What's Your Power Factor....Does it matter in a residential application

[Lloyd]

It probably matters more if the supply is a Co-Gen/Solar/Hydro.

But for sure it can add up to dollars saved, even if you're connected to the cheap grid power.

Just something up for discussion, so that we can be sure to add it to our little co-gen projects.

Lloyd

The length of the #12 gauge cable is 50 ft. Referring to page 60 of Introductory Circuit Analysis by Robert Boylestad, 6th edition, table 3.2 shows that the resistance for #12 gauge copper wire is 1.588 Ω per 1000 ft at 20°C. In our example, 50 ft of wire connect the appliance to the main panel. Therefore, the cable will exhibit a resistance of:

Thus, the power loss in a #12 gauge cable of 50-ft length supplying a 1-hp motor at unity PF is given by:

P = I2 × R = (7.31 A)2 × 0.0794 Ω

P = 4.24 W.

For the same cable, supplying power to a 1-hp motor with a PF of 0.75 will result in a cable power loss of:

P = I2 × R = (9.75 A)2 × 0.0794 Ω

P = 7.55 W.



http://powerelectronics.com/power_management/motor_power_management/705PET23.pdf

[mike90045]

and don't forget, the system alternator (ST-5KW) has it's own Power Factor too, and you will hit an early limit if you are driving large loads that have lame PF.

[mobile_bob]

generally speaking the typical residential customer will save no money on his electric bill by improving power factor

the power company corrects for residential customers in bulk, and adjust the rates for everyone to cover any losses.

all that might well change with the installation of smart grid meters, but i am not sure.

what is very important, is having good power factor if you are "the" power company

bad power factor for an offgrid application requires the genset to cover the reactive component and this leads to reduced capacity
of the genset, however this reduction in capacity does not have a linear relationship to fuel burned.

for instance if a 5kwatt genset, is driving a poor powerfactor load(s) and the effective work being done is only 4kwatts, the genset although
being maxed out will not consume the same amount of fuel as it would if it were driving a unity powerfactor load of 5kwatts.

the apparent loss of 1 kwatt capacity is for the lack of a better way of explaining it, a circulating current, which goes back and forth from the genhead to the load, and back again. the wires have to handle it but because it is recirculating the engine does not have to burn enough fuel to
cover the full kwatt, but only the amount of power needed to cover the resistance losses,

generally speaking the fuel needed to cover that 1 kwatt, might actually be only what is needed to cover 100 watts

so the bottom line is while having a good power factor is desirable, it is not because we are going to save a bunch of fuel, but rather we
will be able to get more nearly the full rated capacity of the genset. the genhead will probably run cooler and last longer providing power to a unity power factor load rather than one that is poor power factor.

bob g

[Lloyd]

generally speaking the typical residential customer will save no money on his electric bill by improving power factor

the power company corrects for residential customers in bulk, and adjust the rates for everyone to cover any losses.

all that might well change with the installation of smart grid meters, but i am not sure.

what is very important, is having good power factor if you are "the" power company

bad power factor for an offgrid application requires the genset to cover the reactive component and this leads to reduced capacity
of the genset, however this reduction in capacity does not have a linear relationship to fuel burned.

for instance if a 5kwatt genset, is driving a poor powerfactor load(s) and the effective work being done is only 4kwatts, the genset although
being maxed out will not consume the same amount of fuel as it would if it were driving a unity powerfactor load of 5kwatts.

the apparent loss of 1 kwatt capacity is for the lack of a better way of explaining it, a circulating current, which goes back and forth from the genhead to the load, and back again. the wires have to handle it but because it is recirculating the engine does not have to burn enough fuel to
cover the full kwatt, but only the amount of power needed to cover the resistance losses,

generally speaking the fuel needed to cover that 1 kwatt, might actually be only what is needed to cover 100 watts

so the bottom line is while having a good power factor is desirable, it is not because we are going to save a bunch of fuel, but rather we
will be able to get more nearly the full rated capacity of the genset. the genhead will probably run cooler and last longer providing power to a unity power factor load rather than one that is poor power factor.

bob g

It probably matters more if the supply is a Co-Gen/Solar/Hydro.

But for sure it can add up to dollars saved, even if you're connected to the cheap grid power.

Just something up for discussion, so that we can be sure to add it to our little co-gen projects.

