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Old 04-28-2018, 08:07 AM   #21
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Originally Posted by Sarmajor View Post
For most electricians VA is easier to measure using simple instruments.
Transformers are rated in kVA mostly for this reason.

For cables carrying current the power factor is for the most part irrelevant as the current measured is the current that will be causing heating effects.

Motors are rated in kW and some will have a PF marked on them which is determined at full load current. If the full load current was not marked on the motor rating plate you could use the kW and PF and Efficiency to work out the theoretical FLC.
Transformers are rated in kVA because transformers work based on current and voltage, not kw. Kw is irrelevant.

NEMA motors are labelled in FLA and HP in North America because engineers can't even fix a decent pot of coffee but they can integrate equations really good so if they care what the kw and pf is let them figure it out! Electricians and mechanics need HP and FLA.

IEC motors are in kw because electrical engineers wrote the specs. It doesn't do the mechanics or the electricians any good. Engineers are cheap and irritating, I mean plentiful, and more unionized, so electricians and mechanics can always ask one there.

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Old 04-28-2018, 08:38 AM   #22
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Originally Posted by paulengr View Post
You got it. Only other tricky part is it isnít just watts + vars. Thereís trigonometry in the math. For most power distribution this is where we stop.

The more advanced part is that capacitors make negative vars, and with generators and synchronous motors you can push or pull the rotor so you can get positive or negative vars. And getting away from watts and vars you can set up
a nasty situation where the inductors and capacitors resonate if you take this too far because we are effectively messing around with filters. This is kind of where the concept falls apart but unless you are working with generators and power factor correction a lot you wonít see this.


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So thatís why I was using the Pythagorean theorem in ac circuits. All I did in that class was plug in variables, but itís starting to come together into something practical.
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Old 04-28-2018, 09:02 AM   #23
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This looks like a job for "ELI the ICE man"! (google it)
I always wondered why that was necessary. You can only have voltage before current and not vice versa.
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Old 04-28-2018, 11:19 AM   #24
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I always wondered why that was necessary. You can only have voltage before current and not vice versa.
Current leads the Voltage in any capacitive load, but it lags in an inductive load.
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Last edited by emtnut; 04-28-2018 at 11:22 AM.
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Old 04-29-2018, 11:07 AM   #25
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I visualize it as a strut in a car.

A spring stores energy and returns it to the system(reactance)

A shock converts energy to heat and dissipates it (watts)

those might not be the best analogies though
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