### Author Topic: Clamp Meters and Apparent Power  (Read 2254 times)

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#### killingtime

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##### Clamp Meters and Apparent Power
« on: March 20, 2023, 07:01:04 pm »
Hi,

Would be grateful if someone would sanity check my understanding of AC current meters..

If you have an AC device that's heavily reactive and consumes a lot of apparent power, am I right in saying that your run of the mill current meter (clamp or moving iron 2 wire) will register the reactive current flowing into and out of it?

So a clamp meter on the primary of an MOT (say) with open secondary would register the 3 amps of reactive current?

How does the real power consumed on top of this show on the meter? It's just a larger value right?

Can't find anything in meter manuals on this. Lots of articles explaining the difference between apparent and real power, but nothing on meters.

And you need a 4 wire setup (voltage and current) with phase measurement for real power measurements - something that clamp meters alone will never achieve.

Thanks.

#### RoGeorge

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##### Re: Clamp Meters and Apparent Power
« Reply #1 on: March 21, 2023, 10:40:57 am »
A clamp-meter only measures the current, so it can not calculate the active power correctly for reactive loads.

If the load is resistive, for example an incandescent light bulb, then the active power can be calculated close enough assuming the nominal mains voltage is known.  Unless the load is a big AC motor, or some heavily switched load without PFC (Power Factor Correction), then measuring only the current (i.e. with a clamp meter) can be close enough to calculate the active power.

For high precision measurements, or in any energy meters, all 3 must be known (U, I, and phase between U and I).

#### killingtime

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##### Re: Clamp Meters and Apparent Power
« Reply #2 on: March 21, 2023, 11:14:32 am »
Hi RoGeorge,

I'm aware of the theory.

My question was; does a clamp meter (or two wire ammeter) register (measure) purely reactive current.

Or to put the question another way, if you were to put a near ideal inductor across an ac supply (theoretically), would a clamp meter  register the current in any one wire feeding it?

In the above example, no real power is expended in the inductor, the current used to magnetize the inductor is fed back to the supply (ignoring resistive losses), so the net current flow is near zero. I think this is why some older mechanical kwh house meters don't record (or bill you) for reactive power used.

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##### Re: Clamp Meters and Apparent Power
« Reply #3 on: March 21, 2023, 11:21:05 am »
If you have an AC device that's heavily reactive and consumes a lot of apparent power

reactive load doesn't consume power, it just get it store for a very small period of time and then returns it back to the mains on the AC next cycle. So, when your device has reactive part of impedance, then there is active part of power which is consumed by device and reactive part of power which flows back and forth on every AC cycle from mains to device and back from device to mains. Usually home power counters doesn't count reactive part of power they should take into account just active one. But industrial power counters (for factory and high power machines) can count both - active and reactive part of power separately. This is because reactive part of power add energy loss in wires (due to wire heating), and when load consumes very high power that losses may be significant and needs to be taken into account.

Since reactive current flows back and forth through wire in both directions, you cannot see it with a clamp meter, just because clamp meter measure average current over some period of time, so when there are two currents +Ir and -Ir which flows in opposite direction during AC cycle, they will cancel each other on average result. But it depends on AC meter implementation, in some cases reactive current can affect measurement result in unpredictable way, so you can see some random error on some meters when there is reactive current is present. Reactive current can distort AC sine wave, so it also can affect measurement, because AC current measurement depends on a waveform.

As said before, in order to measure reactive power, there is need both measurement simultaneously - voltage and current and phase delay between them.
« Last Edit: March 21, 2023, 11:49:30 am by radiolistener »

#### armandine2

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##### Re: Clamp Meters and Apparent Power
« Reply #4 on: March 21, 2023, 11:32:48 am »
CA F205 Clamp Meter

not sure if this is "run of the mill" but it looks like it will give VA var and pf

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#### killingtime

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##### Re: Clamp Meters and Apparent Power
« Reply #5 on: March 21, 2023, 12:34:00 pm »
CA F205 Clamp Meter
not sure if this is "run of the mill" but it looks like it will give VA var and pf
Yes it can, but it needs a voltage measurement to get the relative phase. I was asking after just current clamp meters. Good to know anyway.

.....
Since reactive current flows back and forth through wire in both directions, you cannot see it with a clamp meter, just because clamp meter measure average current over some period of time, so when there are two currents +Ir and -Ir which flows in opposite direction during AC cycle, they will cancel each other on average result.
......

