PULSE WIDTH MODULATION

Upon the completion of this topic, you should be able to

3.2 Understand Pulse Width Modulation 3.2.1 Explain the meaning of Pulse Width

modulation3.2.2 explain the formation of PWM using sawtooth and sinusoidal waveform teqnique3.2.3 Draw the PWM waveform for inverter using sawtooth and sinusoidal waveform

Introduction Pulse-width modulation (PWM), or pulse-duration modulation

(PDM), is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches.

The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast pace. The longer the switch is on compared to the off periods, the higher the power supplied to the load is.

The PWM switching frequency has to be much faster than what would affect the load, which is to say the device that uses the power. Typically switchings have to be done several times a minute in an electric stove, 120 Hz in a lamp dimmer, from few kilohertz (kHz) to tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies.

Definition

Advantages PWM Power loss in the switching devices is very low.

When a switch is off there is practically no current, and when it is on, there is almost no voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases close to zero.

PWM also works well with digital controls, which, because of their on/off nature, can easily set the needed duty cycle.

PWM has also been used in certain communication system where its duty cycle has been used to convey information over a communications channel.

Circuit PWM

The block diagram shows the power conversion unit in Pulse Width Modulated (PWM) drives. In this type of drive, a diode bridge rectifier provides the intermediate DC circuit voltage.

In the intermediate DC circuit, the DC voltage is filtered in a LC low-pass filter.

Output frequency and voltage is controlled electronically by controlling the width of the pulses of voltage to the motor.

Essentially, these techniques require switching the inverter power devices (transistors or IGBTs) on and off many times in order to generate the proper RMS voltage levels

PWM Generator•This switching scheme requires a

more complex regulator thanthe Variable Voltage Input (VVI).

•With the use of a microprocessor, these complex regulator functions are effectively handled.

•Combining a triangle wave and a sine wave produces the output voltage waveform.

Output of PWM Generator

• The triangular signal is the carrier or switching frequency of the inverter.

• The modulation generator produces a sine wave signal that determines the width of the pulses, and therefore

• the RMS voltage output of the inverter.

Types of control• AC drives that use PWM techniques

have varying levels of performance based on control algorithms

• There are four basic types of control for AC drives today:a) Volts per Hertzb) Sensorless Vector Controlc) Flux Vector Controld) Field Oriented Control

• Volts/Hertz control is a basic control method, providing a variable frequency drive for applications like fan and pump. It provides fair speed and starting torque, at a reasonable cost.

• Sensorless Vector control provides better speed regulation and the ability to produce a high starting torque.

Flux Vector control provides more precise speed and torque control with dynamic responses

Field Oriented Control drives provide the best speed and torque regulation available for AC motors. It provides DC like performance for AC motors, and is well suited for typical DC applications.