Electric Device

an electrical engineering apparatus designed to vary, regulate, measure, or monitor the electrical and nonelectrical parameters of a variety of equipment, machines, mechanisms, and so on as well as to provide protection from overloads during prohibited or emergency operating conditions. Electric devices are used in the protection systems of electric mains, in start-control devices for various production processes (especially high-speed processes), in transportation equipment, in automatic and remote-control systems, and in communications. The requirements imposed on such devices depend on the application, purpose, operating modes, and many other factors.

Electric devices may be classified according to their function as switching, start-control, regulating, limiting, measuring, and monitoring devices; however, there are no strict boundaries between the groups. They may also be classified according to the principle of operation, based on what physical phenomenon is used in the device; for example, they may be electromagnetic, thermal, or induction devices. The operation of some devices is based on several physical phenomena. A distinction is also made between automatic and nonautomatic devices. Within any single category, the devices may be subdivided according to the degree of accuracy, voltage (high or low), type of current (DC or AC), method of protection from the environment (open, enclosed, hermetically sealed), and design.

Electric switching devices are designed to switch electric circuits under normal operating conditions when the action of the device involves a change in the operating conditions of the circuit, the application and removal of a voltage, or the disconnection of a circuit under emergency conditions. The class includes relatively simple, nonautomatic devices, such as control push buttons, knife switches, and disconnecting switches, as well as more complicated automatic devices, such as high-voltage circuit breakers. The frequency of operations for electric devices of this type is relatively low—from one operation per year up to several dozen operations per day.

Electric start-control devices are used to start electric machines, regulate the speed of rotation, and stop the machines; to connect and disconnect consumers of electric power; and to regulate power consumption. The class includes contactors, controllers, magnetic starters, rheostats, and reactors. Some devices in this category may be classified as switching devices because of the immediate functions they perform, for example, magnetic starters and controllers. However, they differ in the relatively high frequency of operations performed, which may reach several hundreds of thousands of operations per hour.

Electric regulating devices are used in electric circuits to regulate the values of certain parameters according to a specified law or to maintain the parameters at a specified level. Examples are regulators that maintain a constant voltage or current (electric stabilizers).

Electric limiting devices are used to protect electric circuits under emergency operating conditions from overload currents or to limit the effective value of short-circuit currents. The category includes series reactors, fuses, and dischargers.

Electric measuring devices are designed to measure large currents and voltages with standard measuring instruments. The group includes current and voltage transformers. Electric measuring devices are used to ensure reliable electrical isolation of secondary circuits used for measuring or protection from primary high-voltage circuits.

Electric monitoring devices are used to measure and monitor specific electrical parameters and to provide input to a control circuit. The information about parameter changes is usually fed to the monitoring devices from instrument transformers or transducers.