Created Date: December 16,2011

Created by: dieseltwitch

Its basically a capacitor that captures reactive current for the next cycle. It also goes by the name "Power Factor Correctors "

Do you know what kVA, kW & kVAR are? they are all directly related

kW is the actual power used in a motor

kVA is the apparent power or the power that you pay for from the power company when the motor effency is taken into account

kVAR is the difference between kW and kVA. This is a very very basic explanation, its much deeper then this

The best example I use is the beer glass analogy,

When you go to the bar you pay for a pint of beer, now if the bar tender is "efficient " he will fill the glass with mostly beer but there will be some head or foam left on the top. if he is a crappy bar tender he will flood the glass with a lot of foam and a lot less beer. either way you are still paying for a pint of beer or KVA. The beer is the energy or kW that you get to use and the foam is the KVAR or wasted energy. Like I said this is a very rough explanation but the idea is sound. KVAR Capacitors help correct this problem.

Created Date: December 16,2011

Created by: johnaden

In the VFD class I teach, I use some charts from centrifugal fans and pumps.

The "laws of affinity " for these units show that if a centrifugal fan or pump speed (flow) can be reduced to half, then Horsepower requirement is reduced to one eighth. Horsepower, of course, relates directly to wattage, which equals $$.

The savings is not that the VFD runs the motor more efficiently, but that it allows the whole system to produce only enough flow as needed. This compares very favorably to using dampers, etc.

Created Date: December 17,2011

Created by: milldrone

I'm need some help in calculating the savings of a VFD vs a KVAR,

Have you tried using one of the Kill a watt units? I believe they are sold at Home Depot for around $30.

What I'm trying to figure out is how to calculating loss due to power factor and the amount of energy save by using a PFC (power factor correcting device) I know it can't be a 100% savings of the wasted energy...

I always thought that the energy saved by using power factor correction caps came from the voltage drop caused by the resistance of the conductors in the system. This is because the conductors before the power factor correction device do not need to carry or supply the reactive current for the motor. Keep in mind that you asked about "energy saved ". Normally this is a very small $ value. In the case of a homeowner it's rarely more than 10 cents a month. And could be in the order of .1 cents a month all dependent on where the power factor correction is physically done. The classic example would be placing a power factor correction cap at a barn that was 2000 ' feet from the meter. In this barn example money savings in the order of single digit dollars per month might be possible. But if you were to mount the power factor correction just after the meter base the savings might not be more than .1 cents per month.

But if you really meant "money saved " this may come from a couple of different possible areas.

The classic area that power factor correction is used to reduce power bills is when the utility charges the customer for low power factor. This is normally in the range of megawatt consumers, large sawmills, aluminum smelters, fiberglass insulation manufacture, ect. I doubt that any of your customers are in this category.

Another area. the one that you are working with is to vary the speed of fans and pumps according to the needs of the process. I won't describe this one because you already know it well!

Question: You do know the difference between measuring power by watts and measuring (I hate to use this term) "apparent power " by multiplying voltage and amps?

I go head to head all the time with KVAR guys that thing putting a KVAR on a pump is far better then putting a VFD on one. Need Ammo!

What I'm suggesting is to get one of these Kill a watt units and set up a demo for your customers. Because the Kill a watt unit is a conventional 120V plug in monitor it should be very portable. Get two fans same hp and capacity but one is single phase, one 240 three phase. Set up the single phase unit with the power factor correction box if you can afford it. Also make it so that you can switch the power factor correction in and out of the circuit. Set up the three phase unit with a 120V single phase input VFD to power the motor. let your customer decide from there.

where the KVAR guys come in with a little box that mounts to the wall and call it good . Trying to show them that slowing the motor down the savings is cubed is not easy when often times these guys have no college and the KVAR guys don't want you to slow the motor down. I get asked a lot why they shouldn't just us KVARs.

There are some threads over at Mike Holt's electrical forum that are about these "snake oil " power factor correction devices here is one http://forums.mikeholt.com/showthread.php?t=140830 the thread was just locked by a moderator at over 300 posts because the content had wandered of track but is still interesting reading. Here are some other posts

http://forums.mikeholt.com/showthread.php?t=140087&highlight=power+factor+correction

http://forums.mikeholt.com/showthread.php?t=140830

http://forums.mikeholt.com/showthread.php?t=140295&highlight=power+factor+correction

Created Date: December 20,2011

Created by: dieseltwitch

Have you tried using one of the Kill a watt units? I believe they are sold at Home Depot for around $30.

I have one of these, just don't have a KVAR unit on hand, will be getting one soon.

I always thought that the energy saved by using power factor correction caps came from the voltage drop caused by the resistance of the conductors in the system. This is because the conductors before the power factor correction device do not need to carry or supply the reactive current for the motor. Keep in mind that you asked about "energy saved ". Normally this is a very small $ value. In the case of a homeowner it's rarely more than 10 cents a month. And could be in the order of .1 cents a month all dependent on where the power factor correction is physically done. The classic example would be placing a power factor correction cap at a barn that was 2000 ' feet from the meter. In this barn example money savings in the order of single digit dollars per month might be possible. But if you were to mount the power factor correction just after the meter base the savings might not be more than .1 cents per month.

