Phase monitoring - purpose, principle of operation and connection diagram

There are many different devices, which during their operation often have to be moved from one place to another, each time connecting them to a three-phase network. It is not uncommon for an inexperienced worker to disrupt the phase sequence when connecting equipment, which can lead to equipment failure. To prevent this, it is necessary to ensure phase control by installing a special protection device. In this material we will talk about what a phase control relay is, what is its connection diagram and consider the principle of operation of this device.

Purpose and principle of operation of the phase control relay

Relays for monitoring the phase voltage should be included in instrument circuits, which often have to be reconnected to a three-phase supply network. For example, a screw compressor, which is not a stationary apparatus, is constantly moved from one place to another, each time reconnecting it to the line. If you incorrectly perform the steps to connect it, confusing the phases, five seconds after starting the equipment will be enough for a serious breakdown to occur.

Repair of equipment is associated with considerable costs, therefore, in such devices, phase voltage control is simply necessary.

There are other devices that do not burn out if the wires are incorrectly connected, but simply do not turn on. In this case, the workers usually come to the conclusion that the device is broken, start checking it - and the continuity shows that everything is in order. And it is good if the understanding that the phase conductors were simply mixed up when connected comes quickly, otherwise the working time will be wasted.

What is a voltage relay and how it is configured - in the following video:

Now let's talk about how the monitoring relay works. The main task of the device is to protect electrical devices from damage as a result of exposure to low-quality voltage. This is very important for expensive equipment, therefore imported electrical appliances are installed only together with a control relay. It protects the equipment in the event of phase failure, incorrect connection, as well as asymmetric voltage.

When the phases match the parameters of the control device, the relay contacts turn on, passing a three-phase voltage through the contactor into the circuit. If there is no current on at least one phase conductor, the voltage will not be passed into the line

After the power is restored on the phase conductor, after a few seconds, the load will automatically turn on. So, as you can see, the relay carries out automatic control, turning off the voltage supply in case of an emergency and turning on the load after the parameters of the electrical circuit are normalized.

Relay connection procedure

It is very important that the control device is included in the diagram of any mobile unit, which includes a three-phase electric motor. If such a relay is not included in the equipment, the wrong phase sequence can lead to serious consequences - from malfunction of the device to its failure.

Clearly about the connection on the video:

If at least one phase cable breaks, the power unit will overheat quickly, and the device will become unusable in a matter of seconds. To prevent this, a thermal relay is often installed on the contactor instead of a control relay. But the problem is how to correctly select it and adjust it according to the rated current. This requires a special stand, which not everyone has. Therefore, installing a phase control device is an easier way to solve the problem.

The principle of operation of the RK is based on the fact that the device detects negative sequence harmonics that arise in the event of phase imbalance or when current-carrying wires are broken. The analog filters of the control device isolate them and send a signal to the control board, which turns on the relay contacts after receiving it.

The connection diagram of the phase control relay is not complicated. All three phase conductors and a neutral cable must be connected to the corresponding terminals of the device, and its contacts must be put into the gap of the solenoid of the magnetic starter. If the device is operating normally, the contactor is on, the relay contacts are closed, and the equipment is energized.

In the event of a malfunction, the control device contacts open and the power supply is cut off until the network parameters are restored.

Most often, factory-made relays that are commercially available are used to protect household appliances. But sometimes they are made by hand. Here is a diagram of a simple home-made device, which has graphic designations of the elements included in the circuit.

Conclusion

In this article, we talked about what a phase control relay is, what it is for and how it works. In industrial environments, it protects compressors, motors and other units. In everyday life, they are most often used to protect washing machines and refrigerators.