Dual VFDs versus bypasses: engineers weigh in
Dual VFDs, bypasses, and spare VFDs: a look at costs, benefits, and design considerations.
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Variable frequency drives (VFDs) are electronic devices that start, stop, and control the speed of a motor driving an HVAC pump, fan, or compressor. They have a lot of benefits. For example, they save energy, they provide a soft start that prolongs equipment service life, and their onboard computers can provide operational data that can be used for diagnostics and optimizing operations.
VFDs, however, are electronic equipment that can fail for any number of reasons, such as power surges, board faults, and physical accidents in the facility. If a VFD fails, then what happens to the system?
Traditionally, engineers specify bypasses to provide power to the motor after a VFD fails or is turned off for service or replacement. However, costs and technologies over the years can make a dual-VFD installation cost competitive under certain conditions, or owners and engineers may want to consider having offline spare VFDs available.
The design option of doing without bypasses isn't new. It first showed up in this magazine in a 2002 MEP Roundtable article (“ Constant variables ”) about variable-flow pumping systems. In that article, Mike Kokayko, PE, CIPE, wrote, “Years ago, specifying a drive without a bypass was considered foolhardy. Due to the high reliability and relatively low drive costs, now we typically specify drive bypasses on larger drives or mission critical applications.”
In a March 2009 article, “ Motors and Drives: Repair or Replace ?” by Michael Olson, HVAC Applications Manager, ABB, New Berlin, Wis., wrote that ”...VFDs have become much more reliable over the past 20 years; a bypass on an HVAC application is an investment that needs to be analyzed closely.”
This article provides design considerations that engineers and owners can discuss during the design phase of a new system or a retrofit. Contributing to the article are members of the Consulting-Specifying Engineer's Editorial Advisory Board, who answered questions about their current practices and recommendations for VFD systems.
CSE: Do you prefer specifying a bypass or dual VFDs?
Gerald Versluys, PE, TLC Engineering, Jacksonville, Fla.: VFDs have come a long way in the past 20 years and are essential to satisfying the code for new energy-efficient buildings. They are cost-effective and reliable but they are still not indestructible. The prudent engineer will always have a “plan B” for critical loads served by a VFD. The two most commonly used methods are providing a bypass or providing a dual VFD solution.
Erin McConahey, PE, Arup, Los Angeles: Usually when I'm worried about redundancy and continuous operations, I would handle that with redundant half-sized equipment rather than just redundant VFDs. The conditions of continuous uptime usually require independent airstreams to allow normal maintenance activities such as filter changes. For circumstances when continuous uptime is not as critical, having spare VFDs is a good approach. If, in the specs, you limit the sizes of VFDs to 5 to 50 hp and 50 to 100 hp ranges (which you can easily get) and you require that everyone uses the same VFD manufacturers (for example, for HVAC equipment), then you can buy one spare for each main size to keep in storage and it can be easily retrofitted quickly into any piece of equipment that has an electronic failure.
CSE: Are bypasses required by for life-safety codes for applications such as smoke control?
Pat Banse, PE, Smith Seckman Reid, Houston: In stair pressurization scenarios or smoke control systems, for example, the code requirements state the components must be served by two sources of power and be suitably protected. Although I am not aware of any code requirements for bypasses on VFDs, from a good engineering practice and life safety perspective, the bypass makes sense and adds to the reliability.
McConahey : Occasionally a bypass of some sort is necessary in smoke control situations in which there is not a UUKL-listed building management system. (Note: UUKL is an Underwriters Laboratories (UL) listing for equipment that has been tested and listed for smoke control.) In these cases, it is necessary for the VFD to be bypassed so that the fire alarm system can directly control the fan. These do not need to be manual bypasses but can be done by purchasing special features within the VFD itself.
CSE: Where not required, where are bypasses highly recommended?
Banse : I recommend a bypass for large horsepower motors such as 100 hp and larger due to the cost of an added drive. Large motor drives take up a large amount of space and are difficult to move. The bypass is usually a separate freestanding cabinet, which allows a better working arrangement on the drive itself.
Versluys : Many critical systems cannot endure a shut down—even for short periods of time (e.g., hospital HVAC, central energy pumps). We commonly specify a bypass or two VFDs for these applications. If the VFD fails, then the system can still run on bypass until the VFD is repaired. This also permits the owner to properly maintain the VFD while the system is still operational.
McConahey : We have not found manual bypasses to be particularly useful as they usually come onboard with the VFD. We see that there is also some risk post-balancing, which may have intentionally set the VFDs at a lower speed to support balance based on whatever pressurization you have desired. All of a sudden, bypassing to a 60-Hz operation can cause critical portions of the building to get out of balance, especially if there is no correction on the exhaust or return/relief side of the systems.
CSE: Are VFDs becoming so reliable that bypasses are not really necessary?
Versluys : No, we occasionally have a power supply knocked out by utility power transients or other minor issues. Negative experiences with VFDs are normally blamed on voltage fluctuations or transients of some type. Please recall that most transients are generated within the building envelope and that proper design of the building distribution can minimize transient intrusion into the VFD feeders.
Banse : The drives are very reliable, but redundancy in critical situations such as healthcare and life safety components is still required. Also, some maintenance staff may not be comfortable working on the drives and may need to call in outside sources for service and repairs. Either way, the bypass allows the VFD to be worked on while remaining in service. Also, in certain horsepower ranges, when you have many motors of similar horsepower, then one drive of each hp rating makes sense and can end up being less expensive than the bypass on each drive.
CSE: How would a dual-VFD approach be configured and controlled?
