I am considering installing power factor correction capicitors, either on each individual motor,or at the main service entrance of my plant, to reduce the demand charges on my electric bill. Please tell me how to measure the power factor (PF) of a 3-phase motor.
Specifically,
(1) I think I will need to know the amps, watts and voltage at the load - but, given this information, what is the mathematical formula used to derive PF?
(2) Will I need to measure amps and watts individually on all three legs of the circuit?
(3) Can I take these same measurements on the load side of the service meter to determine PF for the entire plant?
(4) Once I have determined the power factor, how do I calculate the size (kvar) of the power factor capacitor needed to raise the PF to .95 (or higher) without going above a PF of 1?
(5) What problems might I encounter from harmonics, and how do I solve these issues? I realize this is a broad question with many possible considersiderations that may affect the solution. Any info you could furnish will be greatly appreciated.
Here’s a great forum for these types of questions. Hate to steer you away from the SDMB. In any case, if you’re paying a premium for demand current already, maybe your current overall power factor is already known by the power company.
Before acting alone and having your electricians do this work, you may want to consult with an industrial electrical engineer. Again, chances are your power company also does this type of consulting work, because they, too, have a vested interest in increasing your power factor.
Power factor correction can be a rather complex operation. If you use capacitors of the wrong size, when a load is disconnected voltage higher than line voltage can be generated by resonances. This raises the possibility of damage to equipment.
My advice is to contact you power company and ask them to help you do the correction, even if you have to pay.
To *properly * measure PF, you need to use a power analyzer. These are not cheap.
To get a *rough * idea of the PF, you could measure the phase difference between the current and voltage waveforms using a dual-channel oscilloscope. This is a “rough” technique because it doesn’t take into account harmonics.
Nope. As mentioned, you need to either know (or be able to measure) the complex impedance of your circuit load, or be able to measure the phase angle between the applied voltage and the resulting current. Even then, it’s such a complex issue that unless you have a degree in electrical engineering and some expensive equipment, you’re only going to approximate your true PF. Your power utility can best advise you.
If you have a true wattmeter, you can compute your PF. PF = real power divided by apparent power, or PF = Watts/ VA. If you can measure Watts, and also RMS voltage and current, you have everything. Use the wattmeter to get Watts. Use an RMS voltmeter to measure voltage, and your ammeter to measure current. Apparent power = RMS voltage x RMS current. Divide that into the watts reading, and you’ll get PF. Hopefully the load isn’t changing while you’re making the measurements …
Your “wattmeter” must be an actual wattmeter, not something that’s really reading volt-amps. Your cheapest off-the-shelf solution is probably a Fluke 41B, available for about $1000 new, and always handy to have around.
You can do this at the service entrance to get the PF for the entire plant.
Figuring out the proper kVAR capacitor is much more complicated. In theory, assuming just 60Hz current, you can solve for VARs from the formula VA^2 = Watts^2 + VAR^2. In practice, the problems others have mentioned are important. First, the optimal kVAR size changes as the load changes, possibly requiring timers on the capacitors. If you have any harmonic current flowing, the correct kVAR size is also different. Even worse, you can introduce resonances that can greatly increase harmonic currents. This can cause equipment damage, and introduce problems elsewhere on the distribution line. You should definitely work with your local power company before adding PF capacitors. As others have mentioned, they’ll likely bring their own expensive power analyzers to check it out, saving you from buying one.
Arjuna34 is correct… another way to determine PF is by measuring true power and apparent power. Apparent power is simple to measure; true power is not. In order to measure true power, you have to use a wattmeter that measures true power.
Where are you? I have some power analyzers in our lab. I can loan one to you if you’re willing to swing by and pick it up.
I’m in Dallas. I appreciate your offer, Crafter Man - and your input Arjuna 34. However, at this point, I’m just seeking “how to” information for a potential project which has not yet been approved. For the time being, I’m just truing to learn as much as possible about how to approach this project in the event that management decides to go forward.
As I mentioned in my first post, measuring the difference in the phase angle is regarded as an approximation in many instances. Depending on the nature of the load, there may be a lot of harmonics present. If there is, you need to use a true wattmeter or power analyzer.
Yes, this is a valid question, as VFD’s will not allow you to use a typical capacitor bank unless you have some fancy and expensive controls/switching.
I know Square D makes some PF equipment designed to work with a place that uses lots of drives. They are quite pricey and the payback was quite lengthy.
We were looking into using PF correction devices here at work but decided it would be too complicated of an installation and therefore too pricey. Plus this is an R+D type facility and we are constantly changing the electrical environment inside the building.
I have a 41B at my disposal, it’s a neat little meter, and I think it listed at well over $1,000(more like $2,500 (two years ago)).
You can, however, rent them for much, much less than that. I would look into renting one for a week, connect it to a laptop and let 'er record for a week to get a good idea of what you’re dealing with down there.
One way to avoid overly fancy control devices is to have a capacitor (or, rarely, inductor) at each load with the PF corrector on the load side of the switch. Of course this can lead to a lot of capacitors.