Current Transducers vs Current Transformers - What are the Differences?

What is a current transformer?

The primary function of a current transformer is to divide or “step down” very high AC currents to lower levels for safety as well as for easier measurement. They don’t change the current into something else - they simply divide it. Their stepped-down outputs are sent to ammeters and other instruments for monitoring purposes, as well as to relays and other systems for protection applications in power systems.

What is a current transducer?

Current transducers refer to a variety of devices that sometimes convert current into a different form. Typically they convert current to voltage for easier measurement by meters and data acquisition (DAQ) instruments. Conversion to a digital output is also possible in some cases. 

How do current transformers work?

A Current Transformer (CT) operates via the principle of electromagnetic induction.  In the early 1800s, Michael Faraday discovered that a changing magnetic field induced an electromotive force (EMF) in a conductor. Electromagnetic induction is the principle behind the operation of generators, transformers, and other electrical devices. In a transformer, a changing current in one coil induces an EMF (voltage) in another coil by varying the magnetic flux. 

A current transformer has a single-turn primary winding, also known as a “bar primary.” This primary carries the full current. 

The secondary winding (B) has many turns, the output of which is a division of the current. The ratio of primary to secondary windings determines the output level measured at the secondary’s output (A). In other words, the current in the secondary circuit is the current in the primary (assuming a single-turn primary) divided by the number of turns of the secondary. 

The primary winding can take several forms. It may be a bus bar going through the central hole, a single flat turn, heavy wire wrapped around a core, and other variations. 

Current transformers are often used in applications involving very high currents. Note that the primary winding may itself have more than one turn, depending on the application. Again, it is the ratio of windings between the primary and the secondary that determines the output.

Current Transformers are known for high accuracy and linearity within the measurement range designed to handle. As a result, they are used heavily in power generation and transmission applications. Because they are designed to surround the primary winding without touching it, they provide galvanic isolation between the current and meters and other measurement instruments. 

CTs are available in both solid core and split core form factors. A solid core is a toroidal (donut) shape, which means that the bus bar must be disconnected from power to pass it through the core. 

In some applications this is not feasible, hence the invention of split core CTs, where the core is hinged so that it can be opened and closed again to pass the bus bar through. In most cases, a split core causes a small reduction of accuracy, but this form factor is a necessity in some applications.

Inexpensive iron core clamps are another example of current transformers. They work on the same principle and can measure amperes alternating current (AAC). Dewesoft offers inexpensive iron core clamps designed to measure up to 5, 15, 200, and 1000 AAC RMS.

Current transformer key points

• Current transformers can handle very high currents

• Current transformers provide an electrically isolated output

• Current transformers are meant to handle AC (AAC)

• Current transformers provide an output that is a division of the original current

• They are used for safety and some measurement applications

Current transformers applications

• Metering and Billing, in residential, commercial, and industrial settings.

• Power Quality Monitoring, maintaining stable and high-quality power.

• Protection Relays, detecting overcurrent conditions, and tripping circuit breakers

• Fault Detection and Analysis, detecting circuits and other faults

• Energy Management Systems, monitoring current flow and analyzing energy consumption.

• Renewable Energy, monitoring solar and wind power system performance

• Motor Protection, detecting abnormal levels to prevent damage

• Substation Monitoring, ensuring efficient power distribution

• HVAC Systems, optimizing motor performance, and detecting faults

• Industrial Process Control, monitoring proper current levels

What does a current transducer do?

Current transducers are often also referred to as “current sensors”. They convert AC or DC current into a form that can be more easily monitored and recorded - usually a voltage signal. 

Current transducers are based on a variety of principles, including the following:

• Hall effect, 

• Shunts, 

• Rogowski coils, 

• Fiber optics, and 

• other technologies. 

It is beyond the scope of this article to describe each of these current sensing technologies in detail, so to learn more, please refer to our separate article, "Comparing and Contrasting the Major Types of Current Sensors"

What are the Major Types of Current Sensors?This article explores diverse current sensor technologies, including Current Transformers (CT), DC-CT® Flux Technology, Fluxgate, Open-loop Hall Effect, Closed-loop Hall Effect, Fiber-optic Current Sensors, Shunt Resistors, and Rogowski Coils. It compares their features, applications, principles, advantages, disadvantages, and costs.

