Fluke clamp current meters have 2 features that seem similar: peak and in-rush. The older models have Max (some: Peak). The recent advent in Fluke clamps is "In-rush".
How do these differ? Isn't in-rush current the short, max current at motor-turn on? Shouldn't meters with a Max feature capture this accurately?
Compare, for example, my old Fluke 36 (Max):
and the 334 (In-Rush):
(click "Explore" then "Selection Guide").
How do Max & In-rush differ? Only in the marketing department? Or is there a real-world difference?
I found this description on the Fluke 33x virtual demo page:
"Note: In-rush current measurements done with a 330 Series Clamp Meter will differ from Min/MAX, Peak, or Peak-Hold measurements which are not triggered events."
Sounds like other than auto-triggering, the results are the same.
The essence of one of these meters is a current transformer using a core that can be opened and then closed. In the closed condition, the magnetic reluctance of the core is made as small as possible. The secondary coil is connected to a small resistance. The idea is to have the current flow waveform in the secondary duplicate the current flow in the often single turn primary. The voltage on the resistor than duplicates the current waveform.
How this secondary waveform is used and processed will determine the features of the meter. Fluke is one of the big names in hand held oscilloscopes. It is possible to digitize the start of this waveform. Then various algorithms can be used to characterize the waveform including the start of it.
Bill
--
As the years go by, dying just before having to fill out a tax return has merit.
Apply power, inrush current is 10A for .2 seconds. Current drops to 1.2A for 2 seconds. Current rises to 12.5A for 1 second. Current drops to 1.2A until termination.
So the Inrush reading would be 10A, and the Max reading would be
12.5A. I can see some usefulness for this, since a max only meter would falsly read 12.5A which the user might attribute to the inrush current, instead of the event at 2 seconds.
In the usual Max measurement, the input (current) is sampled and the maximum sample is displayed, but the true maximum could have occurred between the samples and in that case you miss the true maximum or inrush current that your looking for. In inrush current measurements first off it's a triggered measurement and measures for a very short period of time and it doesn't depend on samples, I think it's an analog approach.
After a short phone conversation with a tech support person at Fluke, I think I understand the difference: it's the acquisition speed. (The new clamps also have triggered event feature, but that's icing on the cake.)
In the clamp meters in Fluke's present product lineup that have the "In-rush" feature, the acquisition speed is listed as 100 mS. In the older clamp meters (eg. my model 36) that have the "Max" feature, the acquisition speed is listed as 250 mS.
In other words, old (model 36) meters sample 4 times a second. New (model
33x) meters sample 10 times a second (overhead aside).
Help me understand the implications of the faster acq. speed. Obviously for a quick event to be measured, the speed needs to be quick or the event will pass unnoticed. Having said that, as long as the event overlaps *any* period of time with the acquisition window, the peak value will be measured. Yes? It's kind of a random chance of getting the acquisition (for events < acquisition speed) isn't it? But not impossible.
** The term actually used is "integration time " - very important .
** Not at all what Fluke claim.
See page 2 of this pdf.
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The 33x meters are actually sampling the current surge wave a " large number " of times in the crucial first few cycles of applied AC power, so that the peak value can be found.
This is quite unlike your typical DMM that *ANALOGUE * samples a DC input voltage a few times a second - with these, an AC to DC converter ( true rms or average rectified value ) is needed to measure any AC wave.
I recently did some tests on the inrush current on a wound rotor motor. I used two fluke meters, both with inrush capacity. And I also built my own circuit using CT, a few resistors, a couple op amps, and a data acquisition card. The data acquisition card was set to sample at 1000Hz. I ran the tests by starting the data acquisition, and then starting the motor. The samples were taken for 1 second. With the data card I was able to get very good graphs of the asymmetric starting current. However, the max amplitude of the starting current measured by the data acquisition card was remarkably different from that measured by the Fluke. The fluke does not necessarily see the max waveform. The fluke takes a bunch of samples in the first few cycles, and then spits out the max of what it measured. I think the flukes are fine, but it should be noted that they can be off by quite a bit. In my tests, the difference between the fluke and the data circuit ranged from a few percent to almost
Now you've introduced another nuance to the discussion.