Lloyd

The length of the #12 gauge cable is 50 ft. Referring to page 60 of Introductory Circuit Analysis by Robert Boylestad, 6th edition, table 3.2 shows that the resistance for #12 gauge copper wire is 1.588 Ω per 1000 ft at 20°C. In our example, 50 ft of wire connect the appliance to the main panel. Therefore, the cable will exhibit a resistance of:

Thus, the power loss in a #12 gauge cable of 50-ft length supplying a 1-hp motor at unity PF is given by:

P = I2 × R = (7.31 A)2 × 0.0794 Ω

P = 4.24 W.

For the same cable, supplying power to a 1-hp motor with a PF of 0.75 will result in a cable power loss of:

P = I2 × R = (9.75 A)2 × 0.0794 Ω

P = 7.55 W.

It seems it might add faster then we give credit for, just doing the math on the example above.

.75PF           7.55 W
unity PF        4.24 W  
heat loss        3.31 W X 2hrs a day  x 365 = 2.46KWH x 80 million households = 1.9 million KWH

[mobile_bob]

Lloyd:

your point as it applies to the big picture is valid, however
the utilities manage residential power factor fairly well, because it is in their best interests to do so

they don't try to manage industrial power factor because it needs to be done on a case by case basis
and is just simpler for them to use special meters to measure it and make the industrial customer pay a premium
for poor power factor, this effectively makes the industrial user spend the money to correct for his poor power factor.

if the time comes that your meter out on the side of your house starts to measure for power factor and the utility starts
to charge you a line item "powerfactor" then it will be very beneficial to correct for it in some cases.
until that time comes there is no advantage to the residential user to do anything

now if you are offgrid and are your own power company, having good power factor is something to look hard at.

btw,  whats a couple gigawatts in the scheme of things, when the US power consumption per year is likely measured in quads?

bob g

[Tom Reed]

I've got a 1/2hp 120vac well pump that I believe needs the power factor corrected. Since I am the power company and the st5 gets drawn down to about 103 volts when the pump is on I think I need to address this before next winter.

I've seen clamp on meters with power factor for about $200. Anyone know if they work or if there is a way to measure PF with a standard meter? Then once the PF number is known how is it corrected?

Tom

[cgwymp]

I've got a 1/2hp 120vac well pump that I believe needs the power factor corrected. Since I am the power company and the st5 gets drawn down to about 103 volts when the pump is on I think I need to address this before next winter.

I've seen clamp on meters with power factor for about $200. Anyone know if they work or if there is a way to measure PF with a standard meter? Then once the PF number is known how is it corrected?

Tom

If it's 120V, with a little creative wiring you could use a Kilowatt to measure it.  I just installed a 1/2hp, 120V submersible last weekend (to replace a dead 120V jet pump) and in retrospect I wish I had wired it into an outlet to make using the Kilowatt easier.  If you do measure the pf, post it here for our edification!

[mobile_bob]

most modern appliances are now power factor corrected, things like switching power supplies common to puters
once had dreadful power factor but are now typically power factor corrected.

iirc energy star appliances are all power factor corrected and it might be that appliances are mandated to be that way from
the government these days.

power factor correction of a single appliance is relatively easy to do with a capacitor, but  a 5 dollar capacitor at the manufacture level
equates to about 20-30 bucks at the retail level in the price of an appliance, so historically manufactures ever concerned with being competitive
would certainly nix those caps and only offer them as an option.

many older appliances have optional power factor correction caps available from the manufacture, i have seen this going back to a chest deep
freeze from the early 70's where one was listed on the schematic as an "optional" power factor correction capacitor.  presumably the only buyer that would have spec's this option would have been something like an institution kitchen where the commercial nature of the business placed them on the track to be charged for poor power factor. i bet not one was sold to a residential installed application.

bob g

[mike90045]

I've got a 1/2hp 120vac well pump that I believe needs the power factor corrected. Since I am the power company and the st5 gets drawn down to about 103 volts when the pump is on I think I need to address this before next winter.

I've seen clamp on meters with power factor for about $200. Anyone know if they work or if there is a way to measure PF with a standard meter? Then once the PF number is known how is it corrected?

Tom

I just went thru this, and if you can discover the actual pump motor, the mfg often has charts that explain all. 
Motor spec is here: http://www.franklin-electric.com/business/WaterSystems/Service/AIM/page-13.aspx
it's the 240V 3 wire # 214505 At normal loads, it's only 62% efficient, and power factor of .73 My inverter metering shows it pulls 950W.

A 15uF run cap
http://www.franklin-electric.com/business/WaterSystems/Service/AIM/page-15.aspx
is the cure.  BUT, the electronic control box for it (that has the eRelay and starting cap) cannot work with a run cap, and I need to get the old style mech relay, if I want to install the run cap. 

