Measuring RMS current / voltage
Measuring RMS current / voltage
(OP)
We are using a solid state relay to control current / voltage to a resistive load. 480V single phase.
Using a Fluke meter, is there a way to determine the RMS current / voltage?
The meter bounces around. For instance, it will show 0.95 amps to 1.11 amps. Is the mid-point the rms current? The meter will also give a peak amps. If we divide the peak amps by 1.414, is it equal to the rms current?
TYIA,
Heaterguy
Using a Fluke meter, is there a way to determine the RMS current / voltage?
The meter bounces around. For instance, it will show 0.95 amps to 1.11 amps. Is the mid-point the rms current? The meter will also give a peak amps. If we divide the peak amps by 1.414, is it equal to the rms current?
TYIA,
Heaterguy





RE: Measuring RMS current / voltage
But the AC is still 60 Hz and it shouldn't bounce around.
If you're getting a variable reading on a simple resistive load, then something isn't right. Is the voltage steady?
The 1.414 ratio applies only to pure sin waves (like nice clean AC). There is no assurance that your variability is a sin wave (I'd guess it isn't).
Does the meter have an averaging capability? If so, then use it.
RE: Measuring RMS current / voltage
I should emphasize that we are using solid state relays to control the voltage / current. When we measured 0.95 to 1.11 amps, the relay was at 30% power. So we probably had 3 cycles on and 10 cycles off or some multiple of that ratio.
Regards,
Heaterguy
RE: Measuring RMS current / voltage
Next if you are not using a zero crossing relay you cannot play the 1.414 game at all because that only works for a pure sine wave!
If your control signal is relatively steady then you can use a "TRUE RMS" type meter which most of the newer meters are now. It will say that on it somewhere!
But if your control swings a lot then you will need to use some sort of long average meter since typical hand-helds have a fixed sample rate of around a 1/3 of a second. The 1/3 of a second can beat with your control driving you nuts while you try to get the average.
RE: Measuring RMS current / voltage
If your peak current sampling time is less than 1.6 ms, .0016 seconds, you should be able to get an accurate enough peak amp reading to divide it by 1.414 to get the RMS current. If your sampling time is larger your accuracy will vary proportionally.
Barry1961
RE: Measuring RMS current / voltage
Barry1961
RE: Measuring RMS current / voltage
RE: Measuring RMS current / voltage
If the relay is pulsing, and of course the Fluke meter has it's own sampling time, then you may run into all sorts of subsampling issues if the two rates are close. In other words, the periods might be going in and out of phase leading to an apparent variation at a much slower rate than either of them. What a mess...
Can your Fluke average multiple readings?
RE: Measuring RMS current / voltage
No averaging on this meter. It does have peak.
If we were to purchase a meter for this application what feature should we request (rms, averaging, ??)?
Regards,
Craig
RE: Measuring RMS current / voltage
Two ways you can do this. One would be a high speed sampling system. Monitor the current and voltage simultaneously and equate the power averaging it for whatever period you want.
OR
Get a power watthour meter and let it run for say 30 minutes to a stopwatch then divide the watthours by time giving you precisely the average wattage.
OR
If you actually KNOW the percentage of ON time... Just turn the heater on full long enough to measure 100% current. Multiply that by the also measured voltage. This gives you the 100% power. Then it's simply the percentage of the 100% that you are running.
RE: Measuring RMS current / voltage
If you are forced to use a DVOM, then you'll need to average over a sufficiantly long time that the on/off cycles don't leave a significant residual depending on the relative start-stop timing.
If the in-rush current surge is not significant, then perhaps you could just measure the simple steady-state On current and multiply by the duty cycle (simple timing).
RE: Measuring RMS current / voltage
Thanks
RE: Measuring RMS current / voltage
If the current in-rush isn't significant, then it should be easy to measure the On current (back to the DVOM?), perhaps with the relay 100% on for the duration of this measurement (if that is okay?). Then it becomes a simple duty cycle problem.
If there IS a significant in-rush current surge, then things get complicated, and you might want to find a DVOM with an averaging feature over a suficiently long period to average out many of the On/Off cycles.
RE: Measuring RMS current / voltage
If this is a "resistance" load and not a "tungsten" load then the fact that the element is at fairly steady state temperature means any inrush is unlikely.
heater guy; You need to look closely at my last response there in lies your answer.
