RlCcircuit

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Let pp the initial phase of vv

1CQ+RdQdt+Ld 2Qdt 2=ve i(ωt+p)\frac{1}{C} Q + R \frac{d Q}{d t} + L \frac{d^2 Q}{d t^2}= v e^{i ( \omega t + p )}

1C+Rλ+Lλ 2=0\frac{1}{C} + R \lambda + L \lambda^2 = 0

Q=Xe i(ωt+p)Q = X e^{i (\omega t + p)}

1CX+RiωX+L(iω) 2X=v\frac{1}{C} X + R i \omega X + L ( i \omega )^2 X = v

X=v1CLω 2+iRω|X|e iθX = \frac{v}{\frac{1}{C} - L \omega^2 + i R \omega} \equiv |X| e^{i \theta}

Note that XX is always in III and IV quadrant and QQ is before VV by θ\theta. When R=0R=0, QQ is before VV by 0 or 180 degree.

|X|=v(1CLω 2) 2+(Rω) 2 V C=1Cωv(1CωLω) 2+R 2 I=dQdt=iωXe i(ωt+p)=vR+i(Lω1Cω)e i(ωt+p) |I|=v(1CωLω) 2+R 2 V R=Rv(1CωLω) 2+R 2 d 2Qdt 2=ω 2Xe i(ωt+p) V L=Lωv(1CωLω) 2+R 2\begin{aligned}|X| = \frac{v}{\sqrt{\left( \frac{1}{C} - L \omega^2 \right)^2 + (R \omega)^2}}\\V_C = \frac{\frac{1}{C \omega} \cdot v}{\sqrt{\left( \frac{1}{C \omega} - L \omega \right)^2 + R^2}}\\I = \frac{d Q}{d t} = i \omega X e^{i (\omega t + p)} = \frac{v}{R + i \left( L \omega - \frac{1}{C \omega} \right)} e^{i (\omega t + p)}\\|I| = \frac{v}{\sqrt{ \left( \frac{1}{C \omega} - L \omega \right)^2 + R^2}}\\V_R = \frac{R v}{\left(\frac{1}{C \omega} - L \omega \right)^2 + R^2}\\\frac{d^2 Q}{d t^2} = - \omega^2 X e^{i (\omega t+p)}\\V_L = \frac{L \omega v}{\sqrt{\left(\frac{1}{C \omega} - L \omega \right)^2 + R^2}}\end{aligned}

Note that

(V CV L) 2+V R 2=v 2(V_C - V_L)^2 + V_R^2 = v^2

The general solution is for some constants c 1c_1 and c 2c_2 and λ 1\lambda_1 the eigen value of smaller absolute value

Q=Xe i(ωt+p)+c 1e λ 1t+c 2e λ 2tQ = X e^{i ( \omega t + p )} + c_1 e^{\lambda_1 t} + c_2 e^{\lambda_2 t}

I=iωXe i(ωt+p)+c 1λ 1e λ 1t+c 2λ 2e λ 2tI = i \omega X e^{i ( \omega t + p )} + c_1 \lambda_1 e^{\lambda_1 t} + c_2 \lambda_2 e^{\lambda_2 t}

When capacitor does not exist, aka, CC is infinite,

Lω=R(vV R) 21L \omega = R \sqrt{\left( \frac{v}{V_R} \right)^2 - 1}

Note that II is before QQ (therefore V CV_C ) 90 degree.

Typical initial condition is that QQ and II are zero because no charge in the capacitor. If there is no capacitor, initial condition about QQ can be omit, so instead, the initial condition is I=0I =0 only

