Microcontroller Based Variable Voltage Variable Frequency Square Wave Engineering Essay

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Abstract- The present power supply scenario is marred by frequent power cuts, load shedding and low voltage supplies. But power supply dependence in industries and increasing use of electrical and electronics appliances in homes have made uninterrupted power supply a modern day necessity. This has inflated the demand of inverters in both industries and households. The increase in demand has also called for more power efficient and portable designs. Major chunk of the requirement is met using storage batteries and DC to AC Inverters.

As per present scenario large heavy bulky DC motors are being replaced by Induction motors in greater numbers throughout a wide variety of industrial and commercial applications because it is maintenance free, less bulky, reliable, cheaper device to convert the electrical energy into mechanical motion. In some applications, it's desired to control the speed of the induction motor. To achieve this variable frequency and variable Voltage Inverter is the need of future.

The use of micro controller provides the variable frequency pulse width modulation (PWM) signal which gives the variable frequency and variable output voltage as well. Use of PWM reduces the harmonic contents in the output and improves the efficiency of the Inverter.

Keywords-high frequency, Microcontroller,inductiion motor, variable voltage,pulse width modulation,psim

Introduction

The present power supply scenario is marred by frequent power cuts, load shedding and low voltage supplies. But power supply dependence in industries and increasing use of electrical and electronics appliances in homes have made uninterrupted power supply a modern day necessity. This has inflated the demand of inverters in both industries and households. The increase in demand has also called for more power efficient and portable designs. Major chunk of the requirement is met using storage batteries and DC to AC Inverters. Presently 220V/ 50Hz power generation is met using transformer of metals & metal alloys with inherent loss factor of 10 %. However they are heavy in weight, bulky in volume.

To overcome these issues use of high frequency ferrite core transformers provides weight reduction by one fifth of conventional inverters, one third of volumetric reduction along with 25 to 35% cost and the 10% improvement in efficiency. cost of ferrites and growth of ferrite technology will lead to further reduction in cost and more power efficient designs.

Growing dependence of households on inverters calls for more economical, light weight, efficient and portable designs. Switching power supplies are more power efficient. Inverters using switching power supplies are in great demand as they are more compact, more power efficient and low cost.

The inverter design, developed in this work, uses switching technology hence meets all the above requirements needed by the common man.

The variable voltage and variable frequency inverters are widely used in three phase induction drive traction and it is popular in many high power industrial applications, such as speed and torque control. Single phase induction motor (SPIM), has a common using in residential applications, domestic like dish washers, washing machine dryers fans, pumps, etc., are the most common applications and it is possible due to the greater availability of single phase power supply.

This variable voltage variable frequency inverter can be used in hybrid vehicles which is the need of future era.

Microcontroller control can reduce heating of power switches and also can change the switching frequency on the fly; this means that they can use a higher switching frequency to ensure continuous current and smaller magnetic effects for the renewable source interface. This is done while using a lower switching frequency for battery charging, thus minimizing the switching losses and maximizing the system efficiency. The user parameters set-up feature can help to maximize the user control flexibility. Integrated digital control gives the maximized security and faster fault response leads to higher reliability. In addition, a user's interface and communications capability are also easily incorporated.

Another advantage of using a Microcontroller control system is that the PWM converters used for power conversion compose a discrete system and should be treated as such when the control system is designed. Traditionally, a PWM converter (without DSP) is treated as a continuous system when the control system is designed. That means that higher bandwidth and higher switching frequency are required with an analog controller; this leads to high switching losses. With DSP control, PWM converters can be treated as a discrete system when the control system is designed. This means that to achieve the same performance as an analog control inverter, lower switching frequency is allowed with the same filter parameters. This will reduce the switching losses.

block diagram

The block schematic of the proposed scheme is shown in Fig.1.

12 V

Battery

Push Pull Converter

Full Bridge Rectifier with Filter

Full Bridge Converter

Microcontroller

Load

Fig. 1. Block diagram of a Proposed Inverter

details of block diagram

Lead acid battery

Supplies d.c voltage from 10.5 to 13.6 volts to the push pull converter. Battery used may be any battery which supplies an input voltage of 12 V. here as an input voltage source Photovoltaic System can be used.

Push Pull Converter

As shown in the figure 2 Power MOSFETS are used as an switch. It Steps up the dc voltage received on the primary side by a ratio of 28:1. The output obtained is a square wave with average value = 280V.

