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.
On Fri, 11 Jul 2003 09:06:56 -0500,
(John Byrns) wrote:
In article , "Ian
Iveson" wrote:
"John Byrns" wrote in
For mains trannies they require the voltage to be specified, but
it
has no effect on core size.
The input voltage and frequency for a mains transformer are fixed
quantities, so it is only natural to rate the windings in terms of
these.
No, John. The original question demonstrates that
input voltage is not a fixed quantity. That is what the pertinent
parts of the thread are about.
Yes, Ian, the input voltage is a fixed quantity, here in the US the
domestic supply voltage provided to residences is 120 volts at 60 Hz, in
Europe it is something different, but still fixed. The original posters
question was, can I hook the 60 volt winding of a power transformer to
that 120 volt supply without damaging the transformer? It is not
necessary to get the data on the core and calculate current to provide a
reasonably accurate to that question. My point is that I am not a
transformer designer, I am only a user. Sure the transformer designer is
going to do all the current calculations you talk about, but why should I
do that as a user, when I can much more easily determine approximately
what voltage a transformer winding can handle at a given frequency, based
on the original voltage and frequency ratings of the transformer, which in
commercial grade power transformers bear a close relationship to the
saturation point?
The core size has nothing to do with voltage for a given quality
transformer. I'm sure you know this really. You're just trying to
wind me up.
Although I may be mistaken, I don't think I have made any claims in this
thread about core size vs. voltage, I don't think I have even mentioned
core size. I think you are confusing my comments with those of someone
else. You are the one that insists on injecting core size into the
discussion.
I'm not trying to wind you up, I'm simply saying that from the perspective
of a user of power transformers, the voltage spec. of a winding is the
most relevant for determining if a given voltage will saturate the core,
the current is only of interest to the transformer user in calculating the
power the transformer can pass without overheating. As a user,
calculating the saturation point based on current is a much more complex
undertaking than simply looking at the rated voltage and frequency.
Calculating saturation based on current requires me to have knowledge of
several characteristics of the core, and do relatively involved math, why
go to all that trouble when I know approximately how commercial power
transformers are designed with respect to saturation vs. voltage, and
frequency?
Regards,
John Byrns
Surf my web pages at, http://users.rcn.com/jbyrns/
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