The principle of operation and connection diagram of the phase control relay

The phase control relay is a device whose main purpose is to protect linear circuits from overloads and short circuits. In addition, it is able to respond to such a common phenomenon for power grids as imbalance in individual phases. As a result, this device provides comprehensive protection of working circuits and equipment connected to them.

general information

Phase control relays

There are several types of phase imbalance relays that differ in the type of housing and their design features. Despite the large number of designs and the abundance of circuit solutions, the working functions of all models are practically the same. Installation of a phase control relay in 3 phase circuits allows:

  • extend the service life of electric motors;
  • eliminate the need for restoration or repair work;
  • reduce downtime due to three-phase motor failure and the risk of electric shock.

The phase relay installed in the linear circuits guarantees protection of the unit windings from fire and single-phase short-circuit.

What is it for

Application of phase voltage control relay

Special phase controllers are in demand in places where it is often necessary to connect to the mains and where it is important to observe their rotation. As an example, the situation is usually considered when the connected equipment is constantly moved from one place to another. In this case, the probability of mixing up the phase of the line voltages is very high.

In some loads, their incorrect alternation can lead to improper operation of the device and subsequent breakdown. Any unit included in such a network for a long time is likely to fail. When using such a device, it is easy to make a mistake in assessing its condition, considering that the device needs repair.

Features of various designs and their capabilities

There are two types of devices used in linear three-phase systems: phase current relays and voltage switches. They have a standard design determined by the requirements of regulatory documents. Of interest is a comparative assessment of two types of modular devices.

Pluses of current relays

The classic diagram of connecting a phase and voltage monitor to a three-phase motor control circuit

The indisputable advantages of current protective relays (TP) when compared with voltage monitoring devices are:

  • independence from the EMF constantly occurring during phase failures in the event of an overload of the electric motor;
  • the ability to determine deviations in the behavior of an electric machine;
  • admissibility of monitoring not only the line itself (before the branch), but also the load connected to it.

Unlike TP, voltage control devices do not allow most of the listed functions to be realized. They are mainly intended for installation in linear circuits.

Phase failure detection

Failure due to phase loss is a common phenomenon associated with a blown fuse or mechanical damage in the network. Under similar conditions, a 3-phase motor, for example, when one of the phases is lost, continues to operate due to the power taken from the remaining two. Any attempt to restart it in the absence of one of the phases will be unsuccessful.

The duration of its detection (reaction to overload) is so long that during this time the thermal protection simply does not have time to turn off the unit. In its absence, the phase conductor break relay is triggered due to overheating of the motor windings. But this does not always happen, which is explained by the peculiarities of the operation of a device that is underloaded in one of the phases. In this case, the so-called "back EMF" begins to operate in it.

Reverse detection

The use of protective relays is to ensure the safety of working personnel: 1 - interrupted phase; 2 - step voltage

Phase reversal detection is useful in the following situations:

  • the engine is undergoing maintenance;
  • significant changes have been made to the energy distribution system;
  • after the power index is restored, the phase sequence changes.

The need to use a phase rotation relay is associated with the inadmissibility of motor reversal, which can damage the mechanism itself, and also threaten the maintenance personnel. The provisions of the PUE prescribe the use of this device for any equipment, including conveyors, escalators, elevators and other moving systems.

Identifying imbalances

Identifying an imbalance in an electrical circuit

Imbalance in power grids usually manifests itself as a significant difference in the amplitudes of phase voltages coming from the regional substation. Such an imbalance is observed in situations when the uniform distribution of loads on each of the phases is disturbed on the consumer's side. Its presence in the system leads to a spread of currents in individual lines, which significantly reduces the service life of the connected equipment (electric motors, for example).

This is explained by the fact that the so-called "sticking" of phases in the lines of inductive loads causes additional heating of the wires and contributes to the destruction of insulation. All this is the rationale for the need to install the specified model of the phase protection relay in the existing electrical network.

Connection procedure

To understand the procedure for connecting the relay, a preliminary acquaintance with the features of its design will help. This process will greatly facilitate understanding of the principle of operation, as well as the ability to set up the device immediately before starting.

Structural elements

Voltage monitoring relay design

The relay housing is designed to be mounted on a DIN rail or on a pre-prepared flat surface. The external connector allows it to be connected to the mains using standard clamps, to which copper conductors with a cross-section of up to 2.5 mm2 are supplied. On the front panel, there are adjustment elements, as well as a control lamp indicating that the device is turned on.

The operating diagram includes indicators for alarm and connected loads, as well as mode switches, asymmetry and time delay regulators. To connect the device, three terminals are used, designated L1, L2 and L3. Like circuit breakers, they do not provide for the connection of a neutral conductor (this is not true for all relay models).

There is one more contact group of 6 terminals on the body of the device, which is used for connection with control circuits. For this purpose, a wiring harness is provided in the wiring of the power equipment containing the appropriate number of wires. One of the contact groups controls the coil circuit of the magnetic starter, and the second one controls the switching of the equipment connected to the line.

Setting items

The instructions for connecting and setting up assumes the presence of various circuit solutions of the device itself. In the simplest models, no more than one or two controls are displayed on the front panel. This is how they differ from samples with advanced settings. In models with a large number of control elements (they are called multifunctional), a separate microswitch block is provided. It is located on a printed circuit board located directly under the body of the device or in a special hidden niche.

The required relay configuration is obtained by sequential adjustment of each of the available control elements. With their help - by rotating the control knobs while pressing the corresponding microswitch - the required protection parameters are set. The step of their installation or the sensitivity of the device for most samples is 0.5 volts.

Device marking

Relay specifications table

For the purpose of marking control devices on their front or side panel, a sequence of several symbols is applied (sometimes it is indicated only in the passport). As an example, a Russian-made device EL-13M-15 AC400V, designed for connection without a neutral wire, is considered. It is labeled as follows:

  • EL-13M-15 - series name;
  • combination АС400В - permissible voltage.

The marking of imported models is somewhat different. Relays of the "PAHA" series, which have the abbreviation PAHA B400 A A 3 C, is deciphered in more detail:

  • B400 - operating voltage 400 volts.
  • A - type of adjustment.
  • A (E) - mounting method (on a DIN rail or on a connector).
  • 3 - body dimensions in mm.

Symbol "C" means the end of the code combination.

Features of choice

When choosing control devices, first of all, their technical parameters are taken into account. As an example, we consider the case of selecting a model for connecting the ATS, which assumes the following procedure:

  1. The way of inclusion is determined (with "zero" or without).
  2. The parameters of the selected device are found out.
  3. At the same time, it is taken into account that when working with ATS, it will be necessary to control the break and the phase sequence.

To control ATS, the delay time is set in the range of 10-15 seconds.

Acquaintance with individual modifications of control devices will help the performer to take into account the peculiarities of their functioning in specific circuits.

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