If the lights in the control house dim when your plant manager presses the button to start production, you can bet you’re pulling a full load of amps. Whether he runs at full production or half the plant’s rated capacity, the burner, blowers, fans, cold feed belts, pumps, etc. are pulling maximum energy unless you use variable frequency drives (VFDs) to compensate for fluctuating needs.

Proven Concept

We’ve looked at this concept in the pages of AsphaltPro before, so let’s have a quick review of VFD efficacy. Then we’ll explore a new idea, alongside Clarence Richard, consultant and proprietor of Clarence Richard Companies, Minnetonka, Minnesota, for expanding VFD use around the plant.

First, discussion of installing a VFD to reduce energy demand during non-peak production typically focuses on the exhaust fan. This fan pulls the products of combustion out of the drum and to/through the baghouse. That’s heavy duty. And that’s where producers and OEMs have zeroed in on savings in the past. Patrick Ahern of Ahern Industries Inc., San Antonio, shared that the exhaust fan VFD, being the largest motor of the asphalt plant, is sold for all the new and rebuilt plants his company works with.

“It is great for saving energy usage, and helps control capacity,” he explained. “We always put VFDs with the cold feed bins because this is the best way to do an effective mix design and controls tonnage per hour plant capacity.”

Look to other components, now. It doesn’t have to be a daunting task to set up an environmentally friendly, electrical energy saving system all around the asphalt plant, and each high-efficiency motor and VFD you add contributes positively to your bottom line over time. For example, Commercial Asphalt Company, Blaine, Minnesota, has been incorporating more of these elements as of late.

“We have been operating VFDs on our asphalt plants for over 28 years,” Todd Laubis said. He’s the vice president of operations for Commercial Asphalt Co. “The first applications were on feeders and asphalt pumps. In the last five years, we have installed them on our exhaust fans, drums and drag slats with great success. The VFDs of today are a lot smaller and more reliable than their predecessors. Today, a VFD for a 150-horsepower motor is smaller than a 20-horsepower VFD 25 years ago. In the past five years, we have started installing VFDs on our larger motors and have found the return on investment to be two to three years. The other benefits of running machinery more slowly are bearing life and overall metal-on-metal wear.”

Ahern suggested producers be aware that there are amperage minimums on the VFDs. “To avoid prematurely burning up VFDs, see what the manufacturer minimum setting is,” he shared. “The bucket or slat conveyor can get a VFD to speed up the conveyor at highest production and slow down the conveyor at lowest production. Slower slat conveyor speeds are generally better for lower maintenance costs on the floor, chain, slats and bearings.”

With a case study of McNamara Contracting, Rosemount, Minnesota, specifically, we’ll see in this article some optimum ideas from utility and consulting services for putting VFDs, rebates and best practices to work for asphalt producers.

Varied Frequency

First, understand that running a component’s motor at full capacity when the component itself is idle, or being used at only a percentage of its capacity, will cost the same in resources and utility expenses as if you had the component at 100 percent. When you put a VFD on the motor, you can alter the power draw. Producers see the return on their investment through reduced utility rates and bills.

A spokesperson from Maxam Equipment, Kansas City, explained that: “A VFD can, with its reduced voltage starting characteristics, reduce the peak demand by as much as 70 percent. In turn, it lowers the peak demand and the overall rate used by the power company. During online plant production, you could see a 15 to 20 percent reduction in overall demand. Overall demand reduction is the primary approach to take with the power company to reduce their taxed generating systems during high load times.”

Power companies have caught on to the logic. Clarence Richard, consultant and proprietor of Clarence Richard Companies, Minnetonka, Minnesota, is an Xcel Energy trade partner, which means he is rewarded for helping producers install high-efficiency equipment and then submit their rebate applications for the qualifying equipment.

That’s right: Xcel Energy pays rebates to customers who work to lower their power use through VFDs and other efficient componentry.

Richard explained that utility companies “win” three ways when VFDs are used. 1) Demand reduction (kilowatts usage rate) for generating capacity is lowered, which means the utility companies don’t have to build and fire up as many generators. 2) The amount of electricity used (kilowatts per hour—accumulated usage) is decreased. 3)

The power factor is improved because VFDs are not an inductive load. To achieve these wins, utility companies are willing to offer significant rebates to users and incentive programs to trade partners willing to help bring the system online

“Most utility companies have a rebate program to make this a very juicy investment for users,” Richard said. “It’s a complicated process, and not many people know how to navigate the paperwork and calculations to get the utility company to understand what their benefits are. Once the utility company sees enough reduction in demand, as well as consumption, they offer a rebate accordingly.

