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Fluke Scopemeter - overcautious or safety violation?
2

Fluke Scopemeter - overcautious or safety violation?

Fluke Scopemeter - overcautious or safety violation?

(OP)
I am having an ongoing conversation with Fluke about certain aspects of the Series II scopemeters.

One of the aspects is what voltage limits that apply to the isolated inputs. Especially between what Fluke calls 'Probe reference' which is the same thing as the ground clip (or 'cold' side) of the probe and real (earth) ground. There is a diagram in the manual saying that voltage between probe references and and between probe references and real ground must not exceed 30 V when the signal frequency is more than 25 kHz.

I can understand that specification. That is why I always use an isolation amplifier or differential probe when measuring high-power VFDs with PWM inverters.

Fluke top technicians say that I can use the inputs directly (with the 10x probes), without any extra isolation "because the voltage above 1 kHz will be very low - a few volts maximum".

I do not agree. The derating curve and actual measured data can be seen here: http://gke.org/pub/files/Fluke%20Scopemeter%20Series%20II%20maximum%20voltage%20between%20probes%201.pdf

 

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

I don't even see the logic of, "only a few volts a xHz".

Keith Cress
kcress - http://www.flaminsystems.com

RE: Fluke Scopemeter - overcautious or safety violation?

2
(OP)
This is a snip from the answer I got: "Og spenningskomponenten ved 1kHz på et signal fra en frekvensomformer er normalt langt under 1kV, bare noen få volt"

It is in Norwegian. Run it through a translation program.

Word list: Bare=only, Noen få=a few   

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

What is the basis (explanation) for the published voltage limit that decreases with frequency?

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
I cannot find more than what the picture says. The manual is available for download. I haven't been able to find any explanation.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

The curves are labeled Cat III and Cat IV... has  something to do with explosive environments?

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
No Pete. That's quite another thing and does not have much with this question to do.

Cat IV means that you are measuring on transformer secondary or service entrance where you can ecpect transients up to 8 kV while Cat III means that you are measuring inside a building, after disconnects or MCC or VFD and possible/expected transients are lower.

When you measure on an LV motor on a VFD, it is always Cat III.

Read http://gke.org/pub/files/Fluke%20Scopemeter%20Series%20II%20maximum%20voltage%20between%20probes%201.pdf again. The question is in there.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

Your measurment shows 270 volts at 31khz. Using the 10:1 probe, would it be 27 volts seen at the instrument (below the 30 volt limit)?  

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
No. I used the DP120 differential probe. And I did set the probe scale factor to 200:1 - which is the attenuation you get with the DP120.

As you can see, the PWM waveform (the screen dump) has the normal look with PWM pulses going from 0 V to DC link voltage, which is close to 600 V (400*sqrt(2)). The grid was a bit on the high side.

The idea with the post is to discuss if Fluke are overly prudent using this derating curve, where no more than 30 V RMS is allowed at frequencies above 25 kHz or if they are unaware of the fact that PWM motor voltages are as high as they are. The latter seems to be the case.

Remember that they told me that the voltage components above 1 kHz are only a few volts while the truth is that they are several hundred volts.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

On top of page 103, it lists a voltage specification 300 volts direct connected and 1000 volts for 10:1 and continues "for detailed specs, see page 111"

On page 111 is the figure you mentioned.  At low frequency it lists 300 volts, that would seem to correspond to the limit when using direct connection. For 10:1, presumably the curve is somewhat higher?   (And even higher for 100:1 attentuation probes?)

I'm just trying to understand what the limits are, if not why. I agree it does not seem to be well explained whatever is going on with this limit.
 

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

I imagine that the 10:1 probe is a simple 10:1 resistive voltage divider (using very high resistances to keep high input impedance).

In that case, as a first guess, we'd expect the voltage limits are 10 times higher using 10:1 probes, assuming the limit is based on something indside the box. I can't quite reconcile why they mentioned a 3:1 ratio in limits (1000 volts with 10:1 and 300 voltage direct).... maybe the probe becomes limiting.  One would think it would be easier to understand from reading the manual than it is.

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
Pete. Forget about the probes.

The spec is voltage between probe grounds (inputs are isolated) and between probe grounds and 'real' ground. That is where the problem is.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

ok, I missed that part.  

So the problem arises because you're measuring phase to phase voltages?  I'd think you could measure phase to ground voltages and use that to compute phase to phase voltages (if needed), in which case all references would be ground.   Anything wrong with that approach?

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

Page 30 of the user manual discusses the waveform arithmetic available right in the box, including subtraction.

