13.10.2012 Views

boylistad

boylistad

boylistad

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

890 ⏐⏐⏐ RESONANCE<br />

Q L = I 2 X L<br />

S = EI<br />

P = I 2 R = EI<br />

Q C = I 2 X C<br />

FIG. 20.4<br />

Power triangle for the series resonant circuit<br />

at resonance.<br />

The average power to the resistor at resonance is equal to I 2 R, and<br />

the reactive power to the capacitor and inductor are I 2 X C and I 2 X L,<br />

respectively.<br />

The power triangle at resonance (Fig. 20.4) shows that the total<br />

apparent power is equal to the average power dissipated by the resistor<br />

since Q L � Q C. The power factor of the circuit at resonance is<br />

F p � cos v �<br />

and Fps � 1<br />

(20.7)<br />

Plotting the power curves of each element on the same set of axes (Fig.<br />

20.5), we note that, even though the total reactive power at any instant<br />

is equal to zero (note that t � t ′), energy is still being absorbed and<br />

released by the inductor and capacitor at resonance.<br />

Power<br />

supplied to<br />

element<br />

0<br />

Power<br />

returned by<br />

element<br />

p L<br />

p C<br />

p R<br />

p C<br />

P<br />

� S<br />

t 1 t2 t 3 t 4 t 5<br />

p L<br />

FIG. 20.5<br />

Power curves at resonance for the series resonant circuit.<br />

ƒ r<br />

p′ L<br />

t<br />

p′ C = p′ L<br />

A closer examination reveals that the energy absorbed by the inductor<br />

from time 0 to t 1 is the same as the energy released by the capacitor<br />

from 0 to t 1. The reverse occurs from t 1 to t 2, and so on. Therefore, the<br />

total apparent power continues to be equal to the average power, even<br />

though the inductor and capacitor are absorbing and releasing energy.<br />

This condition occurs only at resonance. The slightest change in frequency<br />

introduces a reactive component into the power triangle, which<br />

will increase the apparent power of the system above the average power<br />

dissipation, and resonance will no longer exist.<br />

20.3 THE QUALITY FACTOR (Q )<br />

The quality factor Q of a series resonant circuit is defined as the ratio<br />

of the reactive power of either the inductor or the capacitor to the average<br />

power of the resistor at resonance; that is,<br />

Qs �� reactive<br />

power<br />

�<br />

average<br />

power<br />

(20.8)<br />

The quality factor is also an indication of how much energy is placed in<br />

storage (continual transfer from one reactive element to the other) compared<br />

to that dissipated. The lower the level of dissipation for the same

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!