There is a good deal of knowledge, and more than a little misperception, about the use of variable speed drives (VSDs). The term “variable speed drive” encompasses both variable frequency drives (VFDs) used for controlling AC asynchronous and synchronous motors, and DC variable speed drives for controlling DC motors. In either case, determining if and when to use a drive will be much easier once the facts and myths about these controllers are clear. Let’s examine some of the more prevalent ones.
Myth:“My process flow application is already controlled by control valves, so I don’t need a VFD.”
Fact: In many cases, a VFD can more accurately control flow while reducing power consumed by the pump motor. If the flow control valves (FCVs) are being used mainly to throttle flow, you could instead use a VFD to reduce the speed of the motor, thus reducing the flow at the source. The reduced motor speed means less energy consumed by the system; per the Affinity Laws, power consumption (in watts) in a centrifugal pump application would be reduced by the cube of the reduction in speed (see formula below). You wouldn’t even need to remove the FCVs; simply open them fully and let the VFD handle the rest.
Affinity Law (Power)
If on the other hand your FCVs are being used to regulate flow based on process parameters, VFDs can use the same process inputs to adjust speed in a number of ways, including PID control. This can eliminate the need for additional external controllers such as PLCs.
Myth:“VFDs are too expensive, and I can’t justify the up-front cost.”
Fact: There are two facts that make this statement more myth than fact. First, consider total lifecycle costs of your system. In most industrial motor-driven systems, the initial capital outlay needed is only a small fraction of total operating costs over the life of the system – most estimates find it’s about 3%. That means you’ll spend about 97% of your money after the system is up and running. Add the costs of a VFD into the calculation, and you raise the initial outlay to perhaps 6 – 8%. (This estimate will vary somewhat based on the power rating and complexity of the VFD; nevertheless, it remains a small part of the total cost you’ll see.)
Second, consider maintenance costs over the life of the system. VFDs are capable of tightly controlling starting currents, acting as sophisticated “soft starters” to reduce electrical and mechanical stresses on the system. This means less wear and tear on mechanical components, which in turn means your Maintenance folks can focus more on preventive measures, and less on repairs.
Myth:“My application is simple. I don’t need a complicated VSD to operate.”
Fact: While it’s true that modern VSDs have a diverse array of functions and capabilities, it’s also true that most are equipped with setup wizards, “quick start” guides, and straightforward program presets which make it relatively easy to control a connected motor in a few simple steps. And every application capable of operating at varying rates – be it to control flow, pressure, or velocity; and simple or complex – will benefit in terms of efficiency and energy consumption from the use of a VSD.
Myth:“I have a single-phase power supply, so I have to use a single-phase motor.”
Fact: Even with your site supplied by a single-phase source, you can often benefit from the higher efficiency and starting torque provided by a 3-phase motor. Many VFDs are capable of taking a single-phase, 200-240Vac input and generating a 3-phase output at the same voltage. We see this opportunity often in rural areas. Bear in mind that in this scenario the VFD has to be de-rated to overcome the physics needed to create the additional phase. But even with de-rating the higher efficiency of 3-phase operation sometimes makes this a good decision, and you have the added benefit of being able to control speed as needed. Also, de-rating is made easier by the fact that many VFD manufacturers already de-rate their single-phase to 3-phase models, so selecting the proper drive is as simple as matching the load’s current and voltage requirements.
The above myths are just a few of what we often hear at Joliet Technologies. If you have some of your own to share, let us know in the Comments section. And please contact us if you are uncertain of the facts, or simply not sure where to begin. We’ll gladly assist. Visit us at JolietTech.com, email us (firstname.lastname@example.org), or call 866.492.9888. Thanks for reading!