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Efficient Motor Control for HVAC Appliances

January 1, 2011
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Manufacturers can reduce costs by integrating an entire dual-motor system with active PFC on a single microcontroller.

Shown is the implementation of a typical power factor correction (PFC) stage for an HVAC appliance. The system has two feedback loops and is controlled by the microcontroller using the feedback signals and pulse width modulation (PWM) outputs. Such an implementation can also compensate for differences between the measured and true average current values under discontinuous mode of operation. Not shown is a phase management block which could be added to facilitate phase-shedding and current balancing between multiple phases to increase efficiency and system reliability. Source: Texas Instruments


Appliances including heaters, ventilators and air conditioners provide an opportunity for manufacturers to differentiate their products through increased performance, better power efficiency, and lower prices. Moving to a sensorless implementation, for example, eliminates the need for incremental encoders, resulting in lower component and installation costs.

  Many HVAC systems also need multiple motors to compress and ventilate air and, ideally, developers can implement control of these motors on a single microcontroller to simplify design and further reduce system cost.

  Controlling motors without sensors and with variable-speed drives, however, requires compute-intensive algorithms such as field-oriented control (FOC) to achieve the best efficiency. Adding further complications to design are the increasing number of local regulations that manufacturers must follow, such as the IEC-60730 standard.

    Finally, since motor control must be performed in real-time, lower cost cannot come at the expense of reliability. This article will explore the challenges of real-time motor control and discuss how manufacturers can reduce HVAC appliance cost by integrating an entire dual-motor system with active power factor correction (PFC) on a single microcontroller.

 

Both the motor control and PFC control loops can be managed through the same interrupt clocked at the higher PFC frequency. Each interrupt executes the PFC control loop. Using time-slicing techniques, a portion of the motor control algorithm is completed with each iteration, thus reducing the maximum latency of the motor control loop. Both control loops also complete well within real-time deadlines, allowing a slower, background state machine to manage system-level tasks like instrumentation, soft start/shutdown and communications. Source: Texas Instruments

Increased Efficiency Through Software

Increasing system efficiency often comes at the expense of increased software complexity. For example, simple motor control techniques provide efficient operation for only a limited range of speed. Employing more complex control algorithms, such as FOC, allow for the use of more efficient variable speed drives, and allow manufacturers to design power and drive circuits that optimally match an application’s capacity needs across all speed ranges. FOC also reduces issues such as torque ripple and vibration, resulting in smoother performance and longer operating life.

    Manufacturers also can decrease system cost by shifting functionality from hardware to software. A common method for tracking rotor position is to use an incremental encoder. For many HVAC applications, however, accuracy to within 50 rpm provides sufficient accuracy. As a result, these systems can operate without sensors to reduce both component and assembly costs.

  System processing load is also increased through the need to meet local regulations. The IEC 61000-3-2 standard defines what harmonic components an electronic load can inject into the supply line, thus requiring systems to minimize low-order harmonics. For this reason, PFC has become an integral part of most rectifier designs.

    PFC is an important technology because it allows systems to smooth out their power draw to reduce input current harmonics that do not contribute to active power. It achieves this by ensuring that the current waveform follows the voltage waveform, while also maintaining a constant output DC voltage regardless of any changes in the load. PFC can be implemented in a passive fashion, but such an implementation is inflexible because it locks the system into a single operating mode with limited ability to react to changes in load. Passive implementations also result in bulky designs.

  Given the variable speed and load of appliance motors, PFC is ideally implemented in an active fashion where it can greatly reduce phase shifts between voltage and current for changing operating conditions. It also considerably reduces system size.

    A digitally implemented PFC can intelligently compensate for large dynamic loads, such as when an air conditioner is about to turn on its compressor, as well as reduce the number of power transients generated. For a multi-phase interleaved PFC system, higher efficiency can be achieved by phase-shedding during low-load conditions, which is easy to implement in a digitally controlled PFC system.

  Furthermore, a digitally controlled PFC system can easily adjust its output voltage based on load conditions to improve overall system performance.  By performing tasks such as rotor speed estimation and PFC in a digital fashion, component count can be reduced to decrease cost while eliminating a point of failure and improving system reliability.

  Operating in the digital domain also allows for more efficient control algorithms, which can optimally meet the capacity requirements of an application across the different speed ranges and operating conditions. For applications operating dual motors, having both motors controlled by the same microcontroller also results in better performance and efficiency through coordination of PFC for both motors as well as coordination of how quickly each motor ramps up relative to the speed of the other.



