How to connect a current transformer

Today we will discuss how to connect a current transformer. Consider some of the features of measuring devices. Should call the tool auxiliary. Used in conjunction with electricity meters, protective circuits. The secondary current is proportional to the consumed payload - electric motors, heating devices, lighting. Allows you to evaluate the parameters of a powerful industrial network without the risk of damage to the control equipment. An indirect benefit is the safety of the attendants taking the readings, leading the control. Significantly reduces the requirements for qualification, removes other restrictions.

General information about current transformers

Current transformers are created according to regulatory documentation. Parameters are regulated. For example, standards:

1. GOST 7746-2001.
2. GOST 23624-2001.

The case concerns the transformation ratio. The main parameter showing the relationship between the currents of the primary, secondary windings. The figure will allow you to match the current transformer with a counter, a circuit breaker. Moreover, the requirements are significantly reduced. The network consumes 200 A, the transformation ratio is 100, and the presence of a 2A automatic security switch is sufficient. You see, it is very profitable. Security staff painted.

It turns out that the mains voltage in the secondary circuit. Benefits fail. Actually, therefore, the device is called a current transformer. Does not change the voltage. We remind you that the effective value of the phase voltage of 380 volts is 220 volts. Work with the industrial network resembles a single phase. Three current transformers will be needed. The meter measures the voltage, current, determining the parameters:

• The total power consumption in VA.
• Reactive power var.
• Active watt power.

A neutral wire is often needed( even in three-wire industrial networks).Not applicable to current transformer. It does not turn on like normal. The primary winding of low resistance, so as not to introduce disturbances in the circuit. Turns on in series with the payload( motors).

A typical transformer is switched on as follows: the load is in the secondary circuit. It will allow to untie the consumer, source of direct current( galvanic isolation), to obtain the necessary parameters. In our case( !), There are no manipulations with input voltages, currents.

A measurement and control device is included in the secondary circuit. The meters are equipped with two coils: current, voltage. The first circuit is switched on in the secondary circuit. The voltage coil is connected to the phase with one end, the neutral is fed to the second. An integrated approach will assess the power. Neutral is supposed to start one end of the current coil. How to learn the sequence of actions in more detail? The scheme is given on the instrument of control, measurement. The current transformer is a universal product, the subtleties need to be looked for on the body( nameplate) of third-party equipment.

The primary winding is switched on in series with the payload; the secondary winding is used to introduce monitoring and measurement devices into the network. The detailed wiring diagram depends on the type of mating devices, is given on the case, nameplate, instruction. Consider how a current transformer is indicated by electrical circuits. On the open spaces of the network we will meet a lot of mistakes. In previous reviews, I gave a picture of a current transformer, just copy from the previous location:

1. The primary thick line indicates the primary winding. The phase is connected to one end, the consumer is connected to the other. Refrigerator, air conditioning, factory. The drawing shows a three-phase voltage of 380 volts. Shown is one branch. Others are connected in the same way. In the lower right corner we can see the measuring coils of the counter. One of the possible schemes is not a dogma. In detail electric cards are resulted by cases, nameplates of devices. You can get on a specialized forum.
   

   Connection of current transformer

2. Coiled circuit designates secondary winding. Sometimes in the figures, the inclusion points can lie on a thick line, should not be confused. For greater clarity, the findings of the secondary winding are located below. Measurement and control devices are connected to them. Here, the current is less than the consumed payload( refrigerators, air conditioners) at times. How much - shows the transformation ratio. By the way, according to GOST, can not be arbitrary. The value is selected from the row! According to the requirements for measuring instruments, control, the current of the secondary circuit is equal to 1, 2, 5 A. Counters, other control, accounting devices are calculated for such working conditions. The transformation ratio is selected by varying the current payload flowing in the primary winding. The limits are wide. We give an incomplete series taken from the standards( for measuring laboratory current transformers) mentioned above - in detail, readers can read the document themselves: 0.1;0.5;one;1.5;five;7.5;ten;15;20;25;thirty;800 A;one;1.2;five;6;eight;15;sixteen;18;thirty;32;50;60 kA.From the incomplete list it can be seen that the current transformer is not always downward. It can increase the current value of 0.1 A to 5 A. That will allow the use of powerful meters simplest circuits. The counter should provide an opportunity to take into account the existing state of affairs, some are intended for use only with a certain transformation ratio. In detail about the suitability of the device we judge in each case separately.

