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What is the difference between a current relay and an undercurrent relay?

Current relay:


The overcurrent relay is divided into induction electromagnetic type and integrated circuit type. It has the characteristics of timing limit and inverse time limit. It is used in the main equipment such as motors, transformers and relay protection circuits of power transmission and distribution systems. When the main equipment or the power transmission and distribution system is overloaded and short-circuited, the relay can reliably act or send a signal according to a predetermined time limit to cut off the fault to ensure the safety of the main equipment and the power transmission and distribution system.


Undercurrent relay:


A relay that operates when the current through the relay is reduced below its setting value is called an undercurrent relay. When the detected current is greater than the under-current set value, the under-current output relay is activated, otherwise the output relay is released. The under current setting value is set by the panel keys, the setting range is the measurement range, and the control accuracy is high. The panel has an undercurrent indicator and an internal alarm buzzer.


Working principle of current and undercurrent relay:


The working principle of the electromagnetic over-current relay is a composite type, which is composed of two types of inductive and electromagnetic components that share a coil. When an alternating current is applied to the coil of the relay, two magnetic fluxes with a -definite phase difference are generated in the shielded and unshielded parts of the iron core. This magnetic flux interacts with the eddy currents that it induces in the disc, creating a torque on the disc. At an operating current setting of 20% to 40%, the disc starts to rotate. At this time, because the fan teeth and the worm are not engaged, the relay does not operate.


When the current in the coil increases to the set current, the electromagnetic torque is greater than the reaction torque of the spring, and the frame rotates, causing the teeth of the fan to engage with the worm, and the fan teeth rise. At this time, the moving iron of the relay is pushed by the fan tooth rod to reduce the air gap on the right side of the guide magnet and increase the air gap on the left side. Therefore, the moving iron is attracted by the guide magnet and the relay contact is activated. When the current in the relay coil is a set value, the operating time of the inductive element is inversely proportional to the square of the current. As the current increases, the magnet is saturated, and the time limit for the action gradually approaches a fixed value.


When the current in the coil reaches a certain multiple of the current, the electromagnetic element acts instantaneously, so the action time limit of the relay has the characteristics of limited inverse delay. The relay has a plurality of taps for adjusting the operating current of the inductive element and the electromagnetic element. In addition, the air gap between the moving iron and the electromagnet is changed by a doubler screw to adjust the operating current of the electromagnetic element. The relay has a mechanism for adjusting the setting time of the action time of the induction element and a signal plate for the main contact action. Turning the return mechanism by hand can return the signal board without removing the casing.


Undercurrent relay works:


When the circuit is working normally, the armature and iron core of the undercurrent relay are always attracted. Only when the current drops below the set value, the under-current relay is released, sending a signal, thereby changing the state of the circuit. Undercurrent relays generally draw 0.3 ~ o.65 times the rated current of the coil and release current 0.1-0.2 times the rated current.


Graphic symbols for current and undercurrent relays:


The relay that turns on and off the circuit according to the magnitude of the current in the coil is called a current relay. When used, the coil of the current relay is connected in series with the load, and the coil has a small number of turns and a large wire diameter. There are two commonly used undercurrent relays and current relays.


Undercurrent relay:


When the circuit is working normally, the under-current relay is activated. When the circuit current is reduced below a certain setting value (10% -20% IN), the under-current relay is released to achieve under-current protection for the circuit.


Current relay:


When the circuit is working normally, the current relay does not operate. When the circuit current exceeds a certain setting value (generally 110% -400% IN), the current relay is closed to achieve current protection for the circuit.


Are current relays and undercurrent relays working under normal current?


1. The current relay does not work under normal current, or in other words, the armature is released when normal. This can prolong the service life of the relay and save control power. Therefore, the time that the relay is in the protection state is relatively short compared to the normal working time.


2. The above method is the method that all protective relays hope to adopt, but undercurrent relays cannot be designed in this way, because when the current is small or not, the relay can only release the armature, and cannot catch the armature.


Above we learned the characteristics of the relay at normal current! So what are the operating characteristics of the current relay and the undercurrent relay?


1. Current relay is used for current protection. When the current reaches the set value, it will be closed.


2. The undercurrent relay is generally used for the weak magnetic field protection of the DC motor magnetic field to prevent the motor from overspeeding. The normal work is energized and released when the current is less than the set value.


Difference between current relay and undercurrent relay:


Current relay, used for current protection, when the current reaches the set value, pull in.


Undercurrent relays are generally used to protect the weak magnetic field of the DC motor's magnetic field to prevent the motor from overspeeding. The normal operation is energized and released when the current is less than the set value.