RMIT University © 2021
School of Engineering
Electrical Energy Conversion
Online Laboratory 3: Three-Phase SCR Rectifier
1. Aims

To analyse experimental operational measurements obtained using three phase SCR
rectifiers feeding into various load combinations.

To calculate quantitative output voltages and currents from the rectifier experimental
traces, and to compare these results with theoretical predictions.

To explore and quantify the impact of source inductance on the rectifier operation.
2. Equipment
All experimental data sets provided for this online laboratory have been obtained using the
following experimental equipment:

3 Phase Controlled Rectifier Test Set (see Fig. 5, Left Hand Side Front Panel)

DC Load Box (see Fig. 5, Right Hand Side Front Panel)

Digital Oscilloscope (Keysight Technologies DSO-X 3024A 200MHz 4 Channels)

2 off Differential Voltage Amplifiers (Tektronix P5200 Series)

2 off clamp ON current Probes (Tektronix A600 Series)
3. Software
The following software tools can be utilised to assist and validate experimental results
through simulation:

Powersim PSIM simulation package. Student version download instructions are available
on the EEET2274/2337 Canvas shell.

Mathworks MATLAB can be used to aid calculation and analysis.
4. Background
The general arrangement of a three-phase SCR rectifier is shown in Fig. 1, and consists of a
three phase AC supply (variac) which feeds through line inductors into the rectifier bridge
structure. The DC output supplies a load resistor, optionally through a series filter inductance or
in parallel with a shunt filter capacitance as determined by the switch positions.
~
B
~
R
1
3
5
W
~
3 phase AC
supply (variac)
4
Variable
Inductors
6
2
DC Filter
Inductor
DC Filter
Capacitor
DC
Load
3 phase SCR
Rectifier
Fig. 1: Schematic arrangement of 3-phase Diode/SCR rectifier
Prof. B. McGrath, Dr. R. Wilkinson
EEET2274/EEET2337 Online Laboratory #3 V1
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RMIT University © 2021
School of Engineering
The principle of operation of a three-phase diode rectifier is that diodes 1,3,5 each conduct
for 1200 at the time when their respective input phase voltages are the most positive, and diodes
4,6,2 each conduct at the time when their respective input phase voltages are the most negative.
This means that the line current for each AC phase input has a 1200 positive conduction interval
and a 1200 negative conduction interval, spaced 1800 apart. Fig. 2 shows this conduction
sequence.
Fig. 2: Conduction sequence for 3-phase Diode rectifier
The DC output voltage from the rectifier is the difference between the upper and the lower
rail voltages, which is a waveform which ripples at 6 times the fundamental frequency between
the limits of 1.5V p (1.22Vl-l) and 3V p (1.414Vl-l). The overall average DC output voltage is
calculated using:
π
VDC =
π
3
6
∫
−
π
2Vl −l cos(θ )dθ = 1.35Vl −l
6
Whilst the DC output voltage is not affected by load, the shape of the DC output current
waveform is significantly affected by the load topology, as follows:
•
•
•
For a resistive load, the DC current follows the voltage exactly.
For a load with a DC series inductance, the output current AC ripple component is
reduced, so that the current becomes constant DC in the limit when a load with an
“infinite” inductance is used.
For a load with a large shunt capacitive filter, the rectifier output current becomes
discontinuous whenever the rectified voltage falls below the output filter capacitor
voltage. The point at which this occurs depends both on the average output current and
size of the capacitor.
Fig. 3 illustrates these three conduction conditions.
Prof. B. McGrath, Dr. R. Wilkinson
EEET2274/EEET2337 Online Laboratory #3 V1
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RMIT University © 2021
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Rectifier DC Current - Resistive Load
Rectifier DC Current - Inductive Load
Rectifier DC Current - Capacitive Load
Fig. 3: Rectifier current waveforms for different load types
For operation as an SCR controlled rectifier with a variable output voltage, the conduction of
each SCR is delayed by a firing angle “ α ”. Provided the DC output current remains continuous,
the effect of this firing angle is to delay the transfer of current from the previously conducting
input phase. Hence the rectified output voltage follows the previous AC input voltage further
down the outgoing conducing sinewave, and the average DC output is reduced. Figure 4 shows
this conduction sequence.
