Protect your assets with more functions, fewer wires
04 January 2013
For the majority of processes, motors have a significant influence on production efficiency, so motor protection is of central importance. Stuart Greenwood reviews the latest developments and describes how motor-protective circuit-breaker technologies are increasingly being merged with electronics and lean automation technologies.
The key tasks of motor-protective circuit-breakers include the switching, protecting and disconnection of current circuits primarily for motor loads. At the same time they protect motors from being destroyed by overloads and guarantee safe disconnection in the event of a short-circuit. They are also suitable for the operational switching of motor loads with a low switching frequency, and safe disconnection from the supply for maintenance or modification work.
While development in the past primarily focused on the mechanical side of things, today's development increasingly comes from the field of electronics and automation with regard to communication and fieldbus technology.
Motor-protective circuit-breakers with electronic wide-range overload protection offer a several key benefits compared to devices with thermal releases. These include large setting ranges, low heat dissipation, tripping classes above Class 10, precise and exceptionally long-time stable tripping characteristics, as well as the possibility to read parameter data.
Illustrative of these advances is Eaton's PKE motor-protective circuit-breaker with electronic wide-range overload protection. This device measures the current rise via an integrated current transformer in the trip block, and evaluates it using a special electronic circuit which, in the event of an overload, sends a pulse via an actuator system to the switch mechanism. PKE circuit-breakers operate with a larger current transfer ratio and the device has a wide current setting range in the ratio 1:4 (conventional bimetal motor-protective circuit-breakers have a ratio of 1:1.6).
Wide current setting ranges also reduces the number of product variants needed - reductions of up to 75 percent are possible, compared with bimetal trip type devices. This has a positive impact on both inventory costs and maintenance logistics.
Energy consumption is also important when considering the life cycle costs of an industrial installation. PKE motor-protective circuit-breakers with electronic wide range overload protection feature low heat dissipation (up to 85 percent less than a bimetal alternative), limiting temperature rise in the switching cabinet and often eliminating the need for forced cabinet ventilation.
As well as the purely basic functions of on/off switching under normal operating conditions, and the basic protective functions available from a circuit-breaker, users also want additional information from these devices in order to maximise operational and maintenance efficiency.
While it was previously only possible to evaluate switch states or differentiate overload or short-circuit trip indications, motor-protective circuit-breakers with electronic overload protection offer rather more, including diagnostics, status or overload messages, and even the actual current flow.
The necessary data transmission platform is provided by the SmartWire-DT wiring and communication system, which offers an alternative to point-to-point control wiring, and avoids the laborious process of setting up device addresses via DIP switches. In place of multi-wire control lines there's a single bus cable.
Data from networked motor starters are transferred directly to the controller and made available over the entire system. Indeed, adopting the lean panel wiring capability of SmartWire-DT greatly improves the transparency of the automation system. Since fieldbus and network based field wiring already supports information exchange, the addition of lean panel wiring systems means, in essence, that any information about any aspect of the automation system's operation and status can be made available wherever it is needed.
Stuart Greenwood is with Eaton's Electrical Sector
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