Accuracy of fluke meter to measure inrush current
Accuracy of fluke meter to measure inrush current
(OP)
Hello,
I just did some tests on a 75 Hp wound rotor motor. I wanted to see what the starting currents looked like in both the rotor and stator. I build a data collection circuit with a couple CTs and a few op amps. I measured the gain directly for each of the circuits. My sampling rate was 1000 Hz. I ran each test for 1 second. I would start the sampling, then immediately hit the start button on the motor. The graphs looked very good. I could see the entire current waveform all the way from inrush to steady state.
So, during this test, my electrician friend put two clamp-on fluke meters onto each of the CT circuits which I had been using for my data collection. His measured inrush maximum was always less than what I got in my data collection. The difference varied quite a bit. The fluke measured inrush currents were 50% to 90% of the currents measured by my circuit.
Here's the question(s)... does anyone know how the Fluke measures inrush? And how accurate is it? How likely is it to get or miss the maximum value?
thanks
I just did some tests on a 75 Hp wound rotor motor. I wanted to see what the starting currents looked like in both the rotor and stator. I build a data collection circuit with a couple CTs and a few op amps. I measured the gain directly for each of the circuits. My sampling rate was 1000 Hz. I ran each test for 1 second. I would start the sampling, then immediately hit the start button on the motor. The graphs looked very good. I could see the entire current waveform all the way from inrush to steady state.
So, during this test, my electrician friend put two clamp-on fluke meters onto each of the CT circuits which I had been using for my data collection. His measured inrush maximum was always less than what I got in my data collection. The difference varied quite a bit. The fluke measured inrush currents were 50% to 90% of the currents measured by my circuit.
Here's the question(s)... does anyone know how the Fluke measures inrush? And how accurate is it? How likely is it to get or miss the maximum value?
thanks





RE: Accuracy of fluke meter to measure inrush current
Now when a motor starts there is a high inrush in the first cycle or so that varies greatly depending on the point on the wave that the switch was closed.
The old analog meters could not respond to this short peak and showed the normal starting current that follows the first cycle inrush. This is what an electrician or engineer wants to know for trouble shooting. When you are working on motors you quickly gain a feeling for acceptable starting currents and often more important, the duration of the starting current.
First cycle peak is usually worthless for trouble shooting.
Look at your current graphs after the first or second cycle and see if that correlates with the Fluke. I will be interested to know at what point on your graph you get correlation with the Fluke reading.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Accuracy of fluke meter to measure inrush current
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In general most current clamp-ons don't process fast enough to catch the peaks during inrush. Newer clamp-on designs address this, but "standard" clamp meters usually won't give you all the info you need for a fast transition.
The Fluke forums have some very good discussions on this type topics.
Hope that helps.
RE: Accuracy of fluke meter to measure inrush current
That was a very intuitive suggestion. I looked at the curves and found that the fluke was measuring the current waveform approximately 0.12 seconds later than the highest peak actually occurred. I would like to add that these fluke meters are intended to read inrush. It would seem that they would react faster than 0.1 seconds. I will look on the fluke forums. That's an excellent place to see if someone knows how fast the sampling is.
thanks
RE: Accuracy of fluke meter to measure inrush current
thanks again to both of you for your help
RE: Accuracy of fluke meter to measure inrush current
The maximum peak of inrush is going to occur in the first cycle. That settles out very quickly, within a couple cycles. But depending on when in the inrush cycle the fluke is triggered, it might just miss all the high spots in the first few cycles. That first current surge is the magnetization current, and based on my tests, I would conclude that the Fluke is not reliable for finding the maximum value for inrush current. However it works well for seeing the locked rotor current.
RE: Accuracy of fluke meter to measure inrush current
RE: Accuracy of fluke meter to measure inrush current
Ideally, the sampling should be fast enough to see 10 samples per 1/4 cycle. This is just my opinion, not related to nyquist frequency. But if I do this again, I will run my sampling at 2400 Hz.
RE: Accuracy of fluke meter to measure inrush current
The characterization of an inrush is somewhat ambiguous.... you have to say whether you're looking at true peak, rms over some period (what period), etc. By the way true peak is a number tha surprises people, true peak can approach 2.8 times the numerical value of the rms locked rotor current. There can also be questions about how accurately is that decaying dc comopnent transmitted thru various types of current proble. Programming a delay may just be a way to skirt these issues and give a more well defined number corresponding roughly to the locked rotor current.
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RE: Accuracy of fluke meter to measure inrush current
RE: Accuracy of fluke meter to measure inrush current
I think you misunderstand my test. I did not sample at 1 second intervals. My circuit sampled at 1000 samples per second, for a period of one second. I think I got plenty of resolution, although when I do this again, I'll probably bump that up to 2400 samples/sec. I used a CT with a 1 ohm resistor load. Then I took the voltage across that resistor and scaled it to -9V to +9V to maximize the range of my data acquisition card. I don't remember which Fluke I used; it wasn't mine. For this type of in-shop testing, I prefer to use my own circuits as I can build them to suit the application. In the field, it is usually more practical to just use a meter.
EE
RE: Accuracy of fluke meter to measure inrush current