Microcontroller based PWM Inverter to Control Speed of Rotating Machinery

Introduction

In the industry the addition of speed controlling device based on electronics like Inverter is a significant development. To control the speed of motors in industry the normally electronically based technique used depends upon the control of voltages and/ or frequency to control the speed to desirable results. The control of voltages or current has built in advantages and disadvantages besides the efforts to control the speed. The power electronics system which converts DC (Direct Current) voltages to AC (Alternating Current) voltages at required output voltages level with suitable frequency is known as Inverter. Then the era comes and control of speed task has major development and that is control through variable frequency using PWM (pulse width modulation. The use of Pulse Width Modulation (PWM) is gradually increasing in controller for automation. PWM control signals are also implanted in regulator of signal generators for power inverters. Some method of speed control work on designing to modify the width of signal real time and offline as well. Now PWM is in real world application and can be seen everywhere. It involves a microcontroller which is programmed to generate high frequency, the duty cycle of the high frequency is changed real time which corresponded to the changes in speed of motors. 

There are variety of microcontroller available in market to use for this application, some have built in feature for PWM like microchip PIC microcontrollers and some have timers and other hardware facilities which can be programmed to obtain the PWM like Atmel at89s51 series or simply 8051 family microcontrollers. The microcomputer do the best work to calculate switching timing and frequencies for PWM signals along with harmonic reduction. The microcontroller and digital control components are used for proper shaping the PWM signals. As a result the PWM signals formed on some featured frequencies ranging in 9-35.38 kHz with some limitation at the moment in this project later the range will be revised. The PWM generated control the firing angles (α) which is characterize to voltage difference applied to motors. The microcontroller is programmed inherently to notice the voltage variation in peaks of output voltage to adjust the output through the implementation tabulated information inside microcontroller program in order to keep the output voltages stable. As the motors in industry are mostly designed to work on three phase, therefore the inverter has to design for three phase voltages having 120 degree phase shift among all phases of the line, and it is possible by using the building timers of the microcontroller.

Digital PWM Techniques (DPWMTs) used for Inverter:

The microprocessors and microcontrollers are the great problem solvers in the areas of the traditional Analog to digital converter strategies with the LSI for control of speed which is now being altogether replaced with PWM inverters. Here we which to describe the generation of PWM using digital technique like DPWMTs as The digital control settings are made on the microcontroller to periodically adjustment of the output PWM switching frequency ranging in 12-28 kHz. In the periodic adjustment facility or provision the software performs calculations on duty cycle for the PWM signals and this is done for each of the 3 pins of the microcontroller to achieve the three phase inverter.

Simulation the phase output voltage waveform based PWM scheme with different angles.

The peak value is converted to a digital signal using ADC and the resulted digital signal is read by the microcontroller for voltage regulation. The voltage controller or regulator is designed using simple low price AT8951 microcontrollers along with 12 bits ADC ( analog to digital converter), a keypad of capacity 16 switch for input parameters, the chopper MOSFET transistors IRFZ48 and current sensors. The High frequency switching allows good regulation with lower distortion in the output waveforms of inverters. Soft switching for the power controlled application can be achieved using ZVS and/ or ZCS. Soft switching is an active resource of dropping switching losses hence reflects more efficiency. The implementation of soft switching techniques is required especially where the high power electronic machine is being operated near sensitive communication equipment or medical related equipment which are more sensitive to noises or harmonics. The design is describes in figure below:

Block Diagram of Inverter based on Microcontroller

There are several PWM systems used in inverter for the application of speed control through the implantation of variable voltage , variable current and frequency using many electronics devices like this project, but here we used the microcontroller for the regulation of speed through the variation of frequency. In this project proposal we learnt that how microcontroller can be used for the development of inverter to control single phase or even three phase machines to proper control of the speed through the implantation of variable frequency with less production of noise and harmonics.

Keywords: ADC, Inverter, Microcontroller, PWM, Switching frequency, Voltage Regulation, 8051, block diagram, Phase angle control, Firing angle of SCR, Zero crossing for AC voltages