Question about digital clamp ammeter reading technique/philosophy/principle

[ArchieMedes]

120127-2351 EST

ArchieMedes:

You have stated that your load is a resistance. Thus, the voltage across the load is equivalent to measuring the current. Just a scaling factor. I = V/R. The SCR controlling the current with a resistive load has nothing to do with whether you determine the current my measuring the voltage across the load or you directly measure the current. Measuring the voltage across a resistance as a shunt in series with the load or across the load is probably better than using a current transformer.

What a frequency measurement might tell you is the relation of no current to current, but it might not work well based on how the frequency meter worked. If positive crossings in a given time period are counted then it could work well.

The advantage of using a voltmeter to measure current is that greater overload capability exists. Also for a current probe that is rugged you could use a Hall device probe and it can work from DC to high frequencies with less phase shift problems.

I do not see any particular need to prefer average reading or RMS in your application.

What is the maximum on duration, how many full cycles if more than one, and maximum off duration in full cycles?

From any information you have provided so far you never half cycle, there is no phase shift control of the turn on time within a cycle (turn on is only at a positive zero crossing). Automatically SCRs or Triacs turn off at a zero crossing, and your implication is that this always occurs on a positive zero crossing.

.

We never half cycle, the triggering device makes sure that the SCR is always triggered on the zero crossing and as a minimum should complete one cycle. The maximum or 100% is the mains frequency 50 or 60Hz, it means SCR is always turned ON.
We measure the current in order to VERIFY or TEST the SCR and its triggering device is working properly. The full load current (mains voltage / resistance) is the maximum or 100%. If the SCR is triggered to 50% or one cycle on and one cycle off, the current reading should be half the full load current. That is the method or procedure to test so when we calculate the, let''s say, 50%, we are measuring different reading (mostly higher) than the calculated half of the full load current. Either the SCR or the triggering device is faulty but I am sure it is the measuring equipment as the readings are different with every current ammeter we used.

 

[gar]

120128-1125 EST

ArchieMedes:

In what part of the world are you? The words Mains somewhat implies not US.

Get a Simpson 260. Use the AC range for voltage. Use the 250 V range and an external series resistor that is adjustable so you can make the meter read full scale with your 100% sine wave, assuming your voltage is above 250.

If your voltage is less than 250, then use the 50 V range.

On the meter face you will see AC ohms/volt. One of mine is 5000 ohms/volt AC. A much older one was I believe 1000 ohms/volt.

Assuming 5000 ohms/volt, and for example your voltage was 300, then 300-250 = 50 and you would need to add 250,000 ohms to make the meter read full scale with 300 V input and on the 250 V range. If you were on the 50 V range and still had 300 V input, then the external series resistance would need to be 300-50 = 250, or 250*5000 = 1,250,000 ohms. Use Ohmite 2W potentiometers or equivalent for your adjustable resistor. You can use a fixed resistor in series with the adjustable resistor to improve setability.

Doing this you can then use the 100 scale on the meter as a % scale.

Do try the voltage measurement across your load.

I tried an experiment this morning with a function generator in square wave mode and adjustable duty cycle to drive a Triac switch. But I have no way to sync the function generator to the AC line. Even without sync I was able to get approximately 50 and 25 % readings. Not real close but close enough. I don't have time to setup a synchronized control at this time.

.

 

[ArchieMedes]

120128-1125 EST

ArchieMedes:

In what part of the world are you? The words Mains somewhat implies not US.

Get a Simpson 260. Use the AC range for voltage. Use the 250 V range and an external series resistor that is adjustable so you can make the meter read full scale with your 100% sine wave, assuming your voltage is above 250.

If your voltage is less than 250, then use the 50 V range.

On the meter face you will see AC ohms/volt. One of mine is 5000 ohms/volt AC. A much older one was I believe 1000 ohms/volt.