Fig. 4: Conduction sequence for 3- phase SCR rectifier,  = 400
The DC output voltage once again is the voltage difference between the positive and negative
DC output rails, with a significant 6 pulse ripple. Note also that the AC line input current is
phase delayed by α because of the change in conduction position of the SCRs.
•
For a SCR bridge the DC output voltage is calculated using:
•
π
+α
6
=
2Vl −l cos (θ ) dθ 1.35Vl −l cos α
π
3 ∫− 6 +α
For a pure resistive load, the SCRs turn off naturally when the rectified voltage
instantaneously reaches zero. Hence conduction becomes discontinuous for a firing delay
of more than 60 , and the above equation is no longer valid. No conduction at all occurs
for a firing delay angle of more than 120 .
For a large inductive load with essentially constant current, a 120 conduction interval is
maintained from one conduction period to the next. Hence zero average current occurs
for a firing delay angle of 90 (in practice a zero average current implies discontinuous
operation, so the real firing limit is somewhat less than 90 ).
=
VDC
•
π
Prof. B. McGrath, Dr. R. Wilkinson
EEET2274/EEET2337 Online Laboratory #3 V1
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RMIT University © 2021
School of Engineering
In a practical rectifier, the input current cannot change instantly from one phase to the next as
the diode conduction attempts to crossover, because of the AC source inductance (Ls). Hence as
conduction transfers from one phase to the next, there is a period of time when both the
incoming Diode/SCR and outgoing Diode/SCR are both conducting. This causes the rectified
output voltage to take a value midway between the incoming and outgoing phase voltages until
the current “commutation” process is complete.
For a smooth continuous DC output current, the result is a reduction in the DC output voltage
3ωLs
of:
I DC
VDC = 1.35Vl −l cosα −
π
5. General Test System Description
The Three Phase Controlled Rectifier Test Set shown in Fig. 5 is a custom designed digitally
controlled SCR bridge that can be operated as either a single or a three phase rectifier, in either
half or full wave operation.
The Test Set is powered by an AC supply voltage, either single or three phase, typically
provided by a separate single or three phase variac. It is controlled by an internal Digital Signal
Processor that changes mode between half wave and full wave, single and three phase, as
selected by switches on the front panel. Minor wiring configuration changes are also required to
swap between half and full wave operation. A selectable series inductance can also be included
between the AC supply and the SCR bridge to investigate the effect of source impedance on the
rectifier operation.
The Test Set feeds into a reconfigurable DC Load Box that can switch between low and high
load resistors, with an adjustable inductance added in series as required. A shunt capacitor can be
switched across the load resistor to smooth the output voltage, and a Free Wheel Diode can also
be switched across the input to the Load Box (i.e. the rectifier output as required. The SCR firing
angle can be selected by a multiturn potentiometer. A firing angle of zero places the test set into
“diode” mode, where the SCRs are turned permanently on during their conduction half cycle.
All of the experimental data-sets provided for this online laboratory exercise have been
obtained using this rectifier and load system. Voltages across various sections of the equipment
have been measured using the P5200 Series differential voltage probes, while the rectifier
currents have been measured using A600 Series current probes which have been clamped-on to
the various wire-measurement-loops shown in Fig. 5. The current and voltage probes are
connected to the Keysight Digital Oscilloscope to allow waveform measurements to be obtained.
For all experimental traces the Oscilloscope trigger has been set to “LINE” in order to
synchronise the measurements with the incoming AC mains voltage.
