"Gerald Stombaugh" wrote in message
news
I happened to be designing a transformer when I took a break and
found this discussion. Maybe if I describe this transformer and the
simple math used, it might clear the air.

120v-60Hz Primary input.
Secondary: 12.16V @ 6.61 amps rms for 80Watt lamp load.
Max. temperature rise over 25C = 70C.

Now the most important calculation for any requirement: How
big does the transformer have to be to meet these requirements?

Well, I use: Square root of the watts (va) out divided by 7 equals
the core area. This is an EI transformer so the answer of 1.27 would
equate to a 1-1/8" center leg lam. and a stack buildup of 1-1/8"
This unit would be wound on a three flange glass filled, nylon bobbin.

My curves tell me that in order to meet the 70C heat rise that I can
tolerate 15.20watts combined core and copper losses.

Second calculation: How many primary turns must I use in order to
STAY AWAY FROM SATURATION?? I choose a flux density
of 100,000 lines per square inch density. The formula is :

Ex10^8/4.44 x f x B x A x K.

120 x 10^8/4.44 x 60 x100,000 x .95 = 418 turns on the primary
winding.

4.44 is the factor for sine wave (4.00) for square wave, B is the
flux density, A is the centerleg x the stack in inches and K is the
stacking factor (.95) of the lamination ala that the effective amount
of steel is 95% because of burrs etc.

I can determine the primay current of the transformer by denoting
80 va out plus 15.2 va losses/120v = .793 amps. We determine that
80va/12.16v = 6.61 amperes. SO--
I choose 418 turns of 23 awg wire for the primary and 45 turns 2#17
awg for the secondary.
Rp = 4.25 ohms---Rs = .059 ohms. Pri cu. wgt. is .325# and Sec. cu.
wgt. is .293#

Pri voltage drop is .993a x 4.25 ohms for 3.37 volts. Sec. voltage
drop is 6.61 amps x .059 ohms or .39 volts.

Primary copper losses = .993 x .993 x 4.25 ohms = 2.67 watts:

Secondary copper losses= 6.61 x 6.61 x .059 ohms + 2.58 watts.

Core loss per Tempel steel curves denote 3.5 watts per pound
which is 2.12 lbs for a loss of 7.40 watts.
Total losses of 12.65 gives me a temperature rise of 68C.

The insulation system is rated for 130C total temp. which would allow
me a temp rise of about 100C but the lamp is very close to the
transformer so I design more conservatively.

The transformer, made in Mexico will sell for $6.90 in 2500
quantities.

NOW: If the customer mistakenly applied 140V to the primary
of this transformer you would have HEAP BIG SMOKE AND DEATH
in about 4 min--32 seconds by my curves. We utilize a thermal device
in the primary winding which will immediately open the moment the
copper temp. achives 130C which is usually just over a minute.

That is your lesson for today.

Gerald Stombaugh in hotter than hell Tucson, Az.



****ing period!!

good engineering is not bigger than life.
all we need is few calculations from olde books,few graphs and conditio sine
qua non is also that we don't have short circuit between two earphones.
one of biggest threads lately is about flux or not flux........
o tempora ,o mores!

hehe
cheers to all Rodents!

--
Choky
Prodanovic Aleksandar
YU