# Magnetic Field of an AC generator

Hi
Please help me in understanding the kind the magnetic field used in an AC generator. Most of the figures (one shown in attached file) are shown with concave poles. Hence the field has to be radial. In such a field the angle between plane of coil and the magnetic field remains zero all the time (as in case of galvanometer), so the induced emf will remains maximum and constant.
This contradicts with the waveform of AC which is a sine wave. It is possible only with plane pole faces. Please guide me on this.

DrClaude
Mentor
Most of the figures (one shown in attached file)
You forgot the file...

anorlunda
Staff Emeritus

anorlunda
Staff Emeritus
I can give you a partial answer without any figures.

This contradicts with the waveform of AC which is a sine wave.

An AC signal is just one example of a time-varying DC signal. Ohm's law and Maxwell's Equations apply instantaneously. When you look at one instant of time, there is no difference between AC and DC.

I am really sorry I forgot adding the file. Please have it right now.

#### Attachments

• AC generator.png
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I can give you a partial answer without any figures.

An AC signal is just one example of a time-varying DC signal. Ohm's law and Maxwell's Equations apply instantaneously. When you look at one instant of time, there is no difference between AC and DC.
Thanks for the reply. I agree with you. I am actually worried about the figure shown in most of the books which shows concave poles of AC generator. I wonder how a sinusoidal voltage is produced in the presence of a radial magnetic field which concave poles must produce. Pl reply. Thanks once again.

cnh1995
Homework Helper
Gold Member
Thanks for the reply. I agree with you. I am actually worried about the figure shown in most of the books which shows concave poles of AC generator. I wonder how a sinusoidal voltage is produced in the presence of a radial magnetic field which concave poles must produce. Pl reply. Thanks once again.
The concave pole generator indeed produces a square-ish wave, but I believe what the books want to emphasize is how the polarity of the voltage changes as the coil rotates through 180 degrees. This is why it is called an 'alternator', the one which produces an alternating (not necessarily sinusoidal) voltage . To make this ac voltage sinusoidal, several modifications are needed in this model. You can look up a practical ac alternator and see how they shape the rotor, how they arrange the windings and how they adjust the air gap to make the waveform as sinusoidal as possible.

The concave pole generator indeed produces a square-ish wave, but I believe what the books want to emphasize is how the polarity of the voltage changes as the coil rotates through 180 degrees. This is why it is called an 'alternator', the one which produces an alternating (not necessarily sinusoidal) voltage . To make this ac voltage sinusoidal, several modifications are needed in this model. You can look up a practical ac alternator and see how they shape the rotor, how they arrange the windings and how they adjust the air gap to make the waveform as sinusoidal as possible.
Thank you very much indeed for the answer. I got your point. May we use plane pole faces and parallel magnetic field to produce sinusoidal voltage?

cnh1995
Homework Helper
Gold Member
Thank you very much indeed for the answer. I got your point. May we use plane pole faces and parallel magnetic field to produce sinusoidal voltage?
Yes, flat pole faces will create a sinusoidal ac voltage here.