3 Phase Controlled Rectifier
415V, 5A, 3 phase AC
RED
0mH
DC LOAD
15mH
RED
DC Amps
Meter
RED
0mH
30mH
WHITE
0mH
15mH
WHITE
WHITE
+
30mH
BLUE
0mH
15mH
BLUE
BLUE
30mH
-
FW
NEUTRAL
+
Input
DC Only
Max 350V
-
DC Volts
Meter
HW
+
50mH
200mH
Reverse
Polarity
Protection
+
2200uF 60uF
750W
50 Ω
250W
15
0 Ω
-
RED
NEUTRAL
Full wave
3 phase
Half wave
1 phase
ON
OFF
LCD
SYNCH
Parameter
#1
Parameter
#2
Fig. 5: Three Phase Controlled Rectifier Test Set and DC Load Box
Prof. B. McGrath, Dr. R. Wilkinson
EEET2274/EEET2337 Online Laboratory #3 V1
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RMIT University © 2021
School of Engineering
6. Resistive Load
a) Diode Rectifier
The archive <Data_Set_1.zip> contains 3 oscilloscope images that capture the operation of a
three phase full-wave SCR rectifier supplying a purely resistive nominal 50Ω load. The rectifier
is supplied from a variac set to produce a nominal 100Vl −l (rms ) 50Hz AC source voltage.
The measurements shown in these oscilloscope traces are as follows:
•
•
•
•
•
Channel 1 (Yellow) – Rectified DC Voltage using P5200 probe @ 500X attenuation.
Channel 2 (Green) – AC Supply Voltage (VRW and VBN) using P5200 probe @ 500X
attenuation.
Channel 3 (Blue) – DC Load Current using A600 current probe @ 100mV/A.
Channel 4 (Magenta) – AC Supply Current (IR) using A600 current probe @ 100mV/A.
Reference 1 (Orange) – AC Supply Voltage (VRN) using P5200 probe @ 500X
attenuation and saved to R1.
Analysis:
•
•
•
•
•
•
Draw the circuit topology of the rectifier for this operating condition using a schematic
editor (e.g. PSIM, VISIO, etc.). Show the AC source, the rectifying devices, the DC load
and all measurement locations.
Extract from the data set measurements of the line-to-line AC supply voltage (VRW), the
rectified DC voltage, the AC supply current (IR), and the DC load current. Using VRN as
the reference, compare these voltages and currents in terms of their magnitude and phase
relationships. Note in particular the phase relationship between VRN and IR.
Examine VRN carefully, and compare it against VBN. Locate and explain the four notches
in these voltages over one fundamental cycle, and measure the width of these notches.
Extract from the data set measurements of the average rectified DC output voltage, and
compare this with the theoretical response for a full-wave three-phase rectifier supplying
a resistive load.
Explain any discrepancy from the ideal relationship between the magnitude of the
average DC output voltage and the incoming AC line-to-line voltage.
Using the measured notch width in VRN, estimate the input source inductance of the
incoming AC supply (refer to formula in lecture notes).
b) SCR Rectifier
The archive <Data_Set_2.zip> contains 9 oscilloscope images that capture the operation of a
three phase full-wave SCR rectifier supplying a purely resistive nominal 50Ω load. The rectifier
is supplied from a variac set to produce a nominal 100Vl −l (rms ) 50Hz AC source voltage.
The measurements shown in these oscilloscope traces are as follows:
•
•
•
•
•
Channel 1 (Yellow) – Rectified DC Voltage using P5200 probe @ 500X attenuation.
Channel 2 (Green) – AC Supply Voltage (VRW) using P5200 probe @ 500X attenuation.
Channel 3 (Blue) – DC Load Current using A600 current probe @ 100mV/A.
Channel 4 (Magenta) – AC Supply Current (IR) using A600 current probe @ 100mV/A.
Reference 1 (Orange) – AC Supply Voltage (VRN) using P5200 probe @ 500X
attenuation and saved to R1.
Analysis:
Prof. B. McGrath, Dr. R. Wilkinson
EEET2274/EEET2337 Online Laboratory #3 V1
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RMIT University © 2021
•
•
•
•
•
School of Engineering
Draw the circuit topology of the rectifier for this operating condition using a schematic
editor (e.g. PSIM, VISIO, etc.). Show the AC source, the rectifying devices, the DC load
and all measurement locations.
