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# 3VA PT/VT Measurement and Safety/Accuracy

07/01/2018 5:38 AM

Hello i have a 3VA voltage transformer rated 7V on primary side (actually 2x7V) and 70V on the secondary side. Class 0,5.

This transformer is going to be measuring voltage around 0...7VAC. But in the worst case scenario (if logic fails) it could be measuring up to 36,6VAC. Though logic safety will have it measure around 5 to 7 VAC.

The secondary, 70V secondary side will be connected to an analog input with a internal impedance of 0,103VA.

Rated impedance on the secondary side would be given as Zn=((Un*Un)/Sn). Which gives 1633Ω. What will happen if secondary side becomes short circuited @7V on primary side ? My assumption is that the short circuit current will be limited by 1633Ω, and thus very low. This current will be 10 times as high on the primary side, where i should focus my protections at.

And would my equipment (load) be safe under this measurement ?

The measurement would be fine with regards to http://www.ritzusa.com/resources/43.pdf meaning that the load should be below rated 3VA (burden).

But i also read in a book that for a voltage transformer it should be above the burden (compared to a CT that should stay below). But one should always aim to load equal to burden for best accuracy.

What do you say guys ?

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#1

### Re: 3VA PT/VT Measurement and Safety/Accuracy

07/01/2018 11:50 AM

7V primary, 70V secondary is an odd ratio. Does transfo plate give a definite VT/PT standard?

7V + 20% on "primary" side would usually cause saturation. The "knee"of saturation curve is usually taken as point at which 10% increase of voltage gives 50% increase in magnetising current on no-load test. 37 V gives severe saturation with high current [limited only by winding resistance] which could damage VT if sustained. There will also be extreme voltage spikes on the secondary "70V" side, say 700V, as the core goes out of saturation twice per cycle, which could damage your load.

You would be far better off to reverse the CT - 37V is no problem for 70V winding - & maybe put 7V windings in series, unless something you did not write, like Monitored side being at 1000V. One cannot assume primary & secondary have same insulation thickness to core which is earthed?? (VT secondaries are usually earthed on one side for safety)

The VT will give its rated accuracy at the rated burden, but the difference is usually small. You could find out by varying the resistive load e.g. 1 watt, 2W, 3W & reading with voltmeter.

If you short circuit a VT load winding, then because VTs have very low regulation (low impedance) you will get e.g. 100x rated current and cause damage (supposing source resistance of test supply is negligible).

I suggest you energise 7V winding through a 12V 3 watt bulb or 22 ohm 3W resistor with shorted 70V winding. Voltage/current on 7V winding wiil give short-circuit impedance Z from "7V viewpoint". Then 7V/Z gives current on short circuit @7V, 1/10 of that for 70V winding if turns ratio is 10:1. Z will also give a value for maximum regulation for a given current [Z is both resistance R & inductance L, so you have to measure phase angle to get R & L values to compute regulation with resistive load].

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#2
In reply to #1

### Re: 3VA PT/VT Measurement and Safety/Accuracy

07/01/2018 12:16 PM

Rating plate reads 7V and 70V, and is custom made (to increase measuring resolution). Though i have been promised it will keep its class 0,5 at voltage up to 14V. He said he could have given me a input voltage up to 4x7V, but at that voltage he couldnt keep the accuracy level at 0,5%.

It seems my only option is to measure the transformer, with open circuit and short circuit to get my R' and X' as well as R'' and X''. Or is it even possible to approximate or calculate the short circuit currents ?

To be clear, there is no intention to measure at any voltage above 7V.

And im curious of the load compared to the burden on the secondary side. My burden should be 3x0,1VA plus cables and terminals. And im assuming it, for now to be far less then 3VA.

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#3
In reply to #2

### Re: 3VA PT/VT Measurement and Safety/Accuracy

07/01/2018 6:42 PM

I am "in the dark" because I do not know what meters or instruments you have, or even your mains volts & frequency. It is better to make the measurement with a lamp or resistor as I suggested than to guess. I do know that you have a "high resolution measurement" at about 70V AC full scale but do not know how low it goes or if full scale.

