Motor Starting and Protection Moves Closer to the Process
Your industrial electric motors are in good hands. Motor starter devices and systems provide all the functions necessary for safe starts and stops. They protect against current overload, under voltage, over voltage, short circuit, phase loss, ground fault, etc. Newer products—especially microprocessor-based ones—add more motor protection, diagnostics, and the ability to commun...
Frank J. Bartos, CONTROL ENGINEERING
Your industrial electric motors are in good hands. Motor starter devices and systems provide all the functions necessary for safe starts and stops. They protect against current overload, under voltage, over voltage, short circuit, phase loss, ground fault, etc. Newer products—especially microprocessor-based ones—add more motor protection, diagnostics, and the ability to communicate with the process being controlled. Now protection takes on wider meaning to include the protective devices as well as motors. Most intriguing, the motor is beginning to be regarded as a basic "sensor" on the connected processes.
Besides communication capability, some newer trends are solid-state overload relays and highly modular systems of numerous devices.
An overload relay is a basic system element (see diagram). In the electromechanical type, current flows through the heater element and, during an overload, melts the eutectic alloy to trip a normally closed contact. This drops out the starter coil circuit, removing current from the motor. Similar devices use bimetallic strips that bend under heat to trip the contact. Newer solid-state relays also open a contact during overload, but do so by monitoring motor current directly. An ASIC activates the trip. Solid-state overloads add improved accuracy and multiple protective functions.
Recent product introductions include two major modular systems. Siemens' Sirius 3R line takes the idea of "docking" from space station structures to refine the art of connecting modular pieces. Rockwell Automation/Allen Bradley's Modular Control System promotes the notion of decentralized motor management (see the sidebar for details).
Among trends noted at Cutler-Hammer/Eaton (Milwaukee, Wis.) are motor starters based on the microprocessor unit (MPU). These newer devices provide programmability and added functions for motor protection and diagnostics. MPU-based technology allows actual decisions to be made for motor protection. "Without the MPU, the simple motor starter would require separate remote I/O systems, along with current sensors and trip relays. Now, the advent of the 'micro' in the device makes all those separate devices unnecessary," says Lee Smith, product manager of NEMA Contactors and Starters.
Charles Kane, product manager, IEC Contactors and Starters, points out an important difference between two basic motor fault conditions: overload and short circuit . The latter is more serious. For example, it's unsafe to restart a motor prior to fully checking and clearing the short circuit. In contrast, an overload is often temporary; the motor can be restarted after the load returns to normal. "In an enhanced motor starter, the MPU can determine which event happened or in what order," explains Mr. Kane.
Ability to communicate with "intelligent" devices is a hot topic. All motor starter manufacturers pursue this cause. At Cutler-Hammer, DeviceNet and Profibus DP are online, with other networks in the wings. The objective is to obtain vital information about motor (or process) status—and do so rapidly. Motor currents, cause of trip, motor run time, % thermal capability, etc., can be sent directly to the PLC and/or industrial PC. "The key is using motor starters and drives that incorporate MPUs and communication capabilities," states Mr. Smith.
Some overall trends noted at C-H include:
Blurring of IEC/NEMA design standards for motor starting and protection products;
Smaller devices and simplified wiring via integration of circuit breaker, contactor, and overload relay functionality; and
Preference for 24 V dc control power due to lower cost I/O points, simplified wiring, and safety considerations.
Size of control equipment is steadily trending down, thanks to electronic advances. Though more often associated with miniature devices such as "microdrives," cost and space savings also benefit packaged control cabinets that occupy substantial real estate.
New from Cutler-Hammer is DeviceNet MCC, said to be the industry's first DeviceNet-compatible, motor control center (photo). With DeviceNet, a single trunk cable and one drop cable for each vertical section of the motor control center helps reduce cabinet size. An industrial PC or PLC in the MCC acts as the logic engine. The single connection reduces wiring, installation cost, and troubleshooting. It simplifies adding more devices, as well. David Adams, general manager, refers to DeviceNet MCC as the "next natural step" for Cutler-Hammer.
Solid-state in the forefront
Square D Co. (Raleigh, N.C.), part of Groupe Schneider, sees a wide role for solid-state overload relays. It's part of today's unprecedented need for information as a competitive tool—for flexible manufacturing through streamlined maintenance tasks. Solid-state devices provide specific motor condition data and trim the size of electrical equipment. Tony Lesczyznski, product manager, NEMA Contactors and Starters, states, "Solid-state overload sensing technologies can eliminate the cost and space required for many current transformers and specialty relays typically needed to obtain information that's critical to maintenance and operation of remotely located motors."
Current transformers and simple circuitry within the solid-state overload also eliminate the need for additional phase loss/phase unbalance relays.
Original equipment manufacturers (OEMs) favor solid-state overloads to monitor the condition of motors mounted on their equipment. "Most OEMs do so through modem communications while others have begun to explore the use of the Internet," Mr. Lesczyznski comments.