Extract from the data set measurements of the average rectified DC output voltage, and
plot this response as a function of the SCR firing angle. Compare this with the theoretical
response for a full-wave three-phase rectifier supplying a resistive load.
Similarly, plot the measurements of the average load current as a function of the SCR
firing angle, and compare this response with the theoretical response.
Compare against theory the firing angles at which rectifier conduction becomes
discontinuous, and at which conduction ceases entirely.
Explain any discrepancies that exist between the theoretical rectifier responses and the
measured data.
7. Resistive Inductive Load.
a) Diode Rectifier
The archive <Data_Set_3.zip> contains one oscilloscope image that captures the operation of
a full-wave three-phase SCR rectifier supplying a series connected resistive and inductive load
with the nominal parameters of 50Ω and 200mH . The rectifier is supplied from a variac set to
produce a nominal 100Vl −l (rms ) 50Hz AC source voltage.
The measurements shown in these oscilloscope traces are as follows:
•
•
•
•
•
Channel 1 (Yellow) – Rectified DC Voltage using P5200 probe @ 500X attenuation.
Channel 2 (Green) – AC Supply Voltage (VRW) using P5200 probe @ 500X attenuation.
Channel 3 (Blue) – DC Load Current using A600 current probe @ 100mV/A.
Channel 4 (Magenta) – AC Supply Current (IR) using A600 current probe @ 100mV/A.
Reference 1 (Orange) – AC Supply Voltage (VRN) using P5200 probe @ 500X
attenuation and saved to R1.
Analysis:
Draw the circuit topology of the rectifier for this operating condition using a schematic
editor (e.g. PSIM, VISIO, etc.). Show the AC source, the rectifying devices, the DC load
and all measurement locations.
• Extract from the data set measurements of the line-to-line AC supply voltage (VRW), the
rectified DC voltage, the AC supply current (IR), and the DC load current. Compare the
waveforms for IR and IDC against the results of the previous resistor only load condition in
Section 6a).
• How does the rectified DC output voltage compare with the measured value of Section
6a)?
• Explain the effect of the DC load inductance on the input AC and DC load current
waveforms.
• Explain any variations in DC output voltage compared to the results of Section 6a).
• Explain any discrepancies that exist between the theoretical rectifier responses and the
measured data.
•
b) SCR Rectifier
The archive <Data_Set_4.zip> contains 9 oscilloscope images that capture the operation of a
full-wave three-phase SCR rectifier supplying a series connected resistive and inductive load
with the nominal parameters of 50Ω and 200mH . The rectifier is supplied from a variac set to a
produce a nominal 100Vl −l (rms ) 50Hz AC source voltage.
Prof. B. McGrath, Dr. R. Wilkinson
EEET2274/EEET2337 Online Laboratory #3 V1
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RMIT University © 2021
School of Engineering
The measurements shown in these oscilloscope traces are as follows:
•
•
•
•
•
Channel 1 (Yellow) – Rectified DC Voltage using P5200 probe @ 500X attenuation.
Channel 2 (Green) – AC Supply Voltage (VRW) using P5200 probe @ 500X attenuation.
Channel 3 (Blue) – DC Load Current using A600 current probe @ 100mV/A.
Channel 4 (Magenta) – AC Supply Current (IR) using A600 current probe @ 100mV/A.
Reference 1 (Orange) – AC Supply Voltage (VRN) using P5200 probe @ 500X
attenuation and saved to R1.
Analysis:
•
•
•
•
•
•
•
Draw the circuit topology of the rectifier for this operating condition using a schematic
editor (e.g. PSIM, VISIO, etc.). Show the AC source, the rectifying devices, the DC load
and all measurement locations.
Extract from the data set measurements of the line-to-line AC supply voltage (VRW), the
AC supply voltage (VRN), the rectified DC voltage, the AC supply current (IR), and the
DC load current. Using VRN as the reference, compare these voltages and currents in
terms of their magnitude and phase relationships. Note in particular how these waveforms
change as α increases past 60 , in comparison to the results of Section 6b).
Determine the firing angle ( α ) at which conduction effectively ceases.