Synching the PWMs at every inverter PWM cycle ensures that calculations are finished and duty cycles updated before the time-slicing engine is next executed to guarantee cycle-by-cycle duty cycle control of the motor control stage. Flexible ADCs and PWMs also allow inputs to be precisely sampled at the midpoint of the PWM signal to minimize switching noise. Source: Texas Instruments

Interleaving Motor Control and PFC

Most HVAC applications require a motor control loop with an operating frequency no higher than 20 KHz. A typical operating frequency for PFC, however, is on the order of 100 KHz. The first figure in this article shows the implementation of a PFC stage for an HVAC appliance. The system has two feedback loops and is controlled by the microcontroller using the feedback signals and pulse width modulation (PWM) outputs. The current controller operates at 50 KHz (half the PFC switching frequency) while the voltage controller operates at 10 KHz.

    Such an implementation can also compensate for differences between the measured and true average current values under discontinuous mode of operation. If required, manufacturers can add phase management to facilitate phase-shedding and current balancing between multiple phases to increase efficiency and system reliability.

  To achieve good dynamic performance and input power factor, the current control loop for the PFC stage typically runs at a period equal to the PWM switching period or at some small integral multiple of it. Guaranteeing good dynamic performance to meet system specifications for such a system can be challenging given that the PFC and motor control loops run at different frequencies. Thus, the microcontroller must have the capacity to maintain two, high-frequency, interleaved control loops quickly and efficiently. In addition, the microcontroller must operate with minimal latency to prevent the lower frequency motor control loop from disrupting the higher frequency PFC control loop.

  A common approach to this issue is to dedicate an interrupt for each loop. This approach has its drawbacks, however, since multiple interrupts can occur simultaneously. Even though the microcontroller can prioritize the interrupts, in certain situations the slower motor control loop requires complex calculations that can delay execution of the faster PFC control loop for one or more cycles. The impact of such delays depends upon the application.

An alternative approach is to manage both control loops through the same interrupt. The interrupt is triggered at the rate of the faster PFC control loop, which executes each iteration. Using time-slicing techniques, a portion of the motor control algorithm also is completed.

  In the example system, the interrupt triggers with a frequency of 50 KHz. Time-slicing increases system reliability by reducing the latency impact of the motor control loop. Utilizing a single interrupt also eliminates prioritization conflicts and reduces context-save overhead. 

  Also important, is to maintain ADC signal integrity because low signal integrity can negatively impact performance and efficiency. Even when filtering signals to remove noise, inputs should be sampled at the midpoint of the PWM signal-i.e., as far from the MOSFET switching as possible-to minimize switching noise.

  To achieve this, flexible ADCs and PWMs are required to enable the microcontroller to precisely trigger ADC conversions. The PWMs are synched at every inverter PWM cycle to ensure that the calculations are finished and the duty cycles updated before the time-slicing engine is next executed to guarantee cycle-by-cycle duty cycle control of the motor control stage.

  With today’s highly-integrated microcontrollers and design tools, developers can implement dual motor control with interleaved PFC on a single microcontroller. The result is lower cost systems with fewer critical components and smaller passive devices that provide the long-term reliability required for HVAC appliances.

  In addition, developers can improve efficiency through more complex control algorithms while precisely regulating power through PFC to meet regulatory requirements. Finally, with the extensive development tools and libraries available, companies can shrink development cycles to quickly bring new products to market ahead of the competition.  

System-level utilization @ 60 MHz for sensorless FOC-based control of two permanent magnet motors operating at 10 KHz and active PFC with a switching frequency of 100 KHz and a control loop frequency of 50 KHz. Source: Texas Instruments. Source: Texas Instruments

Maintaining Real-time Performance

Many HVAC applications use a separate microcontroller for each motor, as well as a third microcontroller to manage PFC. Today’s advanced microcontrollers have the capacity to support dual-motor control with active PFC, as well as complex control algorithms such as FOC on a single microcontroller. Such systems are both cost-effective and efficient.

  The above table shows system-level utilization numbers for a dual-motor system with active PFC. For this implementation, Texas Instruments’ (TI) 32-bit fixed-point Piccolo (TMS320F28035) was used to control both motors and the PFC stage. The Piccolo MCU has hybrid ADCs provided the flexible conversion needed while supporting continuous sampling up to 5 Msamples/second. High-resolution PWMs provided duty-cycle modulation up to 150 ps.  The microcontroller also integrates two internal oscillators that provide redundancy and a three-tier clock protection mechanism as required by IEC-60730.

  To speed development and minimize the number of cycles required to execute PFC and motor control, digital motor control libraries were used for functions such as proportional integral derivatives, Park transform, Clarke transform, space-vector generation, and sliding mode observation. In addition, the Code Composer Studio integrated development environment compiler automatically optimized these functions for certain operating conditions, resulting in shorter code that requires fewer cycles to execute.

  Such optimizations are critical because the microcontroller must have enough headroom to reliably control two FOC-based motors and a high-frequency PFC control loop, as well as perform system-level tasks such as monitoring, protection, and signal conditioning.   

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