Current relays respond to the current value and can be:
- primary built-in circuit breaker drive (RTM);
- secondary connected via current transformers : electromagnetic - (RT-40), induction - (RT-80), thermal - (TPA), differential - (RNT, DZT), on integrated circuits - (PCT), filter - reverse current relay sequences - (RTF).
Purpose
Relays are designed to disconnect protected circuits when exceeding the permissible current consumption. It is possible to use relays to protect circuits and power supplies from overcurrent and short circuit .
Application
They are used in cases when the simultaneous operation of all consumers leads to overloading the power supply network (power supply is designed for less power than the power of consumers, the introduction of limits on electricity consumption, etc.). Consumers are divided into two groups: priority ones, disconnecting from the power supply network is extremely undesirable (computers, television and video equipment, data processing systems, etc.) and not priority ones (electric heaters, various auxiliary equipment, electric stoves, electric kettle, etc. .). The response current of the relay is set in such a way as to prevent overloading the mains supply (disconnecting the input circuit breaker).
In relay protection devices, the most common are current relays that respond to an unacceptable increase in current in the protected circuit, and undervoltage relays that respond to a decrease below a certain value or a complete loss of voltage. Current relays are connected in series, and voltage relays are connected in parallel to the protected circuit. The coils of current relays are made with a small number of turns from a wire of large cross section and therefore have a small resistance, and the coils of a voltage relay with a large number of turns of a wire of a smaller cross section than coils of current relays, and therefore have a large resistance.
The overcurrent relay trips when the current passing through its coil reaches a predetermined value called the tripping current. When the current decreases to a certain value, called the return current, the mobile relay system returns to its original position. The ratio of the return current to the trip current is called the return coefficient, which for most modern relays is in the range of 0.8-0.9.
In the relay of the maximum instantaneous current through the windings of the coils 6 located on the poles of the magnetic circuit 7, current flows from the current transformer included in the working circuit of the electrical installation or the operating current of the installation (if its value does not exceed the values allowed for the relay). When the current reaches or exceeds the value of the set operating current, the steel armature 5 under the influence of magnetic flux, overcoming the counteraction of the spring 2, will rotate with the axis clockwise, and the contact bridge 3, mounted on the axis, will close the upper pair 4 and open the lower pair of fixed contacts . When the current in the coils decreases, the movable relay contact system returns to its original position under the action of spring 2. For smooth adjustment of the operating current, lever 1 is used; in addition, the magnitude of this current can be changed by switching the coil windings. When the coils are connected in series, each one flows around twice as much current as in parallel, as a result of which the relay response current will be half as much. - The relay does not have an adjustment of the response time