PF Correction Capacitors

[ascj]

I was looking at a condenser, yesterday. It had a 15hp and a 25hp compressor in it. While checking amperages, I noticed I was reading let amps on the line side then the load side. I saw some wires on the load side and found they went to power factor correction capacitors.

I need some help understanding.....for example Compressor 2. I measured around 16amps on each line side contactor leg. I measured 26amps on each load side contactor leg going to the compressor. If I measure the motor leg and PF cap wire together.......I got 16amps.

Why is this? Is the cap really lowering the amperage 10amps?

[jbarron4]

Yes it is. The motor has inductive reactance and the cap offers offestting capacitive reactance.
The current is lagging due to the inductive reactance of the motor and the capacitor adds capacitive reactance offsetting the "wasted" inductive currents.
This phase shift is what lowers the power factor of the induction motor.
This happens for all induction motors. In industrial plants this phantom current is pretty significant. You have measured current or amps, but if you had measured power with a wattmeter you would not see a difference with the cap or not. This difference in real power and apparent power is again power factor.
All of the current handling components have to handle current both real and reactive either inductive or capacitive.
The power company will charge for this reactive currents along with the real power in watts.
This would be either a power factor penalty or other means.
These currents also cause heating of components so it is best to correct near the source, but usually more expensive than correcting in a big lump in one place in the power system.

[GT1980]

JB, how's that reactance differ from the "back EMF" heard electro-motive-force(s) of a DC with/w/out magnets... etc.

[jbarron4]

Two very different things/ The back EMF you refer to is the voltage generated by a DC motor that is counter to the DC powering the motor. This is why a DC motor runs to some rpm max at some voltage. The counter emf limits the RPM and is a DC motor thing.
The inductive reactance of an induction motor could be considered as a lump sum inductance in the circuit. The correction cap is actually a lump sum capacitive reactance to counter that inductance.
AC theory is pretty complex where DC is fairly straightforward. Inductive reactance causes the current in the circuit to lag the voltage and capacitive reactance causes it to lead. The leading component of the capacitive reactance will offset the laggin of the inductive reactance.
If the load were purely resistive then it would act like just a resistor and exhibit no reactance.
This is the preferred condition. By the way the ECM motors have greater effeciency since they basically have a rectifier circuit the charges capacitors and then the electronics take that DC and electronically switches it to the proper coils to spin the DC motor with no brushes as commutators. The inductive reactance is very low and may actually present a capacitive load to the line. On this I could not say for sure.
A true VFD that generates an AC current to drive an induction motor presents about a 96% power factor to the line since the motor and line have that DC power supply and inverter between them.
orry I can't make it simpler since the whole reactance and AC theory is pretty complex for a forum format. Here is a wiki link:

http://en.wikipedia.org/wiki/Power_factor

[GT1980]

understood to enough... I rewound motors for customers slot cars in 1967 and Dad had the sense to drag G-Pa's engineering guide out about windings and inductance, etc...
T's4all!

and the Horsepower to horsepower vs input wattage paid for in ECM at highest speed to new eff psc blower motors at 1075 rpm psc to 1050 ecm rated was only 3.5a ECM : 3.8a PSC this year... (!)

so I tell folks of always consider small motor + larger = both for a third staging and very VFD-Like results with under say 7 hp... within 8% ( and better ) but years of savings on controls and expensive replacements seen to date (!)

[jayguy]

here is a quick 2 page, real example report i just put together on this issue. see if it clears some things up on PFCC's.

[jbarron4]

Power factor is an intereseting thing. For example one post mentioned the amp draw of a motor vs recovering operating cost.
The amps are mot a very good measure of the operating cost of a split phase variable speed motor.
The power factor varies greatly with the speed and like any motor vs load the amds will only drop to approximately 1/3 of full load when it is producing no power. The power factor is very poor at this point.
If you were to hook up a watt meter you would see relatively little power being consumed at that 1/3 amp draw. You have the heating losses of the motor and the drag of the bearings and that would be all of the actual power. The apparent power would be pretty high, however.
The ECM helps since in this case you are charging the capacitor bank for the DC electronically commutated motor and there is a "disconnect" from the AC reactance at this point and is the reason the efficiency is better.
The only problem I have had with the ECM motors is thery are prone to being damaged by spikes and surges where the PSC is relatively immune.
That being the case I am still planning to replace some of the PSC blower motors in our small 3 5 tom units with ECM motors to improve the humidty control down here in Mobile AL.
I have considered using the Square D ATV12H075F1 drive for 120 VAC i1 phase input and 240 VAC 3 phase output to a 3 phase 240 VAC FR48 motor. A ATV212H075M3X would work for a 240 volt system the same way.
Using these drives you could set any RPM you desired as a digitally selected speed or input an analog speed to vary the speed as desired. This would be a more expensive and complex change that probably the average HVAC tech would nt be familiar with, but reasonable in an industrial setting like we have.
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