Trying to improve power factor 1

[2571]

Hello,
We are attempting to improve or service power factor by installing soft starters on various pieces of equipment with large squirrel cage motors. Starting at the 30hp motors down to the 7.5hp. This is my first time considering something like this and would appreciate any information you are willing to provide to get me started in the correct direction. What types of thing need to be considered and what do we need to pay close attention to? In your opinion how effective are these soft starters. We are currently at 87 percent and need to get into the mid 90 percent to start seeing reward from our service provider. Can we effectively get there by installing these unit through out?
Thank you for taking the time to respond in advanced.
2571

 

[waross]

Capacitors are much more effective than soft starters for improving power factor. Soft starters don't usually have much effect on power factor.
If you only need 3%, try calculating (From the power bill) how many KVARHrs per month you are consuming. Then calculate how many KVARHrs are needed to correct to 90%. Divide this figure by the number of hours in a month and you get the number of KVAR of capacitors needed to correct to 90%.
You then have to evaluate your system to see if permanently connected capacitors will have an adverse affect when your facility is idle.
This approach is not always suitable, but if your utility is charging power factor penalties on the average monthly consumption of KVARHrs and you only need 3% it is often an economical solution.
respectfully

 

[2571]

waross,
could you exsplain this statement better,
"You then have to evaluate your system to see if permanently connected capacitors will have an adverse affect when your facility is idle". Please

2571

 

[dpc]

Can you explain how adding soft starters is going to improve your power factor?

I doubt if you will see any improvement in power factor by use of SS starters. And by having the starters continuously energized, your real power consumption will increase due to losses in the starter. Generally, bypass contactors are used to run the motor after starting.

Waross is correct that the best approach to power factor correction is generally adding capacitors somewhere in the system. If you have a lot of VFDs on your system, you will need to be careful about sticking caps on the system with proper harmonic filtering.

 

[Marke]

Hello 2571

Soft starters will not improve your plant power factor.
Soft starters with energy saving algorithms may reduce the magnetizing current of motors that are operating at very light load only, but will not alter the VARs of loaded motors.
If a significant proportion of your load was small very lightly loaded motors, then you may see some effect, but in general, there will be no PF improvement.

Power factor correction capacitors will correct a displacement power factor under all loading conditions and if pf is the only reason for fitting soft starters, your money would be better spent on capacitors.

Soft starters do however offer many other advantages, just not power factor correction.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jraef]

Soft starters, although wonderful for a variety of other reasons, will definitely NOT HAVE ANY EFFECT on your Power Factor! If someone is trying to sell you on this concept he is a charlatan and a scam artist. Do not fall for this.

To jump in on waross' statement, when your lagging PF is caused primarily by motors, PF correction is best done with what is called "at load" capacitors, meaning the capacitors are only switched on when the motors are running. This is easily accomplished by placing the capacitors near the motors,thus downstream from the starters so they are only on-line when the starter is closed and the motor is running. Leaving capacitors on-line all of the time (called "bulk correction") means that when the motors are not running, the capacitors are still in the circuit and can result in your facility having an overall leading PF, which has almost as many detrimental consequences as too lagging. Automatic PF control systems can prevent this, but they are expensive compared to just placing capacitors downstream from your motor starters.

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[waross]

By evaluating the system I mean considering the effect of the capacitors on your system when the system is unloaded. Too much capacity for the connected load may raise the voltage excessively. I estimate the magnetizing current of the supply transformer and any other transformers in the system. I take a look at how many KVARs it will take to offset this current. In some happy circumstances this will be enough for your 3% increase from 87% to 90%. You can go a few percent leading but be careful. If this doesn't make it, the next step is to follow jraef's suggestions. Put as much bulk corection as you can, it will be active 24/7. Additional correction switched with the motors will only be active when the motors are running. On an 8Hr. 5 day work shift that's a little more than 160 Hs per month. So, a 10 KVAR capacitor will give you about 1600 KVARHrs a month. The same capacitor used as bulk correction will give you 24 Hrs. x 30 days x 10 KVAR = 7200 KVARHrs per month.
That 4505 advantage is why I like to start with bulk correction.
This is the art of power factor correction as opposed to just spending a lot more of the customers money to put in an automatic correction panel. Typically an automatic correction panel will correct to 100% at the worst possible case. There are some places in the world where you are charged for any deviation leading or lagging. You need an automatic panel there.
When you are charged on the KVARHrs over your monthly average and are allowed to drop to 90% AVERAGE, a little art and ingenuity will save a lot of money.
I have learned most of the "ART" in the field, as I suspect jraef has. I've never seen a book or paper with the tricks and art of PF correction. Have you, jraef?
Another exersize, for someone, Estimate the KVARs required to correct the power factor to 100% under the worst case conditions. Now calculate the KVARs required to bulk correct the PF from 87% to 90%. You will be looking at another very large and expensive ratio.
I love the art of power factor correction. It is a rewarding challenge.
respectfully

 

[jraef]

Myron Zucker (a capacitor mfr.) has a series of papers called "Capacitalk" that explain a lot of these principals. I must say however, they are decidedly biased in their opinions and in fact even make a statement in one alluding to PFC caps saving "energy" by showing how they reduce current. They lost my respect on that one, but other papers and information they provide are educational nonetheless.

http://www.myronzucker.com/docs.html

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[davidbeach]

jraef, to the extent that improved power factor lowers current (lower kVA for same kW), the reduced current means less I²R loss. Less loss is less energy. The power consumed by the loads won't change but the power consumed getting the power to the loads will go down. Will it be enough to worry about? I don't know.

 

[jraef]

Thanks, yes I know that, but when you read their paper that is not what they are referring to IIRC (it's been a while since I read it now). They were using the motor current when the capacitor is connected as an argument that energy was being saved.

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