Assuming 5000 ohms/volt, and for example your voltage was 300, then 300-250 = 50 and you would need to add 250,000 ohms to make the meter read full scale with 300 V input and on the 250 V range. If you were on the 50 V range and still had 300 V input, then the external series resistance would need to be 300-50 = 250, or 250*5000 = 1,250,000 ohms. Use Ohmite 2W potentiometers or equivalent for your adjustable resistor. You can use a fixed resistor in series with the adjustable resistor to improve setability.

Doing this you can then use the 100 scale on the meter as a % scale.

Do try the voltage measurement across your load.

I tried an experiment this morning with a function generator in square wave mode and adjustable duty cycle to drive a Triac switch. But I have no way to sync the function generator to the AC line. Even without sync I was able to get approximately 50 and 25 % readings. Not real close but close enough. I don't have time to setup a synchronized control at this time.

.

I will try the voltage measurement across the load. So it means that we will not get the actual current on the load but only theoretical (I=V/R)? Sorry to be a dumb ass but what is the best reason I can tell the client that we should not measure the actual current but only the theoretical current? Thanks...

 

[T.M.Haja Sahib]

Some digital multimeters have the marking'AC+DC'.Using such a meter,you can measure directly the RMS value of current with DC component as shown in post #3.Otherwise you have to measure them separately using a different digital multimeter measuring AC and DC current components separately and you have to find the equivalent current as the square root of the squares of the two current components.

 

[gar]

120130-0813 EST

ArchieMedes:

Measuring current with a clamp-on probe or a shunt resistor is just as much a theoretical measurement as is measuring the voltage across a resistive load where the goal is to measure the current thru that resistive load.

I believe your stated goal is to determine whether the circuit supplying current to the load is working correctly.

At the present the reason to measure the voltage is to provide some readily available alternative instruments to determine what is going on in the system.

.

 

[T.M.Haja Sahib]

Further to post#24,here is link to a fluke make digital multimeter(power quality analyser).See if it can be used to solve the problem mentioned in post #3.

http://www.fluke.com/Fluke/usen/Pow...shooting-Power-Meters/Fluke-43B.htm?PID=56080

 

[GeorgeB]

Sorry to be a dumb ass but what is the best reason I can tell the client that we should not measure the actual current but only the theoretical current? Thanks...

My data acquisition equipment can calculate the RMS amps via a (could be older clamp on) CT and burden resistor.

And for gosh sakes, your waveform is a common situation in temperature control via the common PID process controllers ... 1% to 100% is not unusually number of full cycles out of 100. On periods of (well zero, too) 16.67 ms to 1.667 seconds then repeat.

For the cost of a Fluke 80i-600 or 80i-400, a 10 ohm resistor (4VRMS at 400A), and a Dataq DI-149HS you can do this. The Fluke will have to come from eBay since they were discontinued with the Cat (whatever) requirements.

 

[gar]

120130-2334 EST

ArchieMedes:

I think I forgot to mention that you do want to check for DC current. If is substantial, maybe more than 1% of the AC, then your waveform has a problem. If you use a meter across the load resistance it is easy to do.

You can not do it with an AC current transformer.

.

 

[T.M.Haja Sahib]

120130-2334 EST

ArchieMedes:

I think I forgot to mention that you do want to check for DC current. If is substantial, maybe more than 1% of the AC, then your waveform has a problem. If you use a meter across the load resistance it is easy to do.

You can not do it with an AC current transformer.

.

gar
I do not see the need for improvising the measurement,when the most suitable instrument (post#26) exists to serve the actual purpose.......

 

[quogueelectric]

Hi All,

I would like to ask about the principle on how a clamp ammeter reads the current. This is not about EMF / induction that is sensed by the ammeter. This is about the current digital reading shown by the ammeter.
I understand that there are some ammeter which uses averaging and RMS value of the current flowing. I am guessing this is based on the assumption that the AC current it is reading is a pure/perfect sinusoidal waveform with a frequency of, say, 50 or 60 Hz. So, I am guessing, it will just take ONE cycle of the AC current, computes its average or RMS value and show this value in the digital display. IS THIS THE CASE???
Now, if I have an AC current continuously flowing being measured by the clamp ammeter and it has perfect sinusoidal waveforms (no distortion whatsoever) BUT the current cycles are turning 1 cycle ON and 1 cycle OFF of the usual 50 or 60 Hz (in this case it is half or 25 or 30 Hz)...Will it still SAMPLE ONE cycle and measure its average or RMS value? In this case, it will be the same reading as the former''s case reading BUT I believe this is NOT ACCURATE anymore as the current reading SHOULD be effectively only HALF of the former's reading because it is turning ON and OFF at 1 cycle each.
Are there any clamp meters which can read the average value NOT of the single cycle of the AC waveform but the average of the effective current, say, for 1 second timeframe???

