solve not working for symbolic equation

I'm trying to solve a symbolic eqution for a circuit´s angle but i keep getting the ame error

syms t;

Vsrms=120;
Vm=Vsrms*sqrt(2);
Vdc=100;
f=60;
R=2;
L=20e-3;

w=2*pi*f;
z=sqrt(R^2+(w*L)^2);
tau=L/R;
alpha=asin(Vdc/Vm);
theta=atan(w*L/R);

ifo=Vm/z*sin(w*t-theta)-Vdc/R;
A=(-Vm/z*sin(alpha-theta)+Vdc/R)*exp(alpha/(w*tau));
in=A*exp(-t/tau);

i=ifo+in;
ib=ifo-in;
assume(t>0 & t<2*pi);
beta=vpasolve(ib==0,t)

i get this result

Answer 1

[Star Strider]

When in doubt, plot the function.

The ‘ib’ function has a minimum value of -146.3 and a maximum of -28.2.

It never crosses zero —

syms t;

Vsrms=120;
Vm=Vsrms*sqrt(2);
Vdc=100;
f=60;
R=2;
L=20e-3;

w=2*pi*f;
z=sqrt(R^2+(w*L)^2);
tau=L/R;
alpha=asin(Vdc/Vm);
theta=atan(w*L/R);

ifo=Vm/z*sin(w*t-theta)-Vdc/R;
A=(-Vm/z*sin(alpha-theta)+Vdc/R)*exp(alpha/(w*tau));
in=A*exp(-t/tau);

i=ifo+in;
ib=ifo-in

ib = 

assume(t>0 & t<2*pi);
beta=vpasolve(ib==0,t)

 
beta =
 
Empty sym: 0-by-1
 

figure
hfp = fplot(ib, [0 2*pi]);
[y1,y2] = bounds(hfp.YData)

y1 = -146.3207

y2 = -28.2445

grid
xlabel('t')
ylabel('Amplitude')
title('ib(t)')

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