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Recently, Variable Frequency Drive (VFD) manufacturers have seen a trend to move the VFD specifications from the mechanical portion of the specification (section 15XXX) to the electrical portion (section 16XXX). While this trend may stem from concerns about harmonics, it may also be influenced by certain VFD manufacturers who have a vested interest in having the drives specified in section 16XXX. Another reason for this trend may be that mechanical department personnel at consulting firms are often not comfortable with the harmonics issue. It is the writer's opinion that specifying VFDs in section 16XXX can cause unnecessary concerns. This article will document some of the more compelling reasons to place the drive specification in the mechanical or controls section of the specification. This article will also document some of the potential pitfalls possible when installing the VFDs in MCCs (motor control centers). Finally, recommendations for specifiers will be presented.
The motor control center (MCC) was first introduced at the turn of the century. MCCs allow the designing engineer to maximize the usage of valuable space by putting motor controls and branch circuit protection in a common enclosure. MCCs also allow the end user to consolidate wiring, simplify installations and keep the electrical controls out of the manufacturing environment. With the advent of solid state electronic controls, it seemed natural to incorporate these new controls into the MCCs. However, MCC's were not designed to house electronics. In the case of VFDs and other heat producing, solid-state devices, there are several potential problems that the designer should understand. These pitfalls will be discussed in the following paragraphs.
Why not MCCs?
Today's PWM VFDs all use Insulated Gate Bipolar Transistors (IGBTs) as output switching devices. IGBTs switch 10 to 100 times faster than the previous switches used as VFD output devices. This fast switch "turn-on" time can set up a phenomenon known as standing waves or voltage reflection. Voltage reflection can cause motor end-turn failure in a short period of time. Voltage reflection is a function of the lead length of the power cable between the drive and motor. Voltage reflection can theoretically occur with motor lead lengths as short as 25 feet. Voltage reflection is often predicted by the following formula:
L critical (feet) = V cable X tr (ms) Formula #1
Where L critical = Critical cable length in feet
V cable = Pulse speed from the drive to the motor in feet per microsecond (ms)
tr = Rise time of the output pulses from the VFD under consideration in ms
Voltage reflection may occur when the length of the motor cable is greater than or equal to the critical length. Vcable (sometimes referred to as propagation factor) is the speed that the pulse travels from the drive to the motor in feet per microsecond. The value of Vcable depends on the type of conduit or cable tray and other details of the cable installation. Vcable is often estimated as 500 feet per microsecond (m sec).
A recent independent study of 17 different drive manufacturers suggests that tr may be as short as 0.117 m seconds. Solving for the critical cable length by substituting this value into formula #1, and using the 500 V per m second cable default value, equates to 29.25 feet as the critical distance for voltage reflection. Even when mounting the VFDs on the wall, next to the driven equipment, it is not hard to imagine motor cable lead lengths of over 29 feet. Putting the VFDs in the MCCs will almost assure motor cable lengths of over 29 feet in most HVAC installations.
There are several solutions to the problem of long motor cable lead lengths. The most often specified solution is to supply output dv/ dt filters with the VFD. The dv/ dt filters are fairly large and are not easily installed in MCC enclosures. Definite-purpose Inverter-fed Motors can also be utilized when long motor cable lead lengths are unavoidable. If 230-volt power is available on the job site, a 230-volt VFD and 230/ 460-volt motor can be used. When fed with 230 volt power, the 230/ 460 motor will not be susceptible to voltage reflection failure. All of the above listed solutions will cost the owner extra money and can increase the system complexity. These "solutions" will not be required if the VFDs are located close to the driven motor.
To meet local codes, a motor disconnect switch is typically required within sight or within 50 feet of the motor. Most VFD manufacturers can competitively supply a VFD and disconnect switch in a common enclosure. This combination enclosure, positioned properly, will meet the motor disconnect code requirements. If the drives are mounted in the MCCs, the local motor disconnect will probably still be required.
