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nurdee15 years ago
Only if the length of the windings is the same, which it probably isn't. In step down transformers, the most common type, there are fewer windings in the second coil. What this does is make the voltage of the second winding lower than the primary and the amperage in the second winding higher than the primary. The voltage times current in the primary is the same as the voltage times current in the secondary.
iceng nurdee15 years ago
That is the same mistake I made when I wound my first power transformer.

The voltage times current in the primary is always More then the V*I in the secondary ! !

Why ??  Two reasons ;
  1. Efficiency cannot be 100%  Never Ever even in a transformer.
  2. Every transformer has magnetizing loss in the primary.  This makes the primary V*I  greater then the secondary V*I....
BTW what happens with different length and
different wire diameter windings ?

nurdee1 iceng5 years ago
Ok, This is my understanding of why there is less power in the secondary than in the primary. There are two reasons the first is eddy current. This is when the core picks up some of the charge. This icauses there to be less charge in the secondary. This loss is lessened by making transformer cores out of laminations of thin metal plates. This makes it so the vibration is only set up in one of the thin plates and not in the whole transformer core. This effect is what causes the humming in some transformers the humming is caused because of the microscopic vibrations. The second loss of power is the copper wire. Copper no matter how good of copper it still has resistance. after all anything has some resistance except for some superconducters. But anyway the larger the diameter of the copper wire the less the resistance will be. This is why arc welders have succh huge cables.
iceng nurdee15 years ago
And I can wind coils of different lengths with the same exact resistance 
by using different gauge Cu wire.

While magnetostrictive strain is used in ultrasonic cleaners,
the hum in some transformers is poor shellac or polyurethane potting
of windings and laminations allowing simple magnetic movement
with AC periodic changes in magnetic flux.

There is also stray field coupling acoustic hum to nearby iron objects.

iceng5 years ago
Initially I thought only a single case could be true, then I ran a thought experiment and realized there were two cases quickly  followed by the realization there are thousands of cases where the current is the same
in both primary and secondary

I can say that the current in an EI transformer with a 1:1 winding ratio
of the same wire wound bifilar ( two in hand ) or side by side coils
for guaranteed equal resistance, WILL NEVER BE THE SAME !
Because while the unity ratio would make one believe the in and out
should be the same the magnetizing component of the primary will
always easily take more current then the secondary can deliver.

If you are confused, just recall there is no process that is 100% efficient.

That is why the power input to any magnetic transformer will always be
more then the output power..

Now, if you want to match currents change the winding ratio like a Variac
ore change the wire size ( resistance ) or both there can be thousands
of combinations to get equal current or equal voltage BUT not both at
the same time ever, Period...

jeff-o5 years ago
Basically yes, if the turns ratio is 1:1. For anything else the current will be different. Note that there will be some losses at play (depending on the source, load, and properties of the transformer) so in real life the currents will never really be exactly the same.
Depends on turn ratio and wire thickness.
In general, no.
+1. Assuming the author really means 'current' in amps...then it will only be close if there are a similar number of turns and no losses...

so basically no. :D

The wattage (minus losses) should be conserved.