So in a transformer, when an alternating current flows through the primary, thanks to the Ampere's law, a changing magnetic field is produced around it. This induces an alternating current in the secondary. So far so good. Now my question is, does this induced AC not create a changing magnetic field in the secondary, which in turn induces current in the primary (just like the primary did to the secondary)? And thus, won't we have a self-sustaining circuit where the primary and the secondary took turns to induce each other?
Electrical – Mutual Induction in Transformer
acinductiontransformer
Related Solutions
The output of a transformer is always AC. In the long run, the average current thru a transformer secondary with resistive load is always 0.
For example, consider driving the primary with a 0-10 V pulse train that is low for 9 µs and high for 1 µs. The DC level is 1 V going into the primary. However, that 1 V will be lost coming out of the secondary. The open-circuit voltage you get out is -1 V for 9 µs and +9 V for 1 µs, multiplied by the turns ratio. If the primary has 100 turns and the secondary 300, then you will get -3 V and +27 V, for example.
A good meter set to DC will always show this as 0 V.
Different AC voltmeters will show you different voltages for the -1 to +9 V example pulse waveform. That is because most meters don't read true RMS. Most will measure the average of the absolute value, then apply the correction factor from that to RMS for a sine wave. That correction factor will be incorrect for something like the pulse train in this example.
To get the voltage of non-sinusoid waveforms, you either have to get a true RMS meter, and make sure your signal is within its frequency range, or use a scope and do the math yourself. Some scopes have RMS measurement built-in as a math function, which makes them in effect true RMS meters. Again, keep the limitations of the RMS meter in mind.
Electronic – ny Phase difference between the current in primary and secondary coils of a transformer
This answer explains that for an unloaded secondary, the natural phase relationship between primary voltage and secondary voltage is is zero degrees.
It therefore follows that if there is a secondary load current (due to a resistive load), the current in the primary due to that secondary resistive load must be 180 degrees out of phase with the secondary load current i.e. as current flows into the primary, current flows out from the secondary.
This of course is for an ideal transformer and a resistive load.
If you ignore the leakages and magnetization inductance of the transformer, and the load is reactive, then there will be a 90 degrees phase shift.
Bringing in leakage inductance and DC coil resistance will/can muddy the waters. Bringing in magnetization inductance muddies the water a bit more.
The low frequency transformer equivalent circuit is this: -
As you should be able to see, if you considered all the leakages, magnetization inductance and losses and then added a semi-reactive load, the phase angle is quite complex to calculate.
However I do-not know, in which-way Lenz's law acts in transformer; because the law states the induced current will try to hinder the cause.
Strictly speaking, it is voltage that is induced and any current that flows is subject to the that voltage, the load and the leakage inductance.
but when the secondary circuit of a step-up transformer turned On (closed) , so-far I've know , the current in primary-coil goes-up
In normal usage, for a voltage transformer it is non-ideal to consider the secondary being short circuited. However, it makes no difference to the phase angle providing you obey the rules inherent to the model.
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Best Answer
A primary current flows. This induces a changing magnetic field H around it. The changing H field in the core induces a changing B field in the core.
No. The changing B field induces an alternating voltage in the secondary. It also induces an alternating voltage in the primary of the right magnitude to more or less cancel out the primary applied voltage. The small difference between the applied voltage and the primary induced voltage allows sufficient current to flow in the primary resistance to create enough field for this all to be in balance.
If there's a load connected to the secondary, then the secondary voltage pushes a current through the load, which of course completes its circuit through the secondary.
Don't forget they are wound round the same core. Any current that flows in the secondary creates an H field that adds to the primary's H field. The phase of the secondary current is always to reduce the H field. The reduced H field generates a reduced B field in the core.
more or less correct. The reduced B field doesn't cancel as much of the primary input voltage, so the larger difference between the applied voltage and the primary induced voltage allows (demands) a higher primary current. The net result of the secondary current flow is to cause an increased primary current to be drawn.
As I said, the phase is such to reduce the field. The net result of the two-way interaction is that as the secondary supplies more power to its load, the transformer draws more power from its supply to match it.