Please enlighten me on this. Sorry for the very long narration. Just want to be clear and concise on this question. Thanks in advance!

Archie

I dont see why you need to shut off a cycle I believe it is measured as a field strength which will be bidirectional and in a ratio of the meters windings like a ct. It will be one cycle on in one direction then 1 cycle on in the opposite direction.

 

[gar]

120131-0441 EST

quogueelectric:

The circuit ArchieMedes is working with is used to modulate the energy to the load. With 1 full cycle to the load followed by one complete lost cycle results in an average power in the load of 1/2 that of a continuous sine wave, but no DC component. Make it 1/2 cycle on and 3 1/2 cycles off and the average power is 1/4, but now there is a DC component as well as the AC component.

We can assume with a resistive load and either an SCR or Triac switch that the turn-off will occur at both a current and voltage zero-crossing. Turn-on of the switch could be anywhere in the cycle. A malfunction of the turn-on function could cause a DC component, and an incorrect AC current. A failure of one SCR of a back-to-back pair to turn-on results in half cycling. Same can occur in a Triac,



T.M.:

Is a low resistance shunt with a voltmeter across the shunt in series with the load an improvised current meter? This is usually just called an ammeter. Why can't the load be the shunt? It is just a higher resistance shunt. It is still in series with the load.

A reason to possibly avoid a current transformer in this application is bandwidth and phase shift. Also in some of these meters we have no idea of how the measurement is really being performed.

.

 

[T.M.Haja Sahib]

120131-0441 EST

quogueelectric:

The circuit ArchieMedes is working with is used to modulate the energy to the load. With 1 full cycle to the load followed by one complete lost cycle results in an average power in the load of 1/2 that of a continuous sine wave, but no DC component. Make it 1/2 cycle on and 3 1/2 cycles off and the average power is 1/4, but now there is a DC component as well as the AC component.

We can assume with a resistive load and either an SCR or Triac switch that the turn-off will occur at both a current and voltage zero-crossing. Turn-on of the switch could be anywhere in the cycle. A malfunction of the turn-on function could cause a DC component, and an incorrect AC current. A failure of one SCR of a back-to-back pair to turn-on results in half cycling. Same can occur in a Triac,



T.M.:

Is a low resistance shunt with a voltmeter across the shunt in series with the load an improvised current meter? This is usually just called an ammeter. Why can't the load be the shunt? It is just a higher resistance shunt. It is still in series with the load.

A reason to possibly avoid a current transformer in this application is bandwidth and phase shift. Also in some of these meters we have no idea of how the measurement is really being performed.

.

gar
Thanks for your reply.Since the output waveform from the SCR can become distorted as you indicated above,using average reading meter is not a good idea,because the meter would not then give the actual value for diagnostic purposes.....

 

[gar]

120131-0859 EST

T.M.:

Since the output waveform from the SCR can become distorted as you indicated above,using average reading meter is not a good idea,because the meter would not then give the actual value for diagnostic purposes.....

An average reading meter does what it does. An RMS what it does. What does actual value for diagnostic purposes mean? RMS is not the only way to get useful measurements. In fact an average reading DC meter with a scale of -100 to +100 and a scaling resistor with no calibration except by use of a single diode and the continuous AC sine wave could be very useful. Also the same but with a bridge rectifier and now forget the -100 part and we also have a useful tool for the task at hand.

Maybe I can even analyze a problem without any meter. Put yourself back in 1878. The Weston-d'Arsonval meter did not exist. How are you going to measure voltage or current? How are you going to measure magnetic field intensity? How do you measure the losses in a generator?

