Generation of a Sinusoidal Waveform
In our tutorials about
Electromagnetism, we
saw how an electric current flowing through a conductor can be used to generate a magnetic field around itself, and also if a
single wire conductor is moved or rotated within a stationary magnetic field, an "EMF", (Electro-Motive Force) will be induced
within the conductor due to this movement. From this tutorial we learnt that a relationship exists between Electricity and
Magnetism giving us, as Michael Faraday discovered the effect of "Electromagnetic Induction" and it is this basic principal
that is used to generate a Sinusoidal Waveform.
In the
Electromagnetic Induction,
tutorial we said that when a single wire conductor moves through a
permanent magnetic field thereby cutting its lines of flux, an
EMF is induced in it. However, if the conductor moves in parallel with
the magnetic field in the case of points A
and B, no lines of flux are cut and no EMF is induced into the conductor, but if the conductor moves at
right angles to the magnetic field as in the case of points C and D, the maximum
amount of magnetic flux is cut producing the maximum amount of induced EMF.
Also, as the conductor cuts the magnetic field at different angles between points A
and C, 0 and 90o the amount of induced EMF will lie somewhere between this zero and
maximum value. Then the amount of emf induced within a conductor depends on the angle between the conductor and the magnetic
flux as well as the strength of the magnetic field.
An AC generator uses the principal of Faraday's electromagnetic induction to convert a mechanical energy such as
rotation, into electrical energy, a Sinusoidal Waveform.
A simple generator consists of a pair of permanent magnets
producing a fixed magnetic field between a north and a south pole.
Inside this magnetic field is a single rectangular loop of wire
that can be rotated around a fixed axis allowing it to cut the magnetic
flux at various angles as shown below.
Basic Single Coil AC Generator
As the coil rotates anticlockwise around the central axis which is perpendicular to the magnetic field, the wire loop cuts the lines of magnetic force set up between the north and south poles at different angles as the loop rotates. The amount of induced EMF in the loop at any instant of time is proportional to the angle of rotation of the wire loop. As this wire loop rotates, electrons in the wire flow in one direction around the loop. Now when the wire loop has rotated past the 180o point and moves across the magnetic lines of force in the opposite direction, the electrons in the wire loop change and flow in the opposite direction. Then the direction of the electron movement determines the polarity of the induced voltage.
So we can see that when the loop or coil physically rotates one complete revolution, or 360o,
one full sinusoidal waveform is produced with one cycle of the waveform being produced for each revolution of the coil. As
the coil rotates within the magnetic field, the electrical connections are made to the coil by means of carbon brushes and
slip-rings which are used to transfer the electrical current induced in the coil.
The amount of EMF induced into a coil cutting the magnetic lines of force is determined by the following three factors.
- • Speed – the speed at which the coil rotates inside the magnetic field.
- • Strength – the strength of the magnetic field.
- • Length – the length of the coil or conductor passing through the magnetic field.
We know that the frequency of a supply is the number
of times a cycle appears in one second and that frequency
is measured in Hertz. As one cycle of induced emf is produced each full
revolution of the coil through a magnetic field comprising
of a north and south pole as shown above, if the coil rotates at a
constant speed a constant number of cycles will be produced per
second giving a constant frequency. So by increasing the speed of
rotation of the coil the frequency will also be increased.
Therefore, frequency is proportional to the speed of rotation, ( ƒ ∝ Ν )
where Ν = r.p.m.
Also, our simple single coil generator above only has
two poles, one north and one south pole, giving just one
pair of poles. If we add more magnetic poles to the generator above so
that it now has four poles in total, two north and two south,
then for each revolution of the coil two cycles will be produced for the
same rotational speed. Therefore, frequency is proportional
to the number of pairs of magnetic poles, ( ƒ ∝ P ) of the generator
where P = is the number of "pairs of poles".
Then from these two facts we can say that the frequency output from an AC generator is:
Where: Ν is the speed of rotation in r.p.m. P is
the number of "pairs of poles" and 60 converts it into seconds.
Instantaneous Voltage
The EMF induced in the coil at any instant of time depends upon the rate or speed at which the coil cuts
the lines of magnetic flux between the poles and this is dependant upon the angle of rotation, Theta ( θ )
of the generating device. Because an AC waveform is constantly changing
its value or amplitude, the waveform at any instant in time will
have a different value from its next instant in time. For example, the
value at 1ms will be different to the value at 1.2ms and so on.
These values are known generally as the Instantaneous Values, or Vi
Then the instantaneous
value of the waveform and also its direction will vary according to the
position of the coil within the magnetic field as shown below.
Displacement of a Coil within a Magnetic Field
The instantaneous values of a sinusoidal waveform is given as the "Instantaneous value
= Maximum value x sin θ " and this is generalized by the formula.
Where, Vmax is the maximum voltage induced in the coil and
θ = ωt, is the angle of coil rotation.
If we know the maximum or peak value of the waveform, by using the formula above the instantaneous
values at various points along the waveform can be calculated. By plotting these values out onto graph paper, a sinusoidal
waveform shape can be constructed. In order to keep things simple we will plot the instantaneous values for the sinusoidal
waveform at every 45o and assume a maximum value of 100V. Plotting the instantaneous values at shorter intervals,
for example at every 30o would result in a more accurate waveform construction.
