Current sensor for multi core cable

There are many ways to measure current. The traditional methods of taking current measurements by inserting a shunt resistor or using a current transformer have the disadvantage of having to break the circuit conductor. For current transformer there are also models that can be installed in place without cutting the cable (more expensive and usually less accurate). It is also possible to use Rogowski coil or Hall sensor to measure current.

Conventional current clamp meters have been available for many years and are an accepted method of non-intrusive current measurement. However these instruments can only measure current in a single core cable. If you have current flowing on multi-core cable, you need to do usually more or less intrusive split (remove cable cover) before a core can be measured.

I have seen articles and advertisements on multi core clamp meter that can measure current form cables with 2 or 3 cores passing through it (two cores carry the current). They have been much more expensive than traditional clamp meters (that can be nowadays bought very cheaply). I have wondered about their working principle of those multi core clamp meters.

Multi-Core Digital Clampmeter is designed to be used anywhere where you want to measure current without breaking a circuit or splitting cables. Megger MMC850 has a complex array of Planer magnetic sensor coils to calculate the current flowing in the conductors of multicore cables with either 2 or 3 cores.

Suparule web page AC Current Measurement in Multi-Core Cable gives an introduction how SMF Flexiclamp technology can measure the current in each conductor. Here is an illustrative picture from that web page.

Flexiclamp Technology current sensing that by sensing the magnetic field at multiple points, and combining this information with the information about the distance between the conductors, as well as the distance from each sensor to each conductor. Flexiclamp Technology white paper shows also picture of the sensor. It resembles quite much on the sensor technology used inside open jaw meters.

Then for some time I have wondered if there is a DIY method that could be used to measure current on mains cables like. I have made some experimenting with placing some sensing coils to different places around cable, but I have not came out with anything specifically useful or consistent.

Yesterday I saw Modern Device blog article Current Sensor that seem to be doing exactly what I was looking for. The article tells:

Wouldn’t it be nice if you could measure current without having to mess with cords or plugs at all. There are small hall effect sensors with linear output that can be used to measure magnetic fields. I thought that it might be possible to use these to measure the current in common electrical wires such as lamp cords.

The wires on the cable create opposite magnetic fields (that pretty much cancel each other at long distance). The trick used here is to to get the sensor physically closer to one wire than to the conductor.

Two linear hall effect sensors, in surface mount packages are mounted in close proximity to the conductors. They are oriented so that each sensor is closer to one of the wires in the cable. The AC voltage from the sensors is then subtracted, so that common mode noise will drop out, and filtered with a low-pass filter to eliminate any signals greater than the power line frequency.

The output of the whole sensors is a DC voltage that is proportional to the current flowing in the power cable. The technology implementation used on this sensor seems to be quite different than used on patented Flexiclamp Technology. I consider this as a good thing because if this sensor works well there are no patent lawyers trying to stop you from using in in some real application.

The sensor will sense 10 watts at the low end (83 mA @ 120V) or 1500 watts (12.5 A @ 120 volts) on the high end.

The sensitivity of the meter looks quite OK to me. It is in line with many cheaper clamp meters and open jaw meters.

Here is picture from Current Sensor blog article to illustrate how to use the sensor:

Modern Device shop sells the current sensor device with following description: This a little current sensor we designed with the idea of providing an isolated current sensor for mains voltages. It uses some Hall Effect Sensors with a lot of gain applied, to sense the magnetic field generated by current in a wire. The sensor output is just an analog voltage which is proportional to the current sensed. This sensor runs at either 3.3 or 5 volts and is easy to calibrate. The output is fairly linear.

This looks like product that I need to check out and test how well it performs.

10 Comments

  1. T says:

    Looks definitely interesting! Waiting for your report, before ordering any to for myself :)

    Reply
  2. AC Current Sensor in test « Tomi Engdahl’s ePanorama blog says:

    [...] Current Sensor in test In my previous posting Current sensor for multi core cable is talked about measuring AC current flowing on cable where you don’t get access to [...]