VFDs are heat-producing devices and, as such, need proper cooling and airflow. Most VFDs were not originally designed to be mounted in MCCs. Putting a VFD in an MCC enclosure requires a special MCC enclosure with cooling provisions. Although this can be adequately accomplished by a few companies nationwide, any savings that were expected from reduced wiring costs are quickly eaten up by the special engineering charges and cooling requirements. In the writer's experience, the installed cost of drives in MCCs exceeds the installed costs of wall-mounted drives.
All solid-state switching devices generate losses. There simply is not a perfect semiconductor power switch. With the present generation of power output devices, the system efficiencies are typically in the range of 96 to 98 percent. This means that the heat generated must be dissipated within the MCC section. VFD heat loss generated may be calculated using the following formula:
Heat loss in Watts = HP X 746 Watts per HP X (1 - VFD Efficiency) / Motor Efficiency Formula #2
BTU/Hr load = Heat loss in Watts X 3.41 Formula #3
For example, a 15 HP drive with an efficiency of 96% connected to a 90% efficient motor, will yield the following results:
Heat loss = 15 HP X 746 Watts
per HP X (1 - .96) / .9
= 11,190 Watts X (.04) / .9
= 497.3 Watts lost
BTU Load = 497.3 Watts X 3.41
= 1,696 BTU's/Hr
Since VFD losses are approximately linearly proportional to horsepower, losses can be estimated at 33.3 watts or 113 BTU's per hour per horsepower. For example, the losses on a 30 HP drive would be approximately 1,000 Watts. This may not seem significant, but this is the same as having a 1,000 Watt heater in the enclosure. The challenge, then, is to get the heat out of the MCC bucket without compromising the VFD heatsink design or restricting the drive's cooling air.
One MCC / VFD manufacturer offers front ventilation as a viable solution to prevent heat stacking effect from multiple VFDs in a common vertical enclosure of an MCC. This solution is easy to do if one has a NEMA 1 requirement. Front-mounted heat sinks become more problematic in a NEMA 12 design.
Drive heat sinks are used for cooling the power devices, and the output power devices are connected directly to the load. If the heat sinks are on the door to get the heat out to the front of the enclosure, load conductors must be routed to the door. Prudent control panel design limits the number of conductors going across a hinge, the unwritten rule is to NEVER run power conductors across a hinge. Flipping the drive over to shove just the fins of the heat sink through a gasketed opening in the door is another solution that has been used, but the drive is upside-down when it comes time to service / test.
Many specifications also call for the VFD manufacturer to supply input line reactors and/ or output filters. These are also heat-producing devices and may require large mounting areas. Most VFD manufacturers have pre-engineered designs for placing filters inside of the VFD enclosure. Few, if any, can easily mount and wire these devices in the MCCs. Finally, 12-pulse drives and other forms of harmonic mitigation do not fit well into MCC enclosures. If harmonic concerns are discovered after the installation is completed, it is much easier to retrofit harmonic mitigation devices to free-standing or wall-mounted VFDs than it is to mount and connect these devices to drives inside of MCC enclosures.
In many instances, the MCCs are located in less than ideal environments. Most commercial and industrial building owners have no desire to provide conditioned air for the MCC equipment rooms. Since personnel are not generally expected to work in these rooms, very little attention is paid to the environmental concerns of the MCC area. It is not uncommon for high ambient temperatures to be present in the MCC areas. VFDs only exacerbate this problem. On warm days, the maximum ambient temperature of the VFDs can easily be exceeded. While this excessive heat may not present a problem for the electro-mechanical equipment in the MCCs, it is a potentially large problem for the VFDs in an excessive ambient environment.
