Tutorial 3 - Sinusoidal Waveforms

Learning Objective

Use the idea of a rotating phase vector.

Recognise and use V = V0 sin wt calculate instantaneous values.

Work out the resultant from two phasors.

Modify the equation to take into account the phase angle.

Key Question

What is a Phasor?

What happens if there is a phase angle?

What happens if there are two waves?

Phasor and Algebraic Representation of Sine Waves

Phasors

Alternating currents can be represented by phase vectors or phasors.  A phasor is representation of a sinusoidal wave form as a rotating vector

 

Sinusoidal motion was very closely linked with circular motion and simple harmonic motion.  Phasors are particularly useful when you have two or more alternating currents that are a fixed amount out of phase.  This happens when we put reactive components like capacitors and inductors into an electrical circuit.

 

In our study of phasors, we will assume that:

         The amplitude, A, remains constant;

         The angular velocity, w, remains constant;

         The phase relationship, f, remains constant.

(The curious looking symbol, f, is phi, a Greek lower-case letter f or ph.  It is the physics code for phase angle.  We will look at that soon.)

 

Lets look at a phasor that represents a sinusoidal waveform.  It has frequency f, and an angular velocity of w rad s-1.

 

This diagram is showing how there is a rotating vector that is projected onto a moving piece of paper.  It traces a sine wave.

 

Text Box: p/2
Text Box: w rad/s
The rotating vector is turning at a constant angular velocity of w rad s-1.  By convention it turns anticlockwise.  By convention, the zero point is the 3 oclock position.  Angles are measured in radians.

 

The vector starts on the line OA.

Let us consider the situation after a time t.  The vector has moved to position OB and has passed through an angle q (as q = wt).  The perpendicular line BC is a projection of the vector onto the line OA. 

 

By simple trigonometry, we can write:

We can rearrange this to give:

 

OA is the voltage.  So we can give the instantaneous voltage at any time as:

 

Question 1

How are frequency and angular velocity related?

Answer

Question 2

1.      The maximum voltage of an alternating waveform is 15 V.  It is alternating with a frequency of 120 Hz.

a.       Calculate the angular velocity;

              b.       Work out the instantaneous voltage after 35 milliseconds.

 

Answer

 

Adding a phase difference

In the last piece of theory, we started off with the phase vector starting at 0 rad.  Starting at the beginning is, after all, a very good place to start.  But we dont have to start from zero.  We can start at any point we like.

Suppose we started off with the vector at f radians from the zero point.  After t seconds, the phasor would have rotated through an angle q = wt.

 

So the total angle would be wt + f.

 

If we started below the zero point our total angle would become wt f.

 

So we can rewrite the expression for instantaneous voltage at any time and any phase angle as:

 

 

 

These relationships work with angles in radians.  Therefore it is important to convert any angles that are in degrees to radians.  You must ensure that your calculator is set to radians, not degrees.

 

Worked Example

A sinusoidally alternating voltage is given by this relationship:

 

V = 75 sin(200 pt 0.25)

 

a.       What is the amplitude?

b.      What is the periodic time?

c.       What is the instantaneous voltage 3.7 ms after the AC passes through the zero point going in the positive direction?

Answer

a.       Amplitude occurs when sin(200 pt 0.25) = 1

 

     V0 = 75 1 = 75 V

b.      Angular velocity = 200 p rad/s.

 

     w = 2p/T T = 2p 200 p = 0.01 s = 10 ms

c.       V = 75 sin(200 p 3.7 10-3 0.25)

 

     V = 2.71 V

 

Question 3

1.      The current in a sinusoidal AC at any time t seconds is given by the relationship:

I = 120 sin (100 pt + 0.36)

 

Calculate:

(a)    The peak current;

(b)   The period;

(c)    The frequency;

(d)   The phase angle relative to 120 sin 100 pt;

(e)    The instantaneous current when t = 0 (It is NOT 120 A);

(f)    The instantaneous current when t = 8 ms;

         (g)    The time when the current is first at a maximum. 

Answer

 

Combining two wave forms

In many electronic systems, it is possible to have alternating currents that are not in phase, but a fraction of a cycle apart.  For example, in a capacitor, the current is 90o or p/2 rad ahead of the voltage.  But we could have a phase difference of any angle.  We have just been working with the relationship:

 

 

Like any wave, electrical wave-forms can interact to superpose.  This means that the waves add together vectorially

 

  • If the waves are in the same direction, they add up.  

  • If they are in opposite directions they take away.

 

Here we have two low frequency AC waves that are interacting.

 

 

Wave 1 is ahead of Wave 2 by 0.25 rad.  We say that it leads by 0.25 rad.  If it were behind by 0.25 rad, it would lag by 0.25 rad.

 

We also see that the there is a resultant wave.  This would be the current you would measure if you were using a multimeter.  The resultant lags Wave 1 by about 0.12 rad, and leads Wave 2 by about 0.13 rad.

 

 

Maths Note:

In the formulae, all angles must be in radians.  However when dealing with the angular relationships between phasors, it is perfectly correct to express the angles in degrees.  I have done this throughout my arguments as it's much easier.  You do need to convert into radians before putting any phase difference into the formula.

 

Using Phasors

This is a much easier way of going about things.  We can go about it by accurate drawing, or by finding the resultant by calculation.

 

Accurate Drawing

Whichever method we use, we draw the phasor at t = 0.  Consider two alternating voltages, V1 and V2 that have a phase difference of f.

 

 

Question 4

Which voltage is leading?

Answer

 

Now we use a vector parallelogram to work out where the resultant will go:

Lets have a look at this by accurate drawing.  You will need:

         Graph paper;

         A sharp pencil;

         A ruler.

