Fluke Scopemeter - overcautious or safety violation? 2

[Skogsgurra]

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 Scopemeter Series II maximum voltage between probes 1.pdf

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

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)' ?

 

[Skogsgurra]

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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

I have been working intensely the last twelve hours. The result is here:

http://www.gke.org/pub/files/Using the Fluke Scopemeter 190 math function 1.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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

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)' ?

 

[Skogsgurra]

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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

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)' ?

 

[btrueblood]

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.