Frequency does not matter for the "lag" ?

  • Context: Undergrad 
  • Thread starter Thread starter Yuri B.
  • Start date Start date
  • Tags Tags
    Frequency Matter
Click For Summary

Discussion Overview

The discussion revolves around the concept of phase lag in alternating current (AC) circuits, particularly in relation to inductors and the effect of frequency on this lag. Participants explore the implications of frequency on the phase relationship between voltage and current in inductive circuits.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that frequency does not affect the phase lag in an inductor, maintaining that the phase shift remains constant at -π/2 relative to the voltage.
  • Others provide calculations showing that for a 90-degree phase shift, the time lag can be calculated based on frequency, leading to longer time lags at lower frequencies.
  • One participant questions the assumptions regarding the inductor's resistance and capacitance, suggesting that real-world factors such as copper losses may affect the current.
  • There is a query about the form of the current observed on an oscilloscope, indicating interest in the practical implications of the theoretical discussion.

Areas of Agreement / Disagreement

Participants express differing views on whether frequency impacts the phase lag in inductors. While some argue that it does not matter, others highlight the significance of frequency in determining time lag, indicating that the discussion remains unresolved.

Contextual Notes

Limitations include assumptions about the inductor's ideal behavior (zero resistance and capacitance) and the potential impact of real-world factors such as copper losses on current behavior.

Yuri B.
Messages
137
Reaction score
0
AC, as is known, "lags" voltage in a coil and we are familiar with the drawings of the two sine waves with the 90 degree angular "shift" between them. For 50 Hz the shift translates into 5 µs "lagging" but, for instance, for 0.05 Hz AC it would indeed be 5 s ?

What I mean : frequency does not matter for the "lag" ?
 
Last edited:
Physics news on Phys.org
Yuri B. said:
AC, as is known, "lags" voltage in a coil and we are familiar with the drawings of the two sine waves with the 90 degree angular "shift" between them. For 50 Hz the shift translates into 5 µs "lagging" but, for instance, for 0.05 Hz AC it would indeed be 5 s ?

Well, for a 90 deg. phase shift, you'd have π/2 = ωtlag

tlag = π/(2ω) = π/(2*2π*(0.05 Hz)) = 1/(4*0.05 Hz) = 1/(0.2) s = 5 s.

So, sure.
 
Yuri B. said:
What I mean : frequency does not matter for the "lag" ?

No, it does not matter. Impedance of an inductor is ZL = iωL = ωLei(π/2) or ωL∠π/2 in phasor notation.

So you can see that, for a fixed sinusoidal AC voltage applied to an inductor, the the amplitude of the inductor current changes with frequency, but not the phase, which is fixed at -π/2 relative to the voltage, since I = V/Z, these being complex (phasor) currents and voltages.
 
Roughly speaking, a generator's rotor pole passing the stator pole will indeed cause no current in a (load) coil so far the poles have not passed over their midpoint ? No matter how slow moves the rotor ?
 
Thank you !
 
cepheid said:
No, it does not matter. Impedance of an inductor is ZL = iωL = ωLei(π/2) or ωL∠π/2 in phasor notation.

So you can see that, for a fixed sinusoidal AC voltage applied to an inductor, the the amplitude of the inductor current changes with frequency, but not the phase, which is fixed at -π/2 relative to the voltage, since I = V/Z, these being complex (phasor) currents and voltages.

what is the reactance of this inductor?
Are you assuming the inductor has zero resistance and has zero associated capacitance
 
technician said:
what is the reactance of this inductor?
Are you assuming the inductor has zero resistance and has zero associated capacitance

You are correct. It is always assumed so as you say, but in reality the "loss in copper" current starts immediately the emf is applied.
What is form of the current on oscilloscope ? (I have never had an access to one) A slope of the resistive current, then the sine of the magnetizing current (shifted 90 degrees to the emf)?
 
Last edited:

Similar threads

Undergrad Relationship between frequency and power for sound?
  • · Replies 11 ·
Replies
11
Views
6K
Graduate EM waves, longitudinal EM propagation?
  • · Replies 15 ·
Replies
15
Views
3K
Undergrad Wavelength to Frequency Relationship in Musical Notes
  • · Replies 11 ·
Replies
11
Views
6K
Graduate Electronic Noise General Questions - Laboratory Setting
  • · Replies 3 ·
Replies
3
Views
3K
Undergrad Does Phase Relationship Matter in AC Circuits and SHM?
  • · Replies 2 ·
Replies
2
Views
3K
Finding the resonant frequency of this "discriminator" demod circuit.
  • · Replies 2 ·
Replies
2
Views
3K
Graduate How does phase of merging sines affect overall periodic tones?
  • · Replies 18 ·
Replies
18
Views
3K
Solve AC Circuit Analysis Issues with Intel Galileo Gen2
  • · Replies 4 ·
Replies
4
Views
2K
Undergrad Non-locality by memory (Jung, 2020) applied to stochastic mechanics
  • · Replies 5 ·
Replies
5
Views
2K
What is the relationship between current and voltage in an inductive load?
  • · Replies 9 ·
Replies
9
Views
2K