now, you yourself can find out whether the losses will increase or not.
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nesubra
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#4

Re: VFD application

04/17/2011 2:44 AM

V.f is constant when u lower the speed with a VFD.So Magnetic density is maintained in the motor magnetic circuit.Losses reduce as the cube root of fan speed as pointed out.Fixed losses will remian the same.Sice V/f is maintained constant fl eff of motor is not affected.However the total eff will be the product of VFD and Motor eff .The advantage you derive is when u are operating at reduced speeds and for how long?these studies which depend upon the process requirement need to be studied before applyong VFD .Otherwise if it si only 5 min you operate at lower speed better to use a smaller fan.

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