Alternating Voltage – Chapter 6

Alternating Voltage

• Alternating voltage is produced by

generators like alternators, dynamos, or low

frequency generators LFG.

• Such generators have terminals

that reverse (switch, alternate)

their signs quickly and regularly.

Alternating Voltage Generators

• Thus the polarity (+ and -) of the terminals of

such generators does not matter

• A possible symbol of AC source is

Alternating Voltage

* An alternating variable voltage is

displayed on an oscilloscope screen as a

set of repeating cycles.

* In each cycle the voltage varies from

a maximum to a minimum value and

alternates its sign + then –

* The same cycle then repeats within

the same duration or time needed for a

cycle, called the period of a cycle.

or sinusoidal form

Sinusoidal Alternating Voltage

A common form of AC Voltage.

The Voltage supplied to homes and establishments

by EDL (Electricite du Liban) is sinusoidal alternating

voltage.

Sinusoidal Alternating Voltage

This voltage has four quantitative characteristics:

1- Maximum voltage

2- Effective voltage (or root mean square value, rms value of the

voltage)

3- Period of the voltage

4- Frequency of the voltage.

Sinusoidal Alternating Voltage1 / 4 characteristics:

First : along the vertical axis, voltage axis:

1- Maximum voltage :

U max = Sv x y

Where y is the amplitude of the voltage or the number

of divisions between the base line and the highest or

lowest point.

2- Effective voltage

U eff = U max / 2 or U eff = U max / 1.4

• Effective voltage is the value of the DC voltage that

has the same effect in an identical load.

• It is also known as the root mean square value, rms

value of the voltage.

• Its value can be measured directly by a voltmeter set

on VAC mode.

• Its value can be calculated from U max, as shown

above for an alternating sinusoidal voltage.

• EDL supplies us with sinusoidal alternating voltage

of U eff = 220V

Sinusoidal Alternating Voltage2 / 4 characteristics:

Sinusoidal Alternating Voltage3 / 4 characteristics:

Second: along the horizontal, time axis

3- Period of the voltage

T = X x S h

• Where X is the number of divisions along the x axis

corresponding to one cycle.

• And S h is the horizontal sensitivity in seconds per

division, s/div (also called sweeping speed)

• Then T the period of the voltage is expressed in

the SI unit of seconds

Sinusoidal Alternating Voltage

3 / 4 characteristics:

4- Frequency of the voltage

f = 1 / T

• After calculating the period T of a voltage, we can

determine the frequency of the voltage.

•The frequency is the number of cycles in one

second, it is expressed in units of Hertz or Hz.

• The sinusoidal alternating voltage in Lebanon has a

frequency of 50 Hz.

S h = 10 ms /

div

S v = 5 V /

div

Name the type and determine the four characteristics of the shown

voltage.

Type of voltage is:

Characteristics:

1- Maximum voltage:

2- Effective voltage:

3- Period of the voltage:

4- Frequency of the

voltage:

Y = 3 div

S v = 5 V / div S h = 10 ms / div

S h = 10 ms / divS v = 5 V / div

X = 4 div

Type of voltage is:

Characteristics:

1- Maximum voltage:

2- Effective voltage:

3- Period of the voltage:

4- Frequency of the

voltage:

Alternating sinusoidal voltage

Having the following characteristics:

U max = S v x y = 5V/div x 3 div

= 15V

U eff = U max / 2 = 15V / 1.4

= 10.71V

T = S h x X = 10ms / div x 4 div

= 40ms or 0.04s

f = 1 / T = 1 / 0.04s = 25 cycles per s

f = 25 Hz

Determine the value of x

and of y in the following

three oscillogram displays:

Oscillogram 1

Oscillogram 2

Oscillogram 3

Use the same settings of sensitivities as in the previous slide and

determine:

* Effective voltage in display 1

* The frequency of the voltage in display 3

Thank you

Next: Book exercises

starting on page 79