    Reply
  3. Tomi says:

    You can now find my report on my first tests with the sensor at http://www.epanorama.net/blog/2013/03/10/ac-current-sensor-in-test/

    Reply
  4. kishen says:

    Current sensor, a detection device that can sense information on the measured current, and can detect information on feel, current transducer according to certain rules converted into an electric signal needs to meet certain standards or other desired forms of information output, to meet the information transmission, processing, storage, display, record and control requirements.

    Reply
  5. Tomi Engdahl says:

    MIT researchers develop a low-cost device to monitor home power consumption
    https://techcrunch.com/2016/08/01/home-power/?ncid=rss&cps=gravity_1462_8043715913418486213

    A team of researchers at MIT has developed a device designed to give home owners a better picture of how much power their individual appliances are eating up. The gadget, which was outlined in a paper published in a recent edition of the IEEE Sensors Journal, offers a simple installation process that involves securing it over a power line with a zip tie.

    The stamp-size sensor self-calibrates and is capable of monitoring individual appliances courtesy of software developed by the team, even going so far as isolating time segments, like when a refrigerator goes into defrost mode. It also relies only minimally on cloud-based servers,

    MIT estimates a commercial cost of between $25 and $30 for the sensor

    What’s wasting power at home? Ask your app!
    New system from MIT can identify how much power is being used by each device in a household.
    http://news.mit.edu/2016/wasting-power-home-app-0801

    If you want to save on your monthly electric bill and reduce your greenhouse gas emissions at the same time, you might buy a new, energy-efficient refrigerator. Or water heater. Or clothes dryer. But if you can only replace one of these, which will give you the biggest payback?

    While many groups have worked on developing devices to monitor electricity use, the new MIT system has some key advantages over other approaches. First, it involves no complex installation: No wires need to be disconnected, and the placement of the postage-stamp-sized sensors over the incoming power line does not require any particular precision — the system is designed to be self-calibrating. Second, because it samples data very quickly, the sensors can pick up enough detailed information about spikes and patterns in the voltage and current that the system can, thanks to dedicated software, tell the difference between every different kind of light, motor, and other device in the home and show exactly which ones go on and off, at what times.

    Own your own data

    Perhaps most significantly, the system is designed so that all of the detailed information stays right inside the user’s own home, eliminating concerns about privacy that potential users may have when considering power-monitoring systems. The detailed analysis, including the potential for specialized analysis based on an individual user’s specific needs or interests, can be provided by customized apps that can be developed using the MIT team’s system.

    Tests of the system have showed its potential to save energy and greenhouse emissions — and even to improve safety.

    “For a long time, the premise has been that if we could get access to better information [about energy use], we would be able to create some significant savings,” Leeb says.

    First was the ability to monitor changes in voltage and current without cutting the main incoming power line to a home or business (an expensive process requiring a licensed electrician) or plugging every appliance into a special monitoring device. Other groups have attempted to use wireless sensors to pick up the very faint magnetic and electric fields near a wire, but such systems have required a complex alignment process since the fields in some places can cancel each other out. The MIT team solved the problem by using an array of five sensors, each slightly offset from the others, and a calibration system that tracks the readings from each sensor and figures out which one is positioned to give the strongest signal.

    every motor or device has distinctive characteristics as to exactly how fast and how much the voltage varies, or spikes, at the moment the device switches on, or as it operates.

    Once the system is developed into a commercial product, Leeb says, it should cost only about $25 to $30 per home.

    We’re trying to lower the barriers to installation,”

    Reply
  6. sarfaraz says:

    How to use this sensor? with arduino or any software application? please update

    Reply
    • Tomi Engdahl says:

      The sensor output analogue voltage proportional to the measured AC current (more current, higher voltage).
      The output signal is directly compatible with Arduino analog input.

      Reply
  7. sarfaraz says:

    how to use this sensor for output, with arduino or any other software? have LCD?

    Reply
    • Tomi Engdahl says:

      This sensor gives analog signal and operates with 5V power.
      It is very suitable to be connected to Arduino.
      Connect the signal from sensor to Arduino analog input and power it from Arduino +5V power.
      With Arduino you can do whatever you wish with measurement results.

      It is also possible to do some hardware hacking and connect the output to some voltage meter module that has LCD or LED display if you want.

      Reply

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