RE: Measuring RMS current / voltage
We don't have a high speed sampling system, nor a power watthour meter, nor do we know the percentage of ON time. If the oscillosope method doesn't work, we can invest in either a high speed sampling system or a watthour meter.
The load is resistive and there is little if any in rush current.
We have the oscilloscope hooked up and we are trying to count the on / off cycles. This is similar to your high speed sampling system? Will the oscillosope method work?
Regards,
Heaterguy
RE: Measuring RMS current / voltage
At 60 Hz? I don't think so.
"...cost a bundle..."
I was half-assuming that he might jury-rig the Fluke probe into the 'scope just to check the shape of the current in-rush. There are probably other options, but I can't see his gear through the Internet.
"...inrush is unlikely."
I agree, but worth checking if the in-rush time is a significant fraction of the on-time. It wouldn't take much for this to be a significant source of error. If the heater is a typical enclosed-in-a-tube, then its temperature can vary from ambient to red-hot even if it is immersed in a constant temperature bath. I agree that significant in-rush is not likely, but it could be THE largest source of error if you assume it wrong. I'd check because I just like to be careful...
I think that we both agree that it can be moved to the time domain (duty-cycle) once you know the 100% load (assuming it is flat-topped).
RE: Measuring RMS current / voltage
I am trying to be diplomatic to V1BLL. I have to make the measurements you are talking about on a regular basis since I design heater controls and PIDs for a living. I cannot see how a scope will help you in the least. Unless you don't care if your measurements are off 10-20%. Which I gathered you did care about, as your original measurements are probably 'that' close.
You haven't described anything else about your control system or what kind of heater or what the heater is in, and why you need the measurement so we are all kinda shooting in the dark er... cold here.
But you have stated(up top)that the "relay was at 30%". If you know the percentage and lots of time you can know this, because you can set controllers to fixed power settings, then my third option above works very well! Using a scope will drive you nuts for this kind of measurement and I wouldn't stake a dime on the results.
Another problem, again because we have no details, is the control signal to power translation. Zero crossing SSRs can have unexpected differences. Looking at the control side of an SSR DOES NOT give an accurate picture of what is happening on the power side. There are so many variables here it would take me an hour to describe.. It boils down to: If you want to know the power, measure the power on the power side. If the power control is varying constantly and you want to know the average process power, then take a long baseline average(minutes)(watthour scheme).
RE: Measuring RMS current / voltage
We can count the number of cycles on/off; however, the waveforms are not identical. Sometimes the first wave on is a normal size and sometimes it much smaller (about 10-40%). Therefore, itsmoked is correct. The proper way to measure is with a power meter.
We do not own a power meter. Does anyone have a suggestion of one to purchase?
RE: Measuring RMS current / voltage
Your controlling device
The signal from the controller to the SSR
Your SSR
Ed
RE: Measuring RMS current / voltage
http://www.powermeterstore.com/plug/wattsup.php
I use the standard one. For what you want to do, you may want to drop the additional 40 bucks for software and communications stuff on the "pro".
Check its specs etc. You need to be able to run your heater power wiring through it.
I have used them to audit everything in my house and many work projects and computers. They quickly answer the "how much does it cost" to leave my PC on all the time questions.
If you are running something other than 120Vac we will need to come up with something else.
RE: Measuring RMS current / voltage
RE: Measuring RMS current / voltage
If you need to digitize the signal, use a CT with a burden resistor to convert to a voltage, rectify it and make an RC filter for it with a cap with a resistor in series and another one in parallel. The RC filter will give you a good average of the current value as a voltage level, which you can feed into a meter or an A/D converter.
"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more." Nikola Tesla
Member, P3
Warpspeed (Automotive)
It sounds to me like a conventional solid state relay being driven by some sort of PID on/off temperature controller.
VE1BLL (Military)
The reason that I mentioned using a 'scope is that we've probably all seen cases where a DVOM (even a nice Fluke) is fooled by a waveform that doesn't match the inherent assumptions in reducing a time-varying signal to a simple numerical reading (even with a cute bar graph).

