Motor Speed And Direction Controller Using AC Chopper

ABSTRACT

The speed control of separately excited ac motor is carried out by varying the armature voltage for below rated speed and by varying field flux to achieve speed above the rated speed. This thesis presents the speed control methodology by varying armature voltage using chopper by providing control signal to the switches. Speed can be controlled from below and up to rated speed .The firing circuit of chopper receives signal from controller and variable voltage is given to the armature of dc motor according to the desired speed .There are two controllers we are using here one is speed controller and other is current controller. Both controllers are of proportional – integral type. The reason behind using PI type controller is it removes the delay and provides fast control.

ABBREVIATIONS AND ACRONYMS

AC – Alternating Current

MOSFET-Metal Oxide Semiconductor Field Effect Transistor

DC-Direct Current

PWM-Pulse Width Modulation

MATLAB -MatrixLaboratory

BJT -Bipolar junction transistor

IGBT -Insulated gate bipolar junction transistor

EMF -Electromagnetic field

GTO -Gate turn off thyristor

PI -Proportional -integral

PID -Proportional –integral-derivative

CHAPTER ONE

1.0                                                        INTRODUCTION

A large number of motors are being used for general purposes in our surroundings from house-hold equipments to machine tools in industrial applications. The electric motor is now a necessary and indispensable source of power in many industries. The function and the applications of these motors are wide-ranging. For the many applications, the speed control of the motor plays a major role, which can be done using a wide range of control strategies.

The purpose of a motor speed controller is to take a signal representing the demanded speed, and to drive a motor at that speed. The controller may or may not actually measure the speed of the motor. If it does, it is called a Feedback Speed Controller or Closed Loop Speed Controller, if not it is called an Open Loop Speed Controller. Feedback speed control is better, but more complicated, and may not be required for a simple design.

Generally speed control of the motor can be achieved by varying the input parameters of the motor such as current, voltage etc. This can be achieved by various methods such as field control method, armature control method, Ward-Leonard method etc for DC motors and AC motors.

Now a day, by the improvement of the power electronics devices, the flow of power to the motor is controlled by the switching action of the power switches (MOSFET, IGBT, IGCT etc).

The nature of the speed control requirement for an industrial drive depends upon its type. Some drives may require continuous variation of speed throughout the operation, which ranges from zero to full speed, or a over a portion of the range. For most of the drives, however a control of speed within the range of ± 10% of rated speed may be suitable.

The existing methods for the speed control of induction motor are voltage control, frequency control, vector control, changing stator poles, rotor resistance, doubly fed motor.

Recently remarkable efforts have been made in speed control of single phase induction motor (SPIMD) drives.

There are different types of PWM strategies have been used for speed control of induction motor drive. By using PWM strategies, inverter is used to drive the speed of induction motor, AC choppers are also used for the speed control of induction motor.

Ac chopper are used to obtain variable rms voltage from fixed voltage and frequency sources.

It is used in applications such as light dimmers, industrial heating furnaces. In this paper by using AC chopper, speed control of induction motor is achieved. The input to the AC chopper is pure sinusoidal voltage and the output of the AC chopper i.e. chopped voltage is given to the Induction Motor as the input. By varying the pulses given to the

AC chopper the output voltage of AC chopper varies and by this way speed of induction motor is achieved. Generally for the speed control of induction motor drive inverters are used. For the inverter, a separate DC source or rectifier (for converting AC to DC) is required, which increases the size of the whole system. Also for both single phase and three phases Induction motor control, a three leg inverter is used, which increases the number of components and design complexity will be more.

Thus in this paper, AC chopper is implemented to control the speed of single phase induction motor by using asymmetrical pwm method

1.2                                              PURPOSE OF THE PROJECT

1.4                                         SIGNIFICANCE OF THE PROJECT

Can be used to drive small conveyor belts in 2 directions and with dimmer would be able to handle both directional control and speed control.

It is also possible to use this circuit for any robotic application where directional control is needed with speed control such as RC helicopters’ or aircrafts’ propellers. “Single Phase Induction Motor Speed Control Using PWM AC Chopper Fan Applicaitons”

1.5                                                 SCOPE OF THE PROJECT

Conventional methods for starting and speed control of single-phase induction motor need often quite expensive external electrical equipment. Moreover, they require mechanical switches, which are the most troublesome parts.

These switches suffer from mechanical wear, which affects the motor performance. Electronic methods based on using a triac in series with the stator main winding or using four triac in a bridge topology has been suggested to control the speed of the motor.

Starting is still done by a capacitor in with the auxiliary winding. In the literature many authors have suggested and analyzed the performance of single-phase induction motors fed from inverter circuits with space vector pwm. In this case, either single-phase inverter is used with permanent capacitor motors or two phase inverter is used when the capacitor is omitted. Both methods need complicated circuits with high cost. However, AC chopper seems to be an acceptable solution when a wide range of speed control is required. AC chopper is the advance method of the triac which are used to obtain variable RMS voltage from fixed AC course. It is used in the application such as light dimmers, industrial heating furnaces and induction motor speed controllers. The different type of pwm technique such as conventional pwm, asymmetrical pwm, symmetrical pwm etc, is used to drive the AC Chopper. The hysteresis pwm technique offers a reasonably clean output voltage waveform at high power factor and less harmonic distortion factor of the motor current.

1.6                                          APPLICATION OF THE PROJECT

This work is used in the following devices:

• Electronic bikes
• Swing machines
• Robotic controls
• Mobile air condition fans
• Ac operated drill machines
• Winding machines
• Electronic door controllers
• Toys
• Electronic chairs (beauty parlor. Etc.)

1.7                                           LIMITATION OF THE PROJECT

Drawback of this method of dc motor speed control is that they are not able to control the motor speed smoothly at lower levels, and as the desired speed is decreased, the torque of the motor also decreases proportionately. Due to this, at any unpredictable point the motor may just halt very abruptly. Also, during power ON, the motor may just not start up at lower speed settings and may require an initial boost by increasing the setting. Such situations are pretty undesirable and do not constitute an ideal speed control.