This answers my question. I don't wan't to measure reactive current specifically, I just wanted to know what el-chepo AC current clamp meters actually measured. If it's the slow averaging on the meter side that causes a purely reactive current to read as zero, then using an AC clamp with a much faster response would work - like an O-scope current clamp with a bandwidth of a few 100 KHz. Still just a single clamp over a conductor, but if the bandwidth is there, you'd see the reactive current flowing both ways - for a purely reactive load. In reality there be some resistive component to the load as well, so you'd see the combination of both currents as one waveform.

If my understanding of this is correct, that's a limitation of current clamp meters (and moving iron meters) I wasn't aware of.
« Last Edit: March 21, 2023, 12:45:03 pm by killingtime »

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##### Re: Clamp Meters and Apparent Power
« Reply #6 on: March 21, 2023, 12:44:36 pm »
Some nonsense is written above.  The clamp meter measures current.  It cannot know the phase angle between current and voltage so it cannot know what is real current and what is reactive current.  Real current and reactive current show on the meter just the same.
« Last Edit: March 21, 2023, 12:46:13 pm by wasedadoc »

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#### killingtime

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##### Re: Clamp Meters and Apparent Power
« Reply #7 on: March 21, 2023, 12:52:33 pm »
Some nonsense is written above.  The clamp meter measures current.  It cannot know the phase angle between current and voltage so it cannot know what is real current and what is reactive current.  Real current and reactive current show on the meter just the same.
>>The clamp meter measures current
Yes it does, in one direction or the other, shown as a + or - on the display. For a very reactive load (like a capacitor) with very little real resistance, the current is going to flow back and forth within each cycle, averaging the result out if the bandwidth of the meter is many times that of the AC frequency...
If I'm wrong, please enlighten me. Happy to say I'm wrong.
>> It cannot know the phase angle between current and voltage so it cannot know what is real current and what is reactive current.
Agreed.
>>Real current and reactive current show on the meter just the same.
Not sure what you meant by that.

---EDIT---

Perhaps you are right wasedadoc. After a little more thought, even for a resistive load on an AC supply, the current reverses (flows back and forth) on each half cycle. If the meter were to average these currents out, the net result would be zero, and that would defeat the purpose of an AC ammeter. Guess I'm going to have to run a test to find out.
« Last Edit: March 21, 2023, 01:08:24 pm by killingtime »

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##### Re: Clamp Meters and Apparent Power
« Reply #8 on: March 21, 2023, 01:01:45 pm »
Some nonsense is written above.  The clamp meter measures current.  It cannot know the phase angle between current and voltage so it cannot know what is real current and what is reactive current.  Real current and reactive current show on the meter just the same.
>>The clamp meter measures current
Yes it does, in one direction or the other, shown as a + or - on the display. For a very reactive load (like a capacitor) with very little real resistance, the current is going to flow back and forth within each cycle, averaging the result out if the bandwidth of the meter is many times that of the AC frequency...
If I'm wrong, please enlighten me. Happy to say I'm wrong.
>> It cannot know the phase angle between current and voltage so it cannot know what is real current and what is reactive current.
Agreed.
>>Real current and reactive current show on the meter just the same.
Not sure what you meant by that.
Reactive current is only applicable when AC (alternating current) is involved.  A clamp meter never shows + or - when set to measure alternating current.  Some clamp meters can measure DC current and in that case will indicate + or -.

The reasoning that reactive current is going back and forwards and so cancels out is baloney.  Real current flows back and forth too.  What is the difference?

"shows the same" means that whether the current is N Amps reactive or N Amps real, the clamp meter will show N Amps.

And if there is both real and reactive current the clamp meter will show the vector sum.  Square root of the sum of the squares of the real and reactive currents.
« Last Edit: March 21, 2023, 01:06:46 pm by wasedadoc »

#### killingtime

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##### Re: Clamp Meters and Apparent Power
« Reply #9 on: March 21, 2023, 01:05:16 pm »
Reactive current is only applicable when AC (alternating current) is involved.  A clamp meter never shows + or - when set to measure alternating current.  Some clamp meters can measure DC current and in that case will indicate + or -.

The reasoning that reactive current is going back and forwards and so cancels out is baloney.  Real current flows back and forth too.  What is the difference?

"shows the same" means that whether the current is N Amps reactive or N Amps real, the clamp meter will show N Amps.

