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41 - ����������� - AC ��������

Figure 41-7 Instantaneous values of a sine wave

Finding the instantaneous voltage (e). We know from Figure 41-7 that:

sin {45°} x Emax = 240VAC.
By dividing both sides by sin (45°) we get:
Emax = 240VAC / sin{45°)

Solving we get:

Emax = 339 volts

Which I hope you recognised as the peak household mains voltage in Australia.
Calculate the instantaneous voltage at 210°

e = sin (210°) x Emax
e = sin (210°) x 339
e = -169 volts

Notice how the voltage on the second half cycle of the sinewave is negative. This
tells us that when the sinewave is representative of voltage, it has a polarity reversal.

THE RADIAN

We say that a circle is 360°. Why? We could just as easily define a circle as 100°.
The 360° is totally arbitrary and goes back to the Babylonians who used a base 60
numbering system. While we work in degrees and are familiar with them in daily life,
it really is a silly way to define angles on culture rather than on the circle itself.

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