Motor Control Centers a Must
December’s C-SE broaches the subject of motor control centers: the latest innovations, most current code issues and the most common mistakes engineers make in specifying MCCs.
Editor’s note: December’s issue of Consulting-Specifying Engineer broaches the subject of motor control centers, specifically the latest innovations involving the technology, the most current code issues affecting design and installation, and the most common mistakes engineers make in specifying MCCs. In the meantime, we present additional discussion of the technology, in this case, key electrical protection issues. Participating in the discussion are Syed M. Peeran, Ph.D., P.E., Chief Electrical Engineer, WB Engineering & Consulting, PLLC, Woburn, Mass.; David Ray, MCC Product Line Manager, Square D/Schneider Electric, Seneca, S.C.; and Dave Blair, Marketing Manager, MCC Business, Rockwell Automation, Milwaukee.
CSE: On the subject of electrical protection, what are the most critical issues involving motor control centers?
RAY: Improper adjustment of branch protective devices is a hidden issue. Our surveys indicate that more than half of field installations have serious problems with the adjustment of instantaneous trip points and overload settings. We often recommend the use of thermal magnetic circuit breakers in place of MCPs. The thermal magnetic units provide excellent protection against high level short circuits while also providing back-up thermal protection. The majority of motor applications are below 100 amps and in this range adjustment of the instantaneous trip point is not required.
BLAIR: We've seen several trends:
Selection of automatic shutters—when the unit is withdrawn, the stab opening is automatically closed.
Higher bus bracing of 42kA or 65kA—reflecting the use of larger upstream transformers.
Increased vertical bus ratings of 600A or 1200A to accommodate the higher density of devices).
CSE: How about grounding?
BLAIR: Simply put, users need to make sure each MCC unit is solidly grounded. You might assume that since the MCC is all-metal, grounding is inherent. In fact, it's not. In reality, painted metal surfaces are touching painted metal surfaces. A good way to resolve this is to install a vertical ground bus in each section and ground stab on each unit, making sure this stab engages before the power stabs.
PEERAN: The NEC well directs the selection of the circuit breakers and equipment grounding. Furthermore, most manufacturers now offer microprocessor-based multi-function motor protection relays. These relays are useful additions to the MCC, particularly for large motors. Most manufacturers offer a
RAY: It is also important to consider the type of loads that will be fed from the MCC. Arc welders are notorious for creating interference particularly when the electrical grounded is suspect. But in general, we’ve noted high impedance grounded Y systems are replacing delta systems for high reliability power users. These systems offer many advantages in that they can be first fault tolerant if required for critical process or facility applications such as hospitals. The detection and localization schemes for this approach can be easier than the delta system but it is very important to take this into account when specifying the equipment. High impedance Y systems require similar component and equipment selection as delta systems since the full system voltage can appear between the phases and ground.
CSE: Along the lines of discussing proper electrical schemes, I once heard an engineer describe motor control centers as being available in a combination of class and type. Class referring to basically a pre-wired component package (Class 2) vs. an unwired system (Class 1); Type-A,B,C-referencing terminal block connections, with A referring to centers with no terminal blocks being provided; B, where all connections within individual compartments are connected to terminal blocks; and C, where all connections are made to a master terminal block. Of these permutations, he felt Class 2, Type C was ideal, but Class 1, Type B was the most prevalent. Does this still hold true today, or has there been a shift to the former? Are there any other kinds of wiring or NEMA classifications, among other matters, that specifiers need to be aware of?
RAY: Class 1, Type B is still the most popular construction for MCCs. In this type of construction each unit is prewired but wiring between units is provided in the field. Class 1 Type A is occasionally used by some value added resellers and OEMs who will be integrating application specific equipment. Class 2, Type B and Class 2 Type C wiring have been dramatically changed by the addition of prewired automation, intelligent components and networking within the MCC.
BLAIR: In the 1980's and 1990's we definitely saw a trend toward interwired (Class II) MCCs, meaning that the MCC manufacturer would supply I/O chassis and interwire I/O modules to the devices in the MCC. This trend changed course dramatically over the last couple of years with the advent of intelligent, networked devices. Today customers prefer having a device-level network built into their MCC, with intelligent devices commissioned and plugged into the network. This intelligent MCC approach offers tremendous benefits for improved process monitoring, improved diagnostics, and reduced start-up costs. As an example of the dramatic shift, just the other day I was talking with our Cement Industry manager, and we found that Every major cement job in the last two years involved intelligent MCCs.
PEERAN: The class and type would actually depend upon the application. NEMA Class II Type C offers maximum flexibility, while Class I Type A is the simplest and probably the least expensive. I think that there is a trend towards increasing use of the former.
CSE: What other kinds of wiring or NEMA classifications, among other matters, should specifiers be aware of?
BLAIR: When routing communication cables in MCCs, make sure the cables are rated Class 1, 600 volts. If they're rated Class 2, then separation with power cables is required, and this is impractical in MCCs.
CSE: Shifting gears a bit, what kind of components, such as contactors and overload relays are critical and should be considered as part of a base package, as opposed to "extras," such as lighting panel boards or transfer switches, that might be important to add?
PEERAN: Lighting panels and transfer switches can definitely be considered as options. The MCC should primarily contain motor starters and related equipment. Combination starters and device panels are the most common components of the MCC. Simple device panels with indicating lights start and stop buttons and miniature ammeters are most common for small motors. For larger motors, for compressors and pumps, the device panel can have electronic metering that can monitor all the electrical quantities. Smaller variable-frequency drives and solid state soft starters can be installed in the MCC. In the case of the VFDs the engineer must consider the cable length between the MCC and the motor. It would be prudent to specify VFDs with output dv/dt filters to limit the voltage spikes at the motor due to the PWM switching in the VFD.
BLAIR: I would emphasize the use of intelligent devices—electronic overload relays soft starters, power monitors and communication devices. This reflects the fact that MCCs have always been part of the power distribution system, and now are becoming more a part of the control system. Devices that allow smooth integration into the control system are paramount.
RAY: Basic solid state overloads that provide ambient insensitivity and eliminate thermal elements should also be considered. As the criticality of down time, the complexity of application and the capability of the maintenance staff increases then drives, harmonic abatement and various levels of equipment intelligence can be added.
Other key MCC issues to consider:
1) New seismic standards
2) Arc Flash Mitigation
3) Internet technologies
4) High Impedance grounding
5) Proper adjustment of MCPs
6) Bus plating material
For more on MCCs, See "Gaining a Controlling Edge," CSE 02/98 p. 50, and Motor Control Centers for Industrial Retrofit, CSE 05/90, p. 60.