Current transducer applications

• Motor Control, controlling the speed, torque, and direction of electric motors

• Inverters and Variable Frequency Drives (VFDs), feedback to ensure efficient operation.

• Power Electronics, monitoring converters and rectifiers to optimize power conversion.

• Renewable Energy Systems, solar and wind current monitoring to optimize power outputs.

• Uninterruptible Power Supply (UPS) Systems, monitor current flow during power interruptions.

• Battery Management Systems, monitoring charging and discharging currents to optimize battery performance.

• Energy Monitoring, measuring the energy consumption of electrical systems.

• Induction Heating Systems, monitoring induction coil current for efficient heating performance.

• Power Quality, measuring and analyzing harmonics and distortions, ensuring high-quality power supply output.

• Electrical Safety Systems, monitoring current levels, fault detection, triggering safety relays and systems.

Comparing current transformers with current transducers

FAQ frequently asked questions

Are current transducers and current transformers suitable for both AC and DC measurements?

Current transformers are typically used for AC measurements, while current transducers can be designed for both - AC and DC measurements.

What are the typical applications for current transformers and transducers?

CTs are often used in power metering and protective relays within the electrical power industry. Current transducers, with their ability to provide detailed signals, are capable of serving in research and development, process control, equipment monitoring, and inverter control, including use in both industrial and renewable energy sectors.

How many types of current transformers are there?

Current transformers are available in four main types and many subvariations:

• Window current transformers: These CTs are box-shaped with a central hole for a primary current conductor. They contain a circular secondary winding around a core within the assembly. For installations requiring a non-disruptive setup, split-core models are available. Additionally, zero sequence CTs, which detect ground faults by monitoring current imbalances across multiple conductors, are another type of window CT.   

• Bushing current transformers: These CTs feature secondary windings on a magnetic core, housed in an insulator for high-voltage use with power equipment. Smaller, low-current versions also exist.

• Wound current transformers: These CTs have distinct primary and secondary windings on a laminated core, often round or box-shaped, suited for low-ratio, secondary circuit applications. They have high burdens, requiring cautious selection based on the source CT burden.

• Bar current transformers: These CTs have a box-like shape with an internal bar, serving as the primary current carrier, encircled by the secondary winding within the assembly. External connections are used for the primary current and reading of the secondary's output. They're used in fixed installations.

Can current transducers be calibrated?

Yes, nearly any sensor can be calibrated, where its performance is measured, profiled, and documented according to international standards. Dewesoft DAQ systems can store calibration information from sensors and transducers and can apply correction curves (both linear and polynomial), to their outputs. 

How do the outputs of current transducers and current transformers differ?

Current transformers typically output a scaled-down AC, whereas current transducers output a voltage, current, frequency, or digital signal that corresponds to the measured current.

Can current transducers measure high-frequency currents?

Yes, certain types of current transducers can measure high-frequency currents, unlike traditional current transformers which are generally limited to the power frequency range (50/60 Hz).

In terms of accuracy, how do current transducers compare to current transformers?

This depends on the specific designs, types, and applications. In general, current transducers offer higher precision and the ability to measure both AC and DC with the same device, which is impossible with a traditional CT.

Are there any safety considerations when choosing between a current transducer and a current transformer?

Yes, due to the isolated measurement, current transducers can sometimes provide safer interfacing with low-voltage control systems. CTs, on the other hand, can pose a safety risk if the secondary side becomes open-circuited while the primary is energized.

How do the costs of current transducers compare to those of current transformers?

Generally, current transducers are more costly than current transformers due to the complexity and the technology used in transducers.

How do the sizes of current transducers compare to current transformers for the same current-rating applications?

Generally, current transducers are smaller, but this can vary depending on the application's specific requirements and the device's design.

Do current transducers and current transformers require calibration, and how often?

Both may require calibration depending on the precision required for the application, but due to the complexity of current transducers, they may need more frequent calibration checks.

Conclusions

In addition to current transformers, there are many types of current sensors and transducers.  The choice of which current transformer or transducer to use depends on the application. Each type offers distinct capabilities, advantages, and disadvantages. 

Engineers make selections based on a combination of performance specifications, size, durability, cost, user-friendliness, and product availability. The good news is that there is a broad range of options when it comes to current measurement devices. From a trusty shunt resistor to the most advanced DC-CT transducer from Dewesoft.