1.1 Power Factor Correction with PMSM FOC using PLL Estimator

Power Factor Correction ( PFC ) shapes the input current drawn from the AC mains to be in synchronization with the input AC voltage. This project uses Boost-Converter topology to carry out the Power Factor Correction. Permanent Magnet Synchronous Motor (PMSM) is controlled using Field Oriented Control (FOC). Rotor position and speed is determined using PLL estimator technique. Motor start/stop operation is controlled by the switch and motor speed can be changed by the on-board potentiometer. Waveforms and variables can be monitored runtime using X2CScope.

Key features enabled in this project are:

• Power factor correction
• Dual shunt current measurement
• Speed control loop
• Field weakening

• AFEC Peripheral:

  The AFEC0 peripheral is used to measure analog quantities for motor control operation. Four channels are used to measure the Phase Current A, the Phase Current B, the DC Bus Voltage and the Potentiometer. Conversion is triggered at the PWM (zero match + offset of the switch delay).

  The AFEC1 peripheral is used to measure analog quantities for power factor correction. Three channels are used to measure the input rectified voltage, input rectified current, and the output DC link inverter voltage. Conversion is triggered at the PWM period match.

• PWM Peripheral:

  The PWM0 peripheral is used to generated three phase synchronous PWM waveforms. Fault functionality is also enabled to switch off the output waveforms asynchronously.

  The PWM1 peripheral is used to generate the PWM waveform for the boost converter switch for current waveform shaping.

• UART Peripheral:

  The UART is used for X2CScope communication to observe graphs and variable values in run time.

The Power Factor Correction is a technique of increasing the power factor of the input AC power supply, thereby providing following advantages:

• Low line harmonics.
• Reduced input current.
• Reduced kVA requirement.
• Improved line efficiency, and improved electromagnetic compatibility.

The Power Factor Correction techniques can be classified as - Passive and Active. The Passive Power Factor Correction uses passive components, while the active Power Factor Correction Technique uses power electronic switches to carry out Power Factor Correction. Following figure shows the various Power Factor Correction techniques classification:

This project uses Boost topology for Power Factor Correction. A boost converter is placed between the rectifier and output inverter stage to synchronize input AC current to the input AC voltage. For details, refer to application note AN1106. The block diagram of a typical boost converter topology based Power Factor Correction is shown below.

Block Diagram:

The power factor correction is implemented in following steps:

1. Measure the rectifier stage rectified AC voltage and rectified AC current, and output inverter stage DC bus voltage via high speed ADC channels.
2. Regulate the DC bus voltage using the Voltage Error Compensator.
3. Determine the reference input for Current Error Compensator based on the output of Voltage Error Compensator and Voltage Feed-Forward Compensator.
4. Track the rectified reference AC current using a Current Error Compensator.

The following section describes the software flow diagram for the power factor correction application project. The power factor correction algorithm in this example project is realized by following state machines.

1. PFC Init State:

   In this state following tasks are performed: Initialization and configuration of NVIC, AFEC, PWM peripherals for generation of periodic ADC triggers and ADC conversion interrupt. Calibration of input rectified AC current sense amplifiers. Initialization of reference DC voltage and reference rectified AC current PI controllers used for Power Factor Correction.

2. PFC Start state:

   In this state following tasks are performed: Start PWM peripherals timer to trigger ADC conversion interrupt.

3. PFC Running State:

   In this state, the power factor correction ISR is executed.

   

4. PFC Stop State:

   In this state, the power factor correction is disabled.

The following section describes user specific parameters used the project.

1. Setting the PFC boost converter stage frequency: The boost converter stage PWM frequency ( in Hz ) can be configured by setting the following macro in userparams.h file. This period timer ticks should be configured properly for PWM peripheral in the MCC to get the desired frequency as shown below:

   

   Macro	Description
   PFC_PWM_FREQUENCY	Boost converter stage PWM frequency

2. Setting PI Controller parameters of Boost Converter Controller:

   The voltage and current controllers used in the PFC algorithm has to be tuned to get the optimal control.

   Parameters for reference DC bus voltage PI Controller:

   Macro	Description
   PFC_VOLTAGE_PTERM	Proportional gain of voltage control loop
   PFC_VOLTAGE_ITERM	Integral gain of voltage control loop
   PFC_VOLTAGE_CTERM	Anti-windup term of voltage control loop
   PFC_VOLTAGE_OUTMAX	Maximum controller output of voltage control loop

   Parameters for reference rectified AC current PI Controller:

   Macro	Description
   PFC_CURRCNTR_PTERM	Proportional gain of input current control loop
   PFC_CURRCNTR_ITERM	Integral gain of input current control loop
   PFC_CURRCNTR_CTERM	Anti-windup term of input current control loop
   PFC_CURRCNTR_OUTMAX	Maximum controller output of input current control loop