In his experience, users prefer solid-state overload relays because they eliminate the need for thermal heaters and offer current adjustment across a wide range. Fewer sizes hold down inventory and simplify selection.
Mr. Lesczyznski cites product examples to illustrate typical applications. Square D Co.'s Motor Logic line includes a low-cost, solid-state overload analog module with 4-20 mA measurement output. This accessory module's response and accuracy level suit simple trending and alarming; for example, belt loss on a fan, blocked impeller on a pump, a jammed rotor, or loss of a conveyor shear pin.
A Seriplex bit-level network module is the newest accessory. Its two signals—cause of trip indication and 3-phase average current threshold warning—when used together, help operators quickly find conditions that led to the overload trip. Operators can also get advance warning of an impeding overload to prepare for corrective action.
A communication module, part of Motor Logic, aids troubleshooting. A maintenance technician can be directed where to look: upstream of the overload relay for line faults; downstream to check for overload; or evaluate if a significant change has occurred in the health of the motor's bearings or windings.
Modules that communicate
A relative newcomer to motor starters, Phoenix Contact (Blomberg, Germany; Harrisburg, Pa.) seeks to leverage its device-level communication expertise within this arena. Its Interbus motor starters enable efficient decentralized control of 3-phase ac induction motors up to 2.2 kW. In the Interbus IBS IP 500 ELR Series, all motor control and protection functions—current monitoring, extensive diagnostic features, etc.—are combined into one module. Several models serve specific starter purposes.
Each IBS IP 500 ELR module has solid-state contactors, thermal overload protection, motor brake control, and communicates via Interbus Remote or Installation remote bus. The series also offers high-ingress protection (IP54 and IP65). "This reduces or eliminates the need for motor control cabinets and enclosures," says James Gibson, product specialist, Interbus Hardware at Phoenix Contact. "The motor starters allow for power line busing of up to 20 amps for general distribution." A receptacle on the modules' Interbus remote interface connects a handheld control unit that serves to drive motors without the bus.
Monitoring motor current detects pending failure in an application. For example, a change in the motor current due to increase in friction can be evaluated quickly, allowing for targeted maintenance before a malfunction occurs. "These compact motor starters are unique, cost-effective devices for use directly on the process line," adds Mr. Gibson.
Type 2 protection
Klöckner-Moeller Corp. (Bonn, Germany; Franklin, Mass.) mentions a development in "Type 2'' coordination for motor starters. It is the more stringent of two protection levels defined for starters by IEC standard 947-4-1. Type 2 coordination specifies that the starter must operate after a short circuit occurs. Only minimal damage to the contactor (minor welding, easily separable) and overload relays is allowed. (Under Type 1 coordination, the starter may not be usable without repair or parts replacement.)
"Many consulting engineers are starting to specify Type 2 coordination for motor control centers and starters," says John F. Doda, national marketing manager for Klöckner-Moeller. "Up until now, providing true Type 2 coordination meant that one had to use fuses as the short-circuit protective device."
Fuses offer safety, but add complexity with many type and size choices. For example, to get Type 2 protection, a fuse sized quite close to the motor's FLA (full-load amperes) may be needed. This can result in frequent nuisance tripping with high-efficiency motors.
An extensive test program just finished by Klöckner-Moeller now certifies Type 2 coordination using its circuit breakers— eliminating the need for fuses, according to Mr. Doda. What's different here is that separate, nonintegrated starter elements (circuit breaker, contactor, and overload) can be combined for safety and possible cost reduction. "This allows many of our customers who use circuit breakers for short-circuit protection to stay with those devices and have certified Type 2 coordination," he adds.
New from Klöckner-Moeller is MPS (Motor Protective Switch) System—a small, low-end manual device for simple applications (up to 11 kW at 460 V or 25 A). MPS offers thermal and short-circuit protection; it is group motor rated, finger safe, and designed to replace fuses.
Cutler-Hammer also notes the inconvenience replacing fuses. It sees an industry trend toward resetable short-circuit protection.
Rapid replacement reality
GE Electrical Distribution & Control (GE ED&C, Plainville, Conn.) believes something extra is needed to satisfy industry's latest demand for very rapid supply of replacements of motor starting and protection devices. Past attempts to fix this problem were to shrink manufacturing cycles or set up a network of distributors with assembly capability. "The weakness of relying on such methods is that manufacturing, scheduling, and shipping of product still takes 24 to 48 hours at best," says Marcelo Valdes, product manager, Control Components. These approaches further require an expert staff and sizable investment in inventory located near the user.
GE ED&C's answer to the challenge is a family of modules designed for assembly by distributor personnel into many sizes and types of enclosed motor starters (photo, next page). Assembly takes "just minutes," says GE ED&C. This family, named "Fastrac Now," doesn't need point-to-point wiring or knowledge of control schematics. "What used to take days can now be accomplished on the spot with Fastrac Now motor starters. Assembly is done without special tools or expertise," says Mr. Valdes.
