# Calculation Of Ac Current Input To A Dc Drive

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### #1 AB2005

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Posted 25 June 2007 - 10:41 AM

Dear All,

We have a 220KW DC drive for a 220KW, 440V, 1750RPM DC motor which is driving a 2.5meters wide, 60meters long felt in corrugator machine. As I have test run the machine and checked the 3phase current input to the drive 128-133A, checked the DC current out from drive was 120-124A. Is this difference is ok? Can anyone provide me the formula for calculating the 3phase ac current input to drive if we have the value of dc current from drive out put?

Thanks for any input.

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

### #2 gerryroxas

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Posted 27 June 2007 - 01:24 PM

Hi AB2005,

I am trying to figure out how you arrived with that actual measurement.
Theoretically: Iac = Sq. root of 2/3 multiplied by Id (Dc current)

We may need to start at Power Factor Calculation for SCR Conversion System
Power Factor Formula for Three Phase Bridge Rectifying Circuit is as follows:

Cos phi. = Power of DC Circuit / Power of AC Circuit

= Ed x Id / √3 x Eac x Iac

= Ed x Id / √3 x Eac x √2/3 x Id

Cos phi. = Ed / √2 Eac

Rated Voltage of Ac Circuit = ?
Rated Voltage of Dc Circuit = 440 Volts

Hence:
Cos phi = 440 / √2 x Rated AC Voltage

I hope it may assist to start with the calculation, please provide other available information if there’s any. I apologize for poor presentation. I can not “key- in” the Equation Editor Format for numeral.

Best Regards,

Gerry

### #3 jraef

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Posted 28 June 2007 - 02:07 AM

The problem is that because it is a variable speed drive, the output current has virtually no predictable relationship to the input current, because the output voltage will be varying with speed. Why is this important to you anyway? If you need to know the AC current, just measure it at the input side.
"He's not dead, he's just pinin' for the fjords!"

### #4 marke

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Posted 29 June 2007 - 02:52 PM

Hello AB2005

The difficulty is that
i) the AC current in non sinusoidal and so the average current will be different from the RMS current. This means that the way that you measure the current will strongly influence the AC current measurements.
ii) The ratio between the AC current and the DC current is a function of the conduction angles of the controlled bridge.

I expect that you will find different meters giving you different answers on the AC current measurement with the errors increasing as the DC voltage is reduced.

Best regards,

### #5 AB2005

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Posted 08 July 2007 - 04:08 AM

QUOTE(marke @ Jun 29 2007, 07:52 PM)

I expect that you will find different meters giving you different answers on the AC current measurement with the errors increasing as the DC voltage is reduced.

Yes Marke. You are right. I have measured the current with two different meters and found the different answers.

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

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