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Motor starting power calculation


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#1 newbies2005

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Posted 22 December 2005 - 10:13 AM

Hi everybody,
I'm just a newbies:D, so I feel very excited with all topic in forum, and would like to join with you.
I'm having a new project, provide VFD to system that used DOL for conveyor motor (constant torque). The old system always have 5 times to starting per 8 hour in common. Because I think that using VFD will decrease starting current of motors, it will also reduce the power consumption the system normally uses. So I just curiously want to know that any method or application to calculate the power when motor is started in DOL?

Thank you so much for your advice and attention.

#2 johnbose

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Posted 24 December 2005 - 01:30 PM

whether it is starting or running, Power is Root 3 x Vx I x cos phi. During DOL start conditions power factor would be very low ( as low as 0.2 ) . So reactive Power component would be high .

#3 marke

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Posted 25 December 2005 - 11:32 PM

Hello newbies2005

Welcome to the forum.

The VFD will certainly reduce the start current of the motor and probably keep the current during acceleration below 120% of the rating of the motor compared with about 650% when started DOL.
There will be a little less energy consumed during acceleration, but in the overal scheme of things, this energy saved is very small relative to the total energy consumed by the motor.
If the VFD operates the conveyor at normal speed, (fixed at line frequency) you will have a significant energy loss over a period of running time. This is due to the additional losses in the VFD which are typically 4 - 5%
If the speed of the conveyor is reduced, ther may be some energy savings due to the reduced frictional losses.
The use of VFDs can result in an increased energy consumption rather than a reduction, you need to check the sums.

Best regards,




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