Extract from the data set measurements of the average rectifier DC output voltage, and
plot this response as a function of the SCR firing angle. Compare this with the theoretical
response for a full-wave three-phase rectifier supplying a resistive inductive load.
Similarly, plot the measurements of the average load current as a function of the SCR
firing angle, and compare this response with the theoretical response.
Compare the DC output voltage waveforms as α increases past 600 to the response of a
resistor only load system from Section 6a).
Explain any discrepancies that exist between the theoretical rectifier responses and the
measured data.
8. Capacitor Output Filter
a) Diode Rectifier – No DC Load Inductor
The archive <Data_Set_5.zip> contains 2 oscilloscope images that capture the operation of a
full-wave three-phase SCR rectifier supplying a 150Ω resistive load with a parallel capacitive
filter equal to 2200µF. The rectifier is supplied from a variac set to produce a nominal 100Vl −l
(rms) 50Hz AC source voltage.
The measurements shown in these oscilloscope traces are as follows:
•
•
•
•
•
Channel 1 (Yellow) – Rectified DC Voltage using P5200 probe @ 500X attenuation.
Channel 2 (Green) – AC Supply Voltage (VRW and VBN) using P5200 probe @ 500X
attenuation.
Channel 3 (Blue) – DC Load Current using A600 current probe @ 100mV/A.
Channel 4 (Magenta) – AC Supply Current (IR) using A600 current probe @ 100mV/A.
Reference 1 (Orange) – AC Supply Voltage (VRN) using P5200 probe @ 500X
attenuation and saved to R1.
Analysis:
•
Draw the circuit topology of the rectifier for this operating condition using a schematic
editor (e.g. PSIM, VISIO, etc.). Show the AC source, the rectifying devices, the DC load
and all measurement locations.
Prof. B. McGrath, Dr. R. Wilkinson
EEET2274/EEET2337 Online Laboratory #3 V1
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RMIT University © 2021
•
•
•
School of Engineering
Extract from the data set measurements of the average rectifier DC output voltage.
Compare this with the theoretical response for a full-wave three-phase rectifier supplying
continuous current to the load.
Extract from the data set measurements of the line-to-line AC supply voltage (VRW), the
AC supply voltage (VRN), the rectified DC voltage, the AC supply current (IR), and the
DC load current. Explore in particular the waveform of IR. Is this current continuous or
discontinuous? Examine VRN carefully, and compare it against VBN. Are the four notches
identified in Section 6a) still present in these voltages? If not, explain why they are no
longer present. Explain the waveform of IR, including in particular the reasons for
deviations in magnitude of successive pulses within a fundamental cycle.
Explain any discrepancies that exist between the theoretical and experimental rectifier
responses.
b) SCR Rectifier – DC Load Inductor
The archive <Data_Set_6.zip> contains 5 oscilloscope images that capture the operation of a
full-wave three-phase SCR rectifier supplying a 150Ω resistive load with a parallel capacitive
filter equal to 60µF, and a series connected inductor equal to 50mH. The rectifier is supplied
from a variac set to produce a nominal 100Vl −l (rms) 50Hz AC source voltage.
The measurements shown in these oscilloscope traces are as follows:
•
•
•
•
•
Channel 1 (Yellow) – Rectified DC Voltage using P5200 probe @ 500X attenuation.
Channel 2 (Green) – AC Supply Voltage (VRW) using P5200 probe @ 500X attenuation.
Channel 3 (Blue) – DC Load Current using A600 current probe @ 100mV/A.
Channel 4 (Magenta) – AC Supply Current (IR) using A600 current probe @ 100mV/A.
Reference 1 (Orange) – AC Supply Voltage (VRN) using P5200 probe @ 500X
attenuation and saved to R1.
Analysis:
Draw the circuit topology of the rectifier for this operating condition using a schematic
editor (e.g. PSIM, VISIO, etc.). Show the AC source, the rectifying devices, the DC load
and all measurement locations.
