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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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18th Jul 2017, 9:23 am | #1 |
Retired Dormant Member
Join Date: Jul 2017
Location: Great Malvern, Worcestershire, UK.
Posts: 72
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Measuring AC phases on oscilloscope
Out of pure academic curiousity I have been investigating how people can use oscilloscopes to visualise three phase ac.
I checked out this youtube video. https://www.youtube.com/watch?v=yY3VHfxVnJA Everything is OK until about one minute into the video. Then, when he is describing how to measure voltage potential difference between A and B he describes how he would connect phase B to the ground. Surely that would lead to a BANG? Phase B is shorted straight to ground leading to wire-melting oscilloscope destruction? |
18th Jul 2017, 9:51 am | #2 |
Dekatron
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
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Re: Measuring ac phases on oscilloscope
I haven't looked at the video but it's possible the phases aren't relative to real ground but relative to another phase which is described as ground here. Also the scope might be floating (don't do this!)
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18th Jul 2017, 9:52 am | #3 |
Dekatron
Join Date: May 2003
Location: Edinburgh, UK.
Posts: 3,274
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Re: Measuring ac phases on oscilloscope
If he was talking about mains 3 phase and he hadn't made his scope floating then yes, he would blow something.
He is actually talking about a 3 phase supply that is floating (has not got its star point connected to ground) so he's just creating a measurement reference point using the three resistor star. Peter |
18th Jul 2017, 9:53 am | #4 |
Nonode
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,677
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Re: Measuring ac phases on oscilloscope
It looks like he's using a battery-powered motor controller intended, I suspect, for remote-control models. That means it will have no ground reference and so connecting the scope ground to one of the phases is fine.
If he was working with a 3-phase supply that was referenced to ground, he'd need to use a suitably-rated differential probe instead. Chris
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18th Jul 2017, 10:00 am | #5 |
Retired Dormant Member
Join Date: Jul 2017
Location: Great Malvern, Worcestershire, UK.
Posts: 72
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Re: Measuring ac phases on oscilloscope
Ah yes, thanks for the enlightenment everyone.
I was under the impression he was measuring three phase from the mains, but looking at it again, that probably isn't the case. |
18th Jul 2017, 10:26 pm | #6 |
Nonode
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
Posts: 2,350
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Re: Measuring AC phases on oscilloscope
Yes, he clearly said connect phase B to the ground connector, but he did NOT say 3 Phase mains.
When I started playing about with rotary converters 10 years or so ago, I wanted to compare phase voltages on a single phase 230v in, 3 phase 230v out unit. Obviously I was after optimum phase balance. I thought of doing exactly the same as he did, using three 100K resistors connected in star, and using the star point as my reference "artificial neutral". I was using DVMs, rather than a scope, but I suspect if I set up the scope's controls first, stood it on a rubber mat with ground lead disconnected, I would be fairly relaxed about using it to study phase shapes and angles. (Don't do this at home!) It was instructive to note that as I improved balance with the correct capacitors, the current drawn from mains fell (measured with a clip on ammeter), the "noise" from the motor also fell. Optimum voltage balance across the resistive matrix as measured with DVMs led to "coolest, quietest and best" result all around. Les. |
18th Jul 2017, 11:17 pm | #7 |
Diode
Join Date: Jan 2014
Location: Okehampton. Devon, UK.
Posts: 3
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Re: Measuring AC phases on oscilloscope
Standard practice at college in the 1980s was to have a floating scope (earth disconnected at plug) so that the chassis could be connected to one of the phases. Any adjustments would be made very carefully! Fingertips only to the control knobs. How Times have changed... can't imagine a tutor allowing that now, let alone setting it as standard class practice. Thank you Captain Three Phase.
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20th Jul 2017, 1:42 am | #8 |
Retired Dormant Member
Join Date: Jul 2017
Location: Great Malvern, Worcestershire, UK.
Posts: 72
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Re: Measuring AC phases on oscilloscope
That does sound a bit scary!
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20th Jul 2017, 9:15 am | #9 |
Heptode
Join Date: Nov 2009
Location: Scarborough, North Yorkshire, UK.
Posts: 510
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Re: Measuring AC phases on oscilloscope
We use to used a CRO and the Lissajous configuration when synchronising alternators using a home made high voltage box attached to the scope input. We also had three lamps mounted on a box, lights dark or bright you made your choice and a syncroscope, belt and braces. Never heard of any one getting a shock or any thing else, folk were very careful those days.Ted
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