Sinusoidal Waveform Construction
| Coil Angle ( θ ) | 0 | 45 | 90 | 135 | 180 | 225 | 270 | 315 | 360 |
| e = Vmax.sinθ | 0 | 70.71 | 100 | 70.71 | 0 | -70.71 | -100 | -70.71 | -0 |
The points on the sinusoidal waveform are obtained by projecting across from the various positions of rotation between 0o and 360o to the ordinate of the waveform that corresponds to the angle, θ and when the wire loop or coil rotates one complete revolution, or 360o, one full waveform is produced. From the plot of the sinusoidal waveform we can see that when θ is equal to 0o, 180o or 360o, the generated EMF is zero as the coil cuts the minimum amount of lines of flux. But when θ is equal to 90o and 270o the generated EMF is at its maximum value as the maximum amount of flux is cut. The sinusoidal waveform has a positive peak at 90o and a negative peak at 270o. Positions B, D, F and H generate a value of EMF corresponding to the formula e = Vmax.sinθ.
Then the waveform shape produced by our simple single loop generator is commonly referred to as a
Sine Wave as it is said to be sinusoidal in its shape. This type of waveform is called a sine wave because it
is based on the trigonometric sine function used in mathematics, ( x(t) = Amax.sinθ ).
When dealing with sine waves in the time domain and especially current related sine waves the unit of measurement used along
the horizontal axis of the waveform can be either time, degrees or radians. In electrical engineering it is more common
to use the Radian as the angular measurement of the angle along the horizontal axis rather than degrees. For example,
ω = 100 rad/s, or 500 rad/s.
Radians
The Radian, (rad) is defined mathematically as a quadrant of a circle where the distance subtended on
the circumference equals the radius (r) of the circle. Since the circumference of a circle is equal to
2π x radius, there must be
2π radians around a 360o circle, so 1 radian = 360o/2π = 57.3o.
In electrical engineering the use of radians is very common so it is important to remember the following formula.
Using radians as the unit of measurement for a sinusoidal waveform would give 2π radians for one full cycle of 360o. Then half a sinusoidal waveform must be equal to 1π radians or just π (pi). Then knowing that pi, π is equal to 3.142 or 22÷7, the relationship between degrees and radians for a sinusoidal waveform is given as.
Relationship between Degrees and Radians
Applying these two equations to various points along the waveform gives us.
The conversion between degrees and radians for the more common equivalents used in sinusoidal analysis are given in the following table.
| Degrees | Radians | Degrees | Radians | Degrees | Radians |
|---|---|---|---|---|---|
| 0o | 0 | 135o | 3π 4 | 270o | 3π 2 |
| 30o | π 6 | 150o | 5π 6 | 300o | 5π 3 |
| 45o | π 4 | 180o | π | 315o | 7π 4 |
| 60o | π 3 | 210o | 7π 6 | 330o | 11π 6 |
| 90o | π 2 | 225o | 5π 4 | 360o | 2π |
| 120o | 2π 3 | 240o | 4π 3 |
The velocity at which the generator rotates around its central axis determines the frequency of
the sinusoidal waveform. As the frequency of the waveform is given as ƒ Hz or cycles per second,
the waveform has angular frequency, ω, (Greek letter omega), in radians per second. Then the
angular velocity of a sinusoidal waveform is given as.
Angular Velocity of a Sinusoidal Waveform
and in the United Kingdom, the angular velocity or frequency of the mains supply is given as:
in the USA as their mains supply frequency is 60Hz it is therefore: 377 rad/s
So we now know that the velocity at which the generator rotates around its central axis determines
the frequency of the sinusoidal waveform and which can also be called its angular velocity,
ω. But we should by now also know that the time required to complete one revolution
is equal to the periodic time, (T) of the sinusoidal waveform. As frequency is inversely
proportional to its time period, ƒ = 1/T we can therefore
substitute the frequency quantity in the above equation for the equivalent periodic time quantity and substituting
gives us.
The above equation states that for a smaller periodic time of the sinusoidal waveform, the greater
must be the angular velocity of the waveform. Likewise in the equation above for the frequency quantity, the higher
the frequency the higher the angular velocity.
Example No1
A sinusoidal waveform is defined as: Vm = 169.8 sin(377t)
volts. Calculate the RMS voltage of the waveform, its frequency and the instantaneous value of the voltage after a time of 6mS.
We know from above that the general expression given for a sinusoidal waveform is:
Then comparing this to our given expression for a sinusoidal waveform above of
Vm = 169.8 sin(377t) will give us the peak voltage value of
169.8 volts for the waveform.
The waveforms RMS voltage is calculated as:
The angular velocity (ω) is given as 377 rad/s. Then
2πƒ = 377.
So the frequency of the waveform is calculated as:
The instantaneous voltage Vi value after a time of 6mS is given as:
Note that the phase angle at time t = 6mS is given in radians.
We could quite easily convert this to degrees if we wanted to and use this value instead to calculate the instantaneous
voltage value. The angle in degrees will therefore be given as:
Sinusoidal Waveform
Then the generalised format used for analysing and calculating the various values of a
Sinusoidal Waveform is as follows:
A Sinusoidal Waveform
In the next tutorial about
Phase Difference
we will look at the relationship between two sinusoidal waveforms that are of the same frequency but pass through
the horizontal zero axis at different time intervals.
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