OT: VFD Marketing BS

After seeing many articles in magazines from many differnet vendors proclaiming VFDs as cost savers for items like HVAC (typically claiming 15% savings) do not ever seem to address the power factor issues of running a drive at some fraction of full load. By the time you add all the do-dads to fix the penalty you are going to create (depending upon the % of your total load made up by motors you are adding to VFDs) I have a hard time believing that you will come anywhere near that 15% they like to claim. In addition they do not have the tools for the end user to be able to determine if the upgrade will negatively affect their current power factor.

Or maybe it is just me that gets these stange request from customers that get hair brained ideas from the latest adds in whatever engineering magazine/website?

Darren

I am not exactly sure what you are saying but will respond as I understand it.

If using inductive or capactive loads then you will have a power factor issue to some point. If motors are already in the system adding a drive should not affect the pf factor. My understanding, especially with air flow systems like HVAC is that you may not need to have the motor running all the time at full speed; therefore a VFD when you change the frequency will change the voltage which will in turn decreases the power (wattage) that is used.

Assuming constant current/load if a VFD (at 460vac) running 30HZ would have an output voltage of 230vac (7.67 * 30) this would decrease the power used by half.

The original issue with drives pertained to harmonics more then being a pf factor, newer drives are improving to the point harmonics is not as big an issue.

I doubt a VFD will offer savings in all applications though.

Hi Darren,

I had a similar issue when somebody asked me to prove that our VFD had a power factor better than 95%. This is the response I sent him:

Toshiba, along with other VFD manufacturers, does not publish the Fundamental Power Factor which is above .95 for virtually all drives. This is because Fundamental Power Factor does not account for harmonic distortion. Total Power Factor is the important figure since it measures the total system power factor. The attached VFD test report shows that Total Power Factor varies greatly with load, speed, motor efficiency etc. such that a single Power Factor number cannot be used unless all other parameters are also specified. The attached Siemens Application Note does a good job in explaining the Power Factor issues with VFDs.

I agree with you about the marketing BS. Some people believe a VFD will save you money even if its running at 60Hz.

Yes there is definately some HYPE in advertising, but the 15% is a reality!!!!!!!

Having done actual comparisons, a VFD will cost less to operate than a motor across the line.

A VFD will definately cost less to operate than an equivalent DC drive in a constant torque application. One of my company's offices did an actual recording of power consumption and PF on the DC equipment before it replaced them with VFD's. Then they recorded the VFD power consumption. The savings on both total power and harmonics was worth the change over.

Most HVAC applications with a motor running at 100% speed and using baffles to control air flow will save that 15% just putting a VFD on and slowing the motor 10%. Basic math shows at 90% speed the motors use approximately 72% of the energy needed to run the same motor at 100% speed. Reality factors put the savings at 15% due the motor actually running even slower to control the air. Pumps also benefit by nearly the same amounts.

Typically a VFD runs at 0.9 PF and at low speed high amp levels it can drop to 0.8. Have you ever seen a DC drive run at better than 0.7 PF? Most DC drives run in the 0.3 to 0.5 PF region.

Not mentioned is the mechanical savings of the 'soft' start and stop offered by drives. Extending belt and bearing life can save a lot of labor costs.

While there are always people out there making wild claims about just about anything including VFD's, there has been almost 30 years of experience with these devices along with plenty of hard data to confirm that, for increasing torque loads like centrifugal fans and pumps, the energy savings with even small decreases in volumes can be impressive. Further, there is no other method that acheives comparable or better energy savings available today.

Do you have to know a little about the technology to be able to separate fact from fiction? Sure! Can you expect wild claims when the marketing people are on a long leash? Sure!

But, in what field is that not true? And, none of the "snake oil" changes the hard data. Why not check out some of that and draw your own conclusions?

I agree that in almost all cases, a VFD is the way to go. Most of the pump control systems I design use VFDs which are usually specified by engineering firms. I would also use a VFD in place of a soft starter. VFD prices are coming down such that the difference in cost is not worth trading off the benefits of a VFD.

To define what has gotten me wound up right now is that many drives advertise a PF in the range of .92 - .96. This is simply not the case when the motor is operating at part load, especially on a variable torque application like a fan or pump. Recently after upgrading many systems the customer (& and myself) were expecting improvments in power usage and pf (remember that most claim better than .9 for the drives), but the pf for the MCC averaged around .7.

This was a suprise to me. There were some individual drives that were down around .6. These issues are worse on the variable torque and when the mechanical engineer calculates a 3HP load, but because some time down the road he thinks he may install different pulleys, he purchases a 5HP motor. Now say that the electrical person does not know that the actual load is 3HP, so he purchases a 5HP heavy duty rated drive. On this scenario you may see a PF as low as .5.

Anyway, the above information was initially tracked down because the particular drives would display the PF. Well the numbers being displayed were terrible for what we were expecting. So we installed a 3-phase power monitor that is rated to be used in areas with high harmonics. Besides the bad pf numbers the harmonics were incredibly high, especially since we were under the understanding that these drives were supposed to have front end filters to minimize these harmonics. We have seen %THD in excess of 100% for extended periods (this is for several hundred HP of load). Now we are looking at active filter and power factor correction for the MCC. Anyway this has turned out to be an expensive upgrade. Do not believe what the sales people are spouting and do your homework. Find drives that have LC filters, these will minimize the harmonics the most, but several drive manufactures say the drives have built in filters, but they are actually more like a line reactor which does not do much for harmonics.