“Utility companies are rather united in bringing down the usage—such as kilowatts at the asphalt plant meter or tons of coal at the power plant—and demand—such as the size of generator and transmission lines or coal hauling trains it takes to satisfy our demands—that their large customers are using. Sometimes it seems like too much work to navigate through the necessary engineering and accounting and risk assessment to get to the prize. But the prize can be quite a plum when it comes to any electrical savings when operating a plant.”

McNamara Installs High Efficiency

During 2017, McNamara Contracting, headquartered in Rosemount, Minnesota, worked with Richard to upgrade its VFD electrical energy saving system and install three 200-horsepower, energy efficient inverter duty motors. The utility company chipped in $60,000 toward the upgrade.

The company has a plant rated at 600 TPH. During 2015, from the months of April to November, the plant operated at an average speed of 242.4 TPH. Plant Operator Jay Vivant was matching the production needed, but he knew there was room for efficiency improvements. “The exhaust fan was pulling 180 amps,” Vivant said. “Right now, at a full load, it pulls 70 amps.”

He said the difference between the former configuration and the setup now that the VFDs and high-efficiency motors are in place is noticeable. “There’s no doubt the motors have made improvements,” Vivant said. “When you started up, the lights used to dim. Not anymore. There’s even a noise difference. You don’t even know it’s [the plant] starting up. The extreme noise generated by the blower and exhaust fans was deafening, but not anymore. When first firing up the burner with the fans on, I can’t hear the roar. I have to look at the flame strength meter and watch the dryer exit gas temperature rise in order to tell that. The fans are running at a fraction of the speed they were running before, meaning the life of the fans and their drivers will last a lot longer. It’s a huge difference.”

The accounting department is noticing a difference, too. Mike Tubbs, the CFO for McNamara, shared real numbers to show what kind of savings producers can see. Running six to 12 hours a day for five days per week, they saw the following demand changes from 2016, prior to the upgrade, to 2017, after the high-efficiency equipment install (see table 1).

Richard has been working other projects with McNamara for years. To get the high-efficiency install underway, the best approach was to convince Tubbs of its financial sense. If it made financial sense to management, Richard explained, then the team would convince the plant management that the system would make the plant more reliable and quieter.

Richard said, “We figured between one and two years payback. With Xcel Energy’s rebate help, they are on track with that.” The rebate is only part of the story, of course. Using the numbers from table 1 above, we can figure excellent results for the month of June 2017.

169,200 kWh divided by 61,333 tons = 2.76 kWh/ton (in 2016)
120,281 kWh divided by 66,614 tons = 1.81 kWh/ton (in 2017)
Thus: 1.81 divided by 2.76 = 0.655 kWh/ton

After installing VFDs, McNamara used 65.5 percent of the quantity of electricity they used previously. In other words, they cut their usage by a third.

61,333 tons divided by 759 kW = 80.81 t/kW (in 2016)
66,614 tons divided by 596 kW = 111.77 t/kW (in 2017)
Thus: 80.81 divided by 111.77 = 0.723 t/kW

After installing VFDs, McNamara required a little more than two-thirds the size of generating capacity from the utility company compared to the same month in 2016. In other words, if McNamara was generating their own power, they could have down-sized their generator from 759 kW to 596 kW.

If the crew had produced the 66,614 tons during June 2017 at the 80.81 t/kW rate of 2016—prior to the VFD installation—they would have seen an actual demand of 824 kW. That means they saved 228 kW of power by using VFDs. At $8.21 per kW, they saved $1,871.88 for the month of June. Factor in fuel savings, an energy charge savings, participation in Xcel Energy’s “Peak Control Program” that invoices the majority of demand charges at controllable demand pricing, and Tubbs reports that McNamara actually saved a total of $5,597.69 for June.

Doing the math for July, we see even better results.

186,300 kWh divided by 67,077 tons = 2.78 kWh/ton (in 2016)
132,273 kWh divided by 79,673 tons = 1.66 kWh/ton (in 2017)
Thus: 1.66 divided by 2.78 = 0.597 kWh/ton

After installing VFDs, McNamara used 60 percent of the quantity of electricity.