You can as well short 7V side and apply a current to 70V side of near "3VA" current, which is about 0.42 amp at 7V or 0.042 amp at 70V.

Round here I would get that by putting Two 15 watt 230V pygmy filament lamps in series from 230VAC 50Hz house supply live to one end of 70V winding, other end of 70V winding to neutral. Better still put a 4700 ohm +/-5%, 10 watt resistor in place of one lamp - then you can measure resistor's actual value with ohmmeter and measure voltage across resistor with AC voltmeter (even cheapest VOM have 200V AC range), then calculate current V/R. The voltage on the 70V winding will be about 0.7VAC. A lamp will burn-out on overcurrent long before a copper wire of similar rated current so do not worry abou transfo.

An approximation to saturated current of 7V winding could be got by measuring its resistance [about 0.1 ohm] - any reactance would reduce current. Most VOM cannot measure 0.1 ohm, using 3.33 ohm resistor made of 3 x 10 ohm 1/2 watt metal film resistors (usually 1% tolerance) in parallel as dropper from D size 1.5V manganese battery will give about 50 mV across 0.1 ohm. Measure voltage across 3.3 ohm then "0.1" ohm and use ratio to get value e.g. 3.33 ohm x 0.100V/1.400V = 0.24 ohm.

You could make a bridge with 100 ohm 1% versus O.1 ohm & 4700 ohm 1 watt potentiometer versus 3.33 ohm - measuring pot with ohmmeter after balancing.

Of course, 0.1 ohm @ 7V is 70 amp - but what is possible fusing on "7V supply" to 7V transfo?

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#4
In reply to #2

### Re: 3VA PT/VT Measurement and Safety/Accuracy

07/02/2018 5:05 AM

You asked two questions, what happens if you over-volt your VT & what is the error on a very small load 0.1 VA compared to 3VA rated.

A VT standard usually allows that the stated error is met over a range of load VA & power factor within its ratings. in practice most VTs are lightly loaded. If the error due to resistance & load current were very much it could vary a lot with winding temperature(load) so VTs usually have low magnetising & copper losses.

The main error is just the turns ratio & the error open circuit due to primary leakage inductance (flux not linking with secondary, small) & magnetising current flowing through it (small). Then there is error due to loading & its power factor.

The regulation of a transfo, in volts, for small regulations is...

Er.cos∅ + Ex.sin∅ - where cos∅ is the load power factor.

Er and Ex are the volt drops across resistance in transfo windings and leakage inductance [both primary & secondary]. Of course, for resistive loads (cos∅ =1, sin∅ =0), it all reduces to Er, which is going to be very small at 0.1VA compared to rated 3VA. If your transfo maker measured the no-load ratio & recorded it, he should be able to tell you the value - this should be as accurate as his test equipment & much less than 0.5%.

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#5
In reply to #4

### Re: 3VA PT/VT Measurement and Safety/Accuracy

07/02/2018 1:54 PM

Many thanks for your comments. I am going to try and measure the values tomorrow, and dig deeper. Its going to be a new experience for me. Have not really put my hand on transformers before.

Though, i think i will have to read your second comment twice. Its not easy following it. But it was a good introduction of how to look at something differently from another angle.

There are many online information about these tests. But they also tend to not always saying the same thing. High Voltage side / Low Voltage side. Primary Side / Secondary Side.

And thanks for your input. All small information helps. It seems the same thing can have many names (R', Rp, R1 to mention a few)

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#6
In reply to #4

### Re: 3VA PT/VT Measurement and Safety/Accuracy

07/02/2018 6:06 PM

This is a regulation diagram typical for a distribution power transformer...