Project coordination is more complicated when the VFDs are specified in section 16XXX. Changes in project scope often do not get transmitted effectively from the mechanical contractor to the electrical contractor. For example, if an air handler unit motor changes from 15 to 20 HP during the course of a project submittal, it is no problem to change from a 15 to a 20 HP starter. Both starters typically fit into the same size MCC bucket. However, changing from a 15 to a 20 HP drive will likely change the physical size of the drive. This may require a bigger MCC bucket and will assuredly require more CFM of cooling air be brought through the drive enclosed in the MCC.
Changes such as these are obviously very expensive to accomplish in the field. If the MCCs are not yet released, there will still be a time delay while the MCC line-up is re-engineered. In addition to the normal MCC lead time, VFD lead time could also present a problem in later stages of the project.
Motor control centers have "wire-ways" in which all the control and power wires are routed. This can present RFI and EMI concerns. Most drive manufacturers insist that power wiring (both input & output) and control wiring be run in separate, metal conduits. Therefore, installation of the VFDs in MCCs may void the VFD warranty.
The start/ stop and speed control of the VFDs typically comes from the temperature control equipment. The temperature control equipment is not mounted in the MCCs. Mounting the VFDs in the MCC will further complicate the control wire routing and add cost to the installation. Great strides have been made recently in serial communications capabilities between VFDs and temperature control equipment. Placing the VFDs in the MCCs cancels some of the wiring cost savings these new serial communications interfaces have pioneered.
How does one avoid running output conductors (motor leads) in a common wire-way with the control leads? One benefit promoted by MCC manufacturers for years has been the vertical wire-way to the right of each bucket, as well as the top and bottom wireways along the entire MCC length. With electro-mechanical starters, all of the load conductors and control wires could be routed loosely in the wireways, facilitating the quick change of buckets. This is bad practice for VFD installations, however, and separate metallic conduits (the method preferred by almost all VFD manufacturers) are virtually impossible to install inside of MCCs. I would definitely not want my communications network wiring run in the same wire-way as output conductors from a PWM drive. Because of this mechanical limitation, the concept of "smart" MCCs with VFDs inside is unrealistic. In the writer's opinion, placing all this wiring together in MCC wireways is asking for trouble.
Serviceability of the drive may also become an issue. Placing the drive in an MCC enclosure will typically make the drive components harder to get to, and therefore harder to service or replace. Also, if a drive becomes outdated and the owner wishes to replace the unit with a newer generation of VFD, it is much easier to replace a wall-mounted unit than a unit buried in an MCC bucket. The end user also losses his flexibility in this situation; instead of buying a replacement VFD using owner important criterion such as ease of programmability, serial communications capability, or some other issue, the overriding criterion now is to find a VFD that will fit into the existing MCC enclosure.
Drive feature flexibility is lost because only a manufacturer of MCCs can go through the expense of a separate and additional UL845 listing for drives in MCCs. As it stands, UL 508C covers all stand-alone drives, but UL508C does not cover the MCC mounting issue. If a particular brand of drive works well in your application, you probably cannot get it mounted into an MCC and keep the UL Listing unless it happens to be made by the MCC manufacturer. There are currently only five (5) MCC manufacturers left with viable products in the U.S. who also make drives. Specifying drives in the MCC eliminates all other sources, which, at last count, amounts to over 50 brands in the US with UL508C listing. Is one of those five remaining brands the right one for you?
Many MCC manufacturers promote the "quick change-out" benefit of mounting drives in MCCs. However, most drives over 20 HP require more than the 7" - 9" of mounting depth typically available in MCCs with vertical bus. If the vertical bus is removed to accommodate deeper drives, the flexibility of moving plug-in units to create more space within the enclosure is lost. In other words, many higher HP VFDs in "MCC" enclosures are simply common AC Bus feed drives. They are not mounted in quick change-out buckets; they are really floor-standing enclosures with a common AC feed.
Finally, according to a recent market share study , three out of the top four VFD players in the HVAC market do not even manufacture MCCs. According to the same study, these four players account for approximately 80% of the VFDs sold into the HVAC marketplace. However, some of the major motor control companies are able to offer engineered specials for drives in MCCs. These motor control companies represent a small fraction of the VFDs installed in the HVAC market.