For instruments used outside laboratories, the variation is lower. Please note that the load of the secondary circuit of scientists should be as active as possible. More precisely, if the power factor is less than 1, only inductive resistances should be connected. Mostly performed, especially for three-phase circuits. The welding machine at the input contains a transformer winding, the motor is connected to the stator coil of the rotor. Applies to meters, where the twisted wire will serve to evaluate the parameters of voltage, current. Examples of inductive resistances. In reality, it is better to be reinsured if the power factor is less than 1( the reactance caused the occurrence of losses), even if the impedance( impedance) is better to be inductive, not capacitive.

Marking of Current Transformers

Before connecting the transformer, make sure that it is suitable for the chosen purposes. From the above it is clear how to quantify the parameters; to apply knowledge in practice, you should be able to read the product labeling. The code is governed by the standard. Here is a list of parameters specified by the manufacturer on the current transformer label:

1. Manufacturer logo with the following inscription "current transformer".It's hard enough to miss by choosing another device in the store.
2. The type of transformer is characterized by design features, type of insulation. Decoding is given in the standards mentioned above. Next to the label is climatic performance. There are doubts about the ability to read the nameplate, it is easier at home to print the GOST tables in advance. If necessary, you should explore the design features. It will help to understand how to connect a transformer, assess the suitability for the circuit in principle.
3. The serial number of the register of the manufacturer will be needed when contacting support( foreign companies), is used for reporting if the purchase is not made by an individual.
4. The rated voltage of the primary winding is indicated for all current transformers with the exception of built-in ones. Because in the latter case, the electrical parameters must be observed external to the device device.
5. The nominal frequency may be absent if( by the voltage value) can be understood: standard for the state( RF - 50 Hz).
6. In nature, there are transformers with several secondary windings. It will allow you to get two or three devices in one. Depending on the electrical circuit will change the transformation ratio. The number of the secondary winding is indicated opposite the parameters.
   

   Characteristics of the current transformer

7. The transformation ratio is the most important value, it is far from the first in labeling. It is denoted by the direct, oblique fraction, the numerator is the primary current, the secondary denominator. The transformation ratio is much more than one. Among the laboratory products we find blatant exceptions to the rule. It is planned to connect current transformers to a low-power circuit to use standard metering devices - look for a purchase using another GOST number( 23624-2001).
8. Accuracy class is important for powerful consumers. I hardly want to pay extra money. If necessary, pay attention to the parameter. It is deciphered according to GOST 7746-2001.
9. The rated safety class of the device indicates what was mentioned above: due to milder conditions in the secondary winding, the risk of electric shock decreases. If the requirements are met, no one guarantees 100% that an accident will not occur. The production process immediately lays some scanty probability of deaths, our task is to reduce the figure. About the safety factor of the secondary winding of the current transformer will tell as follows. Suppose the maximum current of the meter is 20 A. The transformation ratio is designated 20/2 A. The safety factor of the product must be 10, not more. In the event of a short circuit of the primary winding, the core will go into saturation, the current of the secondary circuit will not exceed 20 A. The counter will not burn. The safety of workers is calculated in the same way.
10. Limit ratio is closely related to the previous value. The ratio of a certain current at which the error is at least 10% to the nominal. The limit at which the current transformer is able to assist in the measurements, to act as a means of control.

We hope that readers now know how this task differs from the question of how to connect a 220/12 step-down transformer. Completely different things. The windings are in series with the load gauge. The transformation ratio indicates which control device can be used in the secondary circuit.