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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 […]

In the last article, we spoke about the requirements-based design and engineering that go into the fabrication of a high-quality, custom-built control panel. Much of this necessarily happens “behind the scenes”, well before a panel is assembled, but when you’re in the process of choosing a panel fabricator all you may have to go on is the “look and feel” of examples being advertised. So, whether you’re online searching panel builders’ sites, or in the field reviewing build quality, what should you look for in a quality custom-built control panel? Let’s examine the most common things to watch for.

• Workmanship: They say “neatness counts”, and this is doubly true for control panel fabrication. A clean, well-managed, orderly layout makes maintenance and trouble-shooting far easier, saving time and money. Keys to look for:
  • Components properly anchored and laid out in a logical fashion
  • Wires neatly bundled and routed with minimal interference, and meeting minimum bending radius requirements
  • Clearances maintained for ease of access and adequate ventilation
  • Consistent wire coloring scheme
  • Covers in place and fastened properly

What Makes a High-Quality Control Panel?

A high quality, well-built control panel is perfectly adapted to its function and, when well-maintained, will provide many years of good service. Just as there are numerous applications with diverse requirements, so there are all types and configurations of panels out there. See Fig. 1 for an example.

So how do you recognize a quality panel when you see it, and more importantly, how can you be sure that a panel builder will provide a quality product before it’s built? Let’s take a closer look at the elements that go into quality control panel design and fabrication.

Understanding the Application and User’s Requirements

Quality is not just a reflection of the materials used, but also of the ability of a panel to satisfy its intended purpose. After all, a panel is only as good as its ability to work properly. And that means clearly defining what the panel must […]

Due to often high starting torque, heavy loads, and changing viscosities, mixing applications can be challenging. But with the right torque vector variable frequency drive (VFD) consistent batching, equipment life, and energy efficiency go hand-in hand. Let’s take a closer look at the features and capabilities that make these drives a solid choice for process mixing applications.

Starting torque: A number of factors can influence the amount of starting torque needed to get agitators in motion. The sheer volume of materials to be mixed, and their density, can place a lot of stress on vessel mixing components. Tank design also influences this – factors such as tank geometry, agitator design, baffle design and geometry, hub arrangement, motor power output, and tank temperature control all can affect the torque needed to begin a mixing cycle. And of course, batch variability also is a critical factor.

Modern vector control drives are capable of providing full torque at zero speed, which not only ensures sufficient starting power but also allows for efficient power consumption during starting. Where high torque isn’t needed, the soft-starting capability of these drives can save significant wear and tear on mixing components and gear boxes.

Mixing dynamics: […]

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Last issue, we discussed several factors which can influence the decision regarding the type of VSD – stand-alone module or packaged drive – to utilize for your application. These factors included the configuration of the drive you are seeking to replace (if this is not a new application); available space; ambient conditions in/around the installation site; and peripheral equipment to be connected to the drive.

Other factors can play important roles as well. For instance, safety and code compliance can heavily impact the VSD configuration to choose. For instance, packaged drives which are UL labeled provide assurances that the assembly is properly built, tested, and suitable for its rated output. While not all installations require UL labeling, the assurances it provides can more than offset the nominal cost for obtaining it. And while many stand-alone drives are UL labeled, a packaged unit ensures that the upstream disconnecting means, overcurrent protection, and any associated peripherals are specified and installed in compliance with applicable codes – which can translate to less work by field engineering and installation personnel.

The installation of any VSD requires properly rated disconnecting means and over-current protection installed in conjunction with the drive. (A basic VFD power wiring installation is shown in Fig. 1 below.) […]

When selecting a new variable speed drive (VSD), there are a number of decisions to be made based on expected use, performance, and operating life. One such decision involves the choice of a stand-alone VSD module versus a packaged drive. What’s the difference? Well, a drive module is generally considered a self-contained converter-inverter combination (in AC applications) or a self-contained converter (in DC applications), containing internally the appropriate controls and programming to derive an adjustable output from the incoming supply. A packaged drive places this stand-alone module within a suitable enclosure and adds line-side disconnecting means and overcurrent protection, enclosure door-mounted control devices, output power and control terminations, and properly sized means for maintaining  specified operating temperature within the enclosure. There are other options which can be added as well. An example of a drive module is shown in Fig. 1, and a packaged drive example is shown in Fig. 2.

Fig. 1 – Drive Module (ACS880 series) Courtesy of ABB Inc.

There are a number of things to consider before deciding if a variable speed drive (VSD) is right for your application. These considerations can be roughly assigned to one of three groups: energy usage, process/application, and hardware.

Energy Usage:

 With the rising cost of electricity and increasing demands on aging power grids, there are large incentives to reduce power consumption wherever possible. According to US Department of Energy statistics, electric motors consume approximately 68% of industrial energy usage, and a significant majority of these could see energy reductions when controlled by VSDs. Couple this with the fact that nearly 96% of the total lifecycle cost of motor ownership is electricity consumed, and you can see that there can be real cost benefits to VSD use.

There are numerous sources online to assess VSD energy savings. For tools to estimate the energy savings your application might realize, go here or here.

Process/Application:

If the process or application you are considering uses a motor to drive a variable torque load (also referred to as a quadratic torque […]