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Energy Saving By Using Inverters In Place Of Dol


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Dear Friends;


I have recieved a mail from a company that they worked a project where they replaced the DOL and star delta starters with inverters and saved energy. They sent me following example of this change;



"Please find the energy saving report finalized after implementation:


1)- 767M2265


Motor Rating: 37KW

Load taken when DOL: 29Amps

Load taken when Drive: 8 Amps

Saving in current: 21Amps

Energy saved: 1.732*500*11*0.9=16KW


2)- 767M1507


Motor Rating: 22KW

Load taken when DOL: 25Amps

Load taken when Drive: 13 Amps

Saving in current: 12Amps

Energy saved: 1.732*500*12*0.9=9.3KW




Motor Rating: 18.5KW

Load taken when DOL: 19Amps

Load taken when Drive: 3 Amps

Saving in current: 16Amps

Energy saved: 1.732*500*16*0.9=12.5KW"


This is a strange think for me that how can it possible. I would have a meeting with them but first i need to verify from experts.

"Don't assume any thing, always check/ask and clear yourself".

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Hello AB2005


First of all, the calculations are not correct as you can not assume that the power factor is high.

In the calculations, I assume that the supply voltage is 500V ??

That is an unusual voltage.


The application is not described. What is the application or is it an open shafted machine.


A VFD will reduce the energy consumption only if the machine has significant mechanical losses at full speed, and does not need to operate at full speed to do the same amount of work.


The saving is the reduction in mechanical losses when the machine speed is reduced.


Calculated savings based on calculated values from measured Amps never work out correctly.


Many VFD suppliers claim energy savings in ALL applications using VFDs.


If the motor operates at full speed with a VFD in ccircuit, the energy consumed is higher than DOL.

The VFD has losses in the order of 3% - 5% plus the motor losses will increase due to the wave shape of the voltage applied to the motor.

The current into the VFD will be less than the current into the motor because the displacement power factor is better than 0.95 at the input of the VFD, irrespective of the displacement power factor at the motor.


Best regards,


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