Cong suat dinh muc quat $P=120W. Dong dien dinh muc cua quat I$ Dien tro trong cua quat la $R_{o}$
$R_{2}$ la gia tri cua bien tro khi quat hoat dong binh thuong khi U=220V
Khi bien tro co gia tri $R_{1}=70\Omega $ thi $I_{1}=0,75A, P_{1}=0,928P=111,36W$
Co $P_{1}=I_{1}^{2}.R_{o}=> R_{o}=\frac{P_{1}}{I_{1}^{2}}\approx 198\Omega $
$I_{1}= \frac{U}{Z_{1}}=\frac{U}{\sqrt{(R_{o}+R_{1})^{2}+(Z_{L}-Z_{C})^{2}}}=\frac{U}{\sqrt{268^{2}+(Z_{L}-Z_{C})^{2}}}$
Suy ra$ \left| {Z_{L}-Z_{C}} \right|\approx 119\Omega $
De quat hoat dong bing thuong $P=120W=\frac{U^{2}R_{o}}{(R_{o}+R_{2})^{2}+(Z_{L}-Z_{C})^{2}}=>R_{1}+R_{2}\approx 256\Omega => R_{2}=58\Omega $
$R_{2}<R_{1} =>$ Bien tro giam di $12\Omega $
$Dap an C$