It is drawn for rated load current - at 10% load, resistive load, 1.0 power factor the regulation would be about 10% of that on diagram, 0.1%. In small transformers, efficiency is lower and usually Er [font does not do curly E as on diagram] is much greater than reactance drop. Er and Ex are voltage drops in % of rated volts on short circuit test at rated current. Notice that even though this is not a low regulation transformer, the regulation near no-load is negligible.

Using formula I gave in previous post, with resistive load, power factor cos∅ =1.0, sin∅=0 you get 1% volt drop, same as diagram - notice the reactance does not figure in regulation with cos∅=1, because its voltage drop is at 90 degrees to load voltage and very small compared to load voltage. If cos∅=0.5 you get 0.5 + 4*√(1 - 0.52) = 3.96.

This diagram represents the short circuit test which is part of transformer standards. Ignore Vout for present purpose. L1 & R1 represent your 70V winding resistance (AC, a little more than DC measured value) and leakage inductance. R2 & L2 represent the 7V winding resistance & leakage inductance transferred to 70V winding by (turns ratio)2 , 100:1 in your case. The magnetising current could be represented at full voltage by an inductance from R1, R2 junction to common at bottom - however with Vin at a few % of rated voltage it is negligible

The input current (approximately rated current for 70V winding) flowing through the total impedance of L1, R1, R2, L2 produces the volt drop Vin.

Calculating Vin x rated current/test current gives Vin(In) for a test at rated current In. You can do your test at several currents to see if the total impedance can be regarded as constant (long term it will not be, because the copper heats up and increases in resistance, rating plate values are corrected to standard 75'C copper temperature).

Then calculating Vin( In)/Vn, where Vn is nominal voltage of powered winding gives the "impedance voltage" as a fraction of rated voltage, commonly this is multiplied by 100 to give an impedance percentage on data sheets, test documents and transformer rating plates. This is more convenient than ohms, because it does not matter which side primary or secondary you work from & for example if impedance is 4%, then it is easy to find short-circuit current is at normal input voltage is 100%/4% = 25 times rated current without using the voltage value.

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#7
In reply to #6

### Re: 3VA PT/VT Measurement and Safety/Accuracy

07/03/2018 12:30 PM

I have done some measurement regarding short circuit test at HV side.

[1] Vin = 691mV @ 46,3mA

And at another measurement with voltage dividing the primary Vin by a quarter... but i think this would distort the measurement. Because i could not get a lower value then 691mV from the Variac. It showed 46,3mA at first power on.

[2] Vin = 400mV @ 42,0mA (voltage divided by serial resistors).

[1] Calculated: (691mv x 42mA) / 46,3mA ~ 0,63V

0,63V / 70V ~ 0,9%

[2] Calculated: (400mv x 42mA) / 42mA ~ 0,40V

0,40V / 70V ~ 0,6%

Open circuit test at LV side gave:

Unp = 7,02V

I0=0,788A

Uns = 72,2V (didnt see any phase angle compated to Unp)

This seems unreasonable high, given that In=0,42 on LV(7V) side. My wiring were quite long, and i also went through a 1A fuse... and the connections might not have been optimal. But considering 3VA rating @ 7V would give me 16,3Ω, what is seen here this with 0,788A is mostly iron losses then ? Something around 9Ω ? Correct me if im wrong here.

Given the short circuit test, should i expect 100%/0,9% = 111 times In (111x0,042~4,8A on the secondary side, and 111x0,42~48A on the primary side). Or even consider the 0,6%.

I am going to some more tests. Its interesting and a good lesson.

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#8
In reply to #7

### Re: 3VA PT/VT Measurement and Safety/Accuracy

07/03/2018 7:13 PM

Hi Roger,

Whether it is 0.9% or 0.6%, the regulation is going to be negligible at your 0.1 VA loading - 0.6%/30 = 0.02%!!. It is only the ratio that matters.

It would be helpful to know the DC resistance of "70V" winding & 7V winding since the split of impedance between 70V & 7V windings is unknown, as is inductive reactance.

Likely, most of the drop is resistance.

