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Power Electronic Drives (EET421)
Laboratory Module
EXPERIMENT 3
Single-Phase Semiconverter with DC Motor Load
Objective :


To evaluate of a DC motor controlled by adjusting the DC voltage supply, using
a bridge rectifier for full-wave rectification and an SCR.
To analyze its harmonics and power factor characteristics.
Introduction :
A DC motor normally contributes to spikes, voltage disturbances, harmonics , low PF to
the main power system and lower the power supply quality.
In this particular experiment, an AC supply source with different voltage selection is used
to supply DC voltage to a DC motor through a single-phase semiconverter (as an AC to
DC power converter).
The motor with a rectifier bridge should have different performance and characteristics.
The speed, harmonics, total harmonic distortion (THD) and power factor should be
observed as these will have significant impact of affecting the motor and power system.
A single-phase full wave rectifier plus a SCR that supplies a very high inductive load
such as a dc motor load is shown in Figure 1.1. This circuit is known as a single phase
semiconverter, and it is commonly used in industrial applications especially for speed
regulation.
D1
A
C
iD1
is
D3
Field
winding
Vs
D4
L
M
D2
Figure 1.1 Single-phase semiiconverter with DC motor load
With resistive load, the load current is identical in shape to the output voltage. In practice,
most loads are inductive load to a certain extent and the load current depends on the
values of load resistance RL and load inductance L.
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Power Electronic Drives (EET421)
Laboratory Module
The average output voltage can be found from
Vdc 
Vm

(1  cos  )
(1-1)
where Vm is maximum supply voltage and α is firing angle.
The average output current
I dc 
Vdc
RL
(1-2)
The output dc power,
Pdc = Vdc Idc
(1-3)
The rms output voltage is found from
Vrms
V 1 
sin 2 
 m     

2 
2  
1/ 2
(1-4)
The ripple factor is defined then as
Vrms  Vdc
2Vm
2
RF 
2
(1-5)

The instantaneous input current can be expressed in a Fourier series as
is (t )  ao 

 (a
n 1, 2,..
n
cos nt  bn sin nt )
(1-6)
The first part of equation (1-6) is the average input current and the second part is the
ripple content on the input current.
The total harmonic distortion in line current is then
 I rms ,tot
THD  
I
 rms , fund
2

 1


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Power Electronic Drives (EET421)
Laboratory Module
Equipment and Parts :
1) Single-phase 5 kVA programmable power source.
2) SCR Speed Controller ED-5113
3) DC Motor DCM-250
4) Variable Resistance Load ED-5101
5) DC Volt/Ampere Meter ED-5105
6) Tacho-Meter ED-5118
7) Wires
Procedure :
Warning :
High voltage are used in this laboratory experiment
Do not make any connection with the Power ON
Please ask the Teaching Engineer or the Technician to check your
circuits before turning on the supply for all the tests
1) Connect the circuit as given below in Fig 1.0.
2) In the first part of this experiment, connect the primary side of the transformer to the
output of the 5 kVA programmable power source and the output of the converter
to 600 Ohm resistive load.
3) After ensure your connection is correct, turn-on the power of the AC supply to 5 kVA
programmable power source equipment (single phase and three-phase) and the
ac power supply for personal computer.
4) Please inform the teaching engineer or the technician when need to check the
connections.
Transformer
A
~
AC
V
150V
230V
M
=
Rectifier
Saffner
Harmonic
Analyzer
Fig 1.0 : Circuit connection
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Power Electronic Drives (EET421)
Laboratory Module
5) Before Pressing OK button on the Test Screen to start a test of step 5 of
manual, the voltage setting must be suited as that has been decided. (look on
the table 1.0)
6) Adjust the potensiometer (circuit control) on 6 position
7) Referring to the Table 1.0, record the readings accordingly. You will need the
AC supply voltage (Vrms) of 40Vac, 50Vac, 60Vac, 70 Vac, 80 Vac, 90 Vac,
and 100Vac for the data. These voltages are set from single-phase 5 kVA
programmable power source.
Table 1.0 For 300 Ohm Resistive load
Vout
Vrms
40
50
60
70
80
90
100
Iout
THDv
THDi
W
PF
8) In the second part of the experiment, replace the resistive load with DC motor.
9). The firing (delay) angle of the SCR is fixed for all of AC supply voltage.
10) Referring to the Table 1.1, record the readings accordingly. You will need the
AC supply voltage (Vrms) of 40Vac, 60Vac, 80Vac, 100 Vac, and 110Vac
for the data. These voltages are set from single-phase 5 kVA programmable
power source.
Table 1.1 For DC motor load
Vrms
40
60
80
100
110
Vout
Iout
Speed
THDv
THDi
W
PF
11) Using the equation (1-1), prove that the multimeter DC voltage value is similar to
the equation.
12) Is it similar or not ? Make a statement to either of your answer.
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Power Electronic Drives (EET421)
Laboratory Module
13) After you have completed the table, plot graphs of speed against voltage, speed
against current and current THD (THDi) against speed and voltage against power.
14) Explain the behaviours of each graph that you have plotted.
15) Please state your conclusion for the whole of this experiment, including your
comparisons and differences (with reasons) of THDv, THDi and PF between
resistor and DC motor loads.
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