The Basics of Current-Sensing Relays

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 below illustrates the electrical arrangement to accomplish this design.

Fig. 1. The current relay with an overcurrent setpoint provides protection and enhanced functionality for this rock crushing equipment arrangement.

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 wait for the overload relay to cool, then reset the overload relay, and finally restart the motor. In addition, tripping of the relay would cause the crusher to stop with a full load of material, resulting in the lost production time needed to remove the material from the crusher.

With a current-sensing relay (CR), the operation of the crusher can be improved. In this case, when the crusher motor becomes overloaded, CR’s setpoint is exceeded, and 2M (the feeder conveyor’s starter) is temporarily de-energized. When the crusher processes the material, the current level drops, and CR restarts the feeder conveyor. An adjustable time delay relay is sometimes included in this control scheme to delay the restart and allow the material to be cleared from the crusher. The time delay may be included in the overcurrent relay or as a separate device. The same circuit could be used for other applications as well. Coal needs to be washed before it can be pulverized and burned. After washing, it must be dried. A centrifuge (similar to a clothes washer in the spin-dry cycle) can be used for the purpose. A current relay can monitor current for the centrifuge to prevent overloading.