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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I understand the basics of transformer theory: turns ratios, current, b-field, flux linkage through the number of turns. What I am not sure of is this:
Imagine two ideal transformers with a 1:10 ratio. The first has 10v on the primary and 100v on the secondary. The current in the primary creates a magnetic field and the flux cuts through the turns on the secondary to set up a current. The current through a load creates the voltage.
Now let's say our second transformer has a primary with 10x the resistance of the first transformer's primary and a primary voltage that is 100v (10x the voltage of transformer one's primary). The currents through both primaries are the same so the flux should be the same cutting through the secondary; yet, according to theory, the secondary should have a voltage of 1000v.
Can anyone tell me what I am missing here, or is the mistake assuming that my ideal transformer has such a high primary resistance?
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
If your second case is a 1:10 turns ratio and primary voltage of 100V then the secondary voltage will be 1000V under no-load condition.
Transformers are different under load because of resistance, inductance, core loss, winding loss.
I think what you're asking is what happens when the winding losses are large. The answer is that you will loose a voltage in the transformer proportional to IIR in the secondary.
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
The current through both primaries will not be the same as you have raised the voltage through the first tranny, you have dropped the current for a given power input. P=VI
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
"or is the mistake assuming that my ideal transformer has such a high primary resistance?" Yes, the current through a transformer primary with no load is usually determined by the inductance of the primary, rather than its resistance.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yes, BC is right. A transformer whose primary current was limited by its resistance would perform so crappily that it would never be used in practice.
With practical transformers what will happen is: In your second transformer, raising the input voltage from 10v to 100v will cause 10 times more flux to flow in the core, and the output voltage to go up to 1000v.
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transformer theory
