RE: Rewinding BIG transformer

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Duke, Ronn (CCI-San Diego CCC) by way of Terry Fritz <twftesla-at-uswest-dot-net>" <Ron.Duke-at-cox-dot-com>


Jason,

With the formulas I have, assuming your 25 sq. in. core area, I came up with
60 turns on the primary and 7900 turns on the secondary with 240v in.

N = (k x E)/ A, where  k=6.256  when  f=60 Hz., or,  k=7.507  if  f=50 Hz

(6.256 x 240v) / 25 sq. in = 60.06 Primary turns
 
Turns per Volt = N/E

60.06 /  240v =  .25 Turns per Volt

N=Number of primary turns
E=Primary Voltage
f=line-frequency
A=core cross-section Area
B=max.flux, in lines, Assuming 60,000 lines of flux 

Secondary turns are determined with this formula:

ST = SV x 1.05 x TV

30,000v out x 1.05(losses) x .25 (turns/volt) = 7875 Secondary turns
 
ST= secondary Turns
SV= Secondary Voltage
TV= Turns per Volt on the primary
This is assuming an average of 5% core and wire losses.


I'm probably going to get hammered on this but I've consulted several
different sources, and they're all pretty close. Hope this helps.

Sparky

	-----Original Message-----
	From:	Tesla list [SMTP:tesla-at-pupman-dot-com]
	Sent:	Sunday, March 25, 2001 10:22 AM
	To:	tesla-at-pupman-dot-com
	Subject:	Rewinding BIG transformer

	Original poster: "Jason Johnson by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <hvjjohnson13-at-hotmail-dot-com>

	Hello,
	
	Yesterday I got back from a state surplus auction that had equipment
from
	around lower Michigan. I managed to score a huge transformer for
cheap (50
	bucks!) there too. The transformer as it sits is good for stepping
2400 delta
	down to 480 delta at 112.5 KVA (no thats not a typo). This thing is
huge! It
	weighs in at 950 lbs (we had fun getting it off the truck, my back
is still
	sore!) and is the size of a small refrigerator! It has a core area
of about 25
	inches^2 and is the standard EI core type, only the core is about 2
feet long
	and 2 feet tall. 
	
	What I was wondering was if I could rewind this beast for about 30kv
at 24 KVA.
	The only thing is I want to do this as cheaply as possible (read: as
few turns
	as possible). Solving the equation:
	
	A=74*PV/(f*PT) 
	where:
	A= cross sectional area of the core in square inches
	74= guesstimated permeability of the core material (has worked well
for all the
	transformers I have)
	PV= Primary voltage
	f= frequency
	PT= Primary turns
	
	for PT:
	
	PT=74*PV/(A*f)
	And putting 240 in for PV, 25 for A, and 60 in for f; I get 11.84
for PT.
	Is this right? Only 12 turns for 24KVA? Are my calculations screwed
up? By this
	same number I would only need 1500 turns for the secondary! I like
these
	numbers, but I'm afraid that they're not right!
	
	Comments?
	Jason Johnson