Effect of Lagging Power Factor

[pmcrowell]

I am an engineer (mechanical) at a paper mill and I have been wondering about the consequence of have a lagging power factor on the downstream equipment (breakers, starters, cabling, motors etc.)? If I am to believe the the meter on the incoming breaker for one of the four feeds from the utility it is reading 0.5 lagging. This feed has only inductive motors and 8 DC motor drives. There are no capacitors or synchronous motors on this feed.

I realise their maybe a penalty (I need to research if there is one as no one has complained about the bill) from the utility for a poor power factor. I am more interested in the effect on the equipment.

Thanks in advance for any advice

 

[davidbeach]

The biggest downstream affect is that your low power factor means that your circuits are drawing more current than necessary and therefore you have more I²R losses than you would have at a higher power factor. These could be minor or they could be significant. Each load draws the current it needs without being affected by the other loads so long as the voltage remains undistorted.

 

[dpc]

There's really no impact on the equipment, other than maybe a lower operating voltage.

Each load will have its own power factor based on the type of load. This has no impact on other equipment.

 

[waross]

The loads are causing the power factor. Good, bad or whatever.
You may improve the power factor of the circuit feeding an induction motor but the power factor of the motor will remain the same. A cheapest and easiest first responce will be to add capacitors to each large motor to correct the motors to unity power factor. If there are no complications you may connect the capacitors to the load terminals of the motor starters.
Caution, do not restart motors with capacitors connected until the residual current has decayed.
If you have a large number of small induction motors you may have to use a switching power factor correction scheme.
respectfully

 

[BJC]

The problem is electrical equipment is rated in KVA. The folowing link has a good explanation and the "Beer mug analogy". The thing to remember is in an electrical system beer mugs are very expensive. That is the system has to be bigger ( bigger transformers mainly) to handle excessive KVA.
If the utility has to make their system bigger to accomidate you system, they may charge you for the KVA they have to have to handle your load.

http://www.nakahoma.com/POWER_FA.PDF

 

[electricpete]

All good responses. I think davidbeach summed it up best.

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[electricpete]

A little more repetition of what was already said:

A motor doesn't care about the power factor of the system it's hooked to... it only cares about the voltage (which might be influenced by power factor as a second order effect mentioned by dpc)

A breaker or cable cares about the power factor of the load flowing through it becuase a given kw at lower power factor means a higher current and higher I^2*R heating

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[pmcrowell]

I appreciate every ones comments. It has been very helpful and confirmed what I was thinking. I especially like the beer analogy.

Now comes the fun part, convincing management the need to address the problem.

Thanks

 

[moreause]

if there is no $$ gain..or fire possiblity...then forget it lol

some time if you have load expension adding cap can prevent changing cables so $$

over here if your TOTAL power factor is below .95 the electrical company charge a big extra for it

 

[peebee]

Low PF will also suck some extra capacity out of YOUR distribution system (not just the utility's). If you don't need additional capacity, no problem. If you're looking to add some more load, and find you don't have sufficient capacity, then some PF correction might be a much cheaper solution than adding more transformation, distribution, cabling, etc.

Capacitors can be a real headache, though. Especially if they start exploding or blowing fuses or causing harmonics problems.

 

[wfowfo]

The new NERC requirements are calling for Utilities to maintain 97% power factors at the substation power bank with penalties for those who don't comply. Even if you don't pay a penalty now, you will very shortly as the utilities cannot afford to eat the penalties.

 

[apowerengr]

As stated earlier poor powerfactor causes losses and lower than necessary voltage. Consider, a typical transformer loaded to its nameplate rating at unity powerfactor may see up to 1% voltage drop across the transformer windings however the same transformer loaded to its nameplate rating at 80% powerfactor will drop 4+% in voltage. 50% powerfactor might result in 10% volt drop across just the transformer windings which can be an issue.........