power factor

[sparkyboys]

Without knowing the kw from a watt meter....

How do you find the power factor of a motor?
And how do you correct it to 100% unity?

i know- ExIx1.73/1000=kva
hpx746=kw
kw/kw=eff%
kw/kva=PF

 

[chris kennedy]

And how do you correct it to 100% unity?

Power factor correction capacitors.

 

[sparkyboys]

that tells me nothing. be more specific.

 

[electricalperson]

what happens if you have 100% unity power factor and some of the inductive loads shut off at night or whatever wont it become capacitive?

 

[sparkyboys]

how do assume what the power factor of a motor will be?

what happens if you have 100% unity power factor and some of the inductive loads shut off at night or whatever wont it become capacitive?
i am not sure. does it?

 

[electricalperson]

unless the PFCC bank switches capacitors on and off to loads im assuming its not a good thing to have 100% unity PF. i think 95% is acceptable

if im wrong please correct me.

 

[chris kennedy]

what happens if you have 100% unity power factor and some of the inductive loads shut off at night or whatever wont it become capacitive?

Not if the capacitors are are installed in the motor feeder. (I think)

 

[sparkyboys]

unless the PFCC bank switches capacitors on and off to loads im assuming its not a good thing to have 100% unity PF. i think 95% is acceptable

if im wrong please correct me.

ok, now i am getting somewhere. would you rate 95% PF for all 1ph & 3ph motors?

 

[electricalperson]

Not if the capacitors are are installed in the motor feeder. (I think)

well thats one way of correcting power factor but usually dont they install a bank at the main service?

 

[sparkyboys]

Not if the capacitors are are installed in the motor feeder. (I think)

what if there are no capacitors, such as a VFD? Still use the 95% PF?

 

[electricalperson]

ok, now i am getting somewhere. would you rate 95% PF for all 1ph & 3ph motors?

well 95% power factor for the entire service

one of the smarter guys will chime in.

im sure most new motors are high power factor anyway

 

[sparkyboys]

well 95% power factor for the entire service

one of the smarter guys will chime in.

im sure most new motors are high power factor anyway

THANK YOU. i feel like i have actually learned something now about PF.

 

[electricalperson]

THANK YOU. i feel like i have actually learned something now about PF.

all power factor is , is the ratio between watts and volt-amps the power factor gets thrown off by inductive reactance caused by inductive loads like motors, ballasts transformers etc. anything with a coil is basically an inductive load. inductive reactance causes voltage to lead current. in a capactive circuit voltage lags current.

in a pure resistive circuit voltage and current reach the peak at the same time

you can put capacitors in an inductive circuit to bring current and voltage more in phase with eachother

 

[peter d]

im sure most new motors are high power factor anyway

A motor inherently has bad power factor. There is no such thing as a "high power factor motor." In order to correct it, external correction (capacitors) must be used.

 

[electricalperson]

A motor inherently has bad power factor. There is no such thing as a "high power factor motor." In order to correct it, external correction (capacitors) must be used.

i never bothered to look at that stuff when i hook up a motor. i dont even think some of them were labeled with power factor

 

[bob]

Without knowing the kw from a watt meter....
How do you find the power factor of a motor?
And how do you correct it to 100% unity?
i know- ExIx1.73/1000=kva
hpx746=kw
kw/kw=eff%
kw/kva=PF

If you do not know the kw of the motor then you have a problem. 746 watt/hp does not include the inefficiencies of the motor.
There are meters that will measure the motor power factor. You can use a clip on amp meter the get the KVA. If you don't know then you have to estimate. 0.80 is a figure to begin with. It depends on what you want to do. If you are trying to correct the plant power factor then you need to know what the actual PF is. You can get that from the utility. There is more information if you Google "motor power factor'.
I hope you got that problem you had on the other forum with the 0.25 factor corrected

 

[sparkyboys]

There are meters that will measure the motor power factor. If you don't know then you have to estimate. 0.80 is a figure to begin with. It depends on what you want to do. If you are trying to correct the plant power factor then you need to know what the actual PF is. You can get that from the utility.

i do all commercial, mostly schools and hospitals

 

[electricalperson]

i do all commercial, mostly schools and hospitals

anything other than residential gets charged per VA and not KW

you want the KW to be as close to the VA as possible to save money. even if your actual watt usage is lower than volt-amps which with bad power factor it is you pay for the KVA. if your VA is near to apprent watt power the bill will be more for power you actually used instead of power you used and technically "didnt" use

 

[gar]

090607-2024 EST

Refer to "Alternating-Current Machinery", Bailey and Gault, 1951, McGraw-Hill.

Chapter 8, Synchronous Motors.
"Power-factor Adjustment.
The synchronous motor has two unique characteristics:
1. Absolutely constant speed
2. Adjustable power factor ----- "

You can go from lagging to unity to leading by changing the excitation.

Chapter 17, Capacitor Motors.
Maximum efficiency does not occur at best power factor. But you can approach unity PF at full or slight overload at reduced efficiency. See running performance p310 - 313,

.

 

[LarryFine]

well thats one way of correcting power factor but usually dont they install a bank at the main service?

The disadvantage to that, in addition to difficulty of switching correction as loads change, is that your internal wiring system still has to carry the reactive power as well as the real power.

If your correction is near the load(s), the supply system only needs to carry real power. Your system would benefit the same way the POCO's system does with service-located correction.