What is a Current Sensor?

Current measurements are used to monitor and control electrical equipment like blower motors and fan coil units. They can protect a motor drawing too much current and indicate when not enough current is flowing for proper operation. Another important function is sub-metering. To understand the value of these applications, let’s break down how current is measured.

How Does a Current Sensor Work?

Current is measured by sensors that quantify the flow of electrons through wires. Sensors are available for both AC and DC electrical systems. Current sensors can provide either analog or digital outputs.

A digital current sensor acts as a switch that operates when current exceeds or drops below a set limit. This could turn a warning light on or trigger a relay to send a message to another alarm or system. A current switch is configured as either normally open (NO) or normally closed (NC). At a preset current level, (the trip point,) the switch will either open or close.

An analog current sensor indicates the amps flowing in the wire. This is output as a low voltage signal that goes to a display. It can be recorded as a data signal or used to trigger an indicator. An analog current sensor can also indicate the direction of current flow, which is important in applications like motor control.

The choice between digital or analog depends on your project needs.

Direct and Indirect Current Sensing

Current can be measured by applying Ohm’s Law. The current produces a voltage across a resistor placed in series with the load. Measuring this voltage permits calculation of the current. However, this direct method saps some power from the system, (with the additional effect of warming the resistor.)

Indirect current measurement methods do not reduce the power of the system being measured.

Direct current measurement methods are used for lower current flows and where lower accuracy is acceptable. Indirect methods are preferred for amperages above 100A.

Indirect Current Sensing Technologies

Most methods of measuring current use the magnetic field generated by current flowing through a conductor. (The underlying scientific principles are set out in Maxwell’s Equations of Electromagnetism.) Current sensors use one of:

• The Hall effect
• Induction
• The magnetoresistive effect

Hall effect sensors measure the strength of a magnetic field. As more current flows the magnetic field surrounding the conductor grows stronger, and this is detected by the sensor. Hall sensors are generally very small, enabling their use in space and weight-sensitive applications like those in vehicles and aerospace.

In an inductive sensor, a coil surrounds the conductor where it is exposed to the magnetic field generated by an AC current. The changing magnetic flux generates a current in the coil that is proportional to the current being measured. This current is amplified and transformed to a voltage signal for a display or use by signal processing or data logging equipment.

With a solid core inductive sensor, the conductor must be threaded through the center. Split core designs overcome this limitation, but suffer from lower linearity and more drift.

The magnetoresistive effect refers to a property of some materials where electrical resistance varies in proportion to a magnetic field. What happens is the magnetic flux changes the path of a current through the material, with a stronger flux lengthening the path taken, which is measured as higher resistance.

Applications of Current Sensors

Current sensors have many uses but some of the largest application categories are motor control and sub-metering, closely followed by battery charging and solar power systems. Current sensors can:

• Support motor speed control. Large motors, like those used in commercial and industrial HVAC systems, are major energy users, especially at start up. Current sensors support motor speed control.
• Enable energy savings in applications like blower fans, fan coil units (where airflows are too low for accurate differential pressure readings), and pumps for cooling water.
• Empower accurate, real-time sub-metering. Building management systems often incorporate signals and data from current sensors for sub-metering and improved energy utilization.
• Provide information regarding efficiency and operation of solar power deployments.
• Help monitor charging rate — and by extension, charge level — of battery systems.

Other potential applications include elevators, conveyors, and other industrial equipment.

Application Example: Airflow in Underground Working

Imagine you manage an underground mine and need to ensure a certain level of airflow reaches the miners working below. Your engineers have determined that a 17amp flow will generate enough breathable air for the miners. You attach a current sensor to a controller on the wall of the mine to measure the amount of air flow. If the air flow sits at 17amp, your digital switch will stay on or your analog will maintain a steady number. If the air flow goes below 17amp, an alarm will sound.

Is there a difference between current sensors and current transducers?

Yes. The two terms are often used interchangeably, but there are some differences. A transducer takes one input and converts it to a different output. It might for example measure a current nominally of 100A and output a 0 to 5V analog output. A sensor, however, monitors and sends a signal at an action level or trip point. A sensor may incorporate a relay, allowing it to act as a switch.

Muddying the waters further, a current transformer is another term sometimes used when discussing current sensors. This is a form of current transducer that uses the current being monitored to generate an output signal.

Veris, for Your Current Sensing and Measurement Needs

Veris offers a wide range of current sensors, current transducers, and current transformers. Both split-core and solid-core designs are available. To learn more about our current sensing products, visit our website or call to speak with our sales team at 1-800-354-8556 or +1 503.598.4564.