GROUP MEMBERS: PROF: ENGR.

ARNULFO EVANGELISTA

ENCAPAS, KRYZELLE JEAN ECE31

PORTUGALIZA, AREEN ANJANETTE

REYES, MIKAELA ANGELA

VILLANUEVA, DAVE HARVEY

POWER ELECTRONICS EXPERIMENT 8

CIRCUIT DIAGRAM:
MODEL GRAPH:
FORMULA USED:

3√3𝑉𝑚
𝑉𝑜 − ( ) 𝑥 cos 𝑎
𝜋
Where:
Vm = Peak phase voltage. volts
a = Firing angle, degrees

RESULT:
 Figure 1. LTSpice Schematic of Three Phase Fully Controlled Rectifier Circuit

In this experiment, we analyzed a three-phase fully controlled rectifier circuit using LTSpice.
The circuit was constructed with six SCRs (Silicon-Controlled Rectifiers) arranged in a bridge
configuration, which is typical for this type of rectifier. The SCRs are divided into two sets: the
positive group (SCRs U1, U3, and U5) and the negative group (SCRs U2, U4, and U6). These are
triggered alternately, so that only one SCR from each group conducts at any moment, allowing
for proper operation of the rectifier.

To control the firing of the SCRs, we used pulse generators. Each pulse generator produces a
timing signal that turns on specific SCRs. For example, U1 and U6 share the same firing pulse,
defined in LTSpice as PULSE(0, 12, 1.67m 1n 1n 4m 20m). This means the pulse starts at
1.67 ms, has a very fast rise and fall time of 1 nanosecond, stays on for 4 ms, and repeats every
20 ms. Likewise, U2 and U3 are triggered by a pulse that begins at 8.3 ms, and U4 and U5
receive their firing pulse at 15 ms, following the same pulse characteristics.

The delays between each firing signal introduce a 60° phase shift between each conduction
interval, which is necessary for maintaining continuous and balanced conduction. When the
firing angle is set to 0°, this setup mimics the behavior of a full-wave diode rectifier, delivering
a steady DC output.
 Figure 2. Simulated Waveform of the Three Phase Fully Controlled Rectifier

The second figure shows the simulated output waveform of the three-phase fully controlled
rectifier along with the input phase voltages. From the waveform of the output voltage (Vout),
we can observe the characteristics of full-wave rectification. The output appears as a DC signal
with some ripple, which results from the sequential firing and switching of the SCRs. There is a
noticeable delay in the output waveform when compared to the input phase voltages, indicating
the controlled nature of SCR conduction, where the devices are only triggered at specific points
in the input cycle.

The input voltages—va, vb, and vc—are clearly spaced 120° apart, which is expected in a three-
phase system. This phase difference plays an important role, as it ensures that the rectifier
receives continuous peaks from all three phases. These overlapping voltage cycles allow the
output to maintain a relatively stable and smooth waveform. Based on the simulation, the
behavior of the rectifier matches theoretical expectations, emphasizing the importance of
proper SCR triggering to achieve an efficient and continuous full-wave rectified output.

DISCUSSION:

The simulation of the three-phase fully controlled rectifier successfully demonstrates both the
theoretical principles and practical implementation of the circuit. As shown in Figure 1, the LTSpice
schematic confirms the correct setup using six SCRs in a bridge configuration, with alternating
triggering between the positive group (U1, U3, U5) and the negative group (U2, U4, U6). Pulse
generators are used to control each pair of SCRs with precise delays—spaced by 60°—ensuring
continuous and balanced conduction. This design behaves similarly to a full-wave diode rectifier
when the firing angle is set to 0°, validating the circuit's intended function.

Figure 2 supports this by showing the rectified output voltage waveform (Vout) along with the
three-phase input voltages (va, vb, vc), which are clearly 120° apart. The output waveform
displays a full-wave rectified shape with ripple, caused by the sequential SCR switching. The
observed firing delay reflects the controlled nature of the rectifier operation.
The simulation confirms that a three-phase controlled rectifier can convert AC to DC while also
allowing regulation of the output voltage via firing angle control. This highlights the key advantage
of controlled rectifiers over uncontrolled ones—especially in terms of adjustable output and better
harmonic performance. The equation for average output voltage, , aligns with
the simulated behavior when the firing angle is varied.

Additionally, the simulation hints at the impact of source inductance, showing slight commutation
overlaps that slightly reduce output voltage but help smooth current transitions. While not
explicitly shown, the role of freewheeling diodes in handling inductive loads and maintaining
current flow is also relevant to the circuit's stability.

In conclusion, the experiment validates the operation of a fully controlled three-phase rectifier,
offering both theoretical understanding and practical insights into controlled switching and power
conversion in power electronics.

QUESTIONS:
1. WHAT IS A THREE PHASE CONTROLLED RECTIFIER?
a. Using thyristors or SCRs, a three-phase controlled rectifier converts three-phase
AC power into regulated DC power, and the output voltage may be
adjusted by changing the firing angle.
2. WHAT ARE THE ADVANTAGES OF THREE PHASE CONTROLLED RECTIFIER
OVER THREE PHASE UNCONTROLLED RECTIFIER?
a. Firing angle control enables variable output voltage.
b. greater efficiency and control over power
c. Greater compatibility with industrial uses such as motor drives
d. Less output ripple when compared to single-phase systems
3. WHAT ARE THE CLASSIFICATIONS OG THREE PHASE CONTROLLED
RECTIFIER?
a. half-regulated rectifier (uses both diodes and thyristors)
b. Completely regulated rectifier (using all thyristors)
c. rectifier using three pulses (half-wave)
d. Six-pulse rectifier (full wave)
4. WHAT ARE THE ADVANTAGES OF SIX PULSE CONVERTER?
a. lower output voltage ripple
b. Greater average DC output voltage
c. Increased transformer utilization
d. Lower harmonic distortion than three-pulse converters
5. DERIVE THE EXPRESSION FOR AVERAGE OUTPUT VOLTAGE OF THREE PHASE
FULL CONVERTER.

6. WHAT ARE THE EFFECTS OF SOURCE INDUCTANCE IN THE CONTROLLED

CONCLUSION:
The simulation of a three-phase fully controlled rectifier has provided important lessons about
power electronic circuits. The LTSpice simulation analyzed the rectifier's behavior, confirming key
concepts like full-wave rectification and the effects of firing angles on output voltage. A six-pulse
setup showed a smooth DC output with some ripple, indicating successful rectification. The timing
of thyristor activation and input phase separation confirmed effective operation of the regulated
rectifier bridge. The experiment proved that a fully controlled rectifier can efficiently convert AC
to DC and regulate output voltage for industrial applications needing variable DC electricity. The
findings enhanced our understanding of the operation and behavior of regulated rectifiers.

RECOMMENDATION:

Several recommendations for future research can be made based on the experiment's findings
and observations. To comprehend the effects on output voltage, current continuity, and system
stability, it would be helpful to first study the behavior of the rectifier under different load
circumstances, such resistive-inductive (RL) and resistive-inductive-emf (RLE) loads. In addition,
understanding actual issues will be improved by analyzing the effects of source inductance on
commutation overlap and voltage drop. Additionally, gaining practical experience in real-time SCR
trigger management could be gained through the integration of digital control systems, like
microcontrollers or digital signal processors (DSPs). Ultimately, moving the study closer to the
actual implementation of the circuit in a lab environment would help students address non-ideal
characteristics like thermal management, switching losses, and protection circuits, bridging the
divide between theory and real-world application. These investigations will help pupils get ready
for more complex applications in the field of power electronics by increasing their technical
knowledge.