The magnetising current looks high, but a small "plugtop transfo I measured, 220/24V 0.2 Amp - thus 4.8 VA out, draws 61 mA rms @ 244V (true rms meter) which is 15VA un-loaded! Primary resistance is 430 ohm, secondary 6R9. That is 0.56 amp if fully saturated. This was the inrush - 0.6A peak scaled.

As you can see, the settled current is very peaky, a sign of saturation. Admiittedly, it is 244V on 220V nominal.

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#9
In reply to #7

### Re: 3VA PT/VT Measurement and Safety/Accuracy

07/04/2018 7:13 AM

I just checked a Freidland bell transformer. 30 years in porch, barely warm to touch.

10 mA primary current at 244V. 8V 1 amp out. primary 410 ohm, secondary 1R2, DC

67model

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#10
In reply to #9

### Re: 3VA PT/VT Measurement and Safety/Accuracy

07/04/2018 8:32 AM

I checked the resistance, first by direct measurement.

R(7) ~ 0,1Ω

R(70) ~ 3,2Ω

Then in serial with 3Ω (4//12Ω) with a battery(1,5V) i got around:

3Ω = 745mV and R(7) = 25mV (also another test 740mV and 11mV) which would suggests R(7) = 0,1Ω (0,05Ω)

3Ω = 420mV and R(70) = 490mV (also another test 420mV and 440mV) which suggests R(70) = 3,5Ω (3,14Ω)

I would say the measurement was about +/-10% in accuracy, voltage was not stable, or i have measuring equipment.

Also did open circuit test. I0 was same, around 786mA.

And also another short circuit test. Which was a bit tougher, cause of the minimum of voltage output at around 600mV with variac. Though i saw it was a clear linearity with voltage applied towards mA drawn.

I got 41,4mA @389mV as well as 71,2mA @662mV. Which tells me at 1mV i get around 0,1mA on 70V side and 1mA on secondary side until saturation/overload occurs... where it should get distorted. This would also give me 0,5% and short circuit current at 200xIn at nominal voltage. At primary side around 420mAx200=84A and secondary side 42mAx200= 8,4A.

So the ratio of 1mV:0,1mA might be better of with 1mV:0,12mA. Ill check back, with some better conclusion, Just let me now if im far away with my recent thougths.

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#12
In reply to #10

### Re: 3VA PT/VT Measurement and Safety/Accuracy

07/04/2018 6:52 PM

389mV/41.4mA = 9.4 ohms: 662 mV/71.2 mA = 9.3 ohms.

Dc resistance of 70V winding = 3.3 ohm. 9.3 - 3.3 = 6ohms is resistance transfered by 10:1 transfo from 7v winding, 7V winding resistance ≈ 6/(102) = 0.06 ohm - not far off from one of your test results.

N.B. if the inductive reactance were half that of the resistance, the power factor would be 0.87 - but since the total impedance would be 1 compared to 0.87, the effect of the reactance on the impedance you measure is not much, hardly more than 10%. As might be expected with a small inefficient transfo, the resistance dominates.

The problem you seem to have is that the 7V winding, your primary, has very little resistance - but appears to be saturating and drawing a high current - which makes it difficult to protect the transfo and could affect your 7V source, whatever that may be.

Taking the example of the Friedland transfo I measured [size about 70 x 50 x 50 mm], the magnetising current, 10 mA @ 240V would be 10*240/8 = 300mA at 8V.

In comparison, your magnetising current is more than double, even though its VA must be smaller.

It would be useful to have a current versus voltage test [saturation curve], from 10V up] for 70V winding - easier to do than 7V ,if you have variac [unless you have a 230/12V or similar 20VA transfo to hand]. The standard definition of the saturation point is where a 10% increase of voltage causes 50% increase in current.

