Improved energy efficiency and reduced system costs are the driving factors in modern motor control designs used in fans, pumps, compressors or geared motors. The implementation of sophisticated motor control concepts with sensorless Field Oriented Control (FOC) and Power Factor Correction (PFC) help to fulfill these demands. To demonstrate the capabilities of these powerful concepts, Infineon has built a reference design for the motor control of air conditioners using dual sensorless FOC and active PFC based on the 16-bit microcontroller XE164. This innovative approach leads to a significant improvement of the energy efficiency and to a reduction of the Bill of Materials (BoM).
FOC is a method to generate a three phase sinusoidal signal that can easily be controlled in frequency and amplitude in order to minimize the current, and hence to maximize the efficiency. The related vector control is a math technique for controlling brushless DC and AC induction motors that reduces motor size, cost and power consumption. With FOC, the efficiency of a motor can be improved significantly and raised up to 95 percent. This has a big impact on power consumption, motor dynamics, heat dissipation and noise. A sensorless FOC (Figure 1) on BLDC (brushless DC motor) or PMSM (permanent-magnet synchronous motor) provides additional cost benefits compared to sensor-based motor control.
A BLDC has a permanent magnet rotor and a wound stator. The position of the coils (phases), with respect to the permanent magnet field, are sensed and the current switched electronically (commutated) to the appropriate phases. To sense the rotor position, Hall Effect sensors are typically used. But the costs for an encoder or other position sensor can be saved with sensorless approaches using the back EMF (electromagnetic force) of the motor to calculate the rotation angle and rotor position. The back EMF is calculated in the flux estimator, which is based on the voltage model of the system in the two phase reference frame. A single shunt is enough to reconstruct the phase currents.
System manufacturers are looking for cost-effective implementations of these advanced algorithms. This can be achieved by integrating both the sensorless FOC and PFC on a single microcontroller. With this powerful combination the system behavior can be exactly adapted and optimized to the application needs.
In the following application example a dual sensorless FOC with active PFC was implemented with the XE164 MCU (Figure 2). The three PWM units, several timer and two very fast and powerful ADCs make the XE164 a perfect fit for a variety of motor control applications. This application software runs on the XE164, which is part of the drive card of the Dual Motor Drive Application Kit from Infineon.
PFCThe EN-61000-3-2 sets the harmonic regulation standard on any off-line application with power consumption over 75W. This essentially demands PFC, which is used in power supplies from a higher load-on like in PC power supplies or modern inverters, to increase the ratio of the real power and apparent power flowing to the load.
Most motor control systems use PFC as the first stage of the system to reduce the harmonic content and reactive power, thereby improve the power factor and overall efficiency. The power factor of an AC system is defined as the ratio of the real (working) power flowing to the load to the apparent power. Real power is the capacity of the circuit for performing work in a particular time. The apparent power is the total available power. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power can be greater than the real power.
In an electric power system, a load with low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger wires and other equipment.
Active and passive PFCThere are two different kind of power factor correction: passive and active PFC. Linear loads with low power factor (such as induction motors) can be corrected with a passive network of capacitors or inductors. This filter reduces the harmonic current, which means that the non-linear device now looks like a linear load. A passive power factor correction is used for low loads (typically less then 100W) because this filter requires large-value high-current inductors, however, which are bulky, expensive and the power factor at the end is barely acceptable.
Non-linear loads, such as rectifiers or SMPS, distort the current drawn from the system. In such cases, active power factor correction is used to counteract the distortion and raise power factor. An active PFC is a power electronic system that controls the amount of power drawn by a load in order to obtain a power factor as close as possible to unity. In most applications, the active PFC controls the input current of the load so that the current waveform has the same frequency, phase and shape as the mains voltage with no additional harmonics. Active power factor correctors can be single-stage or multi-stage. This approach is more complex but results in a really good power factor (0.99). A power converter adapts the used current to the sinusoidal input voltage taken from the power net. This active PFC circuit is typically a boost converter connected after a rectifier, using a capacitor boosted to a voltage level higher than the nominal rectified voltage (typically 350V to 400V).
In some applications passive and active PFC are combined. For example, motor drives with passive PFC can achieve power factors of about 0.7-0.75 (more for low power applications) and with active PFC of 0.98 and more, while a system without any power factor correction has a power factor of only about 0.55-0.65. With active PFC, variations of the AC line voltage can be balanced and often the related devices can be adjusted to operate on AC power from about 100V (Japan) to 230V (Europe).
