Motors & Drives: Tips and tools for efficient motor management, Part 4
Michael Lyda, motor and drive engineer with Advanced Energy Corp., explains tips and tools for efficient motor management in this transcript from a December 2020 webcast.
Michael Lyda, motor and drive engineer with Advanced Energy Corp., explains tips and tools for efficient motor management in this transcript from a December 2020. Part four focuses on variable frequency drives (VFDs). This has been lightly edited for clarity.
Variable frequency drive tips and advice
So now we’re into our fourth and final topic, variable frequency drives. This content includes VFD basics, identifying potential applications to add VFDs, and some application issues involved with VFDs.
There’s quite a few names for VFDs. Here I’ve listed a few: variable speed drive, variable frequency drive, inverter, just drive.
A VFD is a device used to adjust the speed of an electric motor. Grid power can be connected to the input terminals of the VFD and then the output terminals of the VFD will be connected to the motor. The VFD can be used as a medium to control motor speed and torque by varying the frequency and voltage seen at the motor terminals.
Old speed control methods prior to the VFDs here listed include dual speed machines, gearboxes, pulleys, belts, soft starts, baffles.
What are the advantages of VFDs?
- Speed control. It’s easy to start and stop the motor. No inrush current at the motor terminals means less stress on the motor windings.
- Improved process control for precise applications, especially when you add encoder feedback in your VFD package.
- High quality VFDs will have a high-power factor at all motor loads since the capacitors and the VFD are offsetting the inductance of the motor.
Some disadvantages of using VFDs are additional capital investment. You may have to ask the boss for more money. My experience, it doesn’t always go well.
VFDs also add harmonic distortion to your system due to their capacitive nature. You may experience nuisance tripping in your application if the VFD isn’t programmed or sized correctly. And lastly, the VFD could actually reduce motor life.
A VFD takes in the three-phase AC power, converts it to DC, which then converts back to AC through the form of a pulse-width modulated or PWM signal. It has an AC to DC rectifier on the front end and then a DC-AC inverter on the back end with a DC bus in between the two. VFDs utilize IGBTs for the fast-switching capability.
When you walk into a facility, you may hear some high pitch audible noise if VFDs are being used. This is called the switching frequency or carrier frequency and is typically two to 20 kilohertz for standard commercially available VFDs in industrial applications. Not all VFDs are designed like this diagram, but just the simpler ones.
What applications work with VFDs? Centrifugal loads like pumps, fans and blowers, throttled flow processes and variable processes. New air compressors are popular VFD applications. Any boiler or chiller feed water pumps are good opportunities. And of course, fans all over the facility are good opportunities.
If you have higher operating hours, then you have more time to capture energy savings and offset the cost of the VFD itself. Target larger motors for quicker payback to offset the initial cost. Once you’ve identified potential motor applicants, dig a little deeper to determine the best candidates for a more in-depth analysis.
If you’ve done what we call a motor survey of your facility, you’ll capture a lot of this information. Look at the estimated, or better yet, exact run hours. We recommend looking at all centrifugal loads with operating hours over 6,000 hours annual usage.
Next we’re going to look at three different applications and compare the validity of adding a VFD. First we look at a poor VFD candidate, a very highly loaded application.
You may ask, why shouldn’t we add VFDs to highly loaded applications? The VFD itself does use some energy. If you put a VFD on a motor and the motor is running near full load, you’re adding unnecessary power to the circuit and actually raising your electricity usage. Adding a VFD doesn’t make sense here.
Second example, we see a moderately loaded application. Most of the operating time is spent around 60% to 70% load. This presents a good opportunity to slow the motor and capture some energy savings based on the affinity laws.
Finally, with our third example, we have a moderate to low load at application, even some loading below 50%, but still with high operating hours overall. This is an excellent candidate for adding a VFD and presents a great opportunity for significant energy savings.
A few last slides here about VFDs. There are some disadvantages. I would like to spend a few minutes going over the issues and what you can do to mitigate those. Some well-known application issues include the reflected wave, which we’ll get into more in a minute, and bearing effects, including fluting, grease blackening and potential increased motor failures.
How does this happen? Well, motor bearings typically present the path of least resistance for high voltages coming from the VFD that then get induced across the motor shaft. The voltage will reach a level to overcome the grease layer and the bearings then shaft current will discharge. This is called fluting.
We have some pictures in the next slide of the grease breakdown and bearing fluting. If we were doing an in-person training, we could have some hands-on examples to pass around the group.
Because of the pulse-width modulated signal the VFD is sending to the motor, the motor terminals may actually see a peak voltage of three to four times the rated voltage of the machine. Higher quality VFDs find ways to mitigate this, but not all of them. The issue becomes especially bad when you have long leads between the VFD and the motor, which is very common in industrial facilities.
You may have a control room on one end of the plant where VFDs are housed that are feeding motors all across the facility. This is not good news for the motor windings.
How can we mitigate some of these issues? The bearing issues can be solved by using insulated bearings. You may also want to look at purchasing shaft grounding systems or grounding brushes. These are items that you can get from your motor repair vendor.
When it comes to insulated bearings, this is something to request from your distributor when you purchase a motor or when you get it repaired.
How do we mitigate the harmonics and reflected wave issues? One method is straightforward, use shorter cables between the VFD and motor. But this isn’t always possible. You can also purchase optional line reactors or load reactors with your VFD.
There are also filters available that basically add inductance to the system. Adding filters and reactors can bring harmonic distortion down to more reasonable levels, but you need to weigh the pros and cons.
In summary, VFDs can be beneficial in speed control, process control, eliminating high inrush currents to your motor, keeping power factor high at all loads and adding energy savings potential.
However, some disadvantages include the additional capital investment, the added harmonics to your system, possible nuisance tripping due to incorrect programming and even potential reduction of motor life. VFDs aren’t right for every application. Screen your applications well before making the move to add a VFD. All right. So that concludes our content.
Today we covered the motor basics, motor applications, motor management, and variable frequency drives. I have listed a few references here. If you’re working around motor testing, IEEE standard 112 is very good, mainly for three-phase machines, IEEE standard 114 for single-phase machines. General motor knowledge and specifications, you can use NEMA MG1. I call it the motor bible.
For motor repairs, you see the last standard listed, EASA AR100. And that one actually has a new revision of this year.