Power factor Correction help ??

milmat1

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Join Date
Aug 2005
Location
North Carolina
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Folks,
My boss say's he is paying a large fee due to Mostly inductive loads. He thinks it's because the guys come in and start up all the blowers and compressors at the same time each morning. So they start a lot of 20, 25 and a 50HP motors all within a short time. So he suggested I look at adding some soft-starters or VFD's etc to the motors. However this will really do nothing to correct the power factor other than at that instantaneous time will it ??


This really out of my area as I'm a controls engineer, but If we are being charged a fee for a low power factor, is there really any way to correct this other than adding correction capacitors ??
 
Others will correct me if I'm wrong, but if when you start things matters to your electric bill, that's the demand charge. Its usually based on the highest 15 minute KW demand. The PF is important too, and I think there's a penalty if its too low.
 
electricity bill are different from place to place but it usually reccord the highest average peak demand in VA of the month and regardless if you do that peak only once in the whole month, you get bill on it + your KW/h

So the power factor is important at that time because the closest it would be to 1, the lower would be your VA at that moment...

But it is not a instant peak but some kind of 5-15 minutes averaging...So across the line small motor start inrush is usually not read. This is why soft-start won't help at all for this...
In the other hand, A good VFD with his built in capacitors would usually make a very good power factor regardless of what the motor use. So this would help but it is at some point expensise if you don't gain from speed modulation at the same time.

The cheapest way to address power factor problem is to have an automated capacitor bank. With a dedicated controller, special contactors and static capacitor, we can control the power factor of a low voltage factory by this mean. Controler monitor pf and just add or remove capacitor when needed.

So you need to check electricity bill or any mean to get the KW and KVA amount at time of registering your peak demand of the month. KVA = Square root of (KW_exp2 + Kvar_exp2) Capacitors cancel negative kvars by providing positive kvars to balance the system. By a reverse calculation, you can determine how much kvar inductive you have and know how much capacitors you would need....
 
Others will correct me if I'm wrong, but if when you start things matters to your electric bill, that's the demand charge. Its usually based on the highest 15 minute KW demand. The PF is important too, and I think there's a penalty if its too low.

you aren't so far by sometimes they reccord Kva and charge you 90% of it so if you have let say 100kw but 50%pf, you would get 200Kva and get billed to 180kw. So you can save a demand of 80kw per month if you correct it...
I don't know the rate for electricity in USA but i would be curious to know it...In here you can usually pay back a capacitor bank in 1 or 2 years...There is few different rate according to the size of entry you have...small system have less penalty for power factor but pay kw#h at an higher rate than large one. Large save on kw/h but need to be a lot more caution with the powerfactor and peak demands.

But in some place we get the full detail of peak KW, KVA, and PF but some just show peak KVA and you need to figure out the rest by yourself...
 
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Thanks for the clarifications. It's been a long time since I've dealt with this stuff. Another possibility is the OP could call in his power supplier and ask them to go over their bill and see what they suggest.
 
1) Power factor has nothing to do with power based penalties. If your company is being penalized on power factor, that is real and solvable, but not related to how your facility is started up. Add PFC caps to the motors, starting with the largest ones, until your facility PF is .95 or better, after that, don't waste any more money. That's the typical penalty point.

2) Unless you can somehow stretch the starting time out to more than 30 minutes, adding soft starters will likely have zero effect (directly) on what are called "peak Demand" charges, if that's what he is thinking. It's one of those "alternative facts" that is repeated so many times by people marketing soft starters that it begins to sound true, but from a pure engineering fact based standpoint, it's bogus. 99.000% of power utilities bill for Peak Demand based on what is called a "demand window", a snapshot of a 15 to 30 minute window of time in which the average power for that period is highest. Your utility likely publishes their Peak Demand schedule that will lay out that demand window length. But bottom line, the 20-30 second surge you might see from starting motors across-the-line is actually insignificant, and don't be swayed by people who tell you otherwise. That myth is as old as the hills and many many people have a vested interest in maintaining it, but if you still doubt me, call the utility and ask them how much your bill will drop by adding soft starters. They will tell you zero.