Boost converterThe standard boost converter topology is the preferred method to implement PFC. A power factor corrected boost converter with continuous mode operation can decrease the total harmonic distortion with simple circuit and easy control method. The key principle that drives the boost converter is the tendency of an inductor to resist changes in current. When being charged it acts as a load and absorbs energy (somewhat like a resistor), when being discharged, it acts as an energy source (somewhat like a battery). The voltage produced during the discharge phase is related to the rate of change of current, and not to the original charging voltage, thus allowing different input and output voltages.
The basic principle of a boost converter consists in two distinct states. In the On-state of the PFC transistor, the inductor current will increase. In the Off-state of the PFC transistor the only path offered to the inductor current is through the fly back diode, the capacitor C and the load R. These results in transferring the energy accumulated during the On-state into the capacitor.
When a boost converter operates in continuous mode, the current through the inductor never falls to zero. In some cases, the amount of energy required by the load is low enough to be transferred in a time smaller than the whole commutation period. In this case (discontinuous mode) the current through the inductor falls to zero during part of the period.
Reference designTo show the capabilities of the design concept, Infineon built a demonstration using an air conditioner (indoor and outdoor unit) and the Dual Motor Drive Kit (Figure 3) based on the XE164. To implement the sensorless dual motor FOC (Figure 4), two independent algorithms for the compressor and fan of the outdoor unit were implemented: the CCU60 is used for the compressor with a PWM frequency of 15kHz, while the CCU62 is used for the fan with a PWM frequency of 15kHz (figure 4). Both PWM frequencies can be increased depending on customer needs to above 25kHz. The fast ADC0 handles the precise current measurements for the two motors and the CAPCOM2 timers are used for ramp-up/down.
In the design example, the active PFC (Figure 5) runs on the Dual Motor Drive Kit in a continuous conduction mode. The implementation consists of two parts: voltage boost and stabilization using a PI controller up to 1kHz calculation frequency at low load and the PWM modulation calculation using IPFC and the voltage PI controller output with up to 130kHz calculation frequency. The CCU61 was used for PWM of the PFC transistor gate at a PWM frequency of 130 kHz, while the ADC1 is used special for PFC to measure the PFC current (IPFC) and DC link voltage (VDC).
The Dual Motor Drive Application Kit comes with the DriveCard for the XE164F which provides the Timer (T13) output signal for PWM generation of the PFC transistor gate. The T13 is running in edge aligned mode with a frequency of 130kHz. The compare match of the timer is used to trigger the ADC1 to measure the PFC current. In the current implementation with the XE164F the ADC is triggered in Software. The period match of timer 13 is used for the current loop calculation and updating the T13 compare channel if the PFC control is activated. Timer 12 is used to generate the time base for the voltage loop with a 1kHz frequency. The PI controller calculates the error of the boost voltage as part of the current loop calculation. If only the voltage boost is active the pi output is directly used for PWM generation to stabilize the VDC.
The PFC improvements were measured with a power factor meter. Using the active PFC a power factor of 0.97 was reached, compared to about 0.65 without. In addition the motor control of the original air condition system needs two PCBs and a quite large and expensive inductor. The Infineon reference design (Figure 6) uses a single PCB and a reduced, less expensive inductor due to the active PFC and control of two motors.
ConclusionThe XE164 microcontroller with its enhanced peripherals can be ideally used to improve the energy-efficiency of applications like HVAC (Heating, Ventilating and Air Conditioning) systems by implementing a dual sensorless FOC algorithm and active PFC. Using both FOC and PFC not only helps the system manufacturers to achieve efficiencies of up to 95 percent in BLDC or PMSM motor applications, but also reduces system costs and minimizes torque ripple to create quieter operation. These software-based motor control designs are very flexible and also allow fast and easy customization of models to address multiple markets.
Author InformationRonny Schulze is Senior Engineer, Application Engineering for Industrial Microcontrollers at Infineon Technologies. He can be reached at +49 89 234 86111 or email at ronny.schulze@infineon.com.
Captions
Figure 1: Block diagram of sensorless FOC.
Figure 2: Block diagram of XE164.
Figure 3: Dual motor application kit.
Figure 4: System block of a dual FOC.
Figure 5: System block of an active PFC.
Figure 6: Air conditioner motor control: comparison of standard and Infineon optimized solution.