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
Yes, Pete. Done that, been there. First time back in the sixties (it was thyristor controllers and DC motors then). That is not what I have a problem with. All motor voltage measurements are done phase-phase.

It is the fact that 1) Fluke has the derating curve and that does such measurements impossible and 2) that Fluke doesn't realize that it is a problem because they think that the voltages are 'a few volts' while they are, in fact, several hundred volts, that I try to discuss in this thread.

That waveform arithmetic is of very limited use when rise time is in the 200 ns range. Even Fluke realizes that. Numbers below are from the manual, page 103:

AC or DC coupled, high frequencies:
60 Hz to 20 kHz ......................... ±(2.5 % + 15 counts)
20 kHz to 1 MHz ........................... ±(5 % + 20 counts)
1 MHz to 25 MHz........................ ±(10 % + 20 counts)
For higher frequencies the instrument's frequency roll
off starts affecting accuracy.

 

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

Is that related to subtraction of two channels, or just a limit of the instrument?

I'd think if you are measuring phase to ground voltages of most PWM drives, when Va-g has a steep ramp then Vb-g is relatively constant during that period of time.  So I wouldn't think measuring Va-b during that period would be much less accurate than computing Vab=Vag-Vbg from measured Vag, Vbg

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
We can discuss that in another thread.  

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

Gunnar
I agree with you, I would be very loath to use this without a differential input. Just not safe at all.
In some applications with the ringing voltages present, you can expect 1KV+ at high frequencies.
The problem is not that the xxx harmonic has an amplitude of vvv, but the arithmetic sum of all the harmonics present and coincident creates a dv/dt of many hundreds of volts per uS and this is what causes the potential breakdown with major amps energy behind it.
It is enough to blow the insulation in motors and cables if they are rated at 800V.
So much equipment is designed today by engineers and marketing people who have never been in the field.
Tweak it to work in the lab, cook the specs to suit what is easy to achieve and send it to market, then spend years fixing the problems or quoting the manuals. The question needs to be asked before release, "is it fit for purpose" and "what is the purpose?"

Mark Empson
Advanced Motor Control Ltd

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
Thanks Mark!

That is exactly what I have tried to make clear in this thread.

I agree that the sum (i.e. the square wave and its ringing) is the actual problem. But it is not easy to explain that to the desk officers at Fluke. All I can do is to use their own data. And that derating curve on page 112 is clear enough.

You did check out http://gke.org/pub/files/Fluke%20Scopemeter%20Series%20II%20maximum%20voltage%20between%20probes%201.pdf ?

 

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

Yes I sure did. They are looking at the RMS values of each harmonic, but the RMS value is low because of the square waveshape. The insulation is damaged by the peak "transient" value that initiates the breakdown. The energy is in the lower frequencies present, but once the breakdown has begun, it does not stop!!

Best regards,
Mark.

Mark Empson
Advanced Motor Control Ltd

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
That is also true, Mark.

But it is not as simple as that (or complicated, for some). I haven't even started talking about the transients (they are reaching 900 V and not 750 V, I misread the graph). All I am worried about so far is the waveform with 'body' or energy - the square waves. Transients are probably not a problem because this is a 1000 V CatIV instrument and that means that it can take single 8 kV transients without exploding - which is quite an achievment for a scope.

Back to the issue at hand: If you look at the carrier fundamental, you have close to 270 V at 31.5 kHz. And around 450 V if working with a 690 V motor. That is more than ten times the limit - and around hundred times the voltage Fluke says there is. Remember that they say "only a few volts at 1 kHz"?
 

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

Pete;
Re the 3:1 limit with 10 to one probes. 1000 volts is a safety limit, not a reflection of the probes ability or the ratio of the 1:1 probe to the 10:1 probe.
 
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
Yes Bill. But, still: it has nothing to do with the probes. It is all about voltage between probe references and between probe references to ground. Please try to grasp that.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

Quote:

... The spec is voltage between probe grounds (inputs are isolated) and between probe grounds and 'real' ground. That is where the problem is. ...

Sorry if this point was already discussed: Doesn't it run on batteries?

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
It can run on batteries. But there is also a mains adapter.

The specs do not mention if mains adapter is connected or not. But 'private correspondance' says that it is impossible to use a small, light, SMPS mains adapter with a wide input voltage range because it would not fulfil the CatIV requirements.

I think that is a non-issue. SMPS can be built as sturdy as any scope. The PSU used is a very old design. With a heavy transformer and a switch for 110-ish and 220-ish voltages. That alone is a problem, if I come back from the US and forget to switch back to 230 V, I will fry the mains adapter.