Duty cycle of PWM signal given to input switches of converter is varied from 64% to 84% to maintain an average value of 280V on the secondary of push pull transformer. A feedback from the output voltage through the microcontroller can maintain the duty cycle for constant output voltage. Auxiliary windings on the secondary side have 3 turns. Average value of 13V is obtained on the Output of the auxiliary winding. This auxiliary voltage supplies the dc voltage required by the three optocouplers (LM 817) in the circuit.

Full Bridge Rectifier and Filter

It Removes the high voltage spikes present in the output of push pull converter. The output obtained is square wave with average value of 280V.

Full Bridge Converter

Receives input from the full bridge rectifier and filter. The switching frequency is 50 Hz with a duty cycle of 85%. The output is square wave with rolled ends at a frequency of 50Hz and average value 220V.

Optocouplers

Three optocouplers (PC 817) are used in the circuit. One optocouplers are used in the feedback circuit to the microcontroller it isolates the microcontroller from the analog voltage being fed back from the output of full bridge rectifier and filter. Other two optocouplers are used to isolate the transistors used in the drive circuitry of full bridge converter switches from the fast rising signals received from the Schmitt trigger.

Simulation results of push pull converter

As shown in figure 2 is the Push Pull Converter circuit using PSIM Software with switching frequency of 50 Hz. This switching frequency can be varied from minimum to maximum range.

Fig. 2 Circuit Diagram of Push Pull Converter

Fig. 3. PSIM Result of Push Pull Converter across Load

The Simulation results for the Push Pull Converter for single phase at 50 Hz frequency is shown in figure 3. It indicates an average output voltage of 280 Volts.

As shown in figure 4 is the Power Circuit diagram of Full Bridge Converter using MOFETS

As shown in figure 5 is the output of Full Bridge Converter Circuit at 50 Hz frequency without a filter. this output is a Square Wave with an average voltage of 280 volts and can be converted to Sine wave by the use of low pass filter.

Fig.4 Full Bridge Converter Circuit Diagram

Fig. 5 Square wave output of Full Bridge Converter

Control Scheme

Fig.6 Flow Chart for generating Switching Signal

The flow chart for developing the control scheme is as shown in figure 6. It is used for the control of switichg the Power MOSFETs of Push Pull Converter and Full Bridge Converter.Micronontroller generates two control signals at variable frequency and four switching signals at 50 Hz frequency that can be controlled by the triggering signal for MOSFET. The output voltage can also be controlled by varying the duty cycle of the PWM signal. Microcontroller also provide Shut down signal at over load, low battery voltage conditions. A temperature sensor also can be used to sense the temperature of the switching devices to enhance the overall efficiency of the system.

Testing and Results

Signals from pin numbers 18, 19, 14 and 15 of microcontroller are fed to it. It decreases the rise and fall time of the signals obtained from microcontroller to a few nanoseconds (50 ns to 990ns.) The hardware of the proposed scheme is developed and integrated with software control through microcontroller. The testing of the prototype is conducted in laboratory test bench. The system is switched on by press button switch on no load. The variable output voltage of a.c. (Square wave) is available and is recorded through a digital voltmeter. The circuit was tested for Resistive load.

Following are the laboratory out puts from Microcontroller.

Fig.7 Output of Microcontroller at 50 KHz frequency

Fig. 8 Output of Microcontroller at 50 Hz frequency

As shown in figure 7 is the 50 KHz output and can be used for switching the Push Pull switches. This frequency can be varied by the microcontroller.

As shown in figure 8 is the Microcontroller out frequency for switching the MOSFETs of Full Bridge Converter at 50 Hz i.e. supply frequency of the Supply. This frequency also can be varied by the help of Microcontroller. Similarly the output voltage can also be changed by varying the duty cycle of the PWM Waveform from the Microcontroller. By this way we can obtain a variable voltage and frequency of the output voltage.

CONCLUSION

As a pilot project developed for a variable voltage and variable frequency from a 12V d.c. 250W system is giving the expected result for resistive load. Added protection features like short circuit, over voltage, under voltage and battery drain protection can be easily implemented by the use of Microcontroller.. The use of micro controller for providing protection makes the protection circuit more compact and corrective measures sharper compared to that provided by an analog circuit. The circuit is tested for 250 watt load and the result was satisfactory. The micro controller also provides the switching control of the converters. This eliminates the need for additional analog control circuitry. Use of step up conversion topology is the heart of this Inverter. Use of ferrite core reduces the size of the system. High switching frequency more than 50 KHz-200 KHz for step up topology further reduces the size of the system.

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