VSD vs DOL energy saving?

OK so this is probably more of an electrical question but was hoping someone on here might have an answer for me.

I'm looking at doing some optimization at my company's processing plant and one of the suggestions I've had is to replace DOL's with VSD's to save energy.

Now I'm just wondering how to quantify the potential savings of doing this?

For example - we have some agitators that are 185kW DOL's that are running at roughly 50% FLC on a continuous basis. Can we save money on power by running these off VSD's instead?

ajingham said:

I'm looking at doing some optimization at my company's processing plant and one of the suggestions I've had is to replace DOL's with VSD's to save energy.

Now I'm just wondering how to quantify the potential savings of doing this?

For example - we have some agitators that are 185kW DOL's that are running at roughly 50% FLC on a continuous basis. Can we save money on power by running these off VSD's instead?

Click to expand...

The answer probably depends on how you are using your agitator. Do you need high speed while adding and mixing? If so then can you reduce the speed after the mixing cycle? I'm presuming that the torque required to agitate at fast speeds "cubes" vs. low speeds.

Here is a calculator from Mitsubishi that can help quantify it. Its surprisingly powerful as you can build a report, change the profiles and it will calculate savings and ROI.

http://www.gibsonengineering.com/mitsubishi-energy-savings-calculator/

And an online simulator.

http://www.energysavingdrive.com/

I would assume you can use this an a 'generic' indicator of saving by going with a VFD, there is an energy saver mode with the Mitsubishi drives that has some effect, not sure of the details.

Most of the company's that sell VSD's should be able to offer you a guide to what you can save, choose a local brand or one that has good local support and talk to them.

Alan

The generalized answer is this.
The energy consumed do perform any task can be broken down into two categories: the energy it takes to do the REAL work, and the energy that is wasted in doing it. So with that in mind, know that you CANNOT SAVE ANY ENERGY THAT WAS NOT BEING WASTED.

So, the energy savings that is achievable using a VFD comes from DECREASING the energy that is wasted in VARIABLE flow processes, and ONLY those that use what are called "centrifugal" machines, otherwise known as "quadratic". The quadratic term comes from the fact that in that type of machine, energy used is related to a quadratic equation to to speed of the motor running it, being the cube of the speed reduction. So for example on a centrifugal pump, if I run the pump at 50% speed, the energy NEEDED by the pump decreases to .5 x .5 x .5 = .125, or at 1/2 speed the load requires 1/8 power. Applied to that variable flow application then, if I am using SOME OTHER MEANS to vary the flow from this pump, that other means will have losses associated with it. In pumps where you use valves for example, the valve causes a pressure drop across it, which represents an energy loss in heat in the valve and pipes due to friction and turbulence (which you may never notice because it is carried away with the fluid). Energy use does still reduce by reducing the FLOW, but at a more direct ratio, closed to 1:1 (meaning at 1/2 flow it takes 1/2 power). So by removing the valve and changing the motor speed, you remove those losses in the valve and take advantage of the DECREASED energy input for the same flow rate.

Overall, this is a concept in physics referred to as pne of the "Afinity Laws", which you can look up and research further. A similar principal exists for centrifugal fans as well, SOME types of centrifugal compressors and, perhaps in your case, a CENTRIFUGAL mixer. Whether or not your mixer can be considered a centrifugal load is entirely up to the mixer design. Paddle mixers that are essentially a pump impeller without a volute, are often considered centrifugal. Axial propeller type mixers are often not. You should be able to get your mixer supplier to answer that for you, they will (should) know.

But here is the caveat to this: if you are not VARYING the speed of the mixer, meaning that SOMETIMES you need a higher speed and MOST of the time you run at a lower speed, then you will actually NOT realize any energy savings, compared to just using a CORRECTLY sized motor for the load. In other words if that 185kW motor was sized for a task that you NEVER perform in your process, and you are only using it at 100kW, replacing it with a 100kW motor will save you MORE energy than using a VFD on the larger motor.