I opened the adapter to see what's so special with it. Nothing, I can safely say. The PCB is single sided. No through-hole plating. The mains lead was squeezed when i put the cover back. Then the adapter didn't work. The cable had pressed the soldering pad off the board and separated it from the copper trace going to the transformer. Would not have happened with a double-sided plated-through PCB.

Fluke quality?
 

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

[quote waross]Pete;
Re the 3:1 limit with 10 to one probes. 1000 volts is a safety limit, not a reflection of the probes ability or the ratio of the 1:1 probe to the 10:1 probe[/waross]
At the time I wrote that, I was thinking about the voltage limit between probe and probe reference (page 111) and not the voltage limit between probe references (page 112).   As you can see, the curves look very similar.

The probe is some kind of voltage divider which brings the non-reference lead voltage closer to the reference lead voltage.

ASSUMING that the basis for this limit lives inside the meter and not in the probe (this is open to discussion, no-one has ever defined the basis for this limt), then we should be able to improve the measured probe-input limit between non-reference and reference inputs by close to the probe scaling factor.   However since the common is not scaled, the probe does nothing to address any voltage difference between commons.  That is why I suggested to measure from voltage to ground which can be measured using waveform subtraction.   When I suggested this, Gunnar pointed out a specification in the back of the book some specifications that accuracy decreases with frequency. When I pointed out that those specifications do not mention subtraction so presumably have little to do with phase-to-phase or phase-to-ground hookup, Gunnar said we can discuss that in another thread.   There may be a perfectly good answer. If there is I certainly would like to hear it. Imo, the important issue  should not be what some anonmymous guy told Gunnar on the phone, but what are the capabilities of the Fluke scopemeter to safely measure PWM waveforms (without add-on's like isolation transformer).
 

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

That is why I suggested to measure from voltage to ground which can be measured using waveform subtraction.
should have been:
That is why I suggested to measure from voltage to ground and compute phase to phase voltage using waveform subtraction.

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
Dear Pete.

I always measure between motor phases. Period. If you want to do it otherwise, please do so. But you will be very disappointed with the result.

The transient response is different for different probes. There are no separate DC, LF, MF1, MF2 and HF adjustments on these probes (like some high end Tek probes have) so it is not possible to have identical transient response up into the 100 ns region.

Why can you not let this go? Or at least start an own thread about it? There we can go into that problem in deepest detail. But, please, not here.  

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

"common" was a bad choice. Correction as follows:
However since the common is not scaled, the probe does nothing to address any voltage difference between commons.
should've been:
However since the reference input is not scaled, the probe does nothing to address any voltage difference between reference input.

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
Right! Pete. That's where the problem is. Let's forget about that arithmetic. It doesn't work. I can do some measurements to illustrate that. Stand by.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

Thanks.  Phase to phase measurement vs phase to ground with arithmetic... it will be interesting to see the difference.

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
Yes. But don't have acess to that motor this Sunday evening. Tomorrow, I hope.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

Whenever you get to it would be great.  I have a LPS reserved for you fwiw.

We have a fluke scopemeter. We also have some VFD's. The weird thing is that we only have vfd's on our  small non-critical machines, while my responsibility is the large critical DOL fixed-speed machines...so I have never gotten any opportunity (excuse?) to do measurements on our vfd's.  But I'd like to be ready if the opportunity presents itself.

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
I did a little excersise on the bench. Wanted to do A-B instead of B-A, which I had. That is NOT possible. I cannot select A as operand 1, because I already have that as operand 2. And, I cannot change operand 2 either, because I already have chosen B as operand 1. A catch22 situation that is absolutely unnecessary. Screen dump attached.

The scope is full of such things. They haven't had any software manager, it seems. And no SW QA either. That is what I have been trying to communicate for more than two weeks. They cannot understand what  say. They hence do not believe me. It is a sad situation.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

Gunnar - is a work-around to select C or D as one measurand while you reconfigure it the way you want? Seems like a pain in the ass, but is it possible?
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
Yes. That is what I did. But, to make it real PITA, when you have 'parked' the last channel - you are kicked out and have to find that math menu again and set (the now not grey) channels. Lots of such things. Did I tell you about the inverse movement of channel C when channel B is inverted? There's more. Get a 190-204 and be amazed at what errors a bunch of hired SW patchers with no SW manager or QA can achieve.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

Quote (skogsgurra):

The transient response is different for different probes. There are no separate DC, LF, MF1, MF2 and HF adjustments on these probes (like some high end Tek probes have) so it is not possible to have identical transient response up into the 100 ns region.
For typical PWM output, isn't it the case that during the time  when one phase to ground voltage is switching, the other phase to ground voltages will generally be constant?  (i.e. only one phase switches at a time in space vector control).      In that case,  there should be no need to compare the transient response of two probes since just one phase to ground probe sees switching at any given time. (right?)
 

