Simscape language: AC Voltage Source - variable frequency input - Problem: Number of equations exceeds number of variables.

Hello everyone,

I try to write a file which will produce an AC Voltage Source in Simscape using the Simscape language. My goal is to vary the frequency by using an external signal.

This is my code:

   component AC_frequ
  %AC_frequ
  %
  %AC Voltage source - variable frequencies 
  %
      nodes
          v_p = foundation.electrical.electrical; %+ :top
          v_n = foundation.electrical.electrical; %- :bottom
      end

      variables
                 v = { 0, 'V' };     % Voltage
             omega     = { 0,  'rad/s' }; % angular velocity
      end

      inputs
      f = { 0, 'Hz' } %Frequenz :bottom
      end

      parameters
          amp       = { 1,  'V' };   % Peak amplitude
          shift     = { 0,  'deg' }; % Phase shift
      end

      equations
    v == v_p.v - v_n.v;
    v == amp*sin(omega*time + shift);

  omega == {2*pi,'rad'}*f;
    end
end

When I use the file in my simulation, it tells me the following: "Number of equations exceeds number of variables."

Can anyone spot my mistake?

Answer 1

[Sebastian Castro]

As the error message suggested, you have 3 equations but 2 variables. Two issues:

1. You needed the current to also be a variable, along with its corresponding branches section defining the polarity of the current.
2. You did not need need to define omega as a variable and create a separate equation for it.

This component file worked perfectly for me:

component AC_frequ

nodes
    p = foundation.electrical.electrical; % +:top
    n = foundation.electrical.electrical; % -:bottom
end

variables
    i = { 0, 'A' }; % Current
    v = { 0, 'V' }; % Voltage
end

parameters
    amp       = { 1,  'V' };   % Peak amplitude
    shift     = { 0,  'deg' }; % Phase shift
end

inputs
    frequency = { 60, 'Hz' };  % f: bottom
end

branches
    i : p.i -> n.i;
end

equations
    v == p.v - n.v;
    v == amp*sin(frequency*time + shift);
end

end

- Sebastian

[alty]

Thank you very much! This helped a lot!

Is there any reason why i do not have to define 'omega' as a variable? Is there some kind of general rule for this?

alty

[Sebastian Castro]

No problem!

Omega could be a variable, but it would introduce an extra set of equations into the system that you don't exactly need.

Variables are mostly necessary if you want to keep track of, log, initialize, or measure time-varying dynamic variables. If it's just some internal counter like in this case (frequency*time), a variable isn't necessary.

- Sebastian

[Ali Degan]

I tried to mimic what you have written but I got an error. Can you please help me to resolve this error?

"The following block(s) do not have resolved block library links: 'acfre/AC_frequ' Physical Modeling blocks cannot have broken or inactive library links. Please restore library links for all Physical Modeling blocks."

Also, Can I make my own model in simscape technology instead of simscape component?