[Lloyd]

Remember to add cable loss into your PF...we all think about V-drop when dealing with low voltage especially high amp...but it's also a factor in higher voltages. A quick look at the NEC table will shine some light. V-drop='s wasted engery as heat..in the winter it may not be a loss...

But for sure on air conditioning, V-drop, and PF correction can help...especially if as Bob said you're the Power Company.

I know the rhetoric is that the power company already factors in PF for residential...but every K watt through the meter is charged to the buyer...so if you can lower your bill by 1-5% with PF then it helps you, and really it helps the world.

Sure it seems silly to consider 1%...but here we are a Co-gen forum..and we strive to eek out every bit of performance...so lets lead by example...If it's not energy star v. 2010 it most likely isn't PF corrected..if it was built for the consumer market, that doesn't mean that all v. 2010 is unity.

Of all peps...we should be able to achieve unity at an affordable cost...being the bunch of DIY'ers we are.

Lloyd

[Lloyd]

Hi Jens,

First the .18 is for cable loss only...second they're based on 50 ft round trip...which I know only applies to the the smallest of bungalows...not your average house...which I know for a fact is even greater especially if you consider high resistance cable connections...poor quality outlets...hell I've seen and repaired circuit breakers that were high resistance.

PF correction is coming to the US in the near future..even to residential...it's already on the boards in Caly...and the IEA...to which the US has committed but not joined is already working on the the drafts for commercial with residential not far behind.


If we be SOMRADS...then we should be leading...not lagging...pun intended.

Lloyd

[mike90045]

Remember to add cable loss into your PF...we all think about V-drop when dealing with low voltage especially high amp...but it's also a factor in higher voltages. ......

Which is why I'm pulling #6 alum for a 240VAC 900' run, for power at my tank farm.   Sure, it might be "overkill", but I won't starve the ozone generator and pump for my drinking water either.    And my power isn't $0.16, it's more like $1.6 KWh, so the less I throw away, the happier I am.

[Lloyd]

http://www.electrician2.com/calculators/vd_calculator_initial.html

or this 3-phase calculator with PF

http://www.electriciancalculators.com/calculators/vdpfcalculator_initial.html

Enjoy It.

Lloyd

[Tom Reed]

Mike I have the Franklin control box too. However there is already a relay in the system in a box behind the house. Perhaps I can move the start cap from the box down by the pump to the box by the house (450' away with 10 ga. CU wiring) and start messing with different caps to get the lowest amp reading possible? I've never messed with a starter relay, so this may be way off base.

[mobile_bob]

be very careful not to over correct, because if you do the voltage will start to climb uncontrollably at the load
and damage will likely be the result

you really need to know the powerfactor before you start adding capacitance, and only work to correct "if" the power
factor is lower than 0.90 lagging and correct only up to about 0.95 lagging.

also the motor power factor will improve with added load, so be careful to check the power factor only under its max load.

a pump motor working against a pressure tank might drop to 40 psi before it kicks on and might exhibit 0.90 lagging p/f
and improve to something around 0.95 lagging as it gets close to pressure cutoff.

this is not something to play around with unless you know what you are doing, and/or have some means of accurately measuring
what is happening.

Bill Rogers book, has a section that is excellent in my opinion on how to accurately measure and add capacitance in small steps
while watching the amps fall off and the voltage climb up to a safe point, he also shows how a bit more capacitance allows the voltage
to start to climb, which is what you do not want to happen.

there are other issues, one of which is placement of the capacitors, and knowing where to place them in relation to the contactor and load
so that they are not across the line when the load is offline, because the capacitance can then be available for a near unity load and a dangerous
and damaging voltage rise will likely burn up something expensive.

this is an interesting topic, one i spent years trying to get information on, and even more time trying to get a good understanding of. until i met
Bill Rogers and having read his book, along with a few subsequent conversations where he set me straight, i hadn't made much progress.

about 8 years ago, had anyone googled "residential power factor correction" they would have found the first page and a half of returned links that
were all attributed to me asking questions about the subject on a variety of EE forums, RE forums and elsewhere. 

it was amazing how much conflicting information i got from EE!

as simple a subject as it seems on the surface, it is not something that is universally understood by EE let alone the DIY community.

bottom line?  get Bill's book and read it, utterpower.com sells it.

follow Bills recommendations to the letter, and experiment first with a load that you are not heavily invested in.

last thing anyone wants is a fried submersible that might cost a grand to replace, or damage to other stuff connected on the buss.

bob g