Some of these companies attempt to get their VFDs specified in the MCC sections to gain a competitive advantage at the bidding stage. By providing a packaged price for switch gear, MCCs, VFDs, and other control equipment, they can effectively stifle open competition. As contractors will attest, the VFD "break-out" price in these package situations often leads to pricing "games" from the package suppliers. None of the above issues helps with the owner's desire to get a competitively priced, reliable VFD installation. There are other pitfalls associated with placing drives in MCCs, but the issues just described are viewed as the most important issues.
OK, so drives do not belong in MCCs -- then
what is the problem with section 16XXX?
In addition to the coordination required between the contractors, further coordination difficulty can be anticipated in start-up coordination, control coordination, and lack of system understanding. The electrical contractor may have less understanding of how the fans and pumps are supposed to operate than the mechanical contractor. One of the main reasons that VFDs were put into the mechanical portion of the specification 20 years ago was for single source responsibility. Specifying drives in section 16XXX loses this advantage.
There is a trend among HVAC consultants to have the temperature control contractor supply and coordinate the VFDs. This makes good sense from several standpoints. First, the control contractor has electrical experience and understands RFI/ EMI issues. Most control contractors also have at least a casual familiarity with harmonics. Also, the control contractor has a good understanding of the sequence of operation of the mechanical system. For example, if a VFD vendor bids the project without regard to such items as Electro-Pneumatic (EP) relays or damper end switch contact receipts, the control contractor will probably catch the mistake before the units are ordered.
The control contractor also knows when the system is ready to be commissioned. Often the VFD certified start-up engineer is called by the mechanical contractor to start-up the VFDs, only to arrive at the job site and discover that the control wires have not yet been pulled or connected. Having the control contractor coordinate the VFD start-up insures that, when VFD start-up personnel arrive on the job site, the VFDs are ready to be commissioned. Finally, temperature control personnel are typically still calling on the owner after the first year. The mechanical contractor's incentive to keep the owner happy may expire with the job warranty. The control contractor has a vested interest in continuing to call upon, and service, the owner.
Because of the above concerns, the writer does not recommend specifying drives in MCCs or in section 16XXX. The VFDs belong in the mechanical or controls portion of the specification. It is recommended that the consulting engineer have their electrical department help their mechanical department write the specifications. If the consulting firm is concerned about harmonics or other electrical issues, they should also have their electrical department review the VFD submittals.
Carefully look at your application and the support available from your local vendors. If you know nothing about drives, hire an integrator who is not beholden to any manufacturer. Then decide what to buy based upon what works best and will be reliably serviced and supported in the long run. Where you put the box is a very minor detail.
Finally, the specifier may want to consider moving the VFDs to the controls portion of the specification (especially on projects with new building controls). The controls supplier will then be responsible for purchasing and coordinating the VFD systems. It is also recommended that the consultant firm get help with the VFD specifications from a local VFD representative who provides good support and has a proven track record.
About the author: Michael R. Olson is Manager of Engineered Drives at ABB, Inc., Automation Technology Products Division, Drives & Power Electronics. Mr. Olson has extensive experience in the HVAC, Water/Wastewater Treatment, and Chemical markets. He has been applying adjustable speed drives to these and other markets for over 20 years. Mr. Olson has been published numerous times in trade journal articles discussing energy savings and the proper application of adjustable speed drives. He has a General Engineering degree from the University of Illinois and a Masters of Science in Engineering Management degree from the Milwaukee School of Engineering.
 A. Mansoor, K. Phipps, and R. Ferro, “System Compatibility Research: Five Horsepower PWM Adjustable-Speed Drives,” Electric Power Research Institute - Power Electronics Applications Center, April 1996
 P. Benoit, D. Clayton, and A. Chatha, “AC Drive Outlook for North America - Market Analysis and Forecast Through 2002,” Automation Research Corporation, October 1997
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