Non Invasive current sensing - Page 1
Forum section: electronics / projects

author: dan3460
author_rank: Frequent Contributor
timestamp: April 02, 2017, 01:16:55 pm
content: Hey guys, I posted some time ago about a project to sense when a machine was turned on and off. I had a prototype working with a sensor similar to this one:http://www.ebay.com/itm/100A-SCT-013-000-Non-invasive-AC-current-sensor-Split-Core-Transformer-Tt-/112322914906?hash=item1a26f7aa5a:g:HB4AAOSwh2xYB55Sand I was looking for the possibility of reduce the cost. We are planning to make a couple of thousands of this sensors. After the discussion it looked like this was the cheapest option.After some tests I came out with a problem, this sensor only works when placed on only one of the wires of the power cord. Some of the machines we need to monitor have power cords that do not expose the inner cables and we do not think that the lab will be willing to peal off part of the insulation of the power cord. So I'm back to square 0.I have been trolling the internet for hours with not much luck. I also tested the above sensor over the whole power cord, monitoring the output with my scope but I cant see anything. I did found, from a woodworking shop, a system that senses current directly on the power cord. I'm puzzled on how they are sensing the current, the pictures below are from the IVac Pro Tool. Any ideas how they are accomplishing this? Could not find any teardowns. Thanks

author: SeanB
author_rank: Super Contributor
timestamp: April 02, 2017, 01:33:16 pm
content: They probably use a small hall sensor, so that it detects the field close to the active conductor. I bet it has a need to adjust the cable by turning it to get the most sensitive detection. Just uses a small hall sensor with a magnetic return to the side, to see the localised magnetic field caused by current flow, and probably has a thin copper or aluminium foil shield over the sensor ( not making a shorted turn) to reduce the effect of the electric field causing false triggering.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 02, 2017, 01:47:46 pm
content: For sensing from the outside of a complete power cord (thus 2 or 3 conductors), there is the possibility to use a kind of Rogowski coil:Due to the current going in both directions and one does not know to position of the conductors the magnetic stray field will be weaker and less predictable. But there will still be a detectable magnetic stray field, unless something like a coaxial cable is used.So it might need something like 3 or 4 sense coils on different sides of the cable and than a way of choosing a suitable combination, as to get good suppression of external background and good sensitivity. Using more than 2 coils would be an alternative to adjusting the position and using only 2 sense coils. To get good accuracy / speed, a kind of learning function might be a good idea, not to have much higher sensitivity than needed.AFAIK there are even special clamp meters that can even give a quantity to the current, though with a relatively high tolerance.How much current is the machine consuming ?How much external EMI is expected ?

author: dan3460
author_rank: Frequent Contributor
timestamp: April 02, 2017, 02:17:07 pm
content: I will experiment with hall sensors. I have seen the Rogowski coil but all the applications I have seen requires that one of the wires goes trough the toroid, also is clear that they are not using more sensors around the cord.As for the machines this are typical lab machines, centrifuges, vibrators, etc. that consume some where between 300 and 1000W.Really intrigued, I look on the user guide and SeanB seems to be correct, there are instructions that if the power cannot be sensed you should rotate the sensor around the cord.

author: coppice
author_rank: Super Contributor
timestamp: April 02, 2017, 04:42:09 pm
content: If you are using machines like centrifuges and vibrators you might have some serious magnetic fields near your current sensor. Pretty much all current sensors have some susceptibility to magnetic fields (even shunts can have currents induced in them), but they are not all equal. Things like Rogowski coils, and hall sensors not used in a balanced error cancelling manner can be super sensitive. Things like current transformers much less so.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 02, 2017, 05:13:47 pm
content: Sensing the stray magnetic field is sensitive to external fields. The Rogowski coils is normally build in a way that it does not (in first approximation) respond to a homogeneous external field. Some magnetic shielding (or a highly conductive case like AL) of the sensor part might be a good idea.As this is AC current, there is no real need to use a hall sensor. Inductive sensing with a coil is often easier and more sensitive. With a series (with opposite orientation) connection of 2 coils on both sides of the cable one can usually reduce the sensitivity to an homogeneous external field.

author: dan3460
author_rank: Frequent Contributor
timestamp: April 03, 2017, 12:16:49 am
content: Quote from: coppice on April 02, 2017, 04:42:09 pmI don't think a centrifuge would have more magnetic interference that a table saw, where the sample that I posted is used. The problem that I see with the Rogowski and current transformers is that they need to be placed on only one of the cables in a power cord. A current transformer is what I have already, but the setup is semi-intrusive and I don't think will fly.I have ordered a bunch of different types of Hall Sensors, I will make a few tests to see what works.Thanks everyone for the help.

author: dan3460
author_rank: Frequent Contributor
timestamp: April 03, 2017, 11:27:31 am
content: I agree with all that you have written, there are many ways to skin a cat. But, on this situation the solution has to be similar to what I have pictured on the original post. It is clear that the sensor is over the main power cord.I will make tests once I get the hall sensors.