• Extract from the data set measurements of the average rectifier DC output voltage, and
plot this response as a function of the SCR firing angle. Examine the waveforms of the
DC output voltage at different firing angles ( α ). Identify the firing angle where the
current becomes discontinuous. Note the magnitude of the plateau voltage when no
rectifier current is flowing.
• Explain the changes in the SCR DC link voltage waveform as the firing angle is increased
past the point of discontinuous conduction.
• Explain any discrepancies that exist between the theoretical and experimental rectifier
responses.
•
9. Effect of Source Inductance
a) Diode Rectifier
The archive <Data_Set_7.zip> contains two oscilloscope images that capture the operation of
a full-wave three-phase SCR rectifier supplying a load consisting of a 200mH inductor and a
150Ω resistor. The rectifier is supplied from a variac set to produce a nominal 100Vl −l (rms )
50Hz AC source voltage through a 15mH source inductance (for all three phases).
The measurements shown in this oscilloscope trace are as follows:
Prof. B. McGrath, Dr. R. Wilkinson
EEET2274/EEET2337 Online Laboratory #3 V1
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RMIT University © 2021
•
•
•
•
•
School of Engineering
Channel 1 (Yellow) – Rectified DC Voltage using P5200 probe @ 500X attenuation.
Channel 2 (Green) – AC Supply Voltage (VRW) using P5200 probe @ 500X attenuation.
Channel 3 (Blue) – DC Load Current using A600 current probe @ 100mV/A.
Channel 4 (Magenta) – AC Supply Current (IR) using A600 current probe @ 100mV/A.
Reference 1 (Orange) – AC Supply Voltage (VRN) using P5200 probe @ 500X
attenuation and saved to R1.
Analysis:
•
•
•
•
•
Draw the circuit topology of the rectifier for this operating condition using a schematic
editor (e.g. PSIM, VISIO, etc.). Show the AC source, the rectifying devices, the DC load
and all measurement locations.
Extract from the data set measurements of the average rectifier DC output voltage.
Compare this with the results from Section 7a).
Determine the width of the notches observed in VRN during one fundamental cycle.
Using the measured notch width in VRN, estimate the input source inductance of the
incoming AC supply, and compare it with the identified value from the test set front
panel.
Explain any discrepancies that exist between the theoretical rectifier responses and the
measured data.
b) SCR Rectifier
The archive <Data_Set_8.zip> contains 10 oscilloscope images that capture the operation of
a full-wave three-phase SCR rectifier supplying a load consisting of a 200mH inductor and a
150Ω resistor. The rectifier is supplied from a variac set to produce a nominal 100Vl −l (rms )
50Hz AC source voltage through a 15mH source inductance (for all three phases).
The measurements shown in these oscilloscope traces are as follows:
•
•
•
•
•
Channel 1 (Yellow) – Rectified DC Voltage using P5200 probe @ 500X attenuation.
Channel 2 (Green) – AC Supply Voltage (VRW) using P5200 probe @ 500X attenuation.
Channel 3 (Blue) – DC Load Current using A600 current probe @ 100mV/A.
Channel 4 (Magenta) – AC Supply Current (IR) using A600 current probe @ 100mV/A.
Reference 1 (Orange) – AC Supply Voltage (VRN) using P5200 probe @ 500X
attenuation and saved to R1.
The Data Set 8 oscilloscope traces also contain up to two duplicate measurements at the
same SCR firing angle, but with horizontal cursors enabled to capture time measurements.
Analysis:
•
•
•
Draw the circuit topology of the rectifier for this operating condition using a schematic
editor (e.g. PSIM, VISIO, etc.). Show the AC source, the rectifying devices, the DC load
and all measurement locations.
Extract measurements of the notch width in VRN as the firing angle varies. Why does the
notch width change as the firing angle is increased?
Using the formula given in the background notes, calculate the source inductance. Also
check and verify that:
=
VDC 1.35Vl −l ( cos α + cos (α + γ ) ) / 2
• Explain any discrepancies that exist between the theoretical rectifier responses and the
measured data.
Prof. B. McGrath, Dr. R. Wilkinson
EEET2274/EEET2337 Online Laboratory #3 V1
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