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Topic Title: Volt drop
Topic Summary: Ze
Created On: 29 December 2012 08:36 AM
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 29 December 2012 08:36 AM
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Avatar for ebee.
ebee

Posts: 5620
Joined: 02 December 2004

You know how for a battery we can calculate internal resistance from knowing open circuit voltage compared to voltage when loaded if we know the current (or load resistance and calculate) therefore by transposing we could find the expected voltage when running any load.

Then is it the same (OK them very similar) formula when using a DNO transformer too ?

In other words, say we put a heavy load on such as shower/cooker in a domestic situation and we notice lights dim at the time and check voltage at CU terminals and then check the difference with no load or little load then would the volt drop be as high as we could calculate from this?

I was thinking that basically our Z at our supply transformer would be the impedance of the secondary coil plus some reflection of the impedance of the primary coil (ie the supply leading up to it including transformers/substations and line conductors etc).

Would we just notice this difference that the calculation would suggest or would the fact that extra electricity is generated downstream lift the voltage back up to some degree rather quickly?

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 29 December 2012 09:07 AM
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GeoffBlackwell

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Joined: 18 January 2003

This is basically how your trusty loop impedance tester works - so you have been utilising this for years .

Regards

Geoff Blackwell
 29 December 2012 09:22 AM
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ebee

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Thanks for that Geoff, yes I did know that, nice to sse you here again, Merry Xmas & Happy New Year to You.

Point I was asking was because . as I understand it, someone with a measured Z (Ph to N) of 0.18 running a 10.5KW shower notices lights dim (not unusual I know) but voltage at CU terminals drops from 245 to 235 and I thought this a tadd excessive just off the top of my head. So I thought try the schoolboy battery internal resistance calc on it.

Assuming KW rating is at 240v not 230v I got volt drop of 7.63 but then thought , would voltage rise a bit when generation side puts more KW on the system.

My complete guess in the first instance would have been about 5 volts or less

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik

Edited: 29 December 2012 at 11:10 AM by ebee
 29 December 2012 09:42 AM
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perspicacious

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Joined: 18 April 2006

"Point I was asking was because . as I understand it, someone with a measured Z (Ph to N) of 0.18 running a 10.5KW shower notices lights dim (not unusual I know) but voltage at CU terminals drops from 245 to 235 and I thought this a tadd excessive just off the top of my head. So I thought try the schoolboy battery internal resistance calc on it.
Assuming KW rating is at 240v not 230v I got volt drop of 7.63 but then thought , would voltage rise a bit when generation side puts more KW on the system.
My complete guess in the first instance would have been about 5 volts or less"


Well Ebee, taking the load at a round 40 A with an "external resistance" of 0.18 Ohm and using Ohms Law, the volt drop is 7.2 V so if you put in your actual load current, I would expect to get your measured value.

Regards

BOD
 29 December 2012 09:51 AM
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GeoffBlackwell

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The trouble with any calculation involving heating elements and the like is that, when cold they have a relatively low resistance and therefore, cause a relatively high current inrush. As the element heats the resistance rapidly rises and the current reduces to its steady state value.

Your calculation will not account for the initial conditions and so may under estimate the initial voltage dip.

Regards

Geoff Blackwell
 29 December 2012 09:55 AM
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Avatar for ebee.
ebee

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Now why didn`t I think of that one?

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
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