Introduction to basic knowledge about current transformers
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Introduction to basic knowledge about current transformers

Posted Date: 2024-01-17

A current transformer is a form of transformer used when the current in a conductor needs to be measured while also requiring some isolation (note: it must be AC).

A current transformer produces an alternating current in the secondary that is proportional to the current in the primary. By having a known turns ratio, the currents in the primary and secondary are connected, and if the current in the secondary is measured, the current flowing in the primary can be determined.

The concept of toroidal current transformer

Therefore, current transformers provide an ideal method for measuring current in a variety of situations and with many different electronic circuit designs.

Answer: Although current transformers are basically the same as the more commonly used voltage transformers, their purpose is very different and therefore the design is very different.

Typically, a current transformer primary will be designed to handle large currents, i.e. the full current of the primary circuit used to measure the current. Therefore, the primary can be a single turn or a heavy duty coil. It can even be a single wire running through the center of the transformer core.

Since the current transformer is in series with the load in the primary circuit, these transformers are sometimes called "series transformers".

The secondary will have a larger number of turns so it can produce a large enough current to be easily measured.

Typically the core can be a laminated core of low loss magnetic material, or a toroidal transformer is also popular. Regardless of the exact format, the core has a large cross-sectional area and therefore produces a low flux density. This enables the transformer to output a constant current that is as independent as possible of the connected load.

Current transformers can come in many forms:

Toroidal Transformers: Toroidal transformers are very popular in many applications. These transformers tend to have a secondary winding around the former in the normal way, but the main wire being sensed runs directly through the center of the toroid.

In order for the transformer to be clamped into the wire, the former can be separated and have a clip mechanism that allows it to be wrapped around the wire and then clamped in place so that it does not fall off. These products are very popular in applications where current transformers need to be applied to existing conductors or wires or where changes may be required.

Traditional Wire Wound Transformer: Another form of popular current transformer is the wire wound transformer. This type of transformer is available from electronic component distributors and manufacturers, and many manufacturers carry multiple types. These are often integrated into electronic systems where the transformer is a permanent electronic component and an integral part of the design.

Bar Transformer: This type of transformer is often associated with high current applications where high current levels require a bar. The cable or busbar of the main circuit serves as the primary winding, which is actually a single turn. Auxiliary wrapped around it.

Turns ratio and current ratio

It is known that in order to increase the output voltage of a transformer, the secondary winding should have more turns than the primary winding.

In fact, we can express this fact mathematically by the following formula:

running under load

The current transformer should always operate in the required load i.e. remain open circuit when there is current flowing in the primary. This is probably the main rule when operating this type of transformer.

The reason for this is that when the secondary is open circuit, i.e. there is no load, the magnetic flux increases significantly, and since there is no reverse current in the secondary wiring, the voltage in the secondary becomes very large, often rising into the region of many kilovolts.

This very high secondary voltage can cause electric shock to any user if the terminals of the transformer are accidentally touched, and can also damage the insulation.

Current transformer applications

Current transformers can be used in many areas: in fact, anywhere alternating current is used there is a need for electrical isolation between the primary circuit and the measuring circuit, and a simple way of connecting measuring test instruments without interrupting the power supply.

There are many examples of the use of current transformers:

Measure current in many power circuits

Measuring the output current in solar systems to enable household appliances, for example. g. Immersion heaters use backup energy only.

Current transformers are used in some test instruments as part of the current measurement function.

Current Overload Protection - Current transformers may be used in certain areas to help sense the input current and detect any overload conditions.

Ground fault detection.

Feedback within switch mode power supplies.

There are many applications for current transformers - they actually appear in a variety of different places.

Clamp Meter

One of the common, almost everyday uses of current transformers is in a type of electrical test meter or multimeter called a clamp meter.

Since electricians often need to measure the current flowing in a wire, the only really convenient way to do this is to clamp around the wire a special toroidal transformer that's part of one of these clamp-on ammeters.

Toroidal transformers can be turned on and off around current-carrying wires without having to disconnect them. The clamped toroid acts as a transformer and is connected to the internal circuitry within the meter that measures the current and processes and displays the reading, usually as a digital readout.

These clamp meters also typically have more standard resistance terminals and may even be used for circuit current measurements. But it is as a clamp-on ammeter that this type of multimeter comes in handy.

Current transformer specifications

There are many different specifications to consider when selecting a current transformer. Essentially, many of these are exactly what is expected, but there may be some aspects to keep in mind.

There are many different specifications that can be applied to current transformers. Here are some of the more important and widely used specifications.

Type: It is important to select the type of transformer required. This depends not only on the type of application, but also on the current that needs to be monitored. Check whether the type of transformer is suitable for the specific application. For example, does it need to be added as a retrofit, in which case it needs to be able to clip on to the wires.

Current Level: The current level of the primary and secondary servers must also be specified. The primary must be able to handle the current levels expected in the primary (with enough margin to accommodate overload), and the secondary must provide enough current to drive any form of ammeter. The ratio obviously depends on the turns ratio.

Frequency range: The frequency range of any transformer will be limited and obviously the same is true for current transformers. Typically, they are made for a small frequency range, typically for 50/60 Hz power lines/power supplies, some may be able to operate at frequencies up to 100 kHz or higher. These are often used with switch mode power supplies. In any case, the frequency range must be checked.

Ammeter Loading and Performance: Typically, the output current of a current transformer will be specified for a given load. The instrument used to measure current should match this. Some current transformers are sold with an integrated current measuring device so the two electronic components are matched and any inaccuracies are eliminated.

Specifying a current transformer for the intended application is key to ensuring that the electronic circuit will operate as expected.

Current transformers or current transformers are used in many areas of electronic circuit design to provide current sensing in a variety of different situations. Not surprisingly, there are many different types of these current transformers, and they are available in a variety of formats and sizes to suit a variety of requirements. Therefore, these useful electronic components will almost certainly be available in the required form and in the required specifications.

Review Editor: Huang Fei


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