DEPARTMENT OF ELECTRICAL & ELECTRONIC ENGINEERING
BANGLADESH UNIVERSITY OF ENGINEERING & TECHNOLOGY
COURSE NO.: EEE 168
EXPT. NO.-4
FAMILIARIZATION WITH ALTERNATING CURRENT (AC) WAVES
OBJECTIVE:
To study ac (sinusoidal) wave forms and correlate them with practically measurable
effective values. An understanding on a simple ac circuit is also expected to be developed
in the experiment.
INTRODUCTION:
Any periodic variation of current or voltage where the current (or voltage), when
measured along any particular direction, goes positive as well as negative, is defined to
be an AC quantity. Sinusoidal AC wave shapes are the ones where the variation (current
or voltage) is a sine function of time.
v
Vm
Fig 1. An ac (sinusoidal) voltage waveform
For the wave form in Fig.1,
Time period = T
Frequency f = 1/T
v=Vm sin 2 ft = Vm sin (2 /T)t
EFFECTIVE VALUE:
Effective (rms) values of sinusoidal waveforms are given as:
T
1 2 V
V=
T 0
v dt = m
2
(For sinusoidal wave)
T
1 2 I
I=
T 0
i dt = m
2
(For sinusoidal wave)
These values are directly measured in ac voltmeter / ammeters and can be used in power
calculation as:
P = I 2R =V 2 / R
PHASE DIFFERENCE:
v/i
t
T
Fig 2. Two sinusoidal waves with phase difference
Phase difference between two ac sinusoidal waveforms is the difference in electrical
angle between two identical points of the two waves. In fig. 2, the voltage and current
equations are given as:
v=Vm sin (2 /T)t
i=Im sin ((2 /T)t- )
IMPEDANCE:
Relation between the voltage across and the current through any component of an ac
circuit is given by impedance. For the voltage and current waveforms in Fig. 2, the
corresponding impedance Z is given as:
EQUIPMENT LIST:
Oscilloscope
Function generation
Decade resistor
Capacitor bank
AC voltmeter
AC ammeter
SPST
Breadboard
CIRCUIT DIAGRAM:
1F
10Vp-p
1 KHz Osc. 10Vp-p Osc. 100 Osc.
Ch-1 1 KHz Ch-1 Ch-2
Fig 1 Fig 2
PROCEDURE:
1. Connect the output of the function generator directly to channel 1 of the
oscilloscope as shown in fig 1. Set the amplitude of the wave at 10V and the
frequency at 1 kHz. Select sinusoidal wave shape.
2. Sketch the wave shape observed on the oscilloscope. Determine the time
period of the wave and calculate the frequency.
3. Measure the voltage with an ac voltmeter.
4. Change the frequency to 500Hz and note what happens to the display of the
wave. Repeat when the frequency is increased to 2 KHz.
5. Construct the circuit as shown in Fig. 2. Measure the input voltage with an ac
voltmeter and the input current with an ac ammeter. The ratio between the
voltage and the current gives the magnitude of the impedance, Z.
6. Observe the wave shapes of oscilloscope channels 1 and 2 simultaneously.
Find the frequency of both the waves and amplitude from the display.
Determine the phase difference between the two waves. The phase difference
is given by 360ft, where 't' is the time delay between the two waves. Also
observe which of the two waves lead. Note that the voltage in channel 2 is the
voltage across a resistance and hence this is in phase with the current flowing
in the circuit.
REPORT:
1. Compare the frequency of the wave determined from the oscilloscope with the
mentioned value on the function generator in step 2 of the procedure.
2. Calculate the rms value of the voltage observed in step 2 of the procedure and
compare with that measured in step 3.
3. How does the time period vary when the frequency of the wave is changed in
step 4?
4. Calculate the magnitude of the impedance from the readings taken in step 5.
5. Find the magnitude and the phase angle of the impedance from the readings
taken in step 5 and 6.
