Does power factor affect your utility bill?

[redly]

I bought a Kill-a-watt device that measures power consumption. it has the option to measure power factor. I was testing my washing machine, and I noticed the power factor was .57.

It got me thinking, do washer/dryer motors have capacitors (like the blower motor in your furnace). If not, would there be any benefit to adding one?

I guess I really don't understand power factor or it's affect on my utility bill

 

[harrkev]

Read this: Dan's Data on the subject

But to summarize, it does not generally matter. If the power factor is real bad on your computer's power supply, you might need a larger UPS, but that is about it.

 

[KMurphy]

I don't know how the utility bills residential users, but our plant doesn't get penalized unless it gets below 0.90 lagging. Our monthly useage is usually around 44,000,000 KWH. Residential users don't have much demand, so I doubt we get penalized.

The only benefit a residential user would have for power factor correction is if your incoming feeder conductor is overloaded. PFC would reduce the current flow, increase the power factor (if you have a lot of induction motor load); but the net kilowatts would remain the same (which is how we are billed).

Also, computer power supplies run near unity; they do introduce a lot of 3rd order harmonics into the system though (you don't see any extra cost for this). These harmonics will add more load on the transformer neutral.

 

[redly]

I do know from my techie days that power supplies and such have FCC, CE, ect madated limits to the amount of "noise" harmonics they can put back on to the AC line

Thanks for the replies, guys

 

[KMurphy]

There are limits to harmoics from non-linear power supplies, but they still do emit them. That's why distribtion systems with large numbers of computers or servers (or any large cluster of non-linear power supplies) must have a K-rated transformer. The neutral conductor is slightly larger to handle the additional current from the additive odd order harmonics. There a few other special properties of K-rated transformers, but they are not that outstanding.

 

[KMurphy]

Sorry, I didn't answer your other question.

Yes, all single phase motors have a start capacitor to provide enough starting torque for the motor to spin up to running speed. Some single phase motors have a run capacitor too; varies by motor type. Single phase motors don't typically have power factor correction capacitors though.

Capacitors for induction motors are used in several ways:
Start capacitor
Run capacitor
Surge capacitor
Power Factor Correction capacitor

In short, single phase induction motors are weak and inefficient. 3 phase induction motors are the most elegant invention ever; perfectly balanced, rugged and strong for many applications.

 

[Jassi]

:thumbsup: to everyone in this thread

 

[PowerEngineer]

As you know, power factor is the cosine of the phase angle between the sinusoidal voltage and current waves. The power delivered by down an electrical line is V*I*cos(phase angle), which means that the current needed to deliver that power increases as the phase angle increases. And because line losses are proportional to the square of the current, then poorer power factors do increase losses some. More importantly, it also increases var (volt-amps reactive) consumption which increases the voltage drops from one end of the line to another. These are the reasons why power companies are so interested in controling phase angles throughout their electrical systems (mostly through shunt capacitors).

As KMurphy has indicated, power companies set limits for power factors on their larger industrial loads. Their tariffs usually allow them to charge for low power factor loads so that they can install some shunt capacitors to compensate (rather than roll that cost into the rates for other customers).

Motors work because of a linkage between the sinusoidal magnetic flux in the stator with the flux induced in the rotor. Synchronous machines use DC voltage to excite a fixed magnetic flux that locks in with the varying stator flux and applies a torque to the machine shaft. Induction motors have no exciter on their rotors; they have conducting bars arranged like a "squirrel cage" that circulate current induced by the stator flux that also establishes a flux for the rotor and a torque on the shaft. Induction machines by their nature have "lagging" power factors. All but the largest motors are induction machines. All the motors in your home (like your wash machine) are induction motors. Single-phase induction motors can not start up by themselves; they need two stator fluxes with a phase angle between them (the angle between phases on a three-phase system is 120 degrees). The capacitor is tied in series with a second starter winding. The capacitor shifts the phase angle in that winding enough to provide the torque to get the motor rolling, and is often opened after started.

Anyway, power companies generally don't charge residential customers for vars or power factor. Your house meter doesn't even measure them. Don't worry about power factor -- life is too short!

 

[jagec]

Originally posted by: PowerEngineer
As you know, power factor is the cosine of the phase angle between the sinusoidal voltage and current waves.....

Anyway, power companies generally don't charge residential customers for vars or power factor. Your house meter doesn't even measure them. Don't worry about power factor -- life is too short!

/looks at screen name

aww, what do YOU know!