What is a current sensor?
Current sensors, also commonly referred to as current transformers or CTs, are devices that measure the current running through a wire by using the magnetic field to detect the current and generate a proportional output. They are used with both AC and DC current. Current sensors allow us to be able to measure current passively, without interrupting the circuit in any way. They are placed around the conductor that’s current we want to measure.
Current transformers are essential in many applications. For instance, they are often used in sub-metering to determine energy usage by separate tenants. They can also help with facility regulation by providing information on how much energy is being used and when in order to keep costs down and increase efficiency.
How do current sensors work?
When current flows through a conductor, it creates a proportional magnetic field around the conductor. Current transformers use this magnetic field to measure current flow. If the CT is designed to measure AC current, inductive technology is often used. AC current changes potential, which causes the magnetic field to continually collapse and expand. In an AC current sensor, wire is wrapped around a core. The magnetic field produced by the current flowing through your conductor induces a proportional current or voltage in the wire that is within the current sensor. The sensor then outputs a certain voltage or current that a meter connected to the sensor can read and translate into the amount of current flowing through the conductor. For instance, you could have a current transformer that outputs 333mV (333mV is a common output for CTs) when the current through the conductor is at 400A. Once you configure your meter to read 400A when it receives a 333mV input, it will be able to calculate how many amps are flowing through the conductor based on what input it is receiving. DC current sensors work similarly, but they rely on Hall Effect technology to operate.
Current transformers can either step-up, step-down, or keep the current the same. Sensors that step-up or step-down current are often referred to as transformers. Sensors typically consist of two coils. The coil on which the current passes is called the primary winding and the coil in which voltage is induced is called the secondary winding. For many of the current transformers that we sell at Aim Dynamics, the conductor that the CT is installed around serves as the primary winding and the secondary winding is within the transformer. The core that the secondary winding is wrapped around depends on what the sensor is designed to handle.
The turns ratio of a transformer is the number of turns in the secondary winding divided by the number of turns in the primary winding (). This ratio determines whether a transformer steps the voltage up or down. The ratio of the secondary to the primary voltage is equal to the turns ratio, as given by the equation . So, when the number of turns on the secondary winding is greater than the number on the primary winding, the voltage on the secondary is higher and it is a step-up transformer. The opposite is true for current, where the ratio of the secondary to the primary current is equal to the inverse of the turns ratio: .
How do Hall Effect current sensors work?
Hall effect sensors, or DC current sensors, are capable of measuring both AC and DC current. Hall effect sensors consist of a core, a Hall effect device, and signal conditioning circuitry. They operate based on the Hall Effect.
The Hall Effect is a phenomenon discovered by Edwin Hall in 1879. When current moves through a conductor it creates a magnetic field. If this conductor is positioned within another magnetic field, the magnetic field produced by the electron moving through the conductor will interact with the outside magnetic field causing the electrons to move to one side of the conductor. This creates a voltage on the conductor that is proportional to the amount of current that was running through it and can be measured. A more in-depth description of the Hall Effect can be found here.
Is there a difference between a current transformer, current transducer, and a current sensor?
Technically yes, but these terms are often used interchangeably. These devices are all used to measure current and operate based on the same principles, but there are distinct technical differences between them that are good to be aware of. Technically, current transformers step-down current so it can be monitored efficiently and safely, whereas current sensor is a general term for a device that senses and measures current. Transducers convert one input into a different output. For instance, they may covert AC to DC signals. However, all these devices work similarly to measure current and create an output that is readable by a power meter.
Types of CTs:
- Hall Effect Current Sensors/DC Current Sensors: As explained above, these sensors operate using the Hall Effect in order to measure both AC and DC current.
- Rogowski Coils: Rogowski coils are flexible current transformers that offer a range of benefits for practical use. For one, they are easier to install than traditional current transformers. The thin coil can be threaded easily around the conductor and snapped closed. This makes them ideal for use in situations where installation might be tricky, and you are working with live wires.
- Split-core: Split-core current sensors can be opened and snapped around a conductor making them easier to install in pre-existing configurations. While they are considered to be less accurate than solid-core current sensors, they are accurate enough to be used for nearly all practical applications.
- Solid core: solid-core current transducers are complete loops with no way of opening, so cables must be disconnected and placed through the CT when installing them. This makes them best suited for new installations. They offer high accuracy.
- Open-loop: Hall effect sensors are available in either open or closed-loop. Open-loop sensors offer low insertion loss, fast response time, compact size, and accurate, low-cost sensing.
- Closed-loop: Closed-loop sensors offer fast response, high linearity, and low temperature drift. The current output of the closed-loop sensor is relatively immune to electrical noise. The Closed- Loop sensor is sometimes called a ‘Zero-Flux’ sensor because its Hall-Effect sensor feeds back an opposing current into a secondary coil, wound on the magnetic core to negate the flux produced in the magnetic core by the primary current.
Which companies manufacture current sensors?
There are many different current transformer manufacturers to choose from. Some of the manufacturers that we offer at Aim Dynamics include Magnelab, AccuEnergy, Socomec, and J&D. Each manufacturer has slightly different defaults and conventions, you can browse our website to find one that works best for your specific needs.
What is the VA (volt-ampere) rating of a CT?
A VA rating is an indication of how much power a CT can produce, which is important for accurate reporting of current. If a CT’s VA rating is too low, it can result in under-reporting because the resistance within the circuit is too high for the CT to compensate for. We have an in-depth article on VA rating that you can read here if you want to know more. We also have a great VA rating calculator, if you are looking to figure out what VA rating you need.
Applications of current transformers
Current sensors can be used in a wide range of applications, from facility management to sub-metering and more. They can help detect faults in machinery and prevent damaging equipment. A great place to read more about different applications of current sensors (as well as many of the other products we carry) is on our blog. We often post about different ways customers have used current sensors in various projects.