A current relay is a device that is often used to signal overcurrent in a particular controlled circuit. It is also used to turn off the electrical circuit in the event of short circuits or overloads. The minimum current relay is used much less frequently. Such devices are designed to break the electrical circuit when a certain minimum current value is reached.
There are many different types of electrical devices such as current relays. They differ in design and operating principles. If we talk about the so-called classical device, then it is a movable anchor on springs that controls the contacts, and a coil with a core (usually iron). When current passes through the coil, a magnetic field of a certain magnitude is created. Under the action of this magnetic field, the core of the coil is magnetized and begins to attract the armature. This way contacts will work.
Coilsuch a device contains few turns, but the wire has a large diameter (unlike, for example, the same voltage relay). The diameter of the wire directly depends on the current, more precisely, on the magnitude of the value of the rated current. This results in a slight voltage drop. This is very important, because the coil is connected in series in the controlled circuit.
Some DC relays have an adjustable trip current. Most often this is achieved by changing the tension of the armature spring. An AC current relay that is used to control large currents can be switched on through transformers.
The most important characteristic of such a protective device is the response time. Devices of this type, which can be used for short-circuit protection, have a response time of no more than a few tens of milliseconds.
DC Solid State Relay delays when the circuit is turned off. This eliminates the possibility of false operation in the event of a short-term increase in current. Such a device usually has a response time control.
One of the most common types of protective devices is a thermal current relay. It is a bimetallic plate equipped with a heating element made of a material with a high resistivity value (for example, nichrome). The plate consists of materials with different coefficients of thermal expansion, which bends whenheating and affects the relay mechanism. The response time of such a device depends on the current - the larger it is, the faster the plate will heat up, and the shorter the response time. An electronic current relay is used for virtually instantaneous overload shutdown. The electronic relay circuit can process signals according to pre-set parameters and characteristics. It is possible to set the allowable maximum current and delay time at tripping. Such current relays can be both variable and constant. These devices are often built into many devices.