0=Xe ip+c 1+c 2 0=iωXe ip+c 1λ 1+c 2λ 2 c 1=iωXe ipλ 2Xe ipλ 1λ 2 c 2=iωXe ipλ 1Xe ipλ 2λ 1 c 1λ 1e λ 1t+c 2λ 2e λ 2t=iωXλ 1e λ 1te ipλ 2Xλ 1e λ 1te ipλ 2λ 1+iωXλ 2e λ 2te ip+λ 1Xλ 2e λ 2te ipλ2λ1λ 1e λ 1t =iωX(λ 1e λ 1tλ 2e λ 2t)e ip+λ 1Xλ 2(e λ 2te λ 1t)e ipλ 2λ 1 =iωX(λ 1e λ 1tλ 2e λ 2t)e ip+XLC(e λ 2te λ 1t)e ipλ 2λ 1\begin{aligned}0 = X e^{i p} + c_1 + c_2\\0 = i \omega X e^{i p} + c_1 \lambda_1 + c_2 \lambda_2\\c_1 = - \frac{i \omega X e^{i p} - \lambda_2 X e^{i p}}{\lambda_1 - \lambda_2}\\c_2 = - \frac{i \omega X e^{i p} - \lambda_1 X e^{i p}}{\lambda_2 - \lambda_1}\\c_1 \lambda_1 e^{\lambda_1 t} + c_2 \lambda_2 e^{\lambda_2 t} = \frac{i \omega X \lambda_1 e^{\lambda_1 t} e^{i p} - \lambda_2 X \lambda_1 e^{\lambda_1 t} e^{i p}}{\lambda_2 - \lambda_1} + \frac{-i \omega X \lambda_2 e^{\lambda_2 t} e^{i p} + \lambda_1 X \lambda_2 e^{\lambda_2 t} e^{i p}}{\lambda 2 - \lambda 1} \lambda_1 e^{\lambda_1 t} \\= \frac{i \omega X ( \lambda_1 e^{\lambda_1 t} - \lambda_2 e^{\lambda_2 t} ) e^{i p} + \lambda_1 X \lambda_2 ( e^{\lambda_2 t} - e^{\lambda_1 t} ) e^{i p}}{\lambda_2 - \lambda_1} \\= \frac{i \omega X ( \lambda_1 e^{\lambda_1 t} - \lambda_2 e^{\lambda_2 t} ) e^{i p} + \frac{X}{L C} ( e^{\lambda_2 t} - e^{\lambda_1 t} ) e^{i p}}{\lambda_2 - \lambda_1}\end{aligned}

And:

I=(iωXe iωt+iωX(λ 1e λ 1tλ 2e λ 2t)+XLC(e λ 2te λ 1t)λ 2λ 1)e ip IωX=ie iωt+i(λ 1e λ 1tλ 2e λ 2t)+1LCω(e λ 2te λ 1t)λ 2λ 1 <ie iωt+i(λ 1e λ 1tλ 2e λ 2t)λ 2λ 1+1LCω(e λ 2te λ 1t)λ 2λ 1\begin{aligned}I = \left( i \omega X e^{i \omega t} + \frac{i \omega X \left( \lambda_1 e^{\lambda_1 t} - \lambda_2 e^{\lambda_2 t} \right) + \frac{X}{L C} \left( e^{\lambda_2 t} - e^{\lambda_1 t} \right)}{\lambda_2 - \lambda_1} \right) e^{i p}\\ \mid \frac{I}{\omega X} \mid = \mid i e^{i \omega t} + \frac{i \left( \lambda_1 e^{\lambda_1 t} - \lambda_2 e^{\lambda_2 t} \right) + \frac{1}{L C \omega} \left( e^{\lambda_2 t} - e^{\lambda_1 t} \right)}{\lambda_2 - \lambda_1} \mid \\&lt; \mid i e^{i \omega t} \mid + \mid \frac{i \left( \lambda_1 e^{\lambda_1 t} - \lambda_2 e^{\lambda_2 t} \right)}{\lambda_2 - \lambda_1} \mid + \mid \frac{\frac{1}{L C \omega} \left( e^{\lambda_2 t} - e^{\lambda_1 t} \right)}{\lambda_2 - \lambda_1} \mid\end{aligned}

Note that

e λ 2te λ 1t<2 λ 1e λ 1tλ 2e λ 2t=λ 1e λ 1tλ 1e λ 2t+λ 1e λ 2tλ 2e λ 2t <λ 1(e λ 1te λ 2t)+(λ 1λ 2)e λ 2t <2|λ 1|+|λ 1λ 2|\begin{aligned}\mid e^{\lambda_2 t} - e^{\lambda_1 t} \mid &lt; 2\\ \mid \lambda_1 e^{\lambda_1 t} - \lambda_2 e^{\lambda_2 t} \mid = \mid \lambda_1 e^{\lambda_1 t} - \lambda_1 e^{\lambda_2 t} + \lambda_1 e^{\lambda_2 t} - \lambda_2 e^{\lambda_2 t} \\&lt; \mid \lambda_1 (e^{\lambda_1 t} - e^{\lambda_2 t}) \mid + \mid (\lambda_1 - \lambda_2) e^{\lambda_2 t} \mid \\&lt; 2 |\lambda_1| + |\lambda_1 - \lambda_2|\end{aligned}