heaterguy (Mechanical)
(OP)
You guys are good!
itsmoked (Electrical)
Confirm you are using a "POWER CONTROLLER" or a POWER METER?
heaterguy (Mechanical)
(OP)
We just finished trying a FineTek PF-3050 and guess what...it's bouncing around just like the Fluke did. We are going to dig into it a little further. Perhaps there's an averaging feature.
itsmoked (Electrical)
Blurb

ScottyUK (Electrical)
Web link: http://www.fine-tek.com/Product-2004118142750.html
Barry1961 (Industrial)
A friend on mine got one of those water heaters in his house. He turned off all the power in the house then ran hot water for 10 minutes and read the watt meter outside. This is probably the best test since it is what you will be billed by.
Warpspeed (Automotive)
Another totally different approach would be to measure load voltage (or current), feed this into a true Rms to dc analog converter chip such as an AD536, and use the Rms corrected output to drive a reasonably high frequency voltage controlled oscillator.
heaterguy (Mechanical)
(OP)
WOW Warpspeed, you really are traveling fast. We will probably use an averaging power meter or a true RMS amp / volt meter.
itsmoked (Electrical)
Repeating for us all... A "true RMS amp / volt meter". Is NOT what you need.



Warpspeed (Automotive)
It is if it hooked up to a TOTALISER that accumulates instantaneous power readings over time, hehehe.
itsmoked (Electrical)
grrrrrrrr.
fangas (Electrical)
Found around most factories:
heaterguy (Mechanical)
(OP)
itsmoked, Looking at your suggestion for the watts up power meter, it only takes 120V input. We are using 480V.
LionelHutz (Electrical)
We have used a Fluke 43B power meter. It will give you instantaneous and average power measurements. You can let it record for a period of time and it will tell you the average power. You don't even need to run for the whole time selected since it gives a running average power measurement.
RudyL54 (Electrical)
Normally when you use a multi-meter to measure voltages or currents, the measured values are in RMS. I hope you set it up to the right AC voltage range.
heaterguy (Mechanical)
(OP)
What do you think about this device?
itsmoked (Electrical)
I have no direct experience with it/them. I despise companies that demand information from ME before they will give you specs of their device... In fact that is enough of a reason from me to shop elsewhere.
ScottyUK (Electrical)
Have a look at
/www.volte ch.com/pro ducts/pwr_ anl/pm3000 /index.htm
ttp://www. yokogawa.c om/tm/wtpz /wtpz/tm-w tpz_01.htm
heaterguy (Mechanical)
(OP)
itsmoked, Go to this website for more information on the ION:
ScottyUK (Electrical)
Hi heaterguy,
.geindustr ial.com/cw c/products ?pnlid=6&a mp;famid=2 0&cati d=60&i d=pqm& lang=en_US
heaterguy (Mechanical)
(OP)
ScottyUK, that's way more than we need. What do you think of the ION?
ScottyUK (Electrical)
I think it will struggle with the horrible load you have got.
itsmoked (Electrical)
1'st choice panel, huh...
//www.simp sonelectri c.com/pdf/ GIMA%20dat asheet.pdf
w.grainger .com/Grain ger/wwg/it emDetailsR ender.shtm l?xi=xi&am p;ItemId=1 613496960& amp;ccitem=
heaterguy (Mechanical)
(OP)
A guy in our engineering department knew exactly what we needed. Check this out...
://www.loa dcontrols. com/produc ts/upc_pro ducts.html
VE1BLL (Military)
"Three Balanced Hall Effect Sensors Sample Voltage Directly"
ScottyUK (Electrical)
I lifted the following directly from LEM's website. One way of using Hall Effect to measure voltage:
VE1BLL (Military)
Complicated, but provides isolation. Thanks.
LionelHutz (Electrical)
I guess I'm not sure what you're looking for. Is this for a production run or for a single use application?
itsmoked (Electrical)
Great points LionelHutz..
bill318 (Electrical)
If it is just a one time measurement your after you could possibly place your resistive load in a styrofoam container (cheap camping cooler), fill with distilled water and replace the lid. Then poke a good thermometer in the water and run the device for a specific amount of time and then measure the temperature rise. This method does not care what the wave shape is.
Read the Eng-Tips Site Policies at FAQ731-376
RE: Measuring RMS current / voltage
It can turn on instantly at any time, stay on for some indeterminate number of cycles, then commutate itself off at the end of the last zero crossing. The on/off cyclic rate could be short or long ?? I am not surprised your Fluke meter is jumping around.
