Projected Energy Savings By Switching to VFD

The motor draws around 111A with a 54% efficient motor. Putting the motor on a VFD would need a way to control it. Part winding start, also known as soft start is also available. This would aleviate any light dimming problems. Give these guys a call and they should be able to answer your questions. http://www.clrwtr.com/ABB-Drives.htm

Your motor draws 125 amps at 480 vac, 3 phase, full load. Starting this motor "across the line" will consume 6 to 8 times the full load amps. UGH!

Either a VFD or a "soft start", sized correctly for your motor HP will absolutely reduce your power consumption dramatically. You should see less mechanical damage to your gear reducer as well.

If you are happy with only the 2 speeds that your gear reducer offers, then you only need a "soft start", which is the least expensive of the two.If you want to control the speeds at variable intervals, then I would go with the VFD, which is more expensive of the two.

You are seeking justification of the costs......I'm not sure that can be determined in terms of energy savings. Simply let your boss take a peek at the current that the motor draws on start up and while running, changing speeds, etc. Then show him the mechanical damage the reducer sustains.....both should "speak volumes", as we say.

Contact your local Rockwell Automation Allen/Bradley rep and he should be able to spec the correct size for you.

Good luck!

If I would probably stick with the VFD not the soft Start utilizing PLC control and preset speed on the VFD. I would be using the AB PowerFlex 700. But no one has yet to offer a calculation of applied power savings by utilizing a VFD. Any one have an idea on how to calculate based on info given or do I need to provide more?

You have a two speed motor. You need to estimate the hours of operation at each speed. Multiply these numbers by the kW of the motor at each speed and you have the baseline kWh.

0.746 is the conversion between kW and HP. Thus, the high speed demand is estimated to be 93.25 kW. To be precise, you need to multiply this by the power factor and divide by the motor efficeincy. However, I've neglected these parameters as this factor is often very close to one. The motor specificiations likely has a brake HP for the low speed. Alternatively, if you have a pwer meter, you could get this information from it.

The retrofit consumption is a little more complicated. The VSD slows the speed of the motor. The theoretical power consumption of the motor should follow the cube law. However, in practice you should use 2.5 as your exponent to account for inefficiencies. Thus, a 10% reduction in speed result in a power draw of 70.6 kW. A 20 % reduction is 52.6 kw. Estiamte the total number of hours in each 10% load bin and multiply that by the VSD controlled kW. The total hours may be more in this case then your baseline because the motor will operate a low speed rather than shutting off. Remember that the VSD will have 3-5% ineffciencies, so a VSD driven motor will consume more at 100% load then your two speed.

Download a software caluclator for this and many other energy effciency measures here:

http://www.aesc-inc.com/download/SPC/

Thank you, this is more of what I'm searching for. I tried the link but the site may be down, it didn't find the page. I'll try again later.

You can also generate a spreadsheet fairly easily.

My mistake, your motor draws 156 amps, full load, even worse!

I have studied this many times for pipeline pump operations, up to 10 MW electric motors (slight larger than yours). VFDs through harmonics onto your system so if you have sensitive voltage regulators elsewhere on line it will drive them crazy. VFDs are very nice for soft start, but also have their own efficiency. careful with any claims that they will save money. And they need to be in an air conditioned clean room. Our biggest cost of any VFD is the facility to store it and keep it cool. Another thing is that if it fails, you may not be able to fix it at midnight with your local skill pool.

Ideas to solve 'shock load' is a Eaton air clutch. Leave the motor running and you clutch in the load 8 times an hour. While the air clutch begins to grab, it is somewhat soft and has a rubber element to give a torsionally soft start. They are VERY robust so do worry about 8 cycles per hour.

Another is a variable capacity torque converter between your shafts. The motor starts unloaded and you bring the load up on line via a torque converter (hydraulically). Very nice and smooth, infinitely variable speed and no AC system for the VFD. See Voith for fluid couplings or variable speed drives. http://www.voithturbo.com/startup-components.htm and http://www.voithturbo.com/variable-speed.htm.

Email me for air clutch details. Too complex for this post. Click on my name and find "Contact Member" or something like that.

As for the harmonics this can easily be conditioned and limited by a Line Reactor, which these days is almost a requirement by Drive Manufacturers and has come common practise on all of my drive panels. As for the conditioned environment this would be completely depend on plant ambient and so far I have never had an issue with enclosure's sized accordingly to dissipate heat. And if the plant air can suffice the load, utilize Exair devices if not a push pull filtered ventilation. AC systems never withstand the test of time, are another maintenance issue, and condensation is a pain to plumb into a drainage system. That's a whole other topic for discussion. If it fails, is always an if. Currently the Contactors are in the same boat, it's a next day air item, and I get called to service it as well, same as would be the Drive. Or for the plant a spare parts issue. The plant's I deal with currently are standardizing on the AB PowerFlex Drives. So by standardizing Maintenance personnel are more apt to have knowledge of drive parameters and functions, etc. We can only hope. But for the question at hand is not whether are not to utilize a VFD. It is a question of what are the calculations for determining savings in power consumption by Utilizing a VFD?

Your energy and cost savings will have to do with duty cycle. Most systems are designed with a margin between the motor's nameplate and the actual load's requirement. This allows for wear, shock loads, overloads, colder weather, lower voltages, etc. I suspect that your mixer does not always run with a full batch. If you have any data to tell you the range of batch sizes, then this should allow you to calculate the energy losses for operating the 125-hp motor with reduced loads. Under these circumstances a VFD (variable frequency drive) can give significant savings, as the earlier posts mentioned.

If you don't have data on the batch sizes used, try hooking up a recording ammeter for a day or two. (I could suggest more elegant equipment that would give actual power used as well as reactive power drawn, but usually this is not available.) Although current draw is not easily converted to horsepower loads because of things like the motor's magnetizing current, this graph can show you if the full-load times are always to the same measured current value, or if there are differences between batches. I suspect you have good relationships with the plant operators. They could probably tell you the actual batch sizes which you can correlate with your measured motor currents. If you take this type of data, you should be able to use a spreadsheet to get a % duty cycle or average load %. Knowing the motor load at the different batch sizes should allow you to calculate the apparent energy efficiency of the motor at each batch size. With a VFD, the energy efficiency will be fairly constant for all loads, so you can calculate the energy savings.

Remember also, that you probably should include the cost of a load reactor between the VFD and the motor. Older motors do not have windings with as high an insulation voltage level as the "inverter duty" motors now being made. Typical motor winding failures are in the first few windings, because of the impedance mismatch between the windings and the feeder. Our RF friends know all about this, but us AC power people usually don't appreciate the real problems that a VFD's 5kHz to 20kHz carrier frequency (from synthesizing the 60Hz sine wave) can cause.

In the end, the cost vs. benefit analysis has to look at all the factors discussed by the various contributors to this thread--wear and tear, shock loads, breakdowns, lost production time, energy costs, required service size for the equipment, maintenance skill levels, equipment costs, etc. Happy calculating!