sizing power factor

[Besoeker]

One of the problems with using anything off the nameplate is that it states the PF at full load.

Nameplates don't generally give pf.

 

[ohmhead]

Nameplates don't generally give pf.

Well thanks for the lesson most of our work is new construction but its heavy commercial and most of our motors are 50 hp - 200 hp and above we do low voltage work and medium voltage work both .

Lots of hard work thur the years but i can still ask a question and i see new ways ive never stopped learning .



Good points its very clear take care .

 

[weressl]

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?

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.

To get the power factor at various loads, I would go back to the motor manufacturer and extrapolate between the next highest and lowest datapoints. A 200HP motor would have that data available unless the motor was manufactured a looooong time ago. Even Westinghouse data is available from TECO. Mind you that these would be approximate data for a TYPICAL motor of that size and model - not original test data from the motor on hand - but good enuf' for this horseshoe game.

 

[glene77is]

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.

Besoeker,
Right.
Interesting subject.
I'm still trying to follow the OP's wording.
Guess I need a diagram of what he's saying.

 

[glene77is]

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

Karst,

Good advice.
May save a life, or at least some equipment, along the way.

(1) I have seen lots of loose numbers
come out of calculators and computers.

(2) I have seen a few good solutions
come from experienced men.

 

[Besoeker]

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.

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?

 

[weressl]

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.

You can go to:
http://www1.eatonelectrical.com/calculators/PowerFactorROI/index.html


Here is some data for various motors:
1785 RPM PF range from 100%-25% is .885-.665 (Premium) TEFC
1785 RPM PF range from 100%-25% is .873-.604 (EPact) TEFC
1785 RPM PF range from 100%-25% is .878-.637 (EPact) ODP
3575 RPM PF range from 100%-25% is .877-.706 (EPACT) TEFC
3565 RPM PF range from 100%-25% is .886-.679 (Premium) ODP
1190 RPM PF range from 100%-25% is .856-.566 (EPact) ODP
1190 RPM PF range from 100%-25% is .873-.61 (EPact) TEFC

There is an equal variation in efficiency. SO you can see how great variance there can be on calcualted value between different motors.

The formula would be:

pf= kW/(SQRT)[(kW*kW)+(kVAr*kVAr)]

So if you would KNOW the efficiency and the FLA, you could caclulate the required kW and the kVAr at full load. The formula can not be solved with TWO missing data, eg. the eff. or pf. needs to be known to calculate the otehr missing info.

 

[weressl]

Nameplates don't generally give pf.

The do in the US.

 

[weressl]

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?

Yes, we do it ALL the time.

You are sure that the frequency tolerance is not 0.1%?

The frequency variance will also have effect on the efficieny of the motor.

You don't know the efficiency, or the power factor, or the voltage unbalance, or the impedance imballance between the phases. An accurate stroboscopic rpm reading with 0.1% accuracy will be pretty accurate to determine the actual shaft load. Better than a current reading. If you have 0% and 100% baseline data from the actual motor, the resullt will be even more accurate.

Accurate power measurement and RPM based load reading will be a good crosscheck on your efficiency if you don't have a dyno 'handy'.

Again, for the purpose of the OP this would be a simple way of determining the ballpark load.

 

[Besoeker]

Yes, we do it ALL the time.

When did you last do it?

You are sure that the frequency tolerance is not 0.1%?

Yes.

The frequency variance will also have effect on the efficieny of the motor.

Yes, some. But not a lot.

You don't know the efficiency, or the power factor,

Actually I do for the example I quoted. Had I got that wrong iniquitous financial penaties would have been imposed. They were not.

or the voltage unbalance, or the impedance imballance between the phases.

Quite so.

An accurate stroboscopic rpm reading with 0.1% accuracy will be pretty accurate to determine the actual shaft load.

The strobe will give you rotational speed. Slip is what determines torque.

 

[Besoeker]

The do in the US.

If the motor in the OP had PF given on the nameplate, this thread might have gone in a different direction - if it had gone at all that is.

 

[weressl]

1. Get some real data on the situation instead of guessing or assuming

Interestingly enough guessing and working on assumptions is what propelled the US to where it is today.

 

[mivey]

Interestingly enough guessing and working on assumptions is what propelled the US to where it is today.

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:

 

[mivey]

I just pictured an engineer addressing the shareholders and saying "yeah, we made a bunch of guesses and assumptions but it should work". :grin:

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.

 

[weressl]

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:

All innovation and discovery starts with assumtions and guesses. The question is that when you start verifying your guesses how many conditions will you set? Does it have to be all places all the time and all conditions?

Or if one posits a question as the OP in this case, will you make generic assumtions based on the available data and offer generic solutions or hold back your answer untill all the data is supplied. Which one will move you forward?

How many times was the A-bomb tested before it was dropped on Hiroshima? The German weapons were superbly engineered but many of them were prone to failure. Look at the 65 year old Kalashnikov and the M15 that is barely 40 years old.