Basics of Transformers: Negative Signs in EMF Terms

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    Basics Transformers
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Discussion Overview

The discussion revolves around understanding the negative signs in the induced electromotive force (EMF) terms, specifically in the context of transformers. Participants explore the implications of these signs in relation to the direction of current, magnetic flux, and the winding configuration of coils.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • Some participants seek clarification on why the EMF terms are expressed with negative signs, particularly in the context of N1dΦ/dt and N2dΦ/dt.
  • There is a discussion about the relationship between the direction of induced EMF and the winding sense of the coils, with some suggesting that the polarities depend on the "dot convention" in magnetically coupled circuits.
  • One participant notes that the induced voltages can be either in phase or 180° out of phase, depending on the winding direction.
  • Another participant mentions that the signs in calculations may not be critical, focusing instead on understanding phase relations.
  • There is a consideration of how the absence of a common electrical ground affects the interpretation of the signs in the EMF terms.
  • Some participants discuss the implications of Lenz's law, emphasizing that the induced EMF opposes the cause of its induction.
  • One participant questions whether the polarity of the induced EMF is independent of the winding configuration, leading to further exploration of how winding sense influences induced EMF polarity.

Areas of Agreement / Disagreement

Participants express differing views on the significance of the negative signs in the EMF terms and the influence of winding direction on induced EMF polarity. There is no consensus on the necessity of the signs in calculations or the interpretation of their implications.

Contextual Notes

Participants note that the understanding of induced EMF and its signs may depend on specific assumptions regarding winding configurations and the presence of common reference points in circuits.

Who May Find This Useful

This discussion may be useful for students and practitioners interested in electrical engineering, particularly those studying transformers and electromagnetic induction principles.

  • #31
cnh1995 said:
The net flux is the flux set up by the magnetizing current. It is constant as long as the applied voltage (rms) is constant.

If flux is constant , how is EMF induced in the two coils ?
 
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  • #32
Vibhor said:
If flux is constant , how is EMF induced in the two coils ?
RMS value of the flux is constant. Or you can say, the amplitude of the flux waveform is constant.
Merlin3189 said:
I think it would - so long as by constant you mean, varying sinusoidally with constant amplitude.
A literally constant flux does not cause any emfs. EMF is always associated with changing flux.
 
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  • #33
Ok . I will stop here for the time being :wink:

You and @Merlin3189 both have been amazing . Thanks both of you for your time and patience . I really admire the way you both stuck with me in this thread . Some expert members would have been fretting and fuming by now ?:)

You might have also been irritated at times . But the good part is that ,you didn't let me know o0) .

People might have more knowledge than you , but few have finer qualities like patience :smile:

Keep up the good work :thumbup: .
 
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  • #34
cnh1995 said:
This reflected secondary current cancels the secondary mmf.
We have been using this term frequently in this thread, but I think we haven't discussed its another important physical significance.

This reflected secondary current Ip1 is actually the reflection of the load connected to the secondary.
For a resistive load, the product of applied primary voltage and reflected secondary current gives the power supplied by the source, which is dissipated in the secondary.
Hence,
Vp*Ip1=Vs*Isec.

Other than maintaining the core flux constant by cancelling the secondary mmf, the reflected load current in the primary is responsible for supplying power (active and reactive) from the source to the load on the secondary.

And thanks for your kind words!:smile:
 

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