Redundant VFDs and fan arrays in critical operations

Understand how to use variable frequency drives in fan arrays to enhance the airflow in mission critical facilities.


Learning objectives

  • Understand fan arrays.
  • Learn how VFDs can operate fan arrays.
  • Understand how to best maintain controlled airflow in a critical operation incorporating VFDs and fan arrays.

Figure 6: Shown here is a motor array in an air handler. This has become an increasingly popular sight in the world of HVAC. What’s the best way to power a fan array in a critical operation? For those who may not know what a fan array is, it is exactly what it sounds like —a series of smaller fans designed to carry the load once handled by a single larger fan.

There are several advantages of using a fan array over a single fan. The first advantage is that a motor/fan failure will not shut down the system, which can be extremely important in critical operations. Second, smaller and lighter wheels and motors are easier to replace. Using six 15 hp motors in lieu of one 100 hp motor means when a motor fails, you’re only replacing a 15 hp motor, which is cheaper, and physically easier to replace than one 100 hp motor. More of an “up front” advantage is that shorter fan section lengths can be achieved when using a fan array versus one large fan. Also, a greater operating spectrum can be achieved, which will be particularly useful during periods of low flow and partial loading.

So, what is the best way to power a fan array? Considering we’re dealing with a critical operation that likely requires a speed range a bit more adjustable than an on/off state of 60 Hz/0 Hz, this discussion will focus on methods involving variable frequency drive (VFD) power. While “VFD power” may sound like enough to be a final answer, it isn’t. There are still several factors to consider.

With that in mind, the first decision to be made is typically going to be between powering each motor with its own VFD, or using one VFD to power all the motors in the array. While powering each motor with its own VFD could be seen as ideal in a perfect world, it may not be the most practical choice here on Earth. Adding the cost of a VFD to each motor in a fan array could quickly prove to be cost-prohibitive, immediately negating the “cheaper and easier to replace” advantages previously discussed. Also, it would quickly eat up panel real estate, which is usually already at a premium, as well as increase the number of conduit runs, as each VFD output must be separate. This fact makes it even more comforting to know that one VFD can power multiple motors. However, that may make some people nervous about having all their eggs in one VFD basket—don’t worry, we’ll come back to that.

Figure 1: This is a common method for powering a fan array with a VFD. It incorporates fuses and overloads in order to provide individual overload and branch short circuit protection, but makes no provision for an individual motor to drop out of the circuit. All graphics courtesy: Yaskawa AmericaOnce multiple motors exist on the output of a VFD, overload protection needs to be addressed, because the built-in solid state motor overload protection of today’s VFDs becomes void. The VFD output sees one load, and there is no way the VFD can detect and individually protect multiple motors. Let’s explore a few ways this can be addressed. In the following examples a four-motor array will be used for illustration purposes, but the size of the array could be any number of motors.

The fist method, shown in Figure 1, fuses and overloads, meets the requirements of short circuit and overload protection, but does have its shortcomings.

The overloads can be tied together in series through the N/C contacts back to a digital fault input on the VFD so that when one trips, the circuit is broken and the VFD can fault. The obvious problem with this scenario is that if one motor becomes overloaded and the VFD faults, they all go down—thus defeating a major benefit of choosing to utilize an array in the first place.

Figure 2: Shown here is a modification of the first method, with the addition of motor contactors. The provision of contactors allows for an individual motor to drop out of the circuit, while allowing the others to continue to operate.Another method, incorporating fuses, overloads, and contactors, shown in Figure 2, takes care of the problem of the system going down. By tying each overload to a contactor, an overloaded motor will result in the removal of only that individual motor from the circuit.

While functional, the addition of contactors can expand the amount of required panel space, not to mention the added component and manufacturing labor costs.

Figure 3: This alternative method for motor protection of fan arrays incorporates manual motor protectors that combine the benefits of an overload relay and a contactor all in one compact package.A third method, shown in Figure 3, manual motor protectors (MMPs), also has the built-in ability to remove the overloaded motor from the circuit, allowing the remaining motors to continue operation unimpeded.
Many consider this to be the ideal method when factoring cost, panel space, and overall component count. The advantage of MMPs is the combination of magnetic short-circuit trip protection and adjustable motor overload protection that they provide per NFPA 70: National Electrical Code (NEC) 430.32. This makes them most beneficial for multiple motor operation on the output of a VFD; however, some of that advantage is lost when the VFD package incorporates an across-the-line bypass.

Figure 4: When the array is powered by a variable frequency drive/bypass package, fuses are required to maintain individual branch short circuit protection.A downside is that an MMP cannot provide motor branch circuit short-circuit and ground fault protection. While this isn’t a problem when it is being utilized as a downstream device, it does pose a problem in an across-the-line bypass situation. In that event, shown in Figure 4, fuses are still required to meet UL.

Figure 5: A redundant variable frequency drive (VFD) package where the bypass section is another VFD is an ideal application for fan arrays using manual motor protectors. It’s a winning combination of VFD redundancy and fan motor redundancy.Many critical operations today involving fan arrays use redundant VFD packages, thus replacing the across-the-line option in a typical VFD bypass package with another VFD, illustrated in Figure 5.
With redundant VFD packages, we have the ideal scenario for employing MMPs for overload protection in our fan array application. With no across-the-line bypass option, the need for fuses is eliminated. A redundant VFD cabinet with built-in motor protection for fan arrays can already be pretty crowded with three fuses for each motor taking residence in the panel, and can mean the difference in whether an enclosure size increase is required.

If you’ve got a critical operation, and have chosen to go the route of fan array, the rest of the pieces fall into place. A redundant VFD package incorporating MMP motor protection simply makes sense. You’ll be making a wise choice that saves up-front costs, panel space, and maintenance costs and, most importantly, provides rock solid reliability for many years (possibly decades) to come.

Shane King is an HVAC and building automation application engineer at Yaskawa America Inc. He is a 1999 graduate of Lake Superior State University (BSEE) and has 15 years of experience in the drives industry.

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