Have you ever designed a new circuit and made it work? Have you invented anything? Have you created a new product? Have you ever had to think outside the box?

 

[T.M.Haja Sahib]

120131-0859 EST

T.M.:

An average reading meter does what it does. An RMS what it does. What does actual value for diagnostic purposes mean? RMS is not the only way to get useful measurements. In fact an average reading DC meter with a scale of -100 to +100 and a scaling resistor with no calibration except by use of a single diode and the continuous AC sine wave could be very useful. Also the same but with a bridge rectifier and now forget the -100 part and we also have a useful tool for the task at hand.

Maybe I can even analyze a problem without any meter. Put yourself back in 1878. The Weston-d'Arsonval meter did not exist. How are you going to measure voltage or current? How are you going to measure magnetic field intensity? How do you measure the losses in a generator?

Have you ever designed a new circuit and made it work? Have you invented anything? Have you created a new product? Have you ever had to think outside the box?

gar
Your greatness is readily acknowledged.
I just wanted to make the life of OP easier..........

 

[T.M.Haja Sahib]

What does actual value for diagnostic purposes mean?

Suppose the SCR's output waveform is distorted but has the same peak value as that of the normal sine wave.Then the average reading meter reads the peak value and displays only the RMS value corresponding to that value of normal sine wave, giving a false indication that everything is okay.But if a true RMS reading meter was used, it would have indicated the deviation and thereby the problem.

 

[gar]

120131-2345 EST

T.M.:

Wrong. An average reading meter when referencing a meter such as a Simpson 260 means. The input signal is full wave rectified and the average value of the signal is measured by a DC meter. Thus, for a steady state input signal of any waveform it is full wave rectified meaning the negative components of the waveform are inverted so the signal averaged by the DC meter is the average value of that waveform. In other words the area under the curve.

For a sine wave this average value is 0.636 of the voltage peak. The Simpson meter scale is calibrated so that the meter displays 0.707/0.636 times its actual DC value. This is a ratio of 1.112 . A sine wave with a peak of 10 V produces a full wave average DC value of 6.36 V, but the AC scale is marked 7.07 V.

If the rectifier is half wave, then the readings are halved.

If the input is a square wave of 10 V peak, then the AC reading will be 11.12 V .

If the input is a positive unidirectional pulse of 10 V with a 10 % duty cycle, then the reading in DC position is 1 V, and in AC is 1.112 V.

But the actual RMS value of the pulse is (100/10)^1/2 = 3.16 V RMS. Next remove the DC component, shift the curve down so the + area equals the - area. Now you calculated the RMS value.

For what I believe ArchieMedes needs to do, quality control, the average reading meter should be adequate.

.

 

[T.M.Haja Sahib]

120131-2345 EST


For a sine wave this average value is 0.636 of the voltage peak.........

Thanks.Please consider me your pupil and answer me.

If the waveform is distorted,what effect it will have average/peak reading meters?

 

[ArchieMedes]

120131-0441 EST

quogueelectric:

The circuit ArchieMedes is working with is used to modulate the energy to the load. With 1 full cycle to the load followed by one complete lost cycle results in an average power in the load of 1/2 that of a continuous sine wave, but no DC component. Make it 1/2 cycle on and 3 1/2 cycles off and the average power is 1/4, but now there is a DC component as well as the AC component.

We can assume with a resistive load and either an SCR or Triac switch that the turn-off will occur at both a current and voltage zero-crossing. Turn-on of the switch could be anywhere in the cycle. A malfunction of the turn-on function could cause a DC component, and an incorrect AC current. A failure of one SCR of a back-to-back pair to turn-on results in half cycling. Same can occur in a Triac,



T.M.:

Is a low resistance shunt with a voltmeter across the shunt in series with the load an improvised current meter? This is usually just called an ammeter. Why can't the load be the shunt? It is just a higher resistance shunt. It is still in series with the load.

