Variable Frequency Drives (VFDs)

Brendan Bunker Author

08/31/2022 Added

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Top Recommendations Series For Food Processors

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[00:00:01.465]The implementation
[00:00:02.298]of variable frequency drives or VFDs
[00:00:05.010]and motor-driven processes is a popular recommendation.
[00:00:08.550]However, whether this recommendation
[00:00:10.320]is reasonable is not always clear cut.
[00:00:13.470]In this video, I will discuss what a VFD is,
[00:00:16.410]what situations justify the use of VFDs,
[00:00:19.620]as well as some alternatives that may be considered
[00:00:21.690]when a VFD is not the ideal choice.
[00:00:24.000]A VFD is a device used to control
[00:00:26.310]the rotational speed of motor-driven equipment.
[00:00:29.790]VFDs are particularly advantageous
[00:00:32.310]when a motor regularly runs at less
[00:00:34.200]than its full operating load.
[00:00:36.570]The added control that a VFD provides
[00:00:38.970]allows us extra energy to be conserved,
[00:00:41.790]which translates to cost reductions.
[00:00:44.460]These energy savings depend heavily on application,
[00:00:47.160]but sometimes there can be other cost driven incentives.
[00:00:50.580]For example, many utilities offer rebates
[00:00:53.700]between 16 and $150 per horsepower
[00:00:57.360]for the installation of VFDs.
[00:00:59.670]Facilities often see returns on this investment
[00:01:02.370]in three to five years on a case by case basis.
[00:01:05.730]These incentives typically apply
[00:01:07.320]to VFDs added to existing equipment,
[00:01:10.050]as is most common for VFD implementation.
[00:01:12.990]VFDs can be used with motors
[00:01:14.910]and many different applications,
[00:01:16.590]but the choice to use them is strongly reliant
[00:01:18.900]on the type of load that the motor is driving.
[00:01:21.630]A motor with a variable torque load for instance
[00:01:23.918]is a prime candidate for a VFD,
[00:01:26.070]because the motor's torque decreases
[00:01:28.230]with the square speed.
[00:01:29.850]Fans, blowers and centrifugal pumps
[00:01:32.100]are examples of variable torque loads.
[00:01:34.590]In one case, it was recommended
[00:01:36.480]that an ethanol plant install VFDs
[00:01:39.060]on T100 horsepower pumps as an alternative
[00:01:42.210]to throttling the pumps with control valves.
[00:01:44.819]This change would save $13,300 in annual energy costs
[00:01:49.350]with a payback period of approximately 3.5 years.
[00:01:52.920]Conversely, VFDs may also be recommended
[00:01:55.950]for motors with constant torque loads.
[00:01:58.470]As opposed to variable torque loads,
[00:02:00.570]the torque of these motors
[00:02:01.800]is not proportional to speed.
[00:02:03.900]Belts and conveyors are two examples
[00:02:06.060]of these kinds of loads.
[00:02:07.860]In another case, the implementation of VFDs
[00:02:10.200]was recommended in a grain drying facility,
[00:02:12.870]because the powering of two horizontal conveyors
[00:02:15.180]by 20 horsepower motors was found to be more effective
[00:02:18.540]when running at 80% power.
[00:02:20.580]The proposed change would save the facility $1,600
[00:02:24.360]on energy annually,
[00:02:25.590]with a payback period of approximately 2.9 years.
[00:02:36.150]While these benefits can be attractive,
[00:02:38.340]the implementation of VFDs is not justifiable on every case.
[00:02:42.840]There are many factors that influence
[00:02:44.430]whether using a VFD is the appropriate choice.
[00:02:47.730]For example, if a luster degree of freedom is required
[00:02:50.670]in terms of process control,
[00:02:52.680]other alternatives may be more effective.
[00:02:55.410]Using multiple pieces of equipment
[00:02:57.120]in series may act as an inexpensive alternative
[00:02:59.910]to variable frequency drives.
[00:03:02.220]Using a series of blowers to aerate
[00:03:04.080]a stream of waste water provides the freedom
[00:03:06.450]to turn on the appropriate number of motors
[00:03:08.580]and adjust the air discharge as necessary.
[00:03:13.050]It may also be possible to make mechanical adjustments
[00:03:15.930]to equipment to change their operating parameters.
[00:03:19.170]Changing a transmission ratio or trimming an impeller
[00:03:22.290]could be a more cost effective option,
[00:03:24.660]depending on the type of equipment.
[00:03:26.940]In general, VFDs are not suited for low speed
[00:03:30.120]or high load operations.
[00:03:32.460]Excessively lowering a motor speed
[00:03:34.560]can cause it to overheat
[00:03:35.940]due to decreased cooling within the motor.
[00:03:39.180]This graph illustrates the limitations
[00:03:41.130]of operating a VFD controlled motor.
[00:03:43.980]Instead of using a VFD for a motor running at lower speeds,
[00:03:47.880]it may make more sense to downsize.
[00:03:50.730]Furthermore, it is important to consider
[00:03:52.680]how reducing a motor speed may impact its performance,
[00:03:56.310]as pumping speed may impact other process parameters,
[00:03:59.490]including fluid head.
[00:04:02.820]An alternative technology to VFDs worth considering
[00:04:06.180]is an Eddy current drive.
[00:04:08.340]They provide similar control of VFDs,
[00:04:10.560]but can offer a longer operating life
[00:04:12.600]and lower maintenance costs.
[00:04:14.670]The difference is that while a VFD
[00:04:16.350]uses AC to DC voltage conversion,
[00:04:19.320]high frequency sampling
[00:04:21.060]in a powerful inverter to alter the speed of an AC motor,
[00:04:24.900]an Eddy current drive consists
[00:04:26.430]of a constant speed AC induction motor
[00:04:29.490]and a magnetic clutch speed driven
[00:04:31.380]by a small analog or digital controller.
[00:04:34.830]The clutch uses an Eddy current coupling
[00:04:37.260]to vary the speed of an attached output shaft.
[00:04:40.350]These kinds of drives are used primarily
[00:04:42.630]in constant torque applications.
[00:04:46.080]Finally, newer, intelligent motors
[00:04:48.390]with proprietary operating systems
[00:04:50.520]are also becoming popular alternatives to VFDs.
[00:04:54.420]They offer remarkable efficiencies
[00:04:56.340]and dramatically reduce energy usage.
[00:04:59.040]The careful inspection of the costs
[00:05:01.110]and benefits of using these technologies
[00:05:03.330]should be conducted.
[00:05:04.950]A food processing plant,
[00:05:06.450]when deciding between implementing third party motors
[00:05:09.180]and continuing to use VFDs,
[00:05:11.520]was advised not to install new motors
[00:05:13.830]because of the large capital cost.
[00:05:16.140]The reduction in energy usage
[00:05:17.760]did not outweigh the cost of implementation in this case.
[00:05:21.360]In general, it is crucial to understand
[00:05:23.580]how a motor is being used to determine whether a VFD
[00:05:27.000]or any of the other discussed alternatives are feasible.
[00:05:30.390]Obtaining data that describes the duty cycle
[00:05:32.790]of a given piece of equipment can help provide clarity.
[00:05:36.390]Every application is different,
[00:05:38.100]so thorough consideration of every option
[00:05:40.230]is necessary when making a recommendation.

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Variable Frequency Drives (VFDs)

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