How do electric counters and current transformers work?

Electric counters are intended for accounting of the spent electric power. We will consider the device and the principle of their work on the example of the single-phase CO-2M counter (fig. 1).

Схема включения однофазного счетчика

Figure 2. Scheme of turning on of the single-phase counter.

In the plastic body the steel core 1 supplied with potential winding is located. It is executed from the large number of rounds of the wire of small diameter and joins in the chain in parallel. Current winding 4 is reeled up on the core 5 and consists of small number of rounds of the wire of big diameter. This winding joins in the chain consistently and is expected rated current 5 A.

Between cores there is the air gap in which the aluminum disk 3 fixed on axis 2 can freely rotate. For adjustment of the counter the permanent magnet established on the steel bracket 7 serves. Conclusions of windings are connected to four plugs 6 of the counter which are closed by the cover and sealed up.

At turning on of the counter on its windings the currents creating the magnetic flux in air gap flow.

This flow crosses the aluminum disk and induces in it whirling currents. Interaction of currents in the disk with the magnetic flux in windings causes emergence of the mechanical force bringing the disk into rotation. The disk is connected by the tooth gearing with the calculating mechanism of the counter giving evidences in kW · h.

Устройство и принцип работы однофазного счетчика типа СО-2М

Figure 1. Device and principle of operation of the single-phase CO-2M counter.

In the scheme of turning on of the single-phase counter (fig. 2 and) the phase wire is connected to the first plug G (the phase clip), and the zero wire - to the third plug. The wires departing to electroreceivers are connected to the second and fourth plugs designated by the letter N (loading).

For measurement of the expense of the electric power in three-phase electric units it is possible to use three single-phase counters which are switched on in each phase according to the scheme provided on fig. 2 б. At the same time the power consumption is defined as the sum of indications of three counters.

It is much more convenient to use, however, three-phase counters which represent three single-phase counters assembled in one body and having the general calculating mechanism. In the scheme of turning on of the three-phase three-element CA 4 counter (fig. 2 in) three phases move on clips of, three-phase loading is connected on clips of N, and on clips 0 the zero wire moves.

Schemes of inclusion are always provided on the reverse of the cover of the counter of any type closing contacts.

Current transformer

Трансформатор тока типа ТК-20

Figure 3. TK-20 current transformer.

Current winding of the counter for installation in the apartment is expected rated current 5 A, but in modern houses there are big multiroom apartments which consume considerably big current intensity, and at the enterprises and in institutions the current capacity can reach several hundred amperes. It is clear, that with such currents counters directly cannot be turned on in the chain. For lowering of variable electric current of big force to the value convenient for measurement standard measuring apparatuses, intended the current transformer, or the measuring transformer.

The TK-20 current transformer (fig. 3) has the steel core 2 with windings. The primary winding 3 with L1 conclusions il2 is executed from the heave-gage wire expected current which is necessary for normal work of the electric unit. Secondary winding 4 and conclusions of I1 and I2 of secondary winding are connected to the terminal block 1. It has such quantity of rounds that at rated current of primary winding in it current 5 A was induced.

Current transformers are issued with different transformation ratioes: 10/5, 15/5, 20/5 And are applied depending on the size of the working current of the consumer above.

In the scheme of turning on of the single-phase counter together with the current transformer the primary winding of the L1-L2 transformer is switched on consistently in the line conductor with big current, and current winding of the counter is connected to secondary winding of the current transformer (I1 conclusions - I2). As well as in the normal scheme, potential winding has to be connected to the phase and zero wire. For this purpose on the scheme between conclusions of L1 and I1 the crossing point is made, and the third clip of the counter is connected to the zero wire.

If the counter works with the current transformer, for definition of the valid expense of the electric power it is necessary to increase the expense shown by the counter by the transformation ratio of the measuring transformer.

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