The crude way to measure may be with a rotating disc Kwh meter, or perhaps one of the newer all electronic Kwh meters that can display voltage and current independently. That should have true Rms software that can respond to sudden step changes, or do you need true power input integrated over time?
RE: Measuring RMS current / voltage
The varying reading was actually a lucky break, a lesser DVOM might have just provided the wrong answer with all the self-confidence of a D^4 (dumb digital display device).
The 'scope will show you what the signal actually looks like over any time scale you'd like to dial in. I wasn't suggesting that the 'scope be used to measure the final numbers, just to help figure out what's going on. Looks like it worked...
A power meter sounds ideal for, ah, er measuring power.
RE: Measuring RMS current / voltage
We found a power controller that also measures harmonics, power factors, etc. We are hooking that up to see if we can get a good read on the power.
The downstream heater has constant resistance, so with a power meter we can figure the voltage and current. Actually, it's the power that we want to know anyway.
It's good that we tried the oscilloscope, because we might have been fooled into thinking that each cycle was idential. This kind of information is valuable to us.
Jraef, we initially thought we could average between the peaks and valleys. If this power meter works today, we can see the accuracy of that method.
Yes, the SSR's are zero-cross.
RE: Measuring RMS current / voltage
You say "controller".
What the heck, I'll ask.. What kind are you considering? We might be able to approve or disapprove its suitability for your needs.
RE: Measuring RMS current / voltage
RE: Measuring RMS current / voltage
http://www.fine-tek.com/series5.html
Details
http://www.fine-tek.com/pdf/PF.pdf
{takes about 60seconds}
Huh doesn't seem to indicate the ability to average over X minutes or seconds...
I think you need a long average like 10+ seconds. Or an energy totalizer that you can divide by any time.
RE: Measuring RMS current / voltage
Looks like a low-end instantaneous meter designed for panel mounting. Probably not bad on a load which isn't using integral cycle control, but I can see why it would struggle with this type of load. You need something with an averaging or integrating feature, not just instantaeous readings. This instrument doesn't seem to have one.
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One day my ship will come in.
But with my luck, I'll be at the airport!
RE: Measuring RMS current / voltage
I guess it would change as the incoming water temperature changed but at least it would be a start. The colder the water the more “on” cycles.
I went over to fix it for him about 6 years later. The heating element had burned out. If I remember it was about $10(USD). He loved it and was sure it saved him a lot of money.
Barry1961
RE: Measuring RMS current / voltage
What happens is that as the voltage (or current) goes up and down each half cycle, the frequency of the VCO follows it instantaneously. It could be for example 1Hz per Rms milliamp or 1Hz per Rms volt.
So the number of cycles out of the oscillator correspond to the number of Rms milliamps (or volts) in the load.
All you need then is a long digital divider to turn this into suitable units that can then operate an electromechanical (or electronic) totaliser.
For instance if each oscillator cycle equals 1mA
Divide by 1,000 to get 1 Amp per pulse.
Divide by 60,000 for one pulse per Amp/minute
Divide by 3,600,000 for one click of the counter each amp hour, or whatever.
The output could just as easily be scaled directly in power, provided the load resistance is known and constant. Watt seconds, or Kwh maybe ?
The whole thing can be calibrated by adjustment of the voltage controlled oscillator, so really odd division ratios are not required. Powers of two in the divider chain are always more convenient, but that may require an odd oscillator frequency.
It is just as easy to calibrate something to run at say 4,096 Hz as at exactly 4,000 Hz for example, so that no problem.
I have used a system like this for measuring amp hours of battery discharge, but with an Rms to dc converter it should work equally well with ac.
RE: Measuring RMS current / voltage
RE: Measuring RMS current / voltage
RE: Measuring RMS current / voltage
RE: Measuring RMS current / voltage
RE: Measuring RMS current / voltage
A counter (high speed)
A 460 X 150 transformer
A timer
needed; 115 X 12 doorbell x'former
Parallel the 480 X 115 transformer w/ the load
Feed the secondary to doorbell transformer
Feed 2nd'er of doorbell thru diode to counter, through timer, to counter
Set timer to 1 Hr
come back later, read counter
counter value / 216000 (cycles per hour)
Answer will provide the "On" time percentage over 1 hr
Do the math.