A reason to possibly avoid a current transformer in this application is bandwidth and phase shift. Also in some of these meters we have no idea of how the measurement is really being performed.

.

Gar,

Yes this is absolutely correct. The application is to control the power to the load using the SCR by controlling on and off the pulse anywhere in the continuous cycle. Thereby controlling the average of power over a span of time. This is why I brought up the question on how to quantify the average power that the resistive load has received. This is in order to show that the SCR is controlling the load effectively.
Reading from the replies, I understand that I just need an average responding meter and no need for RMS or other power quality tools.

 

[PetrosA]

I got a response from my Agilent contact today which I'll post here in case it helps. My Extech contact has passed things on to their engineers and I should hear something soon

Hi Peter,


Looking at the post, it seems like Archie is trying to measure the amplitude for the SCR output current as below. I had a quick chat with the engineer and below are some of the key highlights.


Facts and discussion:

1. Almost all clamp meter are calibrated based upon precision sinewave references with RMS reading (both Average responding and True RMS meter are giving the RMS reading).

2. Waveform output from the SCR is NON-sinusoidal. ? True RMS meter should be used as Average Responding might not be able to measure non-sinusoidal signals correctly.

3. AC only jaw can only pick up AC signal based on transformer principal, AC/DC jaw based on Hall Effect principal. Most clamp meters are spec down to just 45Hz for AC current measurement, but AC/DC jaw has better chance to pick up lower frequency signal correctly.

4. The window of measurement could range from about 100ms or more as someone pointed. Bear in mind that the measurements are carried out continuously and it?s not gated. Hence, the reading on the meter will always show the latest RMS value of the input.

5. With the facts above, if accuracy is important, an oscilloscope or power quality analyzer might be a better instrument.

6. If the measurement needs to be reproduced on a Clamp Meter; as someone recommended, on a same output signal, use an oscilloscope to correlate the measurement value with a TRMS AC/DC clamp meter and treat this correlation as a golden value to understand the characteristics on the specific clamp meter to build up the measurement confidence. This upfront experimental step is needed before we can say if the clamp meter can measure the signal.

7. The other consideration is crest factor (CF=Peak to RMS ratio) of a meter. Typical TRMS meter is able to measure signal with crest factor of up to 3 at full-range with additional uncertainty.


Regards,

S. Y.

I think Gar may be better qualified to expand on this than I, but from what I understand from this an SCR signal would generally be outside the design limitations of an AC only clamp meter, and the performance of an AC/DC clamp would need to be verified before use with a possible correction factor applied to future measurements. If your equipment is to be sold in number, it may be worth designing test methods for troubleshooting which would include make and model of known "accurate" meters to use.

Hope this helps

 

[gar]

In response to post #37. Peak reading is just that.

If the signal being measured is full-wave rectified, then the peak reading is the largest value of the absolute value of the waveform. This could result from either the negative side or positive side depending upon the waveform. Have you calculated the RMS value of the pulse waveform after the DC component was removed that I previously presented to you? If so you should be able to tell me what is the maximum positive peak.

Before removing the DC component the peak voltage is 10 V, and I calculated the average at 1 V and RMS at 3.16 V.

I need your calculation of the RMS value after the DC component is removed, and your calculation of the DC component.

After removing the DC component the DC average is 0, and the full-wave rectified average is 1.8 V.

After removal of the DC component what is the peak positive value and the peak negative value.

If the waveform is distorted,what effect it will have average/peak reading meters?

The result is all over the map. It depends upon the waveform.


On to your post numbered 38. Post #1 is concerned with a full one cycle sine wave, no variable starting point in the cycle (not a phase shifted control of the turn-on point), followed by 0 or more complete cycle periods of 0 power to the load. The waveform illustrated is one full cycle on followed by one full cycle off. See post #3 for the waveform.

The reason I said it doesn't matter whether or not an average reading or RMS reading meter is used is because both will read 1/2 of the value of a continuous sine wave when the circuit is working correctly. The meter will read something else when the circuit is not working correctly, and it does not matter that average reading and RMS reading values are different. It just matters that the reading is not correct.

.