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Decrement of the value of varivel with Matlab Function

Asked by Ademir Silva on 17 May 2018
Latest activity Edited by Ademir Silva on 19 May 2018

Hi, I am new user of MATLAB. I need to create a decrement function. Example:

EcAcum = Ec- (Ec- (EcAcum-Input);

At where,

EcAcum: Available kinetic energy;
Ec: Total kinetic energy;
Input: Field instantaneous energy;

My goal is to start the decrease from the EC value and decrease to zero.

I tried using Matlab Function but it did not work.

  8 Comments

Independent of the mathematical problem. how can I work around this problem without having to set EcAcum before? Even though I use the expression you have done,

deltaEc = Ec- (EcAcum-Input); EcAcum = Ec-deltaEc;

the error is the same. The algorithm requires that the variable be defined first. However, if I set it will not work.

I don't understand the question. How can you possibly subtract a value from a variable that doesn't exist? You can't have a delta without an initial condition.

I think you want a loop, not a function. Re-reading that you're dealing with energy, I am guessing you are starting with some energy, and using it in bursts until you run out.

EcAcum = Ec; % The energy available in the system at the beginning
while EcAcum > 0
  % Probably some code here to calculate "Input"
  % which is assumedly the energy used by some action
  EcAcum = EcAcum - Input; % The remaining available energy in the system?
end

Your email:

"I do not understand the question. How can you possibly subtract a value from a variable that does not exist? You can not have a delta without an initial condition." That is exactly my difficulty. Different conditions:

1. Without the loop I need to set the variable to an initial condition. In this way the decrement calculation will not work. Why the defined variable restarts with the initial value during runtime;

2. With the loop I need to set the variable to an initial condition. In this way the decrement calculation works, but the output variable is updated only when it exits the loop. I need the updated value at runtime.

The ultimate goal of all this.

With a kinetic energy (EcAcum) available (flywheel for example) I will define in pu the mechanical power input of a synchronous machine to feed a load (kW). With the reduction of EcAcum the power of the synchronous machine will also decrease until it is exhausted.

Regards

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