There are plenty of other REAL benefits to using soft starters, I happen to really like them, but the reduction of Peak Demand charges is not one of them. The reason the marketing people get away with this alternative fact is that in maybe a small handful (as in less than a dozen across the US) small utilities, mostly that buy power from other larger utilities, they did at one time use what were called "ratcheting instantaneous demand meters". I have not seen one of those in over 20 years now, yet the myth persists.

If you want to help your company avoid penalties the best strategy, and this by the way DOES often involve a controls engineer, is to thoroughly and carefully evaluate the real need to run any and all machines, then adopt control strategies that eliminate wasteful practices. I'll give you an example.

I did work at a wood mill-works, where they make molded wood products for furniture, picture frames, mantles, trim etc. They had dozens of machines like molders, saws, routers, chippers etc., each once had it's own dust collection blower that ran whenever the machine ran, plus many used compressed air and hydraulics, so any running machine was in fact running MULTIPLE machines. The first thing I implemented was that in observing the 2 shift operations for a week, I found that each machine was turned on by the first shift and turned off at the end of the second shift; NEVER off during breaks, lunch, dinner etc. Even if the machine was not being used, it was left running. When I asked operators why, their response was that they were told that "it cost more to re-start a machine than it does to leave it running for an hour, let alone for a 15 minute break." This of course was based somewhat on that same myth about peak demand charges, but also on a real issue of larger machine start-up sometimes causing a voltage drop that made the HID lights re-strike, which meant it was dark for 10 minutes. So we implemented soft starters on every machine over 20HP, mostly just to change their behavior, but also to prevent that voltage drop. That ALONE dropped their demand charges, not because the peak starting current was reduced, but because the OVERALL load demand was reduced and less energy was wasted.

The second phase was that we tied all of the dust collectors together with one common air duct system, then evaluated how much suction was required at each machine. When any given combination of machines were running, the dust collection control system decided which of the multiple dust collectors to turn on in a combination that got the desired effect. This of course could have been easier with a few collectors and VFDs, but they already had all of these multiple smaller collectors installed, so we used them. Overall this then reduced the overall energy consumption another 18% per year. The modifications, PLC and software paid for itself in under 60 days.

The point is, your boss has given you a golden opportunity to be a hero here. Use it, just not in the way he thinks it needs to be done. So my last piece of advice; don't fight him, manipulate him slowly into thinking it was his idea. Ask first for an opportunity to do a thorough study of energy usage so that you can make optimum suggestions without wasting money, then use that information you gather to come up with solutions that, as I said, usually fall into the controls realm anyway.

Good luck!
 
delaying or sequencing start of motors will have little or no affect to your bill. As mentioned above demand charge is another issue. Commercial establishments in the U.S. Have 15 minute demand meters installed but unless you Don't run all of your motors at the same time you are not going to beable to lower your peak demand.

As stated earlier, you need to install a capacitor power factor correction system. Contact your utility for a list of contractors that can size and install it for you.

Actually even residential customers can also benefit from power factor correction.

Caspian
 
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jraef good points. The time when pf is related to peak demand registered is when the energy meter report kVa to the electricity company. I really don't know if it can be the case in your area but in here, you get billed from the higher of the 2 amount: your kW or 90% of your Kva so as long as your PF is above 90% you aren't paying extra but at 89.9% you start to pay for it...
And the savings really depend of the type of bills you have, some small facility here are just billed on kw#h when very large one (Above 5000kw) could pay half the kw#h price but get charges 3-4x the extra of an higher peak demand and they need to maintain above 95%. So capacitor don't worth to be installed in the first one but on the last one, you need them or they get paid in few months...
It is easier to have a central capacitor system to avoid losing track of individual capacitor hidden all around a facility. Exactly like your central dust collector is better than many small ones.
Unless you have a very large motor located far away, it could help to reduce current in the wires to put some capacitor on it...
 
delaying or sequencing start of motors will have little or no affect to your bill. As mentioned above demand charge is another issue. Commercial establishments in the U.S. Have 15 minute demand meters installed but unless you Don't run all of your motors at the same time you are not going to beable to lower your peak demand.