Therefore,

IωX<2+2λ 1λ 2λ 1+2LCωλ 2λ 1\mid \frac{I}{\omega X} \mid &lt; 2 + 2 \mid \frac{\lambda_1}{\lambda_2 - \lambda_1} \mid + \frac{2}{L C \omega \mid \lambda_2 - \lambda_1 \mid}

Transformer example. R=1.5Ω,C=R = 1 . 5 \Omega , C = \infty , rating 3000 watt 220 V, then LΩ=220 23000=16.13Ω,v=2202=311L \Omega = \frac{220^2}{3000} = 16 .13 \Omega , v = 220 \cdot \sqrt{2} = 311

λ 1=0 λ 2=RL ω|X|=v(Lω) 2+R 2 I=ω|X|(ie i(ωt+θ+p)ie RLte i(θ+p)) dIdt=ω|X|(ωe i(ωt+θ+p)+iRLe RtLe i(θ+p))\begin{array}{l}\lambda_1 = 0\\\lambda_2 = - \frac{R}{L}\\\omega |X| = \frac{v}{\sqrt{(L \omega)^2 + R^2}}\\I = \omega |X| \left( i e^{i ( \omega t + \theta + p )} - i e^{-\frac{R}{L} t} e^{i ( \theta + p )} \right)\\\frac{d I}{d t} = \omega |X| \left(- \omega e^{i (\omega t + \theta + p)} + i \frac{R}{L} e^{- \frac{R t}{L}} e^{i ( \theta + p )} \right)\end{array}

Take the real parts,

(I)=ω|X|(sin(ωt+θ+p)+e RLtsin(θ+p)) (dIdt)=ω|X|(ωcos(ωt+θ+p)RLe RLtsin(θ+p))\begin{array}{l}\Re(I) = \omega |X| \left( - \sin(\omega t + \theta + p) + e^{\frac{-R}{L} t} \sin(\theta + p) \right)\\\Re \left( \frac{d I}{d t} \right) = \omega |X| \left( - \omega \cos(\omega t + \theta + p) - \frac{R}{L} e^{\frac{-R}{L} t} \sin(\theta + p) \right)\end{array}

Steady energy power:

E R=ω|X|sin(ωt+θ+p)ω|X|sin(ωt+θ+p)R=v 2R(Lω) 2+R 2sin 2(ωt+θ+p) E L=ω 3|X| 2Lsin(ωt+θ+p)cos(ωt+θ+p)L=v 2Lω(Lω) 2+R 2sin(2(ωt+θ+p)2 E C=0\begin{array}{l}E_R = \omega |X| \cdot \sin(\omega t + \theta + p) \cdot \omega |X| \cdot \sin(\omega t + \theta + p) \cdot R = \frac{v^2 R}{(L \omega)^2 + R^2} \sin^2(\omega t + \theta + p)\\E_L = \omega^3 |X|^2 L \sin(\omega t + \theta + p) \cdot \cos(\omega t + \theta + p) \cdot L = \frac{v^2 L \omega}{(L \omega)^2 + R^2} \cdot \frac{\sin(2 ( \omega t + \theta + p )}{2}\\E_C = 0\end{array}

If θ+p\theta + p is near zero, i.e. the voltage starts applied at peak, it is safe to start the transformer. If θ+p\theta + p is near π2\frac{\pi}{2} or 3π2\frac{3 \pi}{2} , i.e. the voltage starts applied at zero, then it will trigger huge current and fries the electronic parts. See for example of 240 VAC 50 Hz 0.02 R 0.03 L.

The solutions are:

  1. change v gradually so that 2vLω\frac{2 v}{L \omega} is small aka "soft start". For example, to minimize the current  less than 10A, change the voltage by steps of 5Lω5 L \omega and wait for current stabilizing then repeat.
  2. Change RR gradually by some negative coefficient variable resistance. See The Simplest Soft Starter