After a bit more thought I'm inclined to agree with you. It would help if I owned an AC clamp meter. One on order.

#### bdunham7

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##### Re: Clamp Meters and Apparent Power
« Reply #10 on: March 21, 2023, 01:52:58 pm »
And if there is both real and reactive current the clamp meter will show the vector sum.  Square root of the sum of the squares of the real and reactive currents.

To be fair, the terminology itself can be misleading and maybe the term 'recirculating' current captures the reality a bit better.  The point of common misunderstanding (as shown in this thread) is that both real and reactive (or recirculating) currents are entirely real.  Current meters measure them, as they should, and the wires that carry them heat up in proportion to (I2real + I2reactive) * R.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.

#### TimFox

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##### Re: Clamp Meters and Apparent Power
« Reply #11 on: March 21, 2023, 03:56:31 pm »
A clamp or two-terminal AC ammeter will measure the current through the wire.
It may sense true-rms current or mean-absolute-value ("average") current, depending on the ammeter's circuitry.
It can't tell the difference between "real" and "imaginary" components of the current without a phase reference.
For real power measurements, that phase reference is the AC voltage in the circuit, not seen by the ammeter.

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#### BeBuLamar

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##### Re: Clamp Meters and Apparent Power
« Reply #12 on: March 21, 2023, 10:45:53 pm »
The amp meter either clamp or in series meter measures current regardless if it's reactive or not. The meter doesn't know if the current is in phase with the voltage or if it's out of phase with the voltage.

#### MrAl

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##### Re: Clamp Meters and Apparent Power
« Reply #13 on: March 22, 2023, 12:09:07 am »
Hi,

Would be grateful if someone would sanity check my understanding of AC current meters..

If you have an AC device that's heavily reactive and consumes a lot of apparent power, am I right in saying that your run of the mill current meter (clamp or moving iron 2 wire) will register the reactive current flowing into and out of it?

So a clamp meter on the primary of an MOT (say) with open secondary would register the 3 amps of reactive current?

How does the real power consumed on top of this show on the meter? It's just a larger value right?

Can't find anything in meter manuals on this. Lots of articles explaining the difference between apparent and real power, but nothing on meters.

And you need a 4 wire setup (voltage and current) with phase measurement for real power measurements - something that clamp meters alone will never achieve.

Thanks.

Hi,

With a purely real load like a resistor the meter reads the entire real part of the current.
With a purely reactive load like an inductor the meter reads the entire reactive part of the current.
With a partly real and partly reactive load the meter will read the total current as the 'norm' of the reactive and real parts of the current.  This means you take the square root of the sum of the reactive squared and real part squared.  That will show on the current meter either an AC clamp-on meter or a regular AC current meter.

This is best understood using complex numbers.  Complex numbers have a real and what is called an imaginary part.  That just means that it takes two numbers to specify one complex number.  The imaginary part is shown by a multiplication by the lower case "j" in the English alphabet.  For example for X a complex number:
X=2+3*j
Here X is a complex number, and the real part is 2 and the imaginary part is just 3 (not 3*j).
The imaginary part can be negative, and the real part can be negative, so we might see:
X=2-3*j
where the imaginary part is now -3, and we might see:
X=-2+3*j or -2-3*j
where in the first the real part is -2 and the imaginary part is 3, and in the second the real part is -2 and the imaginary part is -3.

This relates directly to the current measurement of a load that is both reactive and resistive so it is quite handy to know.

To understand this clearly it is probably best to start with a resistor in parallel with an inductor.  That way you can measure the current through the resistor by clamping onto only the resistor lead, and you can measure the reactive current by clamping onto only the inductor lead, and you can measure the total current (both reactive and resistive) by measuring the lead that connects them to an AC voltage source such as 100vac at 50Hz.

With that in mind, say we  have a 100 Ohm resistor in parallel with an inductor of 0.3183 Henries (318.3mH).  That's a large inductor but we just want to use that as an example.
It should be obvious that they are both in parallel with the line voltage of 100 volts, so the current in the resistor is:
iR=100/100=1 amp

and the current in the inductor is:
iL=100/(w*0.3183)
where w is 2*pi*f with f the frequency which is 50Hz.
With w=2*pi*50 and that inductor value we get:
iL=1 amp (very nearly so actually)

So the resistive current (real part) is 1 amp and the reactive current is 1 amp.  That is what you would measure with the current meter.

