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PWM control of inverter

Pulse Width Modulation (PWM) control in inverters is a method of controlling the output
voltage by varying the pulse width of a switching signal.

This is achieved by comparing a reference signal with a carrier signal (which is typically a
higher-frequency waveform) and generating switching pulses based on the comparison.

When the carrier signal is less than the reference signal, the inverter output pulse is
"on"; otherwise, it's "off".

By adjusting the duty cycle the average output voltage can be controlled, allowing for efficient
and precise control of AC power.

Higher duty cycles result in higher average voltages, and lower duty cycles result in lower
average voltages.

Advantages:

PWM control offers several advantages, including:

• Precise voltage control: PWM allows for fine-tuning of the output voltage, making it suitable for
applications requiring precise voltage regulation.

• Harmonic reduction: PWM can minimize the production of lower-order harmonics, which can
reduce the need for extensive filtering.

• Efficiency: PWM is an efficient method for controlling output voltage in inverters.

 • Applications:

PWM inverters are widely used in various applications, including:

• Motor drives: Controlling the speed and torque of AC motors.

• Renewable energy systems: Inverters for solar and wind power.

• Uninterruptible power supplies (UPS): Providing backup power during outages.

• Induction heating: Generating high-frequency AC for heating applications.

Current control of VSI:

Current control of a Voltage Source Inverter (VSI) refers to a control strategy where the output
current of the inverter is regulated to follow a desired reference.

This is especially important in applications like motor drives, grid-tied inverters, and active
power filters, where precise current control ensures stable and efficient operation.

A VSI is a power electronic converter that converts DC voltage into AC voltage with controllable
magnitude and frequency.

A current controller is implemented in a feedback loop to regulate the output current. Typically
uses a Proportional-Integral (PI) controller or Model Predictive Control (MPC).

The reference current is generated based on the desired power output or system requirements
(e.g., torque demand in motors or power injection in grid systems).

Pulse Width Modulation (PWM) or Space Vector Modulation (SVM) is used to convert the
controller output into gate signals for the inverter switches.
Applications:

• Motor drives (AC motors, PMSMs, induction motors)

• Grid-connected inverters for solar or wind power

• Uninterruptible Power Supplies (UPS)

• Active filters for harmonic compensation

PWM converter as line side rectifier

A PWM converter can be used as a line side rectifier to convert AC volt*e from the grid to a
regulated DC voltage.

In this configuration, the PWM converter consists of a three-phase bridge rectifier followed by
a DC-DC converter.

The overall operation of the PWM converter as a line side rectifier can be summarized as
follows:

1. The three-phase bridge rectifier converts the AC voltage from the grid to a pulsating DC
voltage.

2. The output of the rectifier is filtered with a capacitor to reduce ripples.

3. The DC-OC converter regulates the output voltage by controlling the duty cycle of the
chopper switch.

4. The regulated OC voltage is used to power the load, such as a motor.

The PWM converter as a Iine side rectifier 0ffers severaI advantages over traditionaI di0de
rectifier such as –

• Provides a regulated DC output voltage essential for high power application such as
motor drives
• Reduces harmonic distortion of input current
• Improves power factor close to Unity
• Offers better control of output voltage

Current fed inverters with self-commutated devices

Current-fed inverters with self-commutated devices are a type of power electronic converter
used in high-power applications such as motor drives, wind and solar energy systems, and
electric vehicles.

The are also known as current source inverters (CSI) or current source converters and they uses
self-commutated devices such as IGBTs Or GTOs to control the output current and voltage.

In a current-fed inverter with self-commutated devices, the input voltage is connected to a DC

source through an inductor, which acts as a current source.

The self-commutated devices, such as IG8Ts or GTOs are connected in a bridge configuration.

The operation of a current-fed inverter with self-commutated devices can be divided into two
modes: current-controlled mode and voltage-controlled mode.

In current controlled mode- the input current is controlled by varing the duty cycle of self-
commutated device. this result in a variable output voltage which is regulated by a feedback
control loop by adjusting the duty cycle to maintain a constant output voltage.

In the voltage-controlled mode, the input Current is fixed, and the Output voltage is controlled
by varying the duty cycle of the self-commutated devices. This mode is typically used When
output voltage needs to be varied over a wide range such as in Wind and solar energy systems.

The main advantages are their ability to handle large DC input voltages and their superior
performance under dynamic conditions. They also offer better fault tolerance and reliability
than voltage-fed inverters.

However, they are more complex and expensive than voltage-fed inverters and require careful
design and control to achieve optimal performance.
Control of CSI
The control of a current source inverter (CSI) involves regulating the input current, which in
turn controls the output voltage and frequency.

The control strategy for a CSI typically involves a closed-loop control system that measures the
output voltage and adjusts the input current to maintain a constant output voltage and
frequency.

The following are the basic steps involved in the control of a CSI:

1. Current Control Loop: The current control loop is responsible for regulating the input
current to the CSI.
2. Voltage Control Loop: The voltage control loop is responsible for regulating the output
voltage of the CSI.
3. Frequency Control Loop: The frequency control loop is responsible for regulating the
output frequency of the CSI.
4. Modulation: The modulation is trees is responsible for generating the gate policies for
the self-commutating devices based on control signal from the current voltage in
frequency control loops.

The control of a CSI is critical for its reliable operation and performance.

H bridge as a 4-Q drive

An H Bridge is a four switch configuration that can be used as a four quadrant drive for
controlling the direction and a speed of motor or other load.

The H-bridge consists of four switches (typically transistors or MOSFETs) arranged in a bridge
configuration with the motor connected across the center of the bridge.
By selectively turning on and off the switches, the polarity of the voltage applied to the motor
can be reversed, changing the direction of rotation.

The H bridge can operate in four modes, which correspond to the four quadrants of the
current-voltage plane:

Four Quadrant Operation:

• Quadrant I (Forward Rotation, Positive Voltage): Q1 and Q4 are on.

• Quadrant II (Reverse Rotation, Negative Voltage): Q2 and Q3 are on.

• Quadrant III (Reverse Rotation, Positive Voltage): Q2 and Q3 are on, then switch Q1 and
Q4 on to brake.

• Quadrant IV (Forward Rotation, Negative Voltage): Q1 and Q4 are on, then switch Q2
and Q3 on to brake.

The ability of an H bridge to operate in all four quadrants makes it ideal for
controlling the direction and speed of a motor as well as for other applications that
require bidirectional power flow.

Applications: Electric Vehicles, Robotics and Industrial Automation etc.