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Patrick

Mon Sept 13 2010, 05:17AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

my question to anyone who can help is this:

how difficult (or easy) are potential transformers to configure for accurate HV measurements, and do i need to use a "circle diagram" to corrrect for Z and phase angle?

i plan to use a PT to figure out my NST (12kv on the name plate) exactly, then use the PT + NST to calibrate my CVD to +-1%

is this possible? do i need a resistive load like 50-100 watts? i cant think of a better way to calibrate my CVD.

...

Mon Sept 13 2010, 05:39AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

As long as you are doing the calibration at 60Hz why not use a simple resistive divider? It sounds like you have a meter that will measure into the high megaohms so it should be trivial to assemble a string of resistors to get a 100:1 or so division ratio. Just don't forget to factor in the equivalent resistance of your voltmeter.

Patrick

Mon Sept 13 2010, 05:44AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

will i have problems with pF capacitence along the resisteve string? will this be significant at the hi z end of the divider?

EDIT: multisim reveals, 525 Mohms in series with 10.7 Mohms of my meter divides 12,000 Vac / 50 = 240 Vac @ 60 Hz also my meter is only 3 - 5 % accurate at AC, but DC is way more accurate.

radiotech

Mon Sept 13 2010, 07:30AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Could you draw a one line diagram of the hookup of the test?

Carl Pugh

Mon Sept 13 2010, 06:49PM

Registered Member #1064 Joined: Tue Oct 16 2007, 05:04PM
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A potential transformer is the most accurate way to measure high voltage 60 hZ AC. (Except for very low power)

You merely connect the potential transformer and the correct voltmeter. The most accurate results are obtained with only the voltmeter as a load.

Google one of the potential transformer manufactures, they will have lots of advice on the correct procedure for making measuremens.

Patrick

Mon Sept 13 2010, 11:26PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

when i measured my fluke 73 III internal series resistence, first on VAC i got no measurement.
When i measured the resistence of the setting VDC i got 11.075 M ohms.

does any one know if the AC resistence will be the same as the Vdc ? all the manual says is that its > 10Mohms.

i have three meters two 73 III's and 0ne 79 III they all measure out about 10.90 to 11.08 M ohms ESR.

radiotech

Mon Sept 13 2010, 11:41PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
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To find the AC impedance of your meter , decide which voltage you want to use as test. Then measure this voltage. Connect a resistance in series with your meter (red lead) and adjust the resistor until the meter reads 1/2 of what it was without. this resistance will be |z|

Patrick

Tue Sept 14 2010, 01:57AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Dam Radiotech !! that was a great idea !! it worked suprisingly well, AC impedance is 10.20 Mohms or very close.

now i have a new problem, when i use a 525 M ohm resistor in series with my 10.20M ohm meter, i should get a 51.47 division ratio, my NST nameplate says 12,000 Vrms but with the variac and my divider i got 100 volts through the primary which caused 51.47 x 330 vac = 16.9 kV secondary IS THAT POSSIBLE?!?!?! or is this bogus?

is it possible for the voltage too jump up due to light loading, will it drop to near 12kv if i start to load it. its a 288 VA NST.

Arcstarter

Tue Sept 14 2010, 03:03AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Patrick wrote ...

Dam Radiotech !! that was a great idea !! it worked suprisingly well, AC impedance is 10.20 Mohms or very close.

now i have a new problem, when i use a 525 M ohm resistor in series with my 10.20M ohm meter, i should get a 51.47 division ratio, my NST nameplate says 12,000 Vrms but with the variac and my divider i got 100 volts through the primary which caused 51.47 x 330 vac = 16.9 kV secondary IS THAT POSSIBLE?!?!?! or is this bogus?

It is measuring peak voltage it seems. Did you rectify the NST output? You mentioned the meters read DC better. But, it's at 100v on the primary? If that is the case, i have no idea!

Patrick

Tue Sept 14 2010, 03:17AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

no rectification is being used. only Vrms on the fluke 73III meter, has anyone else measured the actual NST output, and compared to whats quoted on the nameplates?

"European Methods
I am told that in some areas of Europe, transformers are specified somewhat differently than in the USA. Typically, a transformer is rated in terms of its open circuit secondary voltage, its operating secondary current, and its â€œGâ€ factor. The â€œGâ€ factor is the desired ratio of loaded to open circuit secondary voltage. Typically, the â€œGâ€ factor is around 0.5 (although this may vary), meaning for example that a transformer rated at 9000 volt open circuit should operate with a tube load connected at around 4500 volts. This may be measured conveniently with a voltmeter equipped with a high voltage probe. This is the preferred method of checking the loading on a European transformer"

EDIT: at 120 Vrms input to primary i get 18,700 Vrms out of secondary. nameplate says 12,000 VAC ! tomarrow i may short it with an arc. to see what happens.

ok now with 120 vrms in to primary:
with secondary A putting out 6,230 Vrms, to center-tap ground.
with secondary B putting out 6,292 Vrms, to center-tap ground.
this adds up to nearly 12,500 Vrms which i sort of believe.
but why the 18,700 Vrms number across sec A to sec B? this leaves 6100 Vrms unexplainable

maybe it the capacitence of the meter when its not close to ground, 50pf.?

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