The TOTAL current, the current from the line, would be the square root of the sum of the squares of the two:
iT=sqrt(1^2+1^2)=sqrt(2)=1.4142 amps.  That is the current you would measure in the line itself.  The simple current meter can not distinguish between the resistive and reactive parts once the two components are in parallel, and that is what you get with a load that is both resistive and reactive.

So you see we have three different current levels to consider.

Now the phase difference, if you care to know that or measure that, would simply be the atan() of the reactive part divided by the resistive part.
In this case they are both the same  so first writing it out:
ph=atan(reactivecurrent/resistivecurrent)

so we have:
ph=atan(1/1)=atan(1)=45 degrees.

Note this is the principle angle which does not show if one leads or lags the other. To get that information it is a good idea to use the expressions that have been used since the dawn of time:
ELI the ICE man.
This helps to remember that in an inductor (L) the voltage (E) leads the current (I),
and in a capacitor (C) the current (I) leads the voltage (E).
Since upper case i's can look like 1's i'll write the i's in lower case:
"ELi the iCE man"
and here the lower case 'i' is the current.

The most important part of this though is the way the reactive part and resistive parts are summed.  Note in the above example we did not just add the two 1+1=2 we had to take the square of each one (1*1 and 1*1) and add those together, then take the square root of that result.
This is easy to do on a hand calculator or of course a calculator you have on your computer.

If any of this is not clear just let me know.  We can go though some more examples.

To calculate power, we need to know the phase angle.
In a DC circuit the power is:
P=I*E

but in an AC circuit with both resistive and reactive parts, the power is:
P=I*E*cos(ph)

where ph is the phase angle and I is measured in the line lead which is the total current.
Again this is easy to calculate with a hand calculator.
In the above example we had E=100 and I=1.4142 and ph=45 so we would get:
P=100*1.4142*cos(45)=100 watts.
It is interesting to note that this is ONLY the power in the resistor, as there is no power dissipated in the inductor with an ideal inductor.  That means we could have, in this example, used just the voltage and the current in the resistor to get the real power:
P=100*1=100 watts.
Small side note:
If this was a series circuit instead of parallel, we would have to calculate the voltage across the resistor to do it this way:
P=vR*I
because then the resistor is not connected directly across the line.
« Last Edit: March 22, 2023, 12:24:29 am by MrAl »

#### rstofer

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##### Re: Clamp Meters and Apparent Power
« Reply #14 on: March 22, 2023, 03:50:06 pm »
More about ELI the ICE man...  The attached plot is for a series RC circuit driven at 1 kHz and, I believe, 1V P-P (but I might be off on that)  The R value is 10k and the C value is 0.1 ufd for a time constant of 1 ms the reciprocal of which is 1 kHz.

Note that the current waveform (orange) goes through the X axis before the voltage waveform (blue).  The current is leading the voltage and the circuit is capacitive - hence ICE (I leads E in a Capacitive circuit).  Note that the phase shift is approx 250 us which is about 1/4 of 1 ms which is 90 degrees (1/4 of 360 degrees) and this is what we expect.  A 90 degree phase shift.

This experiment was done with a Digilent Analog Discovery 2 and the related Waveforms software.  The cursors aren't perfectly placed but are close enough for my use.
« Last Edit: March 22, 2023, 04:08:37 pm by rstofer »

#### TimFox

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##### Re: Clamp Meters and Apparent Power
« Reply #15 on: March 22, 2023, 04:14:56 pm »
Again, the illustration in the response above requires measurement of both the voltage (from the wire to some reference point) and the current (through the wire).
This requires two separate devices:  an ammeter and a voltmeter.

#### rstofer

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##### Re: Clamp Meters and Apparent Power
« Reply #16 on: March 22, 2023, 05:17:37 pm »
Again, the illustration in the response above requires measurement of both the voltage (from the wire to some reference point) and the current (through the wire).
This requires two separate devices:  an ammeter and a voltmeter.

I don't have a clamp-on that simultaneously measures voltage and current.  It would be nice but I'm still using the Amprobe meter I bought around 1965.  Still works...

One neat thing about the AD2 is the fact that both input channels are differential.  I can use one channel to measure the voltage across the resistor (hence the current waveform) where neither end is ground and the second channel to measure the voltage across the capacitor.

