My goal is to design a device that generates a three-phase signal starting from a voltage in DC.
I have a 12VDC power supply and through H bridges, power supply of the transformers in the SCOTT configuration, both transformers with their special connection send me the signals R, S, T plus a Neutral.
Between my airplanes is to vary the voltage of the power supply with a reducing reducer converter.
The system will be controlled by a microcontroller Hercules RM46, this internal has an ETPWM module, which allows me to generate PWM signals with special characteristics, such as bandwidth, phase angle, synchronization in PWM, among others.
*The image of the transformer is referential
CIRCUIT ELECTRICAL BACKGROUND - SCOTT CONNECTION
This is a contribution of Frank Rodrigo Coaquira Molleapaza (Electrical Engineer).
It is a connection of two single-phase transformer to convert a system of three-phase voltages in biphasic and vice versa.
This arrangement is due to the American engineer Charles F. Scott, who invented it in 1984 while working at the company Westinghouse and is based on the well-known fact that the three-phase star system composed voltage between two phases is in square with the voltage Simple third phase.
Equation for output V2a:
Equation for output V2b:
Fragment of text, extracted from the book:
Pag 243. “Maquinas Electricas – Jesus Fraile Mora”
Conclusion: from the previous fragment, it tells us that you must have two alternating signals of the same frequency with a phase shift of 90 degrees between them.
|About the Microcontroller|
The Hercules RM46x LaunchPad Development Kit is an inexpensive evaluation platform designed to help you get started quickly in evaluating and developing with the Hercules microcontroller platform. The LaunchPad Development Kit is based on the IEC 61508 SIL 3 certified RM46L852, which is a lockstep ARM Cortex-R4F based MCU with integrated safety features and peripherals such as two 12-bit ADCs, programmable High-End timers, motor control peripherals (eQEP, eCAP, ePWM), USB, Ethernet, MibSPI and serial communication interfaces. Hercules RM MCUs can help reduce the development effort of IEC 61508 functional safety applications.
|ETPWM Enhanced Pulse Width|
The enhanced pulse width modulator (ePWM) peripheral is a key element in Modulator controlling many of the power electronic systems found in both commercial and industrial equipments. These systems include digital motor control, switch mode power supply control, uninterruptible power supplies (UPS), and other forms of power conversion. The ePWM peripheral performs a digital to analog (DAC) function, where the duty cycle is equivalent to a DAC analog value; it is sometimes referred to as a Power DAC.
As we can see in the image, you can see that ETPWM module uses two output pins of the microcontroller, side A and B, both outputs can be with the same polarity or one in reverse of the other.
The resolution of the module is of nanoseconds, generating a high resolution in the dutty cycle and in the dead band.
|Code Example CCSV7|
/* USER CODE BEGIN (3) */
etpwmDeadBandConfig_t pwm_deadband_config; // declaracion de variable
/*Configuracion del angulo de desfase entre los PWM generados*/
etpwmSetSyncOut(etpwmREG2, SyncOut_EPWMxSYNCI); //SyncOut_CtrEqZero);
etpwmEnableCounterLoadOnSync(etpwmREG2, 8300, COUNT_DOWN); // seting del angulo de
/*Configuracion de la banda muerta*/
pwm_deadband_config.halfCycleEnable = false;
pwm_deadband_config.inputmode = PWMA_RED_PWMB_FED;
pwm_deadband_config.outputmode = PWMB_FED_PWMA_RED;
pwm_deadband_config.polarity = Invert_PWMB;
/*Iniciando la carga de los valor de la banda muerta 1000---> 510 microsegundos*/
etpwmSetDeadBandDelay(etpwmREG1, 1000, 1000);
etpwmSetDeadBandDelay(etpwmREG2, 1000, 1000);
/* USER CODE END */
In the schematic there is a simple inverter bridge excited by transistors and using MOSFET to perform the energy switching
Two of these bridges are used for the project.
In the following images you can see the implementation using only one transformer.
Output voltage waveform
|Operation Videos - Spanish|
First part of the operation
Second part of the operation