Granted might need to scale counter, but still pretty cheep & dirty
Ed
RE: Measuring RMS current / voltage
RE: Measuring RMS current / voltage
It is a single phase/balanced 3-phase power meter. By this, I mean it uses 1CT and 2 volt leads to do single phase measurements but it will also calculate 3-phase measurements based on the 3-phases being balanced.
The meter will also do the same measurements with harmonics, voltages, currents, power factor and also works as a simple scope.
The downside is it's not a real cheap meter.
RE: Measuring RMS current / voltage
If the meter reading contantly varies, it is probably the variations of the load that created that.
Are you sure, your meter is not defective?
The best approach is get the average reading.
RE: Measuring RMS current / voltage
http://www.pwrm.com/products/ION6200/
RE: Measuring RMS current / voltage
It does look like it covers everything it may be more than you need.
I would just go to the local electrician supply place and find an electronic "house" meter. Buy the matching socket for it and wire it up.
RE: Measuring RMS current / voltage
http://www.keithley.com/products/dmm/?mn=2000
This instrument measures true RMS and can average multiple TRMS readings. Averaging is user-defined for number of samples and sample rate. HP / Agilent's 34401A is a similar meter with similar features. Both are nice instruments. Retail new is about £650, but there are plenty available on the surplus market.
If you're intent upon a power meter, Yokogawa's WT series are reasonable. Voltech Instruments have some excellent products - their PM3000 is a very capable power meter, although it is no longer young and this shows mainly in the user interface rather than the measuring ability. Your application shouldn't prove too challenging for either manufacturer. Voltech's applications guys are ok to talk to; I haven't used Yokogawa's tech support for test gear.
http:/
h
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One day my ship will come in.
But with my luck, I'll be at the airport!
RE: Measuring RMS current / voltage
http://www.k-tech.com/ION/ION-6200-PriceList.htm
ScottyUK,
Those table top type meters may be our next choice. We are looking for devices that can be panel mounted first, hand held second, and table mounted last. And we are looking for a low cost device.
RE: Measuring RMS current / voltage
Another thought I've just ahd is GE's PQM meter. It is panel mounting and has a lot of capabilites which used to be the domain of bench instruments. I do not know much more about it - it is a fairly new addition to our site and I haven't had a chance to play with one yet.
http://www
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One day my ship will come in.
But with my luck, I'll be at the airport!
RE: Measuring RMS current / voltage
itsmoked, we called the local electronics store and they recommended Grainger. Can you check www.grainger.com for a power meter that is 480V?
cheers,
Heaterguy
RE: Measuring RMS current / voltage
Northern Design in England make a fair range of relatively cheap meters. They used to be very clued up technically - if you can get in touch and describe your application and maybe point them to the discussion in this thread they can probably suggest something. They may still do semi-custom work if they haven't gotten too big.
http://www.ndmeter.co.uk/prodserv.htm
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One day my ship will come in.
But with my luck, I'll be at the airport!
RE: Measuring RMS current / voltage
This Simpson looks pretty good to me..
Seems to cover all the bases and specifically states that it is accurate with screwy waveforms.
Pretty much can't go wrong with Simpson as far as my experience goes with them.
http:
Note: The table of what the different models can do.
Grainger carries the G100,G200,G300,G400.
http://ww
RE: Measuring RMS current / voltage
http
RE: Measuring RMS current / voltage
Hall Effect Sensors to measure voltage?
"ACCURACY - .5% Full Scale"
I wonder how they do that with Hall Effect sensors and adjustable pots?
RE: Measuring RMS current / voltage
Much the same as a standard Hall current sensor, but using a very small current and a lot of turns on the primary winding.
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One day my ship will come in.
But with my luck, I'll be at the airport!
RE: Measuring RMS current / voltage
RE: Measuring RMS current / voltage
That module could work for what you want but it will require calibration for each installation. If you noticed, the site says it uses 2 pots to adjust the scale so you'd either need to put a known power through it or measure the power at a steady operating point to calibrate it. Each time you change a pot you'd have to calibrate the measurement again. It produces 4-20mA output but you won't know how much power 20mA represents when you adjust the pots. Also, that module says 5hp to 150hp. That's 3750W minimum. What size is your heater?
What kind of scope do you have?
RE: Measuring RMS current / voltage
I think it's over kill and a pain-in-the-rear for a production application...
RE: Measuring RMS current / voltage
Probably not what you are after, but I had to add it to the list as food for thought.