Nameplates don't generally give pf.
Then there is the question of how you know the motor is fully loaded. It is quite difficult to measure output power after the motor is installed. Output is torque multiplied by rotational speed. You can fairly easily measure rotational speed to within about 1 rpm. Torque is a whole lot more difficult. So you'd probably take full load to be when the motor is drawing full load current - which you got from the nameplate. Along with nameplate voltage.
Again, where did you get the hp rating?
I would assume that each 12.5 kVAr unit is three-phase.
If they are, each unit is most likely three capacitors in a delta configuration.
Not easy to connect three such units in series.
1. Get some real data on the situation
instead of guessing or assuming
2. It takes maybe 3 minutes to draw a power triangle and perform the above calculation.... until you are adept at this, I would stay away from "easy" methods such as spreadsheets or tables
Yes, you could but you'd need to be able to measure the speed quite accurately.If you know the full load RPM, you can pretty accurately determine the motor shaft loading by measuring the RPM as it is linear with the load as per above.
Is there an easy way to size KVAR cap banks for 480V, 3phase motors?
I have a 200HP motor with an associated 35KVAR cap bank, (2) 12.5KVAR and (1) 10KVAR. How can I verify if this is accurate...going for 95% power factor.
Assume no nameplate data on motor.
Yes, you could but you'd need to be able to measure the speed quite accurately.
I have some good detailed information on a 660V, 625kW four-pole machine that was used on one of our projects.
Full load speed is 1493.0 rpm (50Hz here) and FLC is 648A.
At 1493.5 rpm the corresponding load is 93% of full load torque.
A difference of 0.5 rpm isn't much and it could easily get lost in supply frequency tolerance. Here it's ? 1% which is ?15 rpm of synchronous speed.
So you'd need to measure frequency and consider slip rather than actual rpm.
Doable of course, but would you?
When did you last do it?Yes, we do it ALL the time.
Yes.You are sure that the frequency tolerance is not 0.1%?
Yes, some. But not a lot.The frequency variance will also have effect on the efficieny of the motor.
Actually I do for the example I quoted. Had I got that wrong iniquitous financial penaties would have been imposed. They were not.You don't know the efficiency, or the power factor,
Quite so.or the voltage unbalance, or the impedance imballance between the phases.
The strobe will give you rotational speed. Slip is what determines torque.An accurate stroboscopic rpm reading with 0.1% accuracy will be pretty accurate to determine the actual shaft load.
You are probably right, but it is not the kind of phraseology that instills confidence. I just pictured an engineer addressing the shareholders and saying "yeah, we made a bunch of guesses and assumptions but it should work". :grin:Interestingly enough guessing and working on assumptions is what propelled the US to where it is today.
Which is a fantasy in itself because the engineer doesn't get to address the shareholders: That is reserved for the manager who barely knows what is going on.I just pictured an engineer addressing the shareholders and saying "yeah, we made a bunch of guesses and assumptions but it should work". :grin:
You are probably right, but it is not the kind of phraseology that instills confidence. I just pictured an engineer addressing the shareholders and saying "yeah, we made a bunch of guesses and assumptions but it should work". :grin: