# Impedance Voltage of a transformer.

by Bassalisk
Tags: impedance, transformer, voltage
 P: 951 I know what impedance voltage is. I will state it just make sure we are talking about the same thing. Impedance voltage is relative short circuit voltage, when secondary of a transformer is short circuited, and we are slowly raising the primary voltage "Vpr" until the current at the secondary reaches its nominal value. The ratio of the "Vpr" and nominal voltage of the transformer is called impedance voltage. It usually goes from 3% to 20%. Now, what does this parameter tell me about the transformer? How is this useful? Is it better to have a lets say 20 ohm or 200 ohm impedance of transformer? I know its important for 2 transformers when working in parallel to have matched impedance voltages, to prevent one transformer "hugging" more power than the other. Additional question: When we have a "rush" and a lot of power is taken from transformer, its voltage at secondary tends to fall. Correct? To prevent this, regulation transformers slip one ring up, and change the transfer ratio of the transformer, so we can get more voltage at the secondary. Is this correct?
 Sci Advisor PF Gold P: 3,636 Seems unusual name - we just called it the "impedance".. It tells you how well the transformer "regulates".. ie at rated current it'll give that much voltage drop; which is useful when designing a power distribution system so voltages still match several transformers away from power source; and it tells you how much current the transformer will deliver into a short circuit (rated current X 1/impedance ), useful when selecting downstream breakers.. Now that's interesting- if your pole pig is a 200 amp 3% transformer it'll dleiver over 6,000 amps into a short circuit and that's why the little breakers in your household electrical panel have numbers on the back like 10,000 amps. Don't be surprised when you get to "transient" and "subtransient" impedance - same concept except very short term while magnetic fields inside transformer collapse. equations look scary but you'll have no trouble. "regulating transformers" : - some are motor driven tap-changers as you describe, some are a slider type (Variac) ;; others are "ferroresonant" :: employing a tuned circuit resonant near 60 hz but operating near saturation so load current voltage affects inductance; effectively changing impedance so as to maintain constant output voltage. No moving parts but kinda inefficient at light load - they run hot of course with such high flux.
P: 951
 Quote by jim hardy Seems unusual name - we just called it the "impedance".. It tells you how well the transformer "regulates".. ie at rated current it'll give that much voltage drop; which is useful when designing a power distribution system so voltages still match several transformers away from power source; and it tells you how much current the transformer will deliver into a short circuit (rated current X 1/impedance ), useful when selecting downstream breakers.. Now that's interesting- if your pole pig is a 200 amp 3% transformer it'll dleiver over 6,000 amps into a short circuit and that's why the little breakers in your household electrical panel have numbers on the back like 10,000 amps. Don't be surprised when you get to "transient" and "subtransient" impedance - same concept except very short term while magnetic fields inside transformer collapse. equations look scary but you'll have no trouble. "regulating transformers" : - some are motor driven tap-changers as you describe, some are a slider type (Variac) ;; others are "ferroresonant" :: employing a tuned circuit resonant near 60 hz but operating near saturation so load current voltage affects inductance; effectively changing impedance so as to maintain constant output voltage. No moving parts but kinda inefficient at light load - they run hot of course with such high flux.
I know its a strange name, we call it relative short circuit voltage(in my country).
Can you explain this "regulation" property further?
And at rated current, what will give voltage drop?

I don't understand what "pole pig is" :D

P: 951
hahaha interesting anecdote :D I think I understood everything you said. Boy this HV theory isn't so naive as I thought it was.

I did encounter per-unit system and I am quite comfortable with it, and I am handy when converting things, just never saw the point of it. Now I do.

So basically this % thing is telling me how much voltage drop I will have on my transformer. So the lower the better?

Thank you.
 Sci Advisor PF Gold P: 3,636 So basically this % thing is telling me how much voltage drop I will have on my transformer. So the lower the better? In general , yes.... sometimes in a big installation you use the transformer Z to your advantage. Cost of circuit brakers goes up with interrupting rating so you can use the transformer as an "orifice" to limit fault current, allowing more modest switchgear. some applications use hi-z on purpose , for example an arc welder where you want maybe 90 volts to strike the arc but really bad regulation so it drops to 30 or so to maintain arc. Have Fun!
P: 951
 Quote by jim hardy So basically this % thing is telling me how much voltage drop I will have on my transformer. So the lower the better? In general , yes.... sometimes in a big installation you use the transformer Z to your advantage. Cost of circuit brakers goes up with interrupting rating so you can use the transformer as an "orifice" to limit fault current, allowing more modest switchgear. some applications use hi-z on purpose , for example an arc welder where you want maybe 90 volts to strike the arc but really bad regulation so it drops to 30 or so to maintain arc. Have Fun!
Thank you I will! I have my final exam tomorrow, regarding all these topics we were talking about past 5 days. I think I will do good.

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