Versluys : We interlock the controls via AUX (auxiliary) contacts in the manual transfer switch (MTS) to assure that the correct VFD is being energized by the control system (see Figure 1). The only issues we've encountered with this scenario is assuring that the interlock on the MTS auxiliary controls is properly handled by either the BAS or contacts within the VFD. The correct solution varies based upon the VFD manufacturer and the BAS available.
CSE: What guidance can you provide for specifying VFD/bypass configurations for HVAC applications?
Banse : You need to be aware of the critical nature of the device (pump, fan) the drive is controlling. How long can it be out of service? What is the experience level of my staff to change or service the drive? Keep the space clean and conditioned to within the environmental limits of the drive. Add shaft grounding kits to the motors served by the VFDs to prolong motor life.
Versluys : One of the most common pitfalls we encounter when specifying VFDs is the available withstand ratings (WSR) for the VFD. It is common to find VFD bypasses with maximum WSR ratings of 10,000 A or less (which wouldn't have been adequate for the one-line shown in Figure 1). Be confident that the VFD and the bypass contacts have the proper ratings for the installation.
Also, the right environmental conditions and regularly scheduled preventative maintenance are essential to the long-term reliability of these systems. A bypass or dual-VFD solution is commonly the only way to assure that the owner's staff has the ability to maintain the equipment. It is the reason that we developed the dual-VFD concept for our client; it gives the maintenance personnel a way to completely lock out the device before any maintenance work is performed on the VFD. Maintenance on energized equipment is being recognized as an issue by most facilities operators who are becoming familiar with the language in NFPA 70E and NEC Article 110.16 .
CSE: Are there special control conditions for air systems and bypasses for preventing duct over-pressurization or under-pressurization?
McConahey : The situation has a lot to do with how you have set up the controls to protect the system. For instance, let's say that you have balanced a system and are running at 55 Hz to achieve your flows and pressures. Usually, jumping up to a 60 Hz speed would increase your flow proportionally, which would then increase your system pressure by the square of the rpm ratios. If you had specified your ductwork pressure classes to be close to your anticipated system pressure, it is conceivable that the change in flow and pressure due to using a bypass could cause duct overpressure. If your whole VFD system, inclusive of manual bypasses, is not overridden by a hard-wired automatic shutdown by the high-pressure sensors in the ducts, then you could definitely blow seals on ductwork.
Usually high-pressure feedback is set up as an input to building management systems with a stop direction sent to the VFD. I believe that manual bypasses usually aren't set up to respond to these input signals, as it is assumed that the person putting the system into manual override knows what he is doing. Similarly, if a technician unfamiliar with the system was getting low flow—perhaps due to an unknown number of fire smoke dampers being closed or a duct obstruction—and he chose to try to resolve that by flipping to bypass to get more flow, this could also blow the ducts in positive pressure situations, or in suction conditions, completely collapse the ducts.
Banse : I agree with McConahey and her comments. We typically include a high-static pressure limit in the supply duct near the unit discharge and also on occasion a negative static sensor in the inlet duct to avoid damage to duct systems due to either over-pressure or negative pressure conditions. In critical applications wiring, the sensor to the fan-start side downstream of the hand-off-automatic switch, not just the VFD, can initiate the shut down and system alarm. The building staff has to understand the VFD operation as well as the controls in order to properly maintain and operate the system the VFD is controlling while minimizing potential damage.
CSE: Are redundant VFDs a cost-effective approach for backing up a VFD instead of using a bypass?
Versluys : Yes, and this has become our new standard for several of our clients. As VFDs have become more common in the marketplace, the cost associated with them has dropped dramatically. We've also have seen that a client can purchase two normal VFDs for the same cost as a VFD with a bypass on it. Our clients like the modularity of having two VFDs on critical systems because they can select VFD A or B through the building's control system.
Table 1 is an example of pricing I've received from a local rep.
Keep in mind that when VFDs are provided as part of a package, such as in an air handler, this can drive down pricing on bid day. And quantity helps purchasing power, too. My client's last order was for 10 units and 20 VFDs. As one supplier put it, “I guess when people say that two VFDs are about the same as a VFD with a bypass, that is either partially true; or it depends on what you consider to be about the same. In the 75-hp units, you could say that it is only a 15% premium, but that's still $900.” Table 2 shows pricing from a second contact, which shows greater divergence at the higher horsepower sizes.
Banse : I asked one of my local reps to give pricing on both approaches. The results are in Table 3.
Ken Lovorn, PE, Lovorn Engineering, Pittsburgh: The approach with two VFDs costs more than one VFD with a bypass. For example, checking with one of my contacts, the budget pricing for a single 100-hp VFD would run about $8,000. Add the bypass and the cost goes up to around $10,000. To go with dual VFDs with a manual transfer between the two, you would have two times $8,000 ($16,000), plus the cost of the MTS, which would be on the order of $1,500.
Table 1: Price comparison of VFDs and VFDs with bypasses for a range of sizes
Equipment cost two VFD only
Equipment cost VFD with bypass
An MTS would cost $500 to $1,000 for the size ranges indicated. VFD prices assume line reactors on each, 65K withstand, and UL 508C-listed. Prices include freight costs, startup, two years' parts, and labor warranty.
Table 2: Price comparison of VFDs and VFDs with bypasses for a range of sizes
Equipment cost two VFDs
Equipment cost VFD with bypass
An MTS would cost $500 to $1,000 for the size ranges indicated. VFD prices assume line reactors on each, 65K withstand, and UL 508C-listed.
Table 3: Price comparison of VFDs and VFDs with bypasses for a range of sizes
Equipment cost one VFD only
Equipment cost VFD with bypass
Installation 1: VFD-only installation has one 460 V, NEMA-1 indoor enclosure VFD, which has an LCD keypad, 5% reactor, RFI/EMI filter, electronic overload protection, and network compatibility with BACnet, Siemens, Johnson Controls, and MODBUS.