Phase control relays

To control the quality of three-phase voltage and to protect electrical equipment in case of emergency situations, phase control relays are used. Emergency situations in this case are: violation of phase symmetry, phase collapse, violation of phase sequence, as well as decrease or increase of voltage below the setting level in at least one of the phases of a three-phase network. In addition to protection against poor-quality power supply, the use of such relays greatly facilitates commissioning.

The use of phase control relays is particularly useful in conditions of frequent reconnection of equipment to a three-phase network, especially if this equipment requires strict phasing, that is, compliance with the phase sequence. The correct direction of rotation of the motors of some machines often depends on the sequence of phases, and if it is violated, then the rotation will occur in the other direction, and this can not only violate the correct mode of operation, but also lead to a serious malfunction of the machine , requiring expensive repairs.

A phase control relay will reliably protect against such situations... The relay circuit will determine the phase sequence of the input, and according to it, the output contacts will work correctly. And if the correct order of the phases is broken, the machine simply will not start and will remain intact.

If one of the phases fails, as well as when the voltage of one of the phases falls below the value set by the setting, the relay will turn off the load after 1-3 seconds. When the voltages return to the preset permissible values, after 5-10 seconds the load will reconnect to the network. The relay will automatically detect if the voltage on at least one of the phases is out of tolerance and turn off the load, then track the return to the acceptable level and turn the load back on.

In some models of such relays, the turn-off and turn-on delay times can be adjusted, but the voltage unbalance level is manually adjusted on all phase control relays. The outputs of the phase control relays can switch both the windings of contactors or magnetic starters, for example for starting motors, and a control circuit containing a signal lamp or bell.

The principle of operation of the phase control relay is based on the selection of negative sequence harmonics (multiples of two of the fundamentals). With imbalances and phase breaks, exactly such harmonics appear in the network.For the purpose of isolating these harmonics, negative sequence filters are used, which in the simplest case are passive analog filters with active and reactive elements (RC-circuits) of the two-arm type, at the output of which electromagnetic relays are included. The control circuit can also be assembled on a microcontroller.

The use of such relays for the protection of electrical equipment in three-phase networks with voltage will save the windings of asynchronous motors from burning out, and expensive equipment from premature failure. Refrigerators, washing machines, air conditioners and other household appliances they have movement powered by electricity, can easily fail if the supply voltage suddenly drops, which is why phase control relays are widely used not only in large enterprises, but also in everyday life.