You may not have a wattmeter, but you could measure the "phase angle of short-circuit test current to voltage" by using a resistive divider/potentiometer from the voltage to common - to get a voltage equal to that across transfo winding, then measuring the voltage between the two to find the third side of a triangle - drawing a scale diagram will find the angle, Drawing on a right angle triangle will give a reactance to resistance ratio N.B. Since you are feeding current through a large resistance for the short-circuit test, the transfo current is practically in phase with source voltage. This works if the voltage & current are sinusoidal, which they should be, because voltage on transfo is too small to cause any distortion.

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#11
In reply to #6

### Re: 3VA PT/VT Measurement and Safety/Accuracy

07/04/2018 1:39 PM

Given your picture i approximate/get:

R1 = 3,5Ω (from DC measurement)

R2 = 100*0,1 = 10Ω (from DC measurement on 70V side)

X1 + X2*100 = 9,3Ω (from short circuit test considering most i copper losses) But as i have not wattmeter, and most tests suggests having wattmeter. I dont see how i could find X1 or X2.

Considering i get 786mA@7V in open circuit test. I would get Z=7/0,786~ 8,9Ω which could be approximated to R1+R2(*100)... as 0,1 was not really an accurate measurement. It seemed lower. But it also includes Im (magnetization current)...

Ill copy paste what i have so far:

Is it possible to measure as an wattmeter without having one ? Using two ampmeters ?

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#13
In reply to #11

### Re: 3VA PT/VT Measurement and Safety/Accuracy

07/05/2018 11:37 AM

You may find this of interest.

http://www.danyk.cz/wmetr_en.html

Op-amps could be added, to reduce the shunt volt-drop required & adapt to different input voltages.

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#14
In reply to #13

### Re: 3VA PT/VT Measurement and Safety/Accuracy

07/06/2018 11:48 AM

Many thanks. Sounds interesting setting up "your own" wattmeter. I might dig deeper on that one rainy day.

For now, i looked around, and there are quite big different price tags on them. As im only going to use it on testing for Cos phi and the accuracy for now isnt needed to be high, i will settle with this:

https://www.banggood.com/PZEM-001-AC-80-260V-10A-2200W-Power-Meter-LCD-Digital-Voltmeter-Current-Meter-Monitor-Display-Module-p-1249964.html?gmcCountry=SE&currency=SEK&cur_warehouse=CN

And very nice price tag on that one. Accuracy 1.0.

Also i would like to thank you for your support. Its been an experience and a learning process, still to be commissioned and tested. But at least i got to know the step up transformer a bit more. Normally, one just buy one at standard rate and go with that. Step up transformer... in the end i might realize that buying a standard 12,5 VA rated 1:10 would have gone just fine, but just use it as a step up. That is another test to be done, for sure, if i find someone at a good price.

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#15
In reply to #14

### Re: 3VA PT/VT Measurement and Safety/Accuracy

07/07/2018 4:18 PM

As you suggested, one can work out reactance X, without wattmeter. I made some more measurements on the bell transfo: magnetisation curve - as far as I could - and S/C test; and then drew a scale vector diagram, as below using 245V, 217.5V, 64.8V and compasses to draw arcs for 64.8 & 245V from 217.5V line....

The 217.5V, being across a resistor, is in phase with the current though the transfo primary. I then worked out transfo resistance in primary circuit by using measured DC values & transferring resistance from secondary using turns ratio2 . I then drew the volt drop (40V) of that effective resistance @ 26.9 mA in line with 217.5V, to get the triangle for transfo S/C impedance. As you see, the third side, 65V, is not at "right angle" to 40V resistance vector.

At this point I realised, I had just used ratio 240/8, as marked, = 30 to transfer secondary resistance. So I went back to hardware and measured open circuit voltage ratio [16.89] & re-calculated volt drop 20.2.

As you see, this gives third side 61V, corresponding to inductance drop, convincingly at right angle to 20.2V resistance drop and lagging. That works out as X = 2270 ohms, or L = 2270/(2x3.142x 50) = 7.2 henries.

Note that the impedance for regulation is 64.8V/26.9mA = 2409 ohms in primary terms.