67,077 tons divided by 780 kW = 86 t/kW (in 2016)
79,673 tons divided by 613 kW = 129.97 t/kW (in 2017)
Thus: 86 divided by 130 = 0.66 t/kW

After installing VFDs, McNamara required two-thirds the size of generating capacity from the utility company compared to the same month in 2016.

If the crew had produced 79,673 tons during July 2017 at the 86 t/kW rate of 2016—prior to the VFD installation—they would have seen an actual demand of 926 kW. That means they saved 313 kW of power by using VFDs. At $8.21 per kW, they saved $2,541.56 for the month of July. Factor in fuel savings, an energy charge savings participation in the Peak Control Program, and Tubbs reports that McNamara actually saved a total of $7,447.57.

The demand and cost decreases you see in the table and equations above are in spite of tonnage increases. Richard described it “like running a train down the tracks 1,250 hours a year at 70 miles per hour rather than 100 miles per hour.” The train and the tracks remain more efficient for a longer time at the gentler speed. “Not only are they saving electricity, but they have substantially reduced their maintenance costs. When including improving the equipment life and lower downtime, this project is zeroing in on a one-year payback.”

Tips for Your Best Practice

Be aware that any equipment-change project could see challenges. Commercial Asphalt’s Laubis had a couple of recommendations.

“When installing VFDs, one has to be concerned with electrical noise bleeding over on low voltage equipment,” Laubis warned. “Making sure grounds are properly connected is essential.”

He also mentioned the housing for motors.

“Another concern with VFDs is the amount of heat they generate. In some cases, we have had to go back and install larger air conditioners or a separate air conditioner for the motor control room.”

Ahern spoke to this as well. “Keep the VFD in a dust-free, air-conditioned environment with cooling fans in the enclosure. Keep the amperage minimum well above the manufacturers recommendation. Make sure the installer also installs/wires the VFDs for a hot start and hot stop application. If wired appropriately, start-up and shut-down will be the same or simpler.”

VFDs can produce troublesome harmonics and may need to be addressed. Designing equipment layout to include this after an installation is prudent. Utility companies usually have the equipment to monitor for this if necessary. Someone may suggest you add a harmonic inhibitor besides line and load filter reactors.

Depending on its age, the burner control may need attention.

“Their [McNamara’s] pilot required air from a fan going full speed,” Richard said. “We had reduced the speed to 25 percent. Hauck came through for us to the tune of $800 to replace the pilot mechanism that works with much less air pressure.”

Steve Weidman, a specialist service tech for Hauck products with Honeywell Thermal Solutions, Cleona, Pennsylvania, pointed out that many of the company’s burners have been converted to VFD combustion air. “We have many ESII burners that have been converted to VFD combustion air, and all of our MegaStars and NovaStar burners built now are using VFDs for combustion air.” He was able to assist with the update to the BCS 6000 control panel McNamara used in conjunction with their burner. “If it was purchased after 2009, then all that would be needed would be an analog output card for VFD control.”

The blower damper can be locked completely open. The control signal used to operate the damper actuator would then be used to control VFD speed, therefore controlling the combustion airflow. The damper linkage also triggered a low fire limit switch to enable the burner control to allow the burner to be fired at a lower and safer firing rate.

Because the damper, the linkage and actuator are no longer used, the VFD is programmed to run the fan at a certain low speed for startup.

Be aware that making changes during winter downtime means you’ll probably test the new system during cold ambient temperatures not indicative of regular production conditions. When the team at McNamara put the new components to the test, it was March 2017 and the Minnesota winter air proved problematic at first. “The VFDs faulted numerous over-voltage alarms,” Richard shared. “But that is expected with the cold, heavy air that we were testing under.” The team accomplished starting the burner and raising and lowering the firing rate while adjusting the exhaust fan speed with the updated Hauck burner control, blower fan VFD, updated exhaust damper controls, and exhaust fan VFD in late March 2017.

Once everything is installed and tested, running it takes a bit of training. For example, Richard suggested: “In once instance, we are varying the speed of the centrifugal loads—the fans—at the plant instead of keeping them at a constant speed, while adjusting dampers to vary the air (gas) flow.”

You Don’t Have to Go Alone

It’s no secret that many producers have made the conversion to a VFD on the exhaust fan. Plenty of OEMs offer VFDs on specific components when you buy a brand new plant. Exhaust fan control is fairly easy to incorporate into existing controls, as we’ve seen in the article above. Richard reminds readers that the other components are equally important.

