Micro Controller Based Ac Power Controller

doi:10.4236/wsn.2009.12012

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Wireless Sensor Network, 2009, 2, 61-121

doi:10.4236/wsn.2009.12012 Published Online July 2009 (http://www.SciRP.org/journal/wsn/).

Micro Controller Based Ac Power Controller

S. A. HARI PRASAD1, B. S. KARIYAPPA1, R. NAGARAJ2, S. K.THAKUR3

1Department of Electronics & Communication Engineering, R.V. College of Engi neering, Bangalore, In d i a

2Director, Center for Cognitive Technologies,R.V.C.E. Campus, Bangalore, India

3Deputy Director, Naval Research Board, Defence Research Development Organiz ation, New-Delhi, India

Email: harivat2002@yahoo.co.in

Received April 25, 2009; revised May 8, 2009; accepted May 9, 2009

Abstract

This paper discusses the design and implementation of single phase PWM inverter using 8051 microcontrol-

ler. The main features of 8051 based PWM inverter are simpler design, low cost, maximum range of voltage

control and compact in size. The designed PWM inverter is tested on various AC loads like AC motor and

intensity control of incandescent lamp in a closed loop environment.

Keywords: Gate Signals Generation, Micro Controller, Pulse Width Modulation, PWM Generation

1. Introduction

The pulse width inverters can be broadly classified as

 Analog bridge PWM inverter [1]

 Digital bridge PWM inverters [2]

The advantage of Analog based PWM inverter con-

troller is that, the level of inverter output voltage can be

adjusted in a continuous range and the throughput delay

is negligible. The disadvantages of Analog based PWM

inverters are:

Analog component output characteristics changes with

the temperature and time. They are prone to external

disturbances. Analog controller circuitry is complex and

bulky. They are non-programmable, hence not flexible.

On the other hand Microcontroller based PWM in-

verter controller (Digital bridge PWM inverter) makes

the controller free from disturbances and drift, but the

performance is not very much high due to its speed limi-

tation. However to minimize throughput delay, some

microcontroller based PWM inverters, retrieves switch-

ing patterns directly from memory so that calculation can

be minimized, but this technique demands more mem-

ory. This drawback can be eliminated if switching pat-

terns are generated by executing simple control algo-

rithms [3]. Even after using simple control algorithms,

sometimes throughput delay may be substantial.

With the availability of advanced microcontrollers and

DSP [Digital signal processor] controllers [4], with many

advanced features like inbuilt PWM generator, event

managers, time capture unit, dead time delay generators,

watch dog timers along with high clock frequency, the

limitation of speed, associated with microcontroller

based PWM inverters [5] can be neglected to some ex-

tent.

This paper presents a simple and cost effective tech-

nique of implementing single-phase AC [alternating cur-

rent] voltage controller, used to control AC loads ,which

doesn’t demand very high precisions, using 8051 micro-

controller.

The paper is organized as follows.

 Review of PWM inverters.

 Block diagram of controller.

 Controller implementation (software and hardware).

 Results and Conclusion.

2. PWM Bridge Inverter Review

Inverters can be classified as single-phase and three

phase inverters [6] and they are further classified as

Voltage fed inverter [VSI.], current fed inverter [CFI],

and variable DC [direct current] linked inverter. In Volt-

age fed inverter, input voltage remains constant, in cur-

rent fed inverter [CFI], input current remains constant

and in variable DC [direct current] linked inverter, input

voltage is controllable.

S. A. H. PRASAD ET AL. 77

Figure 1. Single phase inverter .

Figure 2. O/P voltage/gate signals.

Figure 1 shows single phase bridge inverter with

MOSFET switches [6]. In spite of MOSFET switches

having high ON state resistance and conduction losses

[7], in this paper MOSFET switches are used because of

the following reasons. MOSFET being a voltage con-

trolled device, it can be driven directly from CMOS or

TTL logic and the same gate signal can be applied to

diagonally opposite switches. Also the gate drive current

required is very low [7].

The working principle of Single-phase bridge inverter

can be explained as follows.

Positive Voltage ‘Vs’ appears across the load, when

MOSFET Q1 and Q2 conduct simultaneously. Nega-

tive voltage ‘Vs’ appears across the load, when Q3 and

Q4 conduct simultaneously.

To overcome the effect of back emf in case of induc-

tive load diodes, D1-D4 are used. Diode D1 and D2 are

called feedback diodes, because when they conduct the

energy is feedback to the DC source. The RMS output

voltage is given by

VVp





where P is pulse width. The O/P voltage and gate sig-

nals are as shown in Figure 2.

