definition -- power factor / efficiency

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john schroader

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Did a quick search and did not see this problem addressed. I pretty well understand about both power factor and efficiency. My question is one of definition. If a person improves the power factor of a circuit, has this, in effect, improved the efficiency of the circuit? In improving the pf, less power is used, but I was taught to consider efficiency more as reducing friction, heat, etc. Any insights are appreciated.
 
John, welcome to the forum! :)


You're right, you need to define efficiency. Lowering PF can improve the efficiency of a circuit in the sense that more of the power used is converted to useful output.

If you can use smaller conductors to supply a given load, you've created a more efficient circuit.

If you can usde a smaller electrical load to perform the same work, you have a more efficient load.


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100304-1420 EST

From dictionary.com

3. the ratio of the work done or energy developed by a machine, engine, etc., to the energy supplied to it, usually expressed as a percentage.
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The input might be chemical, such as coal and air. Then the energy might pass thru various stages and devices --- boiler, generator, transmission system, branch circuit, and motor --- and energy output to some mechanical device.

So you could measure this overall efficiency, or various parts of the system.

http://dictionary.reference.com/browse/efficiency

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LarryFine said:
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You're right, you need to define efficiency. Lowering PF can improve the efficiency of a circuit in the sense that more of the power used is converted to useful output.
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Actually it is raising the pf increases efficiency ;)

And it is more of the energy is used... the power remains the same (without getting technical about less power as a result of lower I?R losses, for example). ;);)


::: John, welcome to the forum from me, too!
 
robbietan said:
better efficiency by raising power factor means less heat wasted on conductors (heat = power consumption).
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Certainly less heat dissipated in the conductors, at least on the upstream side of the PFC. It usually isn't a whole lot but a saving is a saving.
Whether that alone is enough to justify the cost of supplying and installing PFC is debatable.
Then, just to muddy the waters a bit more, with reduced current in the conductors there would be reduced voltage drop, all other things being equal. The resulting higher voltage might mean that the connected load(s) take more power thus negating some of the savings.

In my experience PFC is generally used for two reasons.
Firstly, at the installation stage it can reduce required supply transformer and conductor size. And costs.
Secondly, it can reduce operational costs if the site is penalised by the POCO for poor power factor.
It can also reduce maximum demand (in kVA). In UK there is a charge for this and usually a huge penalty if it is exceeded. I don't know if it is the same in the USA or elsewhere in the world.
 
john schroader said:
Did a quick search and did not see this problem addressed. I pretty well understand about both power factor and efficiency. My question is one of definition. If a person improves the power factor of a circuit, has this, in effect, improved the efficiency of the circuit? In improving the pf, less power is used, but I was taught to consider efficiency more as reducing friction, heat, etc. Any insights are appreciated.
Click to expand...

please see the below link


http://209.85.135.132/search?q=cach..._Cohen_Lu.pdf+power+factor+/+efficiency&hl=en
 
Besoeker said:
Certainly less heat dissipated in the conductors, at least on the upstream side of the PFC. It usually isn't a whole lot but a saving is a saving.
Whether that alone is enough to justify the cost of supplying and installing PFC is debatable.
Then, just to muddy the waters a bit more, with reduced current in the conductors there would be reduced voltage drop, all other things being equal. The resulting higher voltage might mean that the connected load(s) take more power thus negating some of the savings.

In my experience PFC is generally used for two reasons.
Firstly, at the installation stage it can reduce required supply transformer and conductor size. And costs.
Secondly, it can reduce operational costs if the site is penalised by the POCO for poor power factor.
It can also reduce maximum demand (in kVA). In UK there is a charge for this and usually a huge penalty if it is exceeded. I don't know if it is the same in the USA or elsewhere in the world.
Click to expand...

here, its usually to avoid the low pf penalties of the poco
 
There is another much more important benefit that good PFC can offer. If your PFC is fast enough to follow any load changes and generate reactive energy continuously and accurately, it will be able to stabilize the voltage.

What happens a lot on industrial facilities is that because of the voltage instability people are using to keep the voltage levels higher than nominal (using tap changer on the feeding transformer). Higher than nominal voltage level causes huge losses due to over consumption by all simple loads like lights and inductive motors.
So, IF your PFC is able to fix the voltage level, you will be able to reduce the tap changer back to the nominal value, and this is a huge saving indeed.
 
1. Low PF means higher losses, means lower efficiency.
2. In AC motors, low PF indicates low efficiency of the motor itself. Changing the way the motor work improves its efficiency and the power
3. PF is efficiency of the total power.
 
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