On all other types of machines that are NOT centrifugal, there is NO appreciable* savings that happen by magic just by applying a VFD to the motor, despite what unscrupulous salespeople might try to convince you of. You cannot violate the laws of physics. As I said in the beginning, motors only use the energy required by the load, plus any losses. Just installing a VFD does not remove losses in the motor itself, so if you cannot remove other load related losses, you will not save energy.

* I say "appreciable" here because there are a few "tricks of the trade" used by VFDs and other power electronic devices to reduce a SMALL portion of the magnetic losses inside of the motor, but only IF it is less than 50% loaded for more than 50% of the time. Those savings are very minuscule (a fraction of a fraction of a fraction) and often grossly over stated in marketing literature and "testimonials" by ignorant users, but there are also losses INTRODUCED by the power electronics tht are decidedly ignored in those statements. Don't bother considering those over stated savings, they are not worth the expense of the VFD or other device. In a VFD, those can be used to further ENHANCE the affinity law savings, but are not worth it stand-alone.

First of all, you probably don't want to save energy - you want to save energy cost.

That means that you need to include your actual energy bill in the calculations. If you pay a flat rate per kWh then your savings opportunities are more limited than if you have time of day rates for energy and peak demand charges for power. If you get charged for poor power factor then VSDs can offer savings there as well.

Second, putting a VFD on an under-loaded motor won't of itself save any money - putting on a correctly sized motor might. Now, if your agitators can adequately meet process needs at a reduced speed then you can save energy, reduce demand, and cut cost with VSDs. You need to look at the load, the process, and the billing all together.

Third, if the load is low hp the savings may not be sufficient to justify VSDs. Start with looking at your largest loads, and go for the units/systems with the shortest payback. (Cost divided by savings per year =payback.)

Fourth, talk to your local electric utility. They probably have engineers or servic bureaus that can help you with applications. In the states most also have incentive programs that will pay part of the cost of energy conservation measures like VSDs.

Be careful about applying the affinity laws jraef describes. As stated they only apply to centrifugal pumps and fans, and then only in terms of re-drawing the characteristic curves. Without adding the system restriction to the calculation you can't determine the new operating point and actual power savings. It also matters if you have mostly static head or mostly friction head, and there is a minimum speed when you run out of head or discharge pressure entirely.

VSDs are a great tool, but like any other application the devil is in the details. A thorough analysis by someone that understands the process, the loads, and the VSDs is required to do it right.

In terms of cost saving, some industrial and commercial sites are charged based on the peak figures any of your phases pull from the grid. If you are one of these sites (Check with your energy supplier), then you may benefit from reducing inrush and transient spikes caused by the plant. This won't be an energy saving, but it will certainly be a cost saving.

With figures like 185kW, if you are running DOLs, you may be better off using soft starters than forking out for VFDs.

Generally speaking - yes. One of the big benefits of VSDs is that they improve power factor considerably - DOL and soft starters cannot do this so there is an immediate saving there.
Additionally there is certainly quite a reduction in current over DOL even if running at full speed and load all the time.
If a motor is under say 7.5 kW I would not bother but even 11 kW running 24/7 will save money.
Calculations were done by engineers for a large water park - the company get a HV subsidy - all the pumps are running on VSDs and the largest is 75 kW - they are using so little power compared to the calculations that they are in danger of losing their HV subsidy because they are not using enough power!
Peak figures also come into consideration as mentioned.
I would suggest if you have, say, 2 x 55 kW motors, run one normally and put a VSD on the other - then put a power monitor on each of them, run them for the same hours and then read the kWh usage - nothing like real figures to convince the boss or hide the figures if it does not pan out! LOL

understand the process before apply the cost saving system,

think why them using 1 big motor with DOL system without using softsarter/autotrans
are this cause by budget issue

or

this is the way to got best process - product quality/product profile.