3. Controller Block Diagram

The block diagram of microcontroller based bridge

PWM inverter is as shown in Figure 3. The required four

digit speed in RPM [Rotation per Minute] is entered

through the keyboard and corresponding to the key

pressed, digital equivalent of that RPM is stored in

memory.

Current running speed of the AC motor is sensed

through speed sensor, and the analog output given by the

sensor is converted to digital data using Analog to Digi-

tal converter [ADC].

8051 MICRO-

CONTROLLER

KEYBOARD GATE DRIVER

Figure 3. Block diagram of controller.

SENSOR A D C

PWM BRIDGE

INVERTER

FILTER A C LOAD

D C INPUT

78 S. A. H. PRASAD ET AL.

Figure 4. Flowchart of basic operation.

Figure 5. Flow chart of keyboard logic.

Figure 6. Flowchart of keyboard logic.

Figure 7. Flowchart of A/D converter.

The digital data is accepted through 8051 microcon-

troller ports and is compared with required speed’s

equivalent digital data. In accordance with the error sig-

nal, the width (duty cycle) of PWM signal is varied,

which in turn controls the AC voltage.

From the generated PWM signal, required two gate

signals are generated using external interrupt to drive the

bridge inverter circuit.

Gate signals are boosted up to a sufficient voltage

level by using gate driver circuit, so that it can drive the

MOSFET switches of bridge inverter to the ON

state. User can alter the speed at any instant of time in

accordance to his requirements. Many additional fea-

tures can be further added like sensing the temperature of

room and automatically controlling either the speed of

S. A. H. PRASAD ET AL. 79

the fan or the level of air conditioning required. Figure 4

explains the logic flow of the basic operation.

4. Controller Design

Controller is designed by using simpler low cost compo-

nents like 8051 microcontroller, 8 or 12 bit Analog to

Digital Converter (ADC), 4×4 keypad, 4 chopper MOS-

FET switches (IRFZ48) and speed/Intensity sensor.

The controller design can be explained under 4 sec-

tions as:

 Keypad interface with 8051 μc.

 ADC interface with μc.

 Generating PWM signals and gate signals using

8051 microcontroller.

 Gate driver circuit implementation.

4.1. Keypad Interface

A 4×4 keypad is interface with 8051 microcontroller as

shown in Figure 5, through which four keys are accepted.

After accepting the four keys they are combined to rep-

resent four digit required RPM, which actually represents

the external memory address, in which digital equivalent

of speed is stored.

For example if the keys entered are 1 (01), 2 (02), 3

(03), 4 (04), then they are combined as 1234 (RPM),

which represents External memory address, in which 8

bit digital equivalent of that speed is stored. Higher byte

of the memory address is stored in DPH [data pointer

high byte]. Lower byte of the memory address is stored

in DPL [data pointer low byte]. This method saves time

since it doesn’t require any program execution to convert

the entered speed in RPM into its digital equivalent. The

other method is to enter equivalent digital data of RPM

directly, provided a conversion chart is available [exter-

nal look-up table]. This technique will save some mem-

ory access time, since communication with memory is

avoided.

4.2. ADC Interfacing

Whenever speed varies from zero to maximum, the speed

sensor O/P varies from zero to five volts respectively. An

8-bit ADC with resolution 1/28 is used to convert the

analog voltage to digital data. Minimum of 19.5 mv

change in voltage (corresponding change in RPM) is

required to change the digital state of ADC. This limits

the accuracy of the application. The logic of interfacing

ADC is as explained in the flowchart given in the Figure 7.

4.3. PWM Generation

8051 microcontroller do not have on-chip PWM genera-

tor. It is implemented using ‘A’ register and any other

A count (ON period time) is loaded onto one of the

GPR (General purpose register), which can be called as

Duty cycle register and accumulator (‘A’) is loaded with

zero. Register ‘A’ is incremented in steps of one and

continuously compared with duty cycle register.

Figure 8. PWM generation.

Figure 9. Gate signal generation using interrupt.

Figure 10. Gate signal booster circuit.

80 S. A. H. PRASAD ET AL.

100

120

010% 20% 30% 40% 50%

Duty Cycle

Vrms

100

120

140

50.05150.15 250.25 350.35

Frequency Hz

Vrms

theoritical

WOF L=100 µH,R=50 Ω

WF L=100 µH,R=50

Ω

C=1400

Theoritical

Practical

100

120

010%20% 30% 40%50%

Duty Cycle

Vrms

Theoritical

Practical

100

120

140

50.05150.15250.25 350.35Frequency Hz

Vrms

theoritical

WOF L=100mH,R=50Ω

WF L=100mH,R=50

Ω,C=1400

µF

100

120

010% 20%30% 40% 50%

Duty Cycle

Vrms

Theoritical

Practical

100

120

140

50.05150.15 250.25350.35

Frequency Hz

Vrms

Theoritcal

WOF L=10mH,R=50Ω

WF L=10mH,R=50

Ω,C=1400

Figure 11. Response for various loads with corresponding duty cycles.