As stated earlier, you need to install a capacitor power factor correction system. Contact your utility for a list of contractors that can size and install it for you.

Actually even residential customers can also benefit from power factor correction.

Caspian

How residential customer benefit from i t? In here, residential customers aren't charged for demand and capacitors have no effects on kw#h?
 
A VFD will correct your pf to about 0.98.
It is about 0.97 efficient at full load.
And, if you use less than the motor full load, in vector control it will reduce the voltage (but not the Hz), making your motor run more efficient.
It does however introduce harmonic currents, which both add to your kVA and cause problems for other loads if the harmonic currents get high enough to alter your voltage. These will typically add 5% to your kVA.
A typical 50HP motor has a pf of 0.8
So... If you assume full load and ignore any bonuses due to vector control efficiency.

kVA_VSD = kVA_Motor * pf_motor / (pf_vsd * eff_vsd) *1.05
=kVA_Motor * pf_motor / 0.9
This means for 50 HP,
kVA_VSD = kVA_Motor * .88, or a 12% reduction in kVA.

I think you are already aware of the savings during startup..
DOL startup is about 6 times kVA running
Star Delta about 2.5 times the kVA running, although this will be more if it doesn't have enough power to get up to a decent speed in star.
 
Is Australian meters record fast inrush like a motor start ? in here it don't...So starter choice has no effects for electricity bill.
 
starting motors are very bad on power factor and cosphi, so yes an capacitor bank is a good investment.
a soft starter helps to keep the peaks low, however as the efficiency is all the day so its meaning the eff is lowered, so use VFD only when slowing down is good for the process, so a compressor is on a normal starter, however a fan can be used on a VFD when full speed is not needed all the time.
 
How residential customer benefit from i t? In here, residential customers aren't charged for demand and capacitors have no effects on kw#h?
Nobody here in the US is billed on PF for residential services*.

Well, indirectly we PAY for it because it's buried in the rate structure as fixed costs for the utility to maintain their equipment. But correcting your PF at home does nothing to mitigate that fixed cost built-in to your bill, so there is no point in it.

*Single phase residential services that is. What does happen however is that home machinists sometimes pay to have a 3 phase service dropped to their house, then discover too late that it comes with another meter... a kVAR meter, and they have to then worry about penalties!
 
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1) Get a copy of your electric bill. Some bill for power factor as measured kVA, some as a multiplier on kW demand, some as a penalty on energy charge. Make sure your corrective measures address you actual billing method.

2) Get your utility representative/engineer to call on you. These guys can offer a lot of advice and expertise.

3) Soft-starts won't affect power factor. VFDs and capacitors will. If you combine soft starts and capacitors make sure you talk to your soft start supplier. There are ways of hooking up capacitors that will blow out the soft starters.

4) Read your motor nameplate or motor data sheet. Most will give a max capacitor kVAR allowable.

5) Starting inrush is too short in duration to measurably affect demand charges. However, in many cases the lead (running) equipment is kept on until the lag (new) equipment is running and warmed up. That will increase your demand charges.

6) Most demand penalties only apply during on-peak hours. Simply starting or alternating equipment during off-peak hours can save you money.

7) Sometimes incentives (rebates) are available from your utility to cover the cost of energy conservation and demand reduction. Check with your utility.
 
How residential customer benefit from i t? In here, residential customers aren't charged for demand and capacitors have no effects on kw#h?

Normal Residential customers ARE NOT charged demand charges nor do power factor correction devices do anything for them but relieve some back pain by taking some weight out of their wallets,
Companies selling power factor correction devices to residential customers are scammers regardless of their claims. However if you open a shop or office at your home location, you may have a new meter installed if your peak demand goes over a certain amount. That depends on your utility. Here that is 26KW last time I checked.

There are claims that improving PF to meet energy star compliance in an appliance will lower a residential bill because it makes the appliance run more efficient. I have no idea what it actually saves in year. Probably not much if anything since PF is not part of a residential bill.
 
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