"Easy" question: Calculating Watts on a 3-phase motor

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acro

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Ok, so this one should be easy.

I have a standard 3 phase scroll compressor (460/3/60).
I would like to find out how many kW it is using (roughly, doesn't need to be exact). Pretty much just trying to estimate cost to run per hour.

Using watt's formula, I have Watts = Vavg * Iavg * PF * 1.732

The plan is to just grab a fluke clamp amp meter, measure each of the phases, and take the average. This would give me Iavg.
For Vavg, I am just going to use 460, since I do not need to be exact.
Not quite sure how I would take measurements for PF, but it's probably around .9, right?
Then, I would just plug into the equation, which will give me watts. Divide by 1000 for kW. Then multiply by 0.12 if we pay 12 cents per kWh.

Does this all make sense?


Thanks!
 
acro said:
Ok, so this one should be easy.

I have a standard 3 phase scroll compressor (460/3/60).
I would like to find out how many kW it is using (roughly, doesn't need to be exact). Pretty much just trying to estimate cost to run per hour.

Using watt's formula, I have Watts = Vavg * Iavg * PF * 1.732

The plan is to just grab a fluke clamp amp meter, measure each of the phases, and take the average. This would give me Iavg.
For Vavg, I am just going to use 460, since I do not need to be exact.
Not quite sure how I would take measurements for PF, but it's probably around .9, right?
Then, I would just plug into the equation, which will give me watts. Divide by 1000 for kW. Then multiply by 0.12 if we pay 12 cents per kWh.

Does this all make sense?


Thanks!
Click to expand...
You need to use RMS values which is what your instruments will most likely give you anyway.
You can take the average of each of the three phases but if there is any significant unbalance you may have a motor problem.

PF - well I think 0.9 is probably a bit high.

Oh, and you need to take into account duty cycle if you want to work out running costs.
 
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Besoeker said:
You need to use RMS values which is what your instruments will most likely give you anyway.
You can take the average of each of the three phases but if there is any significant unbalance you may have a motor problem.

PF - well I think 0.9 is probably a bit high.

Oh, and you need to take into account duty cycle if you want to work out running costs.
Click to expand...

Ok thanks.
Do you know which measurements I would need to compute the PF?
I'm pretty sure the duty cycle is 100%, since these are in CRAC units designed for data centers. Some of them never turn off.
 
Most people use a .8PF for guesstimation of motor power consumption, but that assumes the motor is running at 75% load or greater. If your motors are running at 50% load, as is often the case in data centers in my experience because people like to over design for redundancy and reliability, then I'd use .7PF.
 
acro said:
Ok thanks.
Do you know which measurements I would need to compute the PF?
Click to expand...
Volts, amps, and power I suppose is the obvious answer.
I'd use an oscilloscope to get the phase displacement. But I cart one around in the boot (trunk) of my car.
 
Besoeker said:
Volts, amps, and power I suppose is the obvious answer.
I'd use an oscilloscope to get the phase displacement. But I cart one around in the boot (trunk) of my car.
Click to expand...

I'd use the old analog wattmeter in the wooden case with the chromed metal touchable knurled knob connections and instrument transformers to read the real watts not the arithmetic Volts?Amps.
I could also use a KWH meter (even single-phase if load is balanced) and count disk rotations.
I could also use my portable test watthourmeter, again with instrument transformers.

The idea is to get something that does vector multiplication on the volts and amps.
 
fmtjfw said:
I'd use the old analog wattmeter in the wooden case with the chromed metal touchable knurled knob connections and instrument transformers to read the real watts not the arithmetic Volts?Amps.
I could also use a KWH meter (even single-phase if load is balanced) and count disk rotations.
I could also use my portable test watthourmeter, again with instrument transformers.

The idea is to get something that does vector multiplication on the volts and amps.
Click to expand...
I agree.
I just happen to have a scope that will do the calculations and has a coms port so I can download the data to my laptop.
Possibly a sledgehammer to crack a nut, but if I have it and it does the job of several instruments that I wouldn't normally have to hand, then why not?
 
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