But if you really meant "money saved " this may come from a couple of different possible areas.

The classic area that power factor correction is used to reduce power bills is when the utility charges the customer for low power factor. This is normally in the range of megawatt consumers, large sawmills, aluminum smelters, fiberglass insulation manufacture, ect. I doubt that any of your customers are in this category.

Another area. the one that you are working with is to vary the speed of fans and pumps according to the needs of the process. I won't describe this one because you already know it well!

Question: You do know the difference between measuring power by watts and measuring (I hate to use this term) "apparent power " by multiplying voltage and amps?

Yeah the other major area come from reducing demand charge. most of my customs are spending 5,000$ - 10,000$ on their power bills now, Car washes, Hotels, Apts, and some other process type locations. None that I have come across have power quality charges just Demand and Usage Charges.

What I'm suggesting is to get one of these Kill a watt units and set up a demo for your customers. Because the Kill a watt unit is a conventional 120V plug in monitor it should be very portable. Get two fans same hp and capacity but one is single phase, one 240 three phase. Set up the single phase unit with the power factor correction box if you can afford it. Also make it so that you can switch the power factor correction in and out of the circuit. Set up the three phase unit with a 120V single phase input VFD to power the motor. let your customer decide from there.

While a demo is good as showing the difference, they often what to know what the savings difference is per their application, not just theory of operation type stuff. thats why I'm looking for hard math on how to calculate savings by using the different devices

There are some threads over at Mike Holt's electrical forum that are about these "snake oil " power factor correction devices here is one http://forums.mikeholt.com/showthread.php?t=140830 the thread was just locked by a moderator at over 300 posts because the content had wandered of track but is still interesting reading. Here are some other posts

http://forums.mikeholt.com/showthread.php?t=140087&highlight=power+factor+correction

http://forums.mikeholt.com/showthread.php?t=140830

http://forums.mikeholt.com/showthread.php?t=140295&highlight=power+factor+correction

I agree that KVARs on a home is some what pointless, I don't deal with residential, just commercial and industrial. places with much high usages and larger demand, none of the projects I'm working on right now have less then 100 hp of motors. but not all of the motors can be slowed, thus installing a vfd is kind of pointless and a better fit for a KVAR unit.

I also learned that KVAR savings is directly tied to the size of the capacitor its self, not so much the reactive energy produced by the motor, for example if a 100kW motor produces 35 kVar units, but I install a 30 kVar Capacitor then the motor would still produce 5 kVar units. I think my units are a little messed up here but I'm still learning. I don't think its so much of a snake oil product as much as a highly miss under stood and an over estimated product.

Created Date: December 20,2011

Created by: dieseltwitch

No hard numbers here, but remember a VFD can reduce demand charges by reducing the motor starting current surges.

Actually thats some what of a myth.

Power companies determine demand charges based on a the highest 15-min average of power consumed per billing period. not the peak instantaneous inrush power. Because that start up surge only last for a few seconds at most it does not effect the over all demand rate. where a VFD does reduce the demand rate is if you can slow a motor down and can avoid running it 100% ever.

Let me explain, lets say we have only 100kW motor in a building that currently is running full out and drawing 100kW of power and runs for 12 hours a day, then your peak charge would be 100kW times what ever your demand rate is. for my clients is averages about 15-20$/kW. Now lets say we put a vfd on that motor that allows it to operate at 75kW for 8 hours of its 12 hour runtime but still has to run at 100kW for 4 hours of it 12 hour runtime period, then the demand would still be 100kW. However, if we could run that same motor at no more then 80kW for its entire run time period then the demand drops my 20kW saving them 400$ per month (@20$/kW) and 4,800$ per year on their demand charge. and 500$ per month in energy consumed (@0.07$/kWh) and 6,048$ per year. Combined the VFD would save them 10,848$ Per year in both demand reduction and energy consumed if the motor never exceeded 80kW.

While the inrush current does show up on the meter is it averaged out with rest of the 15-min measurement time frame.

lets say its inrush current was 600kW (depends very much on the load) and last only 2 seconds. With the rest of 898 seconds (900 second per 15 min) drawing 100kW it would average out to be 101.11kW over a 15-min period, or 1.1% change due to inrush. not even worth calculating in the savings. and that i believe is even filtered out by the metering system.

This is why soft starters don't do any thing for demand reduction.

Where the ramp up does save money is in belts, bearings, and other mechanical components as well as in insulation, and breaker life.

This took me a little bit to wrap my head around and even longer to explain to all my guys and my clients, but I spent some time working with a rep from the local power company and he explained it just as I have to you.

Created Date: December 21,2011

Created by: milldrone

dieseltwitch,

The way you explained demand is also how I understand it. Do any of your customers have on peak and off peak rates and or on peak and off peak demand charges?

I was able to save a former employer $10,000 a month on a $45,000 utility bill by using VFD's on lumber dry kilns. By reducing the KW during on peak and then speeding up the process during off peak.