“Burner blower fans can have their dampers disabled and benefit from the lower rpms the fan and drive system will be reduced to,” he said. “Reaching out to your electrical contractor, the person who tunes your burner, and the utility company is the place to start. If you have trouble pulling all these people together so everyone is on the same page, industry consultants are waiting by the phone.”

Exhaust fan VFDs are considered “easy” to incorporate and control now. Not so easy is controlling the burner fan VFD. Even as a consultant, Richard said he wouldn’t go it alone without the burner manufacturer being on board. He’s working another application with a Denver producer and Xcel Energy on a Gencor plant. Once the controls are in place, the burner blower speeds must match up with complete combustion as being measured with a combustion analyzer. “It’s all a matter of the right air-to-fuel ratio, and most electrical contractors don’t know where to start with that,” Richard said.

The consultants participate in the process at any number of levels, of course. Ahern described his involvement with clients: “Building the cabinet for VFD off-site and installing on-site usually takes two to three days with training. Lead times in our shop of doing VFDs with ordering components and assembly, one and a half weeks (sooner if expedited).”

Bringing all the team members together is worth the effort when the utility savings end up in the tens of thousands of dollars year-over-year. Add in rebates from energy companies, reduced wear-and-tear on equipment components, and happier neighbors with the reduced noise, and you have a winning combination to consider during the down season.

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Sizing Transformers for VFD Applications

Determining the size of a transformer for VFD loads is a straightforward process. To start, individuals must select an appropriate kVA rating for the isolation transformer, sized to handle the VFD’s continuous input amps. At this stage, it is important to consider that you do not have to oversize the unit for the VFD, since the transformer already has current overload protection.

Keeping that in mind, the next step is to convert the continuous input amp requirements of the VFD to kVA:

kVA = 1.732 x Line-to-Line Voltage x VFD Input Amps / 1,000

*Note: The 1.732 multiplier, which is the square root of 3 [√3], is for three-phase kVA.

Example: Line-to-line voltage = 240V AC, VFD input amps = 240A
kVA = 1.732 x 240V x 240A / 1,000
kVA = 99,763.2 / 1,000
kVA = 99.8 or 100 kVA (rounded up)
For single-phase kVA computations, remove the 1.732 multiplier:
kVA = Line-to-Line Voltage x VFD Input Amps / 1,000
kVA = 240V x 240A / 1,000
kVA = 57,600 / 1,000
kVA = 57.6 or 75 kVA (rounded up)

After determining the appropriate kVA rating of the transformer, individuals should also double-check the primary and secondary voltages for compatibility. The primary voltage of the unit must match the input/supply line; while the secondary voltage must match the input requirements of the connected VFD.

Mitigating Harmonics

VFDs can contribute to harmonic distortion, which is defined as electrical pollution due to deviation from a pure sinusoidal wave form to a non-sinusoidal wave form. Harmonic currents consist of greater (odd or even) multiples of the fundamental, sinusoidal wave form. For example, the 5th harmonic of a 60 Hz network is 300 Hz. Even harmonic currents contribute to less machine performance issues, than odd harmonic currents.
The presence of harmonic currents is a concern for VFDs, because the units generate non-linear loads. When visually presented, the waveform of a non-linear load appears random and staggered. It is important to highlight that harmonics are only an issue when the amount of current exceeds certain limits or thresholds.
Since harmonic currents are considered to be unusable, the energy is converted to heat. This is one of the main negative effects of harmonic currents. Other crippling effects associated with harmonics include the following:

• Vibration
• Noise (humming)
• Malfunction due to distorted voltage
• Premature equipment failure
• Flickering (for lights and displays)
• Capacitator performance
• Generation of false readings (for meters)

Isolation transformers can be used to decrease the creation of harmonic currents, by rectifying input current from the supply line to the VFD. For protection against unwanted electrical disturbances, the unit’s primary and secondary windings may be covered with an electrostatic shield.

Alternatively, it would also be possible to use multi-pulse drives (12, 18 or 24-pulse), trap filters and broadband filters to decrease harmonic distortion, when using VFDs. ANSI/IEEE Standard 519 and IEEE C57.110 provide recommendations for mitigating harmonic currents in industrial facilities.

Source: Larson Electronics