Three-phase electricity meters: choosing the optimal circuit for 3 phases

Often, the wiring in household premises is carried out according to a single-phase power supply scheme and household appliances use a voltage of 220 V. For everyday use that's quite enough. But sometimes you have to use high-power devices: machine tools, welders, etc. They require a power supply of 0.4 kilovolts. To control the consumption of resources, you need a three-phase electricity meter.

Types of electric meters

The fixation of electricity costs is carried out using a special metering device - counter. It works with an alternating voltage electric current.

The main difference between the two is current measurement with one or three phases. Single-phase ones are usually found in home dwellings and offices, garage buildings and summer cottages. They are used for a voltage of 220 volts with a frequency of 50 hertz and a maximum load of up to 10 kilowatts.

For more powerful equipment you need a three-phase voltage of 380 volts. It is mainly used in manufacturing plants. There, 3-phase meters are installed. Their feature is the ability to connect to a single-phase power supply scheme. This property is often used by owners of large households with powerful and numerous electrical equipment.

Modern meters are equipped with induction or electronic data counting scheme. ​The former use the properties of inductance and have a rotating disc. Electronic ones have a light board available.

Today, electricity meters can record readings in one or more tariff zones. The greatest demand is for three-phase meters with a two-tariff control system.

The need to install one or another accounting module is selected according to the individual parameters of each supervised object.

Selection criteria for a three-phase meter

Before purchasing such a device, you must consider some factors for an optimal choice.

If there is an already installed meter, then according to its passport located on the case, you need to determine the parameters of the voltage applied in the house.

When it is planned to install the device in cold conditions, its characteristics should include permissible temperature deviations. As a rule, mass instruments are not suitable for use with negative thermometer readings.

When purchasing a metering device, you must make sure that there are sealing marks: one for an electronic device and at least two for an inductive one. The first testifies to the state verification of the meter, the second will be the manufacturer's mark. They are made of black or red mastic for indoor seals or lead or plastic for outdoor seals. Seals are fixed screw connections to prevent unauthorized access for design changes.

An important parameter is the verification period of the device. For models of the previous generation, it was no more than eight years, modern ones require control once every sixteen years. The shorter verification period indicates the low quality of the device and it is better to refrain from buying.

When installing a new meter, you need to notify the accounting authority for the timely sealing of the instrument for fixing the readings.

Benefits of three-phase power supply

There are many equipment that operate on a 380V network. The operation of three-phase meters has some features. Excessive congestion of single-phase lines leads to voltage dips and the failure of home appliances. Sharing the load across the phases will help avoid this.

A cable with a smaller cross-section is used than with a voltage of 220 volts. This corresponds to the provision of Ohm's law about a greater conducted current at the same load.

The principle of operation of three-phase meters

Such devices consist of the following main parts:

• Core coils connected in parallel with the mains.
• The voltage windings are slightly larger in diameter.
• A mechanical worm gear that transmits changes to a scoreboard or digital signage.
• A disc made of aluminum, rotating due to the electromagnetic fields generated during the operation of the windings.
• A magnet that controls the readings of the device.

All components of the meter are placed in a sealed plastic case for protection from external influences. The conclusions of the coils go to the terminal block of the device and locked with a lid with a seal.

Varieties of connection schemes

Mainly a three-phase metering device is selected depending on the option of its connection. There are three such ways:

• Direct connection of the meter.
• Accounting modules for indirect introduction into the circuit.
• Devices with semi-indirect connection.

Three-phase meter of direct connection

Such devices are designed for installation in power grids with a current of no more than 100 amperes. A similar condition limits the total power electrical equipment working with such metering devices up to 60 kilowatts. The contact groups of these meters are not suitable for connecting cables with a large cross-section of conductors. The scheme of their inclusion in the work is simple and is shown on the cover that closes the group of connections in the terminal block.

Connection manipulations are performed in the following order:

1. Insulation is carefully removed from the wires by about five millimeters, the oxidized layer is removed with a solvent and connected to a three-pole automatic machine located according to the diagram to the meter itself. This is necessary to protect the device from short circuits on the supply line.
2. The next operation for connecting the device is carried out in the terminal compartment of the device. The odd-numbered contacts are connected to three phase wires from the supply circuit breaker. Zero input and output pins are connected to the seventh and eighth terminals, respectively.
3. After the accounting module, an automatic machine is installed, similar to the input one, the connection of which is carried out respectively to the even terminal contacts. It is already possible to connect the corresponding electrical appliances to it.

To connect power lines for devices using a voltage of 220 volts, are installed automatic machines for each phase, and the consumers themselves are divided into groups in phases.

Semi-indirect connection of metering devices

Three-phase meters of this switching method require a metering transformer in the circuit. Such equipment can be used in high-power electrical circuit. When fixing the consumed electricity, you need to know transformation index used converter.

The main schemes for connecting a semi-indirect device are:

• Star connection method.
• Turning on the meter with ten wires.
• Application of test terminal blocks.
• By connecting current circuits with voltage circuits.

The main disadvantage of such devices is complexity of periodic testing in controlling organizations.

Indirect way of connecting meters

With a significant amount of energy consumption, increased currents pass through the metering devices. In such cases, an isolating current transformer is installed. To connect such a converter, the current winding is broken.

It has two circuits - primary and secondary. The primary is made from a thick conductor and runs through the middle of the transformer like a core. Broken current conductors are attached to its ends. Power is supplied to the converter using a secondary winding, wound from a large number of turns with a wire of small cross-section, directly from the meter. Such a transformer is connected for each phase. The whole structure is placed in a special cabinet.

This option requires a good knowledge of electrical engineering from the master, and if you doubt your abilities, you need to contact a specialized organization.

The connection of the circuit takes place in several stages:

• First of all, transformers for each phase are connected, fixed to the back wall of the cabinet. The ends of the primary winding are connected to the input machine.
• The second contacts of the primary with the help of separate conductors with a cross section of at least one and a half squares are connected to terminals 2, 5, 8 of the meter.
• Secondary coils are connected with the same wire to the terminals of the meter in the following sequence: 1 and 3, 4 and 6, 7 and 9. Compliance with the scheme is strictly required. Otherwise, the accounting module will not work correctly.
• Terminal blocks 10 and 11 remained for neutral wire connections.

Three-phase devices with indirect inclusion are mainly used for electricity metering in high-voltage networks.

Features of modern three-phase meters

Durability, reliability and accuracy are the main requirements for metering devices. But modern manufacturers do not stop at them and offer a wide arsenal of additional features:

• Fixation of active and reactive electricity.
• Self-control system.
• The ability to account for several tariffs.
• Notation (registration of an incident log).
• Remote reading.

The correct choice and installation are the principles for the operation of modern metering devices. Knowledge of the connection features of each type of such devices guarantees long-term operation, timely and accurate recording of electricity consumption, and in many cases significant financial savings plan.