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

I know you are already familiar with this, but for the sake of clarity, the PWM that I am picturing is as shown in Figures 11 and 12 here:
http://www.google.com/url?sa=t&source=web&cd=1&ved=0CBgQFjAA&url=http%3A%2F%2Fwww.infineon.com%2Fdgdl%2FAP1609710_different_PWM_for_three_phase_ACIM.pdf%3FfolderId%3Ddb3a304412b407950112b408e8c90004%26fileId%3Ddb3a304412b407950112b40a1bf20453&rct=j&q=3-phase%20pwm%20waveforms&ei=AJgjTtOPHIq5twfmz63CAw&usg=AFQjCNE1b1RcAabrwxu2NBCzMKKUTceXKQ
The point again is that only one phase switches at a time.

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

Above I was also making an assumption that the DC buses in the drive remain a fixed voltage difference from ground.   That would seem to be the case if dc buses were fed by full wave rectifier which in turn is fed from a power system fed by grounded-wye transformer winding.   

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
Re. two phases not switching and the other two not.

There are different switching techniques. The simple ones do what you say. There are others that try to reduce common-mode voltage and current by synchronized switching. And others still, like the thre-level neutral clamp swithers and the polylevel switches. There is also the ABB DTC, which is difficult to predict since switching occurs as a result of the flux vector geting close to an inner or outer tolerance circle. Matrix inverters are evolving. And so on...

I do not think that I am interested in measurement techniques where I need to evaluate what kind of switching I have before I can do the measurements. My work is 'fixing problems'. I cannot spend hours discussing with my client what sort of switching he has. I need to get the work done. Find a sloution. Write a report.

Your situation is probably quite different. Known equipment, time to plan, a budget. In such a situation, you can afford to be philosophical about things. I cannot. My planning is minutes and hours. Phone call, pack what you need in the car. Off you go. I need proven techniques that work every time. Like fused probes and differential probes.

The DC link voltage vs ground is not constant. It is the sum of AC incoming and rectified and smoothed DC.

Anyhow. We can forget this. It is not possible to have more than one math channel. So, the simultaneous measurement of three PWM, using the math, is not possible. That is also something that you cannot read from the specs. That is something you find out when you try to do a three-phase measurement. The specification does not say how many math channels there are. Another 'surprise' when you try to use the 190-204 IRL.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
I have been working intensely the last twelve hours. The result is here: http://www.gke.org/pub/files/Using%20the%20Fluke%20Scopemeter%20190%20math%20function%201.pdf

The probe and the math work better than I thought at lab signal levels. But need careful calibration.

The three-phase measurements can not be done. Simply because there is not one math function on every channel - just one math function common to all four channels. That is a big disappointment.

The switching transients can not be measured reliably with ground as a reference. You can see what is going on if you do such a measurement. But there are errors in the 20 percent magnitude. So not useful in most investigations.

Read the pdf. It has all the details.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

Thanks.  LPS for that.

Can you explain why the B phase to ground voltage changes when A phase switches?  

It seems to me as if the dc bus is floating with respect to ground, and that floating behavior is affected by the capacitance to ground of the load.  Specifically when A switches to higher voltage, it causes the dc bus voltage with respect to ground to increases, and therefore voltage to ground on B initially decreases (followed by oscillation).
 

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

(OP)
At these time scales, you cannot assume anything. We have a few hundred nanoseconds during which heavy transients flow just about anywhere in the system. Fifty metres cable is almost the same thing as not connected and transient voltage changes are everywhere. So, yes - your explanation is right.  

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Fluke Scopemeter - overcautious or safety violation?

Also it's weird that there appears a dc steady state error (between green and black of your last figure) which persists long after the transient.   I agree, it's tough to analyse.

Thanks again for all your help.
 

=====================================
(2B)+(2B)'  ?

RE: Fluke Scopemeter - overcautious or safety violation?

LPS from this side too.  Thanks for the persistence in explaining your reasoning (and backing it up with data!), Gunnar.  What a painful experience it must be to have to try and explain these things to an uncomprehending instrument manufacturer.

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