# Flux because of secondary coil in transformer

In a Transformer - Alternating current flows in primary winding

Causing alternating magnetic field in Iron core

Which causes alternating voltage in secondary coil.

But I am confused that what would be the polarity of the Flux caused by the secondary winding voltage.

Won't the Flux caused by the secondary voltage cancel the Flux caused by the primary voltage and thus making the net Flux zero in the coil if both the primary and secondary windings have the same number of turns?

• Voltage does not cause flux current does.
– RoyC
Commented Nov 21, 2021 at 9:28

## 3 Answers

Two differing currents flow in the primary winding of a secondary loaded transformer. The first is the magnetization current and this is purely due to the primary being regarded as an inductor. That current is 90 degrees lagging the applied primary voltage. That current sets up the magnetic field and that current induces the secondary voltage.

The other primary current occurs when the secondary is loaded. This new additional primary current produces a magnetic flux that is exactly opposite to the magnetic flux caused by the loaded secondary current. In effect, the net flux addition due to loading the secondary is zero; fluxes due to loading are cancelled leaving only the original magnetic flux due to a voltage being applied across the primary.

• So when transformer is not loaded the Flux due to the primary current lags 90 degree behind the primary voltage. And this Flux cause secondary voltage which also lags behind the primary voltage by 90 degree but in phase with primary current. That secondary voltage won't also cause a magnetic Flux in the core? What would be the phase of that secondary Flux caused by secondary voltage?
– Alex
Commented Nov 21, 2021 at 10:23
• When the secondary is unloaded the only current in the primary is the magnetization current and that lags the primary by 90 degrees. Just like it would in an inductor because the unloaded secondary can be totally disregarded; it plays no role when unloaded. Magnetization flux is in phase with magnetization current. The secondary voltage (when secondary is unloaded) plays no role in making any flux. Commented Nov 21, 2021 at 11:35
• The secondary voltage does not lag by 90 degrees behind the primary voltage. The magnetizing current lags 90 degrees behind the primary voltage and then, due to Faraday and lenz, the secondary voltage lags the primary magnetization current by 90 degrees and is inverted by lenz's law to make secondary voltage exactly in phase with the primary voltage. Commented Nov 21, 2021 at 11:41
• OK deteled but there is one thing more. So it means that the magnetic field is formed in the secondary electric field or voltage even when there is no secondary current but its of so much less magnitude to have any significant effect? Right?
– Alex
Commented Nov 21, 2021 at 14:09
• It will be very very small and generate currents probably a million times smaller than what flows in the primary of a regular mains transformer. Commented Nov 21, 2021 at 14:17

The excitation flux is 90 degrees out of phase VS. primary voltage. When the current flows, both primary and secondary flux due the load current is cancelled as you noted, but the magnetization flux remains unchanged (90 deg. off).

We can say that the total flux in the transformer core is the magnetization flux and it is constant. The only factors that play are primary voltage and frequency. No matter of the load current the flux remains the same.

In a Transformer - Alternating current flows in primary winding

Yes, but how much flows? See later.

Causing alternating magnetic field in Iron core

Yes, but that's not the entire current-causing-flux story, see later

Which causes alternating voltage in secondary coil.

Yes

But I am confused that what would be the polarity of the Flux caused by the secondary winding voltage.

The secondary winding voltage does not cause a flux. The only thing the secondary winding voltage might do is push a secondary current through an external load. That secondary current will generate a flux.

Won't the Flux caused by the secondary voltage cancel the Flux caused by the primary voltage and thus making the net Flux zero in the coil if both the primary and secondary windings have the same number of turns?

One way to look at it is that the flux caused by the secondary winding current cancels some of the flux caused by the primary current, leaving a residual flux that magnetises the core, and whose changes generate the primary and secondary voltage.

Another way to look at it is that the net magnetising flux in the core has to generate the primary voltage. As the primary voltage is constant, the amplitude of the magnetising flux in the core has to stay constant.

When the secondary is open circuit, the primary current equals the magnetising current and creates the core flux. When the secondary gets loaded, a secondary current flows, which would decrease the core flux, were it not for the fact that the primary current increases to cancel out the secondary current, leaving a difference between the primary and secondary fluxes that is the magnetising flux.

In a good transformer, the magnetising current required might only be a few percent of the full load currents to generate the magnetising flux. In an ideal transformer, the magnetising flux is still finite, but the magnetising current is zero, because the ideal transformer has an infinite permeability core (or approaches zero as the permeability approaches infinity)

• So when transformer is not loaded the Flux due to the primary current lags 90 degree behind the primary voltage. And this Flux cause secondary voltage which also lags behind the primary voltage by 90 degree but in phase with primary current. That secondary voltage won't also cause a magnetic Flux in the core? What would be the phase of that secondary Flux caused by secondary voltage?
– Alex
Commented Nov 21, 2021 at 10:24
• isn't the changing electric field causes the changing magnetic field? When the Flux change because of primary current cause voltage in the second winding then won't that secondary voltage will produce the magnetic field or magnetic Flux of its own as the change in voltage in the secondary winding is also a changing electric filed in secondary ? Won't that secondary electric filed or electrical voltage will produce the changing Flux of their own in the core?
– Alex
Commented Nov 21, 2021 at 10:41
• Electric field does not cause the magnetic field. Current, ie movement of charge carriers, is what causes magnetic field. get the idea that electric field causes magnetic field totally out of your head, that's the main cause of your confusion. Current causes magnetic field. Change in magetic field causes voltage in loops around the changing field. Commented Nov 21, 2021 at 10:45
• @Alex A changing electric field will produce a displacement current in a polarisable medium that it acts across. This current will then produce a magnetic field, and that's what EM waves are. However a transformer is a simplified model of reality, in which this effect is negligible due to the ratios of material properties, we just concentrate on the conducted currents, which can explain the ideal operation of the transformer in full. A real transformer has many extra terms, and we can lump any EM wave contribution into one of the smaller error terms. They are relevant at GHz and with air cores Commented Nov 21, 2021 at 14:48
• @Alex Thanks for the thought, but that pattern of behaviour is so easy to spot that the site has a name for it, and reverts the effect of it when found. Votes are for answers, not authors. Commented Nov 21, 2021 at 20:45