Measuring 460V from a VFD with a scope - Page 1
Forum section: electronics / beginners

author: SteveG63
author_rank: Newbie
timestamp: May 05, 2017, 01:32:25 pm
content: Hi guys and gals. My background - industrial electrician in my 50s currently troubleshooting some VFDs at a local facility. Mainly looking for additive harmonics etc.Forgive me if this is a newb question but I'm happy to defer to you electronic gurus for guidance.In the past I have always used my trusty Fluke scopemeter. I wanted to pick up a modern entry level digital scope (Rigol maybe) but I'm not sure how to read the specs. My concern of course is that don't I fry the thing when I connect it to the VFD's output.I have several old school tube scopes that happily read up to 600V but sick of carting them around.So, looking at this:https://www.screencast.com/t/LwA8qWFlG3a(from Rigol), can I or can I not use an DS1102E to view and measure a 460V VFD output voltage with X10 probes?Thanks very much.Steve

author: Jeroen3
author_rank: Super Contributor
timestamp: May 05, 2017, 02:04:33 pm
content: Yes, with appropriate 100x probes.Or better 1000x probes, since I expect the VFD output not to gently stop at 460, but to be rather choppy with high spikes.A x10 will work with 200V-ish signals, but you immediately destroy the compensation at the slightest spike.Not to mention that they are suicide probes because of all the exposed metals.However!The common ground will be a problem.Fortunately, Dave started selling these.

author: SteveG63
author_rank: Newbie
timestamp: May 05, 2017, 02:08:12 pm
content: Thanks for the reply. Can you elaborate on your last comment regarding common ground?

author: rstofer
author_rank: Super Contributor
timestamp: May 05, 2017, 02:13:29 pm
content: As long as you measure between the phase and neutral/ground.Do NOT, under any circumstance, attempt to measure phase to phase because the coax shield is grounded to the BNC at the scope which is tied to earth ground through the power cord. Connecting that probe ground to a phase will be spectacular! I'm not sure I would ground all 3 probes. I might just do one and remove the leads on the other 2. The absolute last thing I want is a dangling ground wire flopping around near 277V (to ground).Your floating handheld scope didn't have this problem, there was no earth ground. There may have been other issues if the BNC had exposed metal...There are differential probes that make for a much safer test but they are pricey:https://www.eevblog.com/product/hvp70/While you are there, check out the video on "How Not To Blow Up Your Oscilloscope"If I did a lot of this stuff, I would be looking at some potential transformers with adequate frequency response such that I could reduce the 480VAC down to 10VAC (for common potential transformers, I couldn't get that low, I would be stuck at 120AC).ETA: The 100x probe idea is a good one.

author: Jeroen3
author_rank: Super Contributor
timestamp: May 05, 2017, 02:15:28 pm
content: You can't simply attach the scope reference to one of the output phases. That would short it to ground, mains earth, pe, whatever you call it.Perhaps vaporizing the probe lead in the process. Covering you in copper dust.The proper way to do this is to measure everything with common ground, or by using differential high voltage probes.The improper way to do this is by using an isolation transformer to isolate and remove the ground from the scope. But beware, inside the scope it's still all chassis. There will not be inter-channel isolation. Please don't do this. The common mode of the VFD spikes will mess up the power supply and your transformer.

author: SteveG63
author_rank: Newbie
timestamp: May 05, 2017, 02:41:42 pm
content: Quote from: Jeroen3 on May 05, 2017, 02:15:28 pmOh absolutely, I get that.Ok, so thoughts on the Rigol DS1102E? Anything else at that price point that I should consider?Thanks.

author: SteveG63
author_rank: Newbie
timestamp: May 05, 2017, 02:58:46 pm
content: Also, I assume that I could measure phase to phase using two channels with one set to inverted and then measure differential?

author: MagicSmoker
author_rank: Super Contributor
timestamp: May 05, 2017, 02:59:58 pm
content: Also note that it's exceptionally difficult to get something meaningful looking at the phase voltage waveforms from a VFD as they will be high frequency square waves whose duty cycle varies sinusoidally (ie - at the "motor" frequency). Much more meaningful is to look at the current waveform, which is easily obtained in an isolated fashion using clamp-on current probes (Rogowski or Hall or even CT). Obviously, a 4 channel scope is especially handy for this sort of thing.

author: oldway
author_rank: Super Contributor
timestamp: May 05, 2017, 03:23:40 pm
content: For your safety, I strongly recommend that you never measure directly with a probe on a power circuit.In case of probe defect, there is a very high risk of flash, burning and even blindness.Always use 4 100W 220V lamps in series as voltage divider and measure the waveform on the lamp connected to the neutral.Always check for the absence of ac and dc voltage between the ground of the probe and the neutral of the lamp on which the voltage is measured before you connect the ground clip.If not absolutely necessary, do'nt make such measurements.....Check instead the low voltage PWM drive signals at ground voltage level.NB: if the voltage divider trips the protection relay, try using lower power lamps (25W 220V)

author: rstofer
author_rank: Super Contributor
timestamp: May 05, 2017, 03:53:57 pm
content: Quote from: SteveG63 on May 05, 2017, 02:41:42 pmThis is a different conversation and the DS1054Z (hacked) is probably a better deal. Four channels, 100 MHz, etc. Discussions are all over this forum but over in Test Equipment the scope is covered in depth. And don't bother reading the 150 page bitch list. Other than a spelling error, all the complaints have been fixed.

author: mmagin
author_rank: Frequent Contributor
timestamp: May 05, 2017, 05:36:48 pm
content: AFAIK, people who do stuff like this all the time seem to love the Scopemeters. I would either do that or use a properly CAT rated differential probe.

author: max_torque
author_rank: Super Contributor
timestamp: May 05, 2017, 06:44:15 pm
content: The problem you have is that a suitable set of HV differential probes will cost more than the scope!However, let me ask a question: "How much if your life, or that of someone near you, worth?"In my industry (EV passenger car) i simply would not let ANYONE measure any thing to do with an inverter / VFD in my test facility without a suitable rated Diff probe. Even if you think you know that a certain voltage is ground (for example the star point of a motor) you don't actually know that it is for various reasons too complex and boring to go into here.3 diff probes will cost you around £1000 (uk pounds) and probably a similar amount in other countries. My life is worth WAY more than £1000..........

author: Jeroen3
author_rank: Super Contributor
timestamp: May 05, 2017, 08:46:45 pm
content: Quote from: max_torque on May 05, 2017, 06:44:15 pmWell, you're measuring it, so anything can be wrong.I sometimes measure directly on the busbars of ships. And you'll always pay extra attention when doing that.