Why Not MCC’s?
Today’s PWM vfd’s 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 ft. Voltage reflection is often predicted by the Formula 1.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 ft per microsecond (µsec); tr is the rise time of the output pulses from the vfd under consideration.
A recent independent study of 17 different drive manufacturers1 suggests that tr may be as short as 0.117 µ seconds. Solving for the critical cable length by substituting this value into Formula 1, and using the 500 V/µ-sec cable default value, equates to 29.25 ft as the critical distance for voltage reflection. Even when mounting the vfd’s on the wall next to the driven equipment, it is not hard to imagine motor cable lead lengths of over 29 ft. Putting the vfd’s in the MCCs will almost ensure motor cable lengths of over 29 ft 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-V power is available on the jobsite, a 230-V vfd and 230/460V motor can be used. When fed with 230-V 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 vfd’s are located close to the driven motor.
To meet local codes, a motor disconnect switch is typically required within sight or within 50 ft 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.
Thermal Considerations
Vfd’s are heat-producing devices and, as such, need proper cooling and airflow. Most vfd’s 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 may quickly be eaten up by the special engineering charges and cooling requirements.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%. This means that the heat generated must be dissipated within the MCC section. Vfd heat loss generated may be calculated using Formulas 2 and 3.
Since vfd losses are approximately linearly proportional to losses can be estimated at 33.3 W or 113 Btu/hp. For example, the losses on a 30-hp drive would be approximately 1,000 W. This may not seem significant, but this is the same as having a 1,000-W 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.
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 vfd’s than it is to mount and connect these devices to drives inside of MCC enclosures.
Environmental Concerns
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 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 and it is not uncommon for high ambient temperatures to be present there. Vfd’s only exacerbate this problem. On warm days, the maximum ambient temperature of the vfd’s 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 vfd’s in an excessive ambient environment.Project Coordination
Project coordination is more complicated when the vfd’s are specified in Division 16. Changes in project scope often do not get transmitted effectively from the mechanical contractor to the electrical contractor. For example, if an air-handling 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 lineup 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.
Other Concerns
Motor control centers have “wireways” in which all the control and power wires are routed. This can present RFI and EMI concerns. All drive manufacturers insist that power wiring (both input and output) and control wiring be run in separate, metal conduits. Installation of the vfd’s in MCCs may void the vfd warranty.The start/stop and speed control of the vfd’s typically comes from the temperature control equipment. The temperature control equipment is not mounted in the MCCs. Mounting the vfd’s 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 vfd’s and temperature control equipment. Placing the vfd’s in the MCCs cancels some of the wiring cost savings these new serial communications interfaces have pioneered. 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 enduser also loses 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.
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 vfd’s were put into the mechanical portion of the specification 20 years ago was for single-source responsibility. This advantage is lost by specifying drives in Division 16.
New Trends
There is a trend among hvac consultants to have the temperature control contractor supply and coordinate the vfd’s. 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 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 startup engineer is called by the mechanical contractor to start up the vfd’s, only to arrive at the jobsite and discover that the control wires have not yet been pulled or connected. Having the control contractor coordinate the vfd startup ensures that, when vfd startup personnel arrive on the jobsite, the vfd’s 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 on, and service, the owner.
Recommendations
Because of the above concerns, specifying drives in MCCs or in Division 16 is not recommended. The vfd’s 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, it should also have their electrical department review the vfd submittals.Finally, the specifier may want to consider moving the vfd’s 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. ES