This document alludes to what I am saying. http://www.bchydro.com/rx_files/psbusiness/psbusiness1531.pdf

Basically you may not get gigged by the utility if they are still using an older meter that measures the diplacement power factor (older rotary style meters). This apparently is what is used by VFD companies to provide their rating (which is over stated as compared to true power factor). When you use a more modern electronic meter that measures your true power factor you will be shocked at how low it is on a VFD (at least I was). Anyway as the utilities get around to upgrading their meter technology industrial sites will need to upgrade their power factor correction technology to match all the VFDs that are being added.

100% thd?

dash said:

To define what has gotten me wound up right now is that many drives advertise a PF in the range of .92 - .96. This is simply not the case when the motor is operating at part load, especially on a variable torque application like a fan or pump. Recently after upgrading many systems the customer (& and myself) were expecting improvments in power usage and pf (remember that most claim better than .9 for the drives), but the pf for the MCC averaged around .7.

This was a suprise to me. There were some individual drives that were down around .6. These issues are worse on the variable torque and when the mechanical engineer calculates a 3HP load, but because some time down the road he thinks he may install different pulleys, he purchases a 5HP motor. Now say that the electrical person does not know that the actual load is 3HP, so he purchases a 5HP heavy duty rated drive. On this scenario you may see a PF as low as .5.

Anyway, the above information was initially tracked down because the particular drives would display the PF. Well the numbers being displayed were terrible for what we were expecting. So we installed a 3-phase power monitor that is rated to be used in areas with high harmonics. Besides the bad pf numbers the harmonics were incredibly high, especially since we were under the understanding that these drives were supposed to have front end filters to minimize these harmonics. We have seen %THD in excess of 100% for extended periods (this is for several hundred HP of load). Now we are looking at active filter and power factor correction for the MCC. Anyway this has turned out to be an expensive upgrade. Do not believe what the sales people are spouting and do your homework. Find drives that have LC filters, these will minimize the harmonics the most, but several drive manufactures say the drives have built in filters, but they are actually more like a line reactor which does not do much for harmonics.

Click to expand...

is it current thd or voltag thd ???

raviv said:

is it current thd or voltag thd ???

Click to expand...

I'll have to look at the meter documentation. It is only displayed at %THD & I do not know if that is for voltage or current. I would assume voltage, but I will have to verify.

I am not sure what the rant is about. Power companies charged based on power, as far as I know they all do that. If your place of business has poor power factor you can expect to be charged more. They will also charge more if you have a service that you do not use a certain percentage; an example would be where a 3000 amp feed is provided and you use 800 amps but may be charged for 1200-1500 amps minimum.

Whether it be a VFD or any other device, the specifications state capabilities, it is up to the designer to make sure they are used properly to obtain those capabilities....geez a manufacturer can not know when a 5HP motor/drive is used when a 3HP should have....

I know that it is possible in certain situations to lower power consumption in some situations. I can state issues like Leadfoot refered too where we changed numerous systems from DC to AC and had significant differences. I will admit I have run into harmonic issues but sometimes you have to deal with something to understand what can happen, there is no guarantee there will not be issues, just that it is not as bad as it once was.

The rant is that VFDs typically are listed as having a .9 or much greater power factor. That is the marketing baloney. They use a much older method to measure that value and give you the displacement power factor. When you measure the TRUE power factor using newer power meters you get much less than what was advertised. That is the rant. They also state that this power factor is over all operating conditions. Again, only for the displacement power factor.

This does not bother you as being grosly misleading?

I guess it depends on how you look at it and situations you have dealt with.

My meager electronics background tells me capacitance can affect pf BUT in most plants you have inductive loads that create power losses, the common cure is to use capacitors to store energy instead of losing it.

Some things are set in stone, others have variables. A motor is 1HP therefore the drive is 1HP, the pf can depend on variables i.e. a motor with a pf of .8 will not likely obtain a pf of .9 just because you use a drive. I know I am not an Engineer BUT doesn't this make sense? The pf on any inductive system can be affected by other variables too like design, efficiencies etc.

Like I said, I guess it is how you think about things. I never read a drive stating it guaranteed a .9 pf (since pf is known as a variable) but that under properly engineered conditions it should be capable of obtaining it. You can not arbitrarily take a .6pf situation using a .8 motor and achieve a .9pf...in my way of thinking; you have to determine why you have the .6pf to begin with.

I am not an Engineer so maybe I think too different. As Dick mentioned there is "snake oil" for just about everything but, regardless of measuring device, there are proven situations. When in TN we did not have any rotary devices being used. We had situations where we had problems obtaining what we desired.

Hell, I do not take my own word for what I say, I double or triple check it.

dash said:

This does not bother you as being grosly misleading?

Click to expand...

Not to me, VFD's are used to vary the speeds of motors, reduce startup current, reduce mechanical forces, provide overload protection... I have never seen a drive manufacturer claim that a drive was a power factor correction device?

The PF specifications are simply a guideline and if I were a manufacturer I would publish these figures based on an ideal condition. How else would you do it?

Most VFD's have a dumb front end. Simply a diode controlled rectifier. There is no magic there that is going to help the PF.