# Adding sinusoidal functions using phasors

• Vishera
In summary: But if one gives you a result that doesn't agree with the other, then it's probably the angle that you're calculating that's off.Trig is certainly one way, another is simply using the rectangular to polar function on your calculator. Or do them both. If they both agree, better yet. But if one gives you a result that doesn't agree with the other, then it's probably the angle that you're calculating that's off.
Vishera

## The Attempt at a Solution

$$3cos(20t+10°)-5cos(20t-30°)\\ =3\angle 10°-5\angle -30°\\ =-1.376+3.0209j\\ =3.32\angle -65.51°$$ In the last step, the textbook actually got ##3.32\angle 114.49°##.

I checked both answers and it seems that the textbook's answer is correct. All I did to get -65.51° is using the arctan function on my calculator. It looks like their angle is 180° greater than that. How did they choose the correct angle? I'm rusty on my trigonometry.

Don't phasors need to be put in RMS before you work with them in this case?

Last edited:
Vishera said:

## The Attempt at a Solution

$$3cos(20t+10°)-5cos(20t-30°)\\ =3\angle 10°-5\angle -30°\\ =-1.376+3.0209j\\ =3.32\angle -65.51°$$ In the last step, the textbook actually got ##3.32\angle 114.49°##.

I checked both answers and it seems that the textbook's answer is correct. All I did to get -65.51° is using the arctan function on my calculator. It looks like their angle is 180° greater than that. How did they choose the correct angle? I'm rusty on my trigonometry.

The arctan() function doesn't discriminate between the cases where the minus sign is to be associated with the numerator or the denominator of the argument; it only sees the overall sign of the single number it is given. To check what quadrant the correct angle should lie in, do a quick sketch of the x (real) and y (imaginary) values on the complex plane. Adjust the result by 180° if required.

If your calculator has an atan2() function, use that instead as it takes two arguments, one for the x component and one for the y, and sorts out the correct angle automatically. Failing that, your calculator might have rectangular to polar conversion built-in, which will also give the result in the correct quadrant automatically.

Draw a picture of your combined phasor. See the quadrant where the components locate the resultant?

Generally, the inverse trig functions on your calculator only return the principal angles, which for the tangent is in the range -π/2 ≤ θ ≤ π/2. When doing these types of calculations, you must give your result a separate check to make sure you obtain the correct angle.

Thanks for help guys. I completely forgot this part of trigonometry and I get it now. It's actually kind of scary that I forgot this because it's not like I stopped doing Math. Too much calculus has made me a bit rusty on my algebra (or at least trigonometry).

-1.376 + 3.02J never equals -65 degrees...check your math there...it always equals 114.49 degrees.

psparky said:
-1.376 + 3.02J never equals -65 degrees...check your math there...it always equals 114.49 degrees.

Weird. My calculator is always giving me a principal value of -65.

Vishera said:
Weird. My calculator is always giving me a principal value of -65.

Yes. Because you are calculating:
$$\phi = arctan\left( \frac{3.02}{-1.376} \right)$$
Which, after resolving the argument to -2.195, the arctan() function places in the fourth quadrant. It cannot distinguish ##\frac{3.02}{-1.376}## from ##\frac{-3.02}{1.376}## or ##-2.195##, and can only return a corresponding angle that lies within its principal range of +/- 90° .

gneill said:
Yes. Because you are calculating:
$$\phi = arctan\left( \frac{3.02}{-1.376} \right)$$
Which, after resolving the argument to -2.195, the arctan() function places in the fourth quadrant. It cannot distinguish ##\frac{3.02}{-1.376}## from ##\frac{-3.02}{1.376}## or ##-2.195##, and can only return a corresponding angle that lies within its principal range of +/- 90° .

Trig is certainly one way, another is simply using the rectangular to polar function on your calculator.
Or do them both. If they both agree, better yet.

## What is a phasor?

A phasor is a mathematical representation of a sinusoidal function that takes into account both its amplitude and phase. It is typically represented as a complex number in the form A∠θ, where A is the amplitude and θ is the phase angle.

## How do you add sinusoidal functions using phasors?

To add sinusoidal functions using phasors, you first convert each function into its corresponding phasor form. Then, you can simply add the phasors together algebraically. Finally, you convert the resulting phasor back into its sinusoidal form to get the sum of the two functions.

## What are the advantages of using phasors to add sinusoidal functions?

Using phasors to add sinusoidal functions allows for a simpler and more efficient calculation, as it involves basic algebraic operations rather than trigonometric functions. It also allows for a better understanding and visualization of the combined function in terms of amplitude and phase.

## Are there any limitations to using phasors to add sinusoidal functions?

Yes, phasor addition can only be used for linear combinations of sinusoidal functions. Nonlinear combinations, such as multiplication or division, require a different approach. Additionally, phasor addition assumes that the frequencies of the functions being added are the same.

## Can phasors be used for any type of periodic function?

No, phasors are specifically designed for sinusoidal functions. Other types of periodic functions, such as square waves or sawtooth waves, require different mathematical techniques for addition.

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