VFD Cost Savings

[wzrdstrm]

I've been doing some research for my company and have to present how much I can save the company by going to VFD's instead of direct induction (starter application). I don't get the option of a test bench or wind tunnel, so please help me if you can. We have approx. 36 fan applications, dust collection, and from what I've beed finding is that if I go to VFD's without varying the speed (Working on this later) there would be a cost reduction in KW used. Tests I've seen online suggest that a VFD is approx %40 more efficient, delivering the same HP at a reduced cost. Please respond.

 

[davidbeach]

No. If you're running a motor at a constant load all the time you are likely to maximize your efficiency by selecting the right motor/sheave combination and letting it run. On the other hand, if the load varies, a VFD will be much more efficient than other means of controlling fan throughput such as dampers or inlet vanes. The VFD itself has losses that would not exist in its absence so you have to be able to improve the process efficiency by more than those losses. If you have a system that spends most of its time at well less than full load, then go for the VFDs but if it spends most of its time at its maximum load you will be better off with the right motor/sheave combination.

 

[ozmosis]

I'll second davidbeach's NO.
A typical efficiency of a VFD is between 96~98% (and that is tops). So that's an additional 2~4% loss you would be adding into the system (at best), and that's without the effects of adding VFD's to the process (increased EMC, increased harmonic content, cost of installing, cost of cabinets[dust=tight control of enclosure] as well as the [potential] impact on the motors). However, all things considered, I would look at your application a little more carefully and investigate the possibilities of varying the speed if it is fans you have. I've worked on dust extraction systems in aggregate quarries and there are savings to be had as long as you have good control over the pressure differential of your dust extr. system and feed this into your VFD's rather than dampers (if it's a similar system).

 

[wzrdstrm]

Thanks Guys, Ok, Here is what I inititally intended to do to save money. As you say, ther are possible considerable savings by varying the speed. This dust collection unit closes a damper to pulse what amounts to vacuum bags and knock the dust off of them. They know when to pulse by differential pressure. The one unit I targeted, pulses for about 15 seconds(motor dampered off, no other use at this time) aprrox 210 times a day, or about 30 hours a year. If I were to ramp down the motor during a pulsing sequence (15 sec), and ramp it back to required speed, woud it be worth it an save money?

 

[ozmosis]

What ratings are the motors?

 

[jraef]

Do you see any value in leaving the motors running at all during the pulsing? Remember, there is no better energy saver than the OFF button! 210 cycles of 15 seconds comes out to just under an hour of running time in a 24 hour day, so having them completely disconnected will add up. In pulsed air dust collection systems that I have worked on, the motor is just turned off during the cleaning cycle and turned back on again when done. They use soft starters to avoid the problems of mechanical shock on the restart because soft starters are usually 1/5th the cost of VFDs, which makes the payback a little faster. Generally the blowers barely slow down more than 50% during the pulse cycle anyway, so using a VFD would have no tangible benefit other than a slightly reduced re-starting current.

The only similar application I have seen where the VFD made sense is in dryer blowers at car washes. If the dryers have electric heating elements, as some do in colder climates, you need to keep a minimum amount of air flow going across the heaters in between cars to avoid having to either take too long to re-heat, or burn up when the blowers are off. In older designs then, the blowers were just left running. In that case, they could save energy by leaving them running but at a reduced speed; just enough to protect the heater elements, then ramp them back up as the next car became ready. In your case though, there is no benefit to keeping the bowers running is there?

 

[CheeseSarge]

The only other consideration is what type if billing does your poco use with your facility. In mine we have an industrial rate with means that we have three different charges. There is the acutal usage in kwhr both in on peak and off peak. We have an actual demand charge that is the maximum demand in a 15 minute increment for the month which is billed at $X /kw. The second demand charge is the customer demand charge that is the maximum demand in kw that was seen in a 15 minute period fo the past 12 months that again is charged in $Y / kw. It may be more cost effective to correctly match the motor/sheave ratio and keep them running rather than turn them off and on which can depending on the size of the motors increase your demand charges. Just a thought that you may want to consider.

 

[wzrdstrm]

Ok Thanks people. aside from the calculations I did to show savings. We had our power company come and talk about "Focus and Energy" They will foot the bill to install up to %30, which in this case was about 120,000. They also will foot %50 to have an outside source evaluate the application.
For those looking for other areas to save on energy, HVAC has huge savings, now if I can only find the time to do those calcs.