Common mode of mains measurement - Page 1
Forum section: electronics / projects

author: Jeroen3
author_rank: Super Contributor
timestamp: December 28, 2015, 09:07:32 pm
content: Hello EMI wizardsFor decades measuring single or three phase mains voltage has been done using PCB transformers. Works great, but they have several disadvantages. Such as being heavy, expensive, pretty tight in their operational range (voltage/frequency) and somewhat non-linear. But they do work great as a filter.Now the time has come to move to measuring without transformers. This is easy, you'd think.I've made a measurement prototype PCB to do some testing on AC measurement without transformer. The board is based on a typical AC attenuator with two 1Megs and 1k, giving a 2000:1V ratio.This is fed into an AMC1200 100kHs analog isolator, passively. Thus the input impedance of the amc will matter. It also has some filter caps and ferrites empirically chosen.This works great, when testing from DC, the variac, rotary convert or mains.This does not work great when testing from a filtered VFD. The VFD with a transformer, a filter, another transformer and then the DUT is used to have a benchtop AC source. Greatconcept, also fully isolated.When attached to the VFD-source, powered from a 9V brick (through Wurth 760390015 trafo) and measuring using an battery powered isolated scope. All works great. No noise, conversion factor is ok. No weird phase issues or anything.When powered from a linear lab supply, or measured with a mains-earth referenced scope, or even with the isolated scope on charger. There are all these nasty high frequency spikes, >10Mhz. They are not coming from the AMC, which is 100Khz, and the output is also filtered with an RC at 16Khz knee. So they must come from the vfd, two transformers back. Capacitively coupled? This makes RMS the measurement on the scope the slightest bit higher. But if I build/sell it like this, somewhere sometime a customer will have a bunch of unfiltered (like they always have) VFD's on their mains and the thing will just give false readings. Not good...I assume this is common mode noise. I've tried putting some ceramic Y caps to earth on it, which helps a bit.But eventually not all devices will have earth. Then I looked to Common Mode chokes. But these seem to be made for either a power supply (huge), or a hvds signal (ethernet/can).I've 0 prior experience with common mode filtering like this, and therefore I ask here.What are you supposed to do with circuits like this?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 29, 2015, 12:00:24 pm
content: Have you traced where, in the signal path, the common mode noise is being introduced? Using the grounded scope system as an example.If your circuit is doing its job, with high CMRR, this of course shouldn't happen. Either that, or the CM signal is really freaking huge, and it's blowing out the CM range despite the filtering (which is set low at 16kHz, but is only a first order RC, mind).Tim

author: Jeroen3
author_rank: Super Contributor
timestamp: December 30, 2015, 03:51:18 pm
content: It's perfectly possible to measure the switching frequency of the VFD when connecting the isolated scope to only the hot side ground. I did not try that before.The CM noise frequency changes when changing the VFD switching speed. Obviously, where else would it come from.The signal is significantly reduced when using 3n ceramic Y caps. I'll try a common mode choke tomorrow.Because not all applications have earth.I just realized this is an common mode noise generetor test setup. Could be useful sometimes.