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Old 02-18-2015, 03:45 PM   #1
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Default calculating consumption of DC motors?

As a plant manager (I'm not an electrician) I was tasked with conducting a study on the inefficiency of our current DC motor/ drives, for the purpose of calculating an estimate saving on switching to more modern AC motors.
Currently we use 150HP motors with fincor 3130 adjustable drives. we have 8 of them, driving plastic extruders.
my question is: what would be the correct way to measure the consumption of my system? my electrician tells me to measure the current on each of the 3 phases before the drive, extrapolate the Kw by doing Volts x current average of 3 phases x 1.73. But my limited electrical understanding tells me that that would only be true if the power factor was 1, which is unlikely. am I right?
keeping in mind that:
1. we have a power stabilizing system at the electrical main entrance that tells me that overall our power factor is about 95-96%. aside from lots of big motors all our machines use a lot heaters, which I believe don't affect the PF, and we have fluorescent lighting everywhere.
2. the 3 phases entering the drive are not equal: a test reading for example told me that 2 measured 120A and one 190A. Electrician says itís because there is some transformers in the drive cabinet that pull from the 3rd phase.

3. 90% of the time our motor are running within the same speed range, which is abot 50-60% of motor's full speed.

I would consider purchasing a power meter like the fluke 43B or fluke 345, but from the literature it appears that they only give accurate measurements if the phases are balanced. can I somehow measure the KVA, Kw and PF on the phases separately and then average them or something?
should I just ignore the power factor all together? at the end of the day, what I need to know is how much does it cost us to run our motor in a typical scenario.

please keep in mind that I'm not an electrician, I'm a plant manager, and all work will be performed by our 2 electricians who have 20 yearsí experience working with our current equipment and tools, but less "book knowledge" and no experience when it comes to power factor etc.
I tremendously appreciate your help, thank you very much.
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Old 02-18-2015, 05:06 PM   #2

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You can convert HP to kw. 1hp = .7457

So 150 HP = 150*0.7457 = 112 kw
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