Since rated secondary current is 1 amp, this is primary current of 1/16.9 = 0.059 A, so worst volt drop would be 2409 x 0.059 = 142 volts. That leaves 103 volts for the secondary or about 6.1 volts, applying transfo ratio. In the usual percentage terms given on transfo rating plates that is 100% x 142/245 = 58% - so the resistance is 58% x 20.2V/64.8V = 18.1% and the inductive reactance 58% x 61V/64.8V = 54.6%.

That would only occur with a very low load power factor, equal to power factor of transfo impedance - which 20.2V/64.8V = 0.31 from the diagram.

In usual transfo parameter terms one has Er = 18.1%, Ex = 54.6%, X/R = 54.6/18.1 = 3.0 and regulation (N.B. sin∅ = √(1 - 0.312)...

Ercos∅ + Exsin∅ = 18% x 0.31 + 54.6% x 0.95 = 5.6% + 51.9% = 57.5%

N.B. for large impedance like this, there is a more complicated, accurate, regulation formula, but 0.5% is not much!

At 0.8 p.f. lagging one would get 47% regulation or 14.5 volt x 53% = 7.7 volts (8V nominal winding), while unity power factor would give 82% x 14.5V = 11.9 volts.

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#16
In reply to #14

### Re: 3VA PT/VT Measurement and Safety/Accuracy

07/08/2018 2:39 PM

To complete the picture, I tried shorting the 8V winding with 245 volts on primary. One might expect 26.9 mA x 245/65 = 101 mA in proportion - surprisingly, I got 104 mA.

After 1 hour 20 min, primary winding temperature had risen 73'C from 30'C initial ambient [410 to 523 ohm]. One would expect insulation max temperature to be 120'C class B - it could well be class H, 180'C.

So it appears this double insulated safety transfo can also tolerate a short circuit output for a long time

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#17
In reply to #16

### Re: 3VA PT/VT Measurement and Safety/Accuracy

08/05/2018 10:48 AM

Hello again. I just wanted to update and summarize. As of now, im still waiting for that wattmeter... new customs standard and all sorts of problem having it delivered, of some new system at the post office.

Anyways. As this was my first hands on test of a transformator, i learned that the no load current, magnetization current never actually transforms to the other side, which was kind of hard to grasp/understand, until i actually saw it on the instruments.

And about the very high no load current, given it is only a 3VA transformer, i suspect it will help with the device under test, and get a better reading (accuracy). I tried simulate a 100m wire@1.5mm2 attached to 7V... meaning i put a couple of parallel resistors to mimic the resistance of approximately 2 ohm (should actually be another PF though). And from what i saw, the sinus curve started to distort the more resistance i put, and i had somewhat good reading below 3 ohms. My feeling was that this had something to do with the magnetization current. I had around 785mA @ 7V, but with the voltage drop, i had to increase voltage to get the same current of 785mA (which offcourse will not happen with the device i will actually measure, but at least i get a clue of how much of the measurement that will get lost in the length of the cable and be able to compensate/calibrate in the software). The cable of 1,5mm2 at aprox. 100m is really horrible way of measuring things.

I really would have liked to test the same thing with another transformer with another magnetization current, just to see if my thoughts was right about this. That with a higher magnetization current, the more cleaner the waveform will be... 7/0,785=8,9Ω compared to cable impedance of ~1,8Ω.

I also have thought about, what if i had a transformer of much higher burden ? For example 10VA or 20VA. For all that i have read that will affect the secondary side load. And you should stay within 25-100% of that burden to be inside nominal accuracy. But... how can i compensate for "load" on the primary side ? I know there is a 100m cable there, and another transformer at 100VA and Imax at 3A. So the circuits are safe enough. But i have a good feeling that the pure waveform will be quite distorted because of the impedance of the cable as well as the secondary winding of the other transformer.

My conclusion so far is that if i had a big magnetization current enough, it would overshadow the impedance enough to give a good waveform.

If it is possible, could i send you a private message 67model ? Some information is of sensitive nature, so to speak.

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