 

We will consider two voltages of 20 V and 30 V that are 30o out of phase.  The 20 V vector lags the 30 V vector.

 

You need to choose a scale.  Lets do 1 cm = 2 V as there is room on the A4 paper.  Yes, I have used a black biro so that it stands out better on the graph paper.

 

The 20 V vector is at the zero position, so that it goes from left to right.  We can see that below:

 

Now we need to do some construction lines to make it a parallelogram, so that we can draw the resultant vector:

 

 

Now we can measure the final vector using a ruler.  The distance is 24.2 cm, and using the scale 1 cm = 2 V, our voltage is 48.4 V.  Using a protractor, we can measure the phase angle as well.  The 30 V vector leads the resultant by 12o.

 

 

 

Before we put the numbers into of these equations, we must convert the angle from degrees into radians.

 Angle (rad) = (angle (degrees) p) 180

 

Question 5

What is the phase difference between the resultant voltage and the 20 V vector?

Answer

 

What is the phase difference between the resultant voltage and the 20 V vector?

Now lets apply a formula to these vectors in the form of:

Lets make the frequency 1 Hz.  So the angular velocity is 2p rad/s.  The relationship for the 20 V vector is easy, as its at the 0 point when t = 0.  The equation for this vector is:

 

V1 = 20 sin (2pt)

 

Now we need to write the equation for V2.

 

Question 6

Why is v2 = 30 sin (2pt + 30) wrong?

Answer

Question 7

Show that 30o = 0.524 rad

Answer

 

So our relationship for V2 is:

V2 = 30 sin (2pt + 0.524)

Mathematically the equation can be left in surd form:

 

V2 = 30 sin (2pt + p/6)

 

Relative to V1, we can write an equation for the resultant voltage:

 

VR = 48.4 sin (2pt + 0.314)

 

 

We can also derive an expression for the resultant relative to V2.

 VR = 48.4 sin (2pt 0.209)

Maths Note

A surd is a number that is irrational, i.e.does not have an exact value, for example p, or 2.

p rad = 180o.  So 30o = 1/6 p  rad

p rad = 180o.  So 18o = 1/10 p  rad

p rad = 180o.  So 12o = 1/15 p  rad

 

 

 

 

Calculation

The problem with accurate drawing this that it is anything but accurate.  It is much less error-prone to do the working out of the resultant by calculation.  Let us suppose we had our 20 V and 30 V vectors with a 90o (p/2 rad) phase difference.  The phasor diagram would be this:

 

The resultant can simply be worked out using Pythagoras.  We can write equations for the instantaneous voltages.

 

Question 8

Show that the resultant voltage is about 36 V

Answer

Question 9

Write down the expression for the 30 V vector (V2) relative to 20 V (V1)

Answer

 

We can use trigonometry to work out the phase angle relative to V1.  Use the tan function

 

Angle = tan-1(30/20) = +0.983 rad (= 56.3o)

 

The plus sign tells us that the resultant phasor is leading.

 

Question 10

Work out the phase angle of the resultant relative to V2.  State whether the resultant is leading or lagging V2.

Answer

 

What do we do if the phasors are not at 90 degrees?

Maths Note

 

Any vector can be broken down into two components at 90o to each other.

 

We usually talk about the horizontal and vertical components.

We can resolve any vector into two components at 90o to each other.  They are called the vertical and the horizontal components.

           

  Vx = VR cos q

            Vy = VR sin q

Pythagoras only works if the angles are at 90o.   However any vector can be resolved into vertical and horizontal components.

 

Lets try this method on the problem we have looked at previously.

 

We can see the horizontal and vertical components for V2.  The vector V1 is horizontal, so has no vertical component.

 

We can also show the vertical and horizontal components for the resultant voltage VR.  We will use a separate diagram to show this.

 

We can see easily that the horizontal component is given by:

The vertical component is simply:

This is because V1 sin q = 0

 

So lets put some numbers in:

 

Resolving horizontally:

Resolving vertically:

We can use Pythagoras to calculate VR:

So our resultant voltage is actually 48.4 V

 

Now we can use the phase angle.  Remember that tan = opposite adjacent.

f = tan-1 0.326 = 18.1o

 

Subtracting phase vectors

In the example above, we added the phase vectors V1 and V2.    What do we do if we want to subtract them?  Instead of V1 + V2, we do V1 V2.

 

The minus sign means that the vector V2 is going the opposite direction.

 

We can put on the vertical and horizontal components for V2:

 

And for VR:

 

In this diagram we have angle f which is the phase angle.  We also are using f' (phi-prime) which is the angle we will use with our resolutions:

 

f = f' + 90

 

The symbol f' is not in italics to make it clearer which term we are talking about.

 

As before we have the vertical components equal as V1 has no vertical component.

The horizontal components add up:

 

Notice that for VR the sine and cosine are the opposite way round to what we had before.  It wont affect the result.  This way, its easier to work out f'.  The minus sign indicates that the term V2 cos q is in the opposite direction to V1.

 

Lets put the numbers in.  Resolving vertically we can write:

Resolving horizontally:

 

 

Now we can work out vR by Pythagoras.  (Note that the square of a negative number is positive):

 

Now we can work out f'.

Now, eventually, we can work out the phase angle:

 

f = 90 + 21.7 = 111.7o = 112o to 3 significant figures

 

To put this phase angle into an equation, we have to convert it into radians

 

In this case f = 1.95 rad, lagging.

 

We can write an expression for this:

 

Question 11

Two alternating currents have instantaneous values of current given by the expressions:

Work out by calculation of phase vectors an expression for IR = I1 + I2.

 

Answer

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