#### Ground_Loop

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##### Re: Clamp Meters and Apparent Power
« Reply #17 on: March 22, 2023, 07:33:22 pm »
A clamp on ammeter measures current and current only.  There is no differentiation between real or reactive current.
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#### rstofer

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##### Re: Clamp Meters and Apparent Power
« Reply #18 on: March 22, 2023, 07:39:46 pm »
A clamp on ammeter measures current and current only.  There is no differentiation between real or reactive current.

Maybe some are better than that:

https://www.amazon.com/Extech-380976-K-Single-Phase-Three/dp/B008465KMU

Quote
Measures True Power, Apparent Power, Reactive Power, Horsepower, Power Factor, and Phase Angle with Lead/Lag indicator

#### Ground_Loop

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##### Re: Clamp Meters and Apparent Power
« Reply #19 on: March 22, 2023, 07:48:47 pm »
A clamp on ammeter measures current and current only.  There is no differentiation between real or reactive current.

Maybe some are better than that:

https://www.amazon.com/Extech-380976-K-Single-Phase-Three/dp/B008465KMU

Quote
Measures True Power, Apparent Power, Reactive Power, Horsepower, Power Factor, and Phase Angle with Lead/Lag indicator

It still has to measure both voltage and current.  Power cannot be measured.  It is calculated based on voltage and current measurements.  A simple ammeter only measures current.
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##### Re: Clamp Meters and Apparent Power
« Reply #20 on: March 22, 2023, 07:49:53 pm »
A clamp on ammeter measures current and current only.  There is no differentiation between real or reactive current.

Maybe some are better than that:

https://www.amazon.com/Extech-380976-K-Single-Phase-Three/dp/B008465KMU
Measures True Power, Apparent Power, Reactive Power, Horsepower, Power Factor, and Phase Angle with Lead/Lag indicator

Maybe so but:

1.  To do those extras you need to contact the leaded probes to the voltage points at the same time as putting the clamp around the current carrying wire.

2.  The clamp does not measure Direct Current, only AC.

3.  Small DC currents can only be measured by putting the meter in series in the same way as a conventional multimeter.
« Last Edit: March 22, 2023, 07:52:43 pm by wasedadoc »

#### coppice

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##### Re: Clamp Meters and Apparent Power
« Reply #21 on: March 22, 2023, 07:56:30 pm »
A clamp on ammeter measures current and current only.  There is no differentiation between real or reactive current.

Maybe some are better than that:

https://www.amazon.com/Extech-380976-K-Single-Phase-Three/dp/B008465KMU

Quote
Measures True Power, Apparent Power, Reactive Power, Horsepower, Power Factor, and Phase Angle with Lead/Lag indicator
That's not a current meter. Its a power meter. Connect only the clamp and it measures only the current. Connect only the voltage sense leads and it measures only the voltage. Connect the clamp and the leads and it uses both the instantaneous voltage and the instantaneous current, and can give you all the measurements you listed.

#### rstofer

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##### Re: Clamp Meters and Apparent Power
« Reply #22 on: March 22, 2023, 08:29:55 pm »
No, the referenced meter is NOT a simple clamp-on ammeter but it does answer the questions re: power factor, etc.  Adding the voltage probes is all it takes and they have done just that.  I'm sure there are many other meters that do the same thing.

And it will still operate as a simple clamp-on ammeter if that is all that is required.

#### coppice

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##### Re: Clamp Meters and Apparent Power
« Reply #23 on: March 22, 2023, 08:48:31 pm »
No, the referenced meter is NOT a simple clamp-on ammeter but it does answer the questions re: power factor, etc.  Adding the voltage probes is all it takes and they have done just that.  I'm sure there are many other meters that do the same thing.

And it will still operate as a simple clamp-on ammeter if that is all that is required.
The utility meter market has driven the cost of making a meter like that Extech one, with accuracies down to 0.1%, very low. There are a few other similar products, but not many. Its a niche market compared to multi-meters or clamp on current meters. Oddly there are multi-meters with a current clamp, which have all the sockets and leads, where the incremental cost of accurate active, reactive, apparent power, and power factor would be small, yet they haven't all migrated in that direction. I guess there is low demand.

#### BeBuLamar

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##### Re: Clamp Meters and Apparent Power
« Reply #24 on: March 22, 2023, 10:48:02 pm »

2.  The clamp does not measure Direct Current, only AC.

3.  Small DC currents can only be measured by putting the meter in series in the same way as a conventional multimeter.

Many clamps can measure DC current. Fewer clamps can measure small DC current but some can measure down to 5mA.

Smf