Solid-state starters are also well thought of at GE ED&C. They're often used together with contactors that close, when the motor comes up to speed, and shunt current around the solid-state elements. Why use such a combination? Since the starter doesn't need to run continuously, it remains cooler and its solid-state power elements live longer.
ASTAT IBP—new from GE ED&C—provides the best of both worlds: a starter optimized to start the motor and a contactor optimized to run it. According to GE ED&C, the new device delivers most of the capabilities of the original starter/bypass contactor combination, but in a smaller, cooler, and lower cost package. And with the silicon-controlled rectifiers doing only temporary duty, no SCR heatsink fan is needed.
Threat from microdrives?
No part of technology remains static. AC microdrives, with their decreasing costs and attractive features, begin to be competitive as replacements for motor starters in certain applications. Reversing and two-speed starters are the most price sensitive. And newer microdrives offer enclosures typical of starters ( CE , Feb. '98, pp. 95-103). Will "micros" present a serious threat? Not for all applications of motor starters. Overall, it's just too early to tell.
Major manufacturers opt for more modular motor controls
Users' choices for motor starting and protection have widened because of recent IEC product introductions by Rockwell Automation/ Allen-Bradley (Milwaukee, Wis.) and Siemens (Erlangen, Germany; Alpharetta, Ga.).
Siemens calls its "new generation low-voltage switching devices" the Sirius 3R family. Introduced with great fanfare at Hannover Fair '97 under a space-age motif, Sirius evokes images of "the brightest star in the heavens" (also known as Dog Star). Its official North American debut was at National Industrial Automation Show (Chicago) in March '98.
Sirius is a complete modular system of motor starter protectors (MSPs), contactors, overload relays, control circuit relays, timers, and accessory parts. All modules match in form, fit, and function. Product aesthetics was a design criterion. "Six years in development, Sirius 3R represents the largest single product launch for Siemens in this arena," says Joe Rogers, product marketing manager at Siemens-Furnas Controls (Batavia, Ill.).
The system promotes ease of assembly and servicing virtually without tools. Space saving comes from narrow-width devices (45, 55, and 70mm) that can be densely packed into a cabinet. Just four frame sizes simplify panel layout.
Contactors (3RT) have ac or dc coils and can switch motors up to 55 kW at 600 V. Auxiliary contacts mount at the front or side. Units rated 3.7 kW or higher allow top, bottom, or diagonal connection of the coil. ("3R" is merely a common catalog tag for current and future devices.) MSPs (designated 3RV), function as thermomagnetic circuit breakers, offering current rating up to 100 A and short-circuit withstand of 50 kV at 480 V.
A contactor and MSP easily combine by way of a connecting block to form a complete self-protected manual combination starter or to protect a group motor installation. The contactor provides remote start/stop capability for the combination starter.
Overload relays (3RU) offer built-in bimetallic heaters, class 10 tripping, full-load amperes (FLA) adjustment, and trip test. Control relays (3RH) also come with ac or dc coils; they have 4 poles, extendible to 8. Numerous accessory components and timing relays (3RP)—with on/off delay (0.05 sec to 100 hr selectable) or with multifunction time delay—round out the family.
Other features are higher temperature rating and optional cage-clamp wire termination. This is an alternative to screw-type terminals. Importantly, Profibus DP and AS-interface communication are also part of the system.
Sirius is an evolving product line. Solid-state overload and soft-start devices will be introduced in the October 1998 time frame.
More modularity, less parts
Introduced in Europe earlier, Allen-Bradley's MCS (Modular Control System) product line likewise has "heavenly" connotation. It flies under the banner of "motor management in the Automation Universe." Fully available now, MCS had its unofficial launch in North America at year-end 1997.
The system offers short-circuit protection and coordination (Type 1 and 2), while stressing simple, quick installation with configurable components. Its major elements are manual MSPs, contactors, solid-state and bimetallic overload relays, and soft starters—sized in three widths (45, 54, 72 mm). Ratings range up to 45 kW.
Contactors (MCS-C) come with reversible coil terminals for best access of connections: load side for starters with manual MSPs or line side when using overload relays. Two wire clamps per pole make for flexibility to terminate different wire sizes and types. Ratings are up to 85 A for ac and dc coils. Control relays (MCS-F) have low-power contact ratings and a variety of pole arrangements.
Self-powered solid-state Overload Relays (MCS-E) offer various trip classes (times), phase-loss protection, manual or manual/automatic reset, 3.2:1 current adjustment range, and visible trip indicator. Some models have selectable trip class, jam/stall protection, and ground-fault trip features.
Contactors and overload relays link up to make a compact Motor Starter/Protector device (MCS-M) that meets circuit breaker requirements of IEC 947-2. For North American applications, it meets manual motor control requirements and is rated for "group motor" installation.
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