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A range of manufacturing and production processes rely on current-operated relays to provide a continuously adjustable trip-current setting. They are able to protect mechanical apparatus from jam-up or other overloading conditions that result in measurable increases in motor current. Functionally, they sense current levels and provide an output signal when a specified current level is reached. Current-sensing relays are used to:
Signal high-current conditions, such as a clogged grinder.
Identify low-current conditions, such as a pump that has encountered a low-water condition. Sense the current a motor is drawing to feed the current to a programmable logic controller (PLC).
To meet the unique requirements of a diverse set of applications, a wide range of devices and options is currently available to designers, installers, and maintenance professionals, including plug-in style, base-mounted, DIN-rail mount, and donut-style. These types of devices offer the following capabilities:
Sensing both AC and DC current - from milliamperes to several amperes.
Sensing thousands of AC amperes with a current transformer (CT).
Current setpoints can be fixed or adjustable.
Input AC or DC.
Analog output - voltage or current - or contact closure.
Self- or looped-power units.
Fixed or adjustable internal time delays.
Protecting equipment from overcurrents. Crushers come in several forms and are used to reduce the size of materials. Typical applications include shredding wood as well as crushing rock, coal, and other minerals. They have electric motors sized to the capacity of the crusher. Material is usually fed to them by a conveyor at a rate that provides maximum production capacity and prevents overloading. In the event of an overload, a crusher can stall and jam, resulting in significant downtime to clean the material from the crusher and return it to operation.
A current relay with overcurrent setpoint provides protection and enhanced functionality for machinery that may otherwise become overloaded. The circuit in Fig. 1 illustrates the electrical arrangement to accomplish this design.
Material is fed to the crusher by the feeder conveyor, which can overload the crusher with excess material. The crusher starter's overload relay could be used to protect the motor from overloading, but the operation would prove inefficient. The overload relay would trip the starter off, and then personnel would need to first...