Reading the app note it seems that Fluke designed there 33 meters to read the symetrical currents for motor starting.
But as you mentioned, large inductive loads often have a DC offset component to their starting current. This comes as an artifact of closing the starter when the sine wave is not at zero-crossing (inevitable in a three-phase motor).
You can see this in oscillograph traces, or hi frequency samping such as your set up. But it's hard to get repeatability unless you have a zero-crossing motor starter. Each time the motor/transformer is energized, it's likely to be at a different point on the sine wave.
(some large motor/transformer protection schemes avoid false-tripping on this in-rush by using various techniques such has harmonic-restraint, or simple time delays)
I wonder if Fluke deliberately filter out the DC offset just so they don't have to explain why the reading changes each time you start the motor :-)
The way to *guarantee* very large inrush surges ( with transformers and transformer based PSUs) is to switch on at the zero crossing of the AC voltage.
Cos doing this generates the maximum degree of magnetic saturation in the core.
** Fluke make no specific claims about the accuracy of their "inrush surge" detection circuitry.
But I would not doubt is does the job required, as far as motors and circuit breakers are concerned.
Switch on power at or close to the voltage maximum on the AC sinewave creates the biggest transient ( surge current) in equipment with a magnetic core. Think about it.
Shaun is correct in his assessment of inrush current being at a maximum if the breaker is closed at a zero crossing of voltage. Because the magnetization current is inductive, the current lags by 90 degrees. So, if the breaker is closed at V=0, and current is lagging by 90 degrees, then this timing corresponds to a max value of current.
A bit off topic.... At first thought, it seems counter-intuitive that zero-crossing is the worst case for inrush current, so I used LTSPICEIV to simulate the situation. As usual Phil is correct! here's the .asc file for the simulation, note that you must set the inductor current to zero for the initial condition.
Version 4 SHEET 1 880 680 WIRE 192 80 48 80 WIRE 336 80 272 80 WIRE 48 112 48 80 WIRE 336 144 336 80 WIRE 48 224 48 192 WIRE 336 224 48 224 WIRE 48 256 48 224 FLAG 48 256 0 SYMBOL voltage 48 96 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value SINE(0 160 60 0 0 0 10) SYMBOL res 320 128 R0 SYMATTR InstName R1 SYMATTR Value 5 SYMBOL ind 176 96 R270 WINDOW 0 32 56 VTop 0 WINDOW 3 5 56 VBottom 0 SYMATTR InstName L1 SYMATTR Value 1 TEXT 80 272 Left 0 !.tran 0 0.2 0.00001 0.00001 TEXT 88 304 Left 0 !.IC i(L1)=0
In the sim above, is a 1H inductor in series with 5 ohms, with
110vac (160v peak) applied. The voltage is set to start at zero crossing. Afterward, set the phase of the voltage to 90degrees, and you'll see that the current is less. In the above sim, the AC current has +/- 400 mA peaks (800mA p-p) when you start at sinewave of 90 degrees. Starting at sinewave zero degrees, the current STARTS at 0.0 mA,then goes to about +800mA, then gradually loses the DC component. When the AC supply is switched off, you also get interesting results! When switched off at zero-crossing, there is still considerable DC current flowing in the inductor. It will cause a lot of trouble if not taken into account. When switched off at the +/- peak of the sine wave, the inductor current will be zero. In other words, inductive loads are happier being switched at the +/- peaks, which is not what you might expect, if you are used to capacitive input power supplies.
Phil has pretty good advice, but he sure gets irritated if you don't know what you're doing. You'll get absolutely nowhere biting him back, unless you can decently support what seems to be your mistake. There's a lot to learn from his "critiques", but some of the back & forth rants are pretty silly. It's always fun to watch a "Phil" thread!
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