If the ‘A’ contents are less than duty cycle register,

high level is maintained at port line P1.1. When ‘A’ is

higher than duty cycle register content a low level is

maintained on port line. The alternate technique is to use

Timer as Counter by applying clock pulses externally

and comparing the count present in the counter with ‘A’

clock source, since 8051 do not have any clock out pin.

Since the maximum time period is limited to 256 mi-

croseconds, the minimum frequency of PWM signal will

S. A. H. PRASAD ET AL. 81

be 4 KHz, but this can be changed using software delays.

The AC signal frequency generated by PWM bridge in-

verter depends on PWM signal frequency. The error sig-

nal is generated by comparing the required speed with

accepted digital equivalent speed divided by two. In

proportionate with the error signal, PWM duty cycle is

varied. When the required speed value is less than the

accepted one, duty cycle register value and accepted

value is decremented by one continuously till accepted

value is equal to the required speed’s digital value. When

the required speed value is more than the accepted one,

duty cycle register values and accepted value is incre-

mented by one continuously till accepted value is equal

to the required speed digital values.

4.4. Gate Signal Generation

The generated controlled PWM signal itself will be one

set of gate signal (g1, g2) and other set of gate signals (g3,

g4) is generated using interrupt technique. The controlled

PWM signal generated is given to the external interrupts,

which is initialized as falling edge sensitive interrupt

type. When controlled PWM signal’s falling edge occurs,

an interrupt service routine meant for that particular ex-

ternal interrupt is executed.

In the interrupt service routine, a delay is created equal

to the time, 7FH minus duty cycle register content, after

which, the port line is made high and is retained high for

the time duration decided by the contents of duty cycle

The gate signal (vg1 vg2, vg3, vg4) are boosted to a

sufficient voltage level by Gate drive circuitry as shown

in Figure 10, so that they are capable of driving MOS-

FET’S to the ON state, when the gate signals are high.

A transistor switch (with inverted gate signals as in-

put) is made used to boost the gate signal. The same DC

supply, which is used for inverter is also used to drive

the transistor by reducing the DC level using voltage

dividers. The other technique is to use opto-isolators.

Both of these techniques use the same inverter DC

source to boost up the gate signals, thus avoiding more

usage of DC sources.

5. Results and Conclusions

The designed application is tested by designing 60V

MOSFET bridge inverter.

Harmonics are removed by using simple capacitor fil-

ter and the AC voltage is stepped up to 220 V using

step-up transformer. The performance of application is

tested on various A.C loads and the plots of the same are

as shown in Figure 10. The design exhibits good results

for the load values of 50 ohm and 100 mH/ 10mH. A

simple PWM technique with 100% duty cycle variation,

which reduces hardware and software complexity, is

used rather than using the most often used complex si-

nusoidal PWM technique (For Single-phase inverters).

Required dead time is generated through interrupt, which

avoids the usage of dead time delay generators. With

minor modifications the same work can be used to con-

trol light intensity, temperature etc., The accuracy can be

further improved by using high resolution ADC’s and the

delay involved in the software can be overcome using

higher versions of controllers.

6. References

[1] H. Parasuram and B. Ramaswami, “A three phase sine

wave reference generator for thyristorized motor control-

lers,” IEEE Transactions on Industrial Electronics, Vol.

IE-23, pp. 270–276, August 1976.

[2] J. M. D. Murphy, L. S. Howard, and R. G. Hoft, “Micro-

processor control of PWM inverter induction motor

drive,” in Record of the 1979 IEEE Power Electron Spe-

cialist Conference, pp. 344–348.

[3] G. S. Buja and P. Fiorini, “Microcomputer control of

PWM inverters,” IEEE Transactions on Industrial Elec-

tronics, Vol. IE-29, pp. 212–216, August 1982.

[4] G. S. Buja and P. De Nardi, “Application of a signal

processor in PWM inverter control,” IEEE Transactions

on Industrial Electronics, Vol. IE-32, No. 1, February

1985.

[5] Y. K. Peng, et al., “A novel PWM technique in digital

control,” IEEE Transactions on Industrial Electronics,

Vol. 54, February 2007.

[6] M. H. Rashid, “Power Electronics Circuits, Devices and

Applications,” 3rd Edition, Prentice-Hall of India, Private

limited, New-Delhi, 2004.

[7] V. Jagannathan, “Introduction to power electronics,”

Prentice-Hall of India, Private limited, New-Delhi, 2006.