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Pig Impedance. What difference does it make?

 

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From:  Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent:  Friday, August 14, 1998 12:24 AM
To:  Tesla List
Subject:  Re: Pig Impedance. What difference does it make?

Tesla List wrote:
> 
> ----------
> From:  Michael Baumann [SMTP:baumann-at-proton.llumc.edu]
> Sent:  Thursday, August 13, 1998 6:13 PM
> To:  Tesla List
> Subject:  Pig Impedance. What difference does it make?
> 
> I am about to drop the change to upgrade my power source to a
> more robust critter than the NSTs I use now.
> In looking over the T-R specs, I see they list impedance
> as a range from about 1.2 to 2.5 for the same KW rating.
> I am WAGing that this is the inductance of the secondary (Low volt)
> windings?
> 
> What does this mean to me WRT my coil operations? Is lower
> better/worse/makes no difference?
> 
> Thanks for the info.
> --
> Michael Baumann  Optivus Technology Inc.|Loma Linda University Medical
> Center
> San Bernardino, California. (909)799-8308 |Internet: baumann-at-llumc.edu

Michael,

Distribution transformers are rated in "% impedance". This is a measure
of their short-circuit impedance, and is used for calculating fault
currents, and also used in balancing transformers when connecting them
in parallel configurations to increase total output current. 

A transformer rated at 3% impedance will deliver 1/(3%) or about 33
times it's rated output current under short-circuit conditions. A 1.5%
impedance transformer will deliver 66 times the rated output current.
Generally speaking, a lower impedance percentage rating means a bit
beefier transformer (with lower winding resistance). Practically
speaking, either one will work the same in Tesla Coil use, since we will
always be adding external ballast inductance/resistance to control
short-circuit currents to MUCH lower values. A short-circuited 10 KVA
transformer rated at 1.2% impedance and no external ballast could draw
as much as 83 times the rated current... or around 3500 amps off your
240 volt mains... until you took out your breaker or took down the
neighborhood! 

-- Bert --