Improving Energy Efficiency Using DSP Intelligent Motor Control

With motors now responsible for two-thirds of industrial electricity consumption and up to a quarter of residential electricity consumption, the efficiency of motors continues to receive greater attention. While standard motor applications are perfectly capable of operating at a higher energy efficiency, most motors are inefficient in terms of converting electrical energy into mechanical energy. This means that they waste a lot of energy, which is dissipated as heat rather than converted into useful mechanical energy.

Furthermore, since an uncontrolled motor must overcome the effects of transient mechanical loads, the designer has little choice but to oversize the motor. An oversized AC induction motor (the most common motor type) is necessarily less efficient because the motor is operating at a load less than it was designed for.

Improve the efficiency of the motor

These problems can be overcome by intelligent control, which can greatly improve the efficiency of the motor in two ways. First, intelligent control uses advanced algorithms to improve the operating performance of the motor. The most common method is to use vector control to operate the AC induction motor, which allows the motor to be reasonably sized to achieve optimal efficiency. In addition, adjustable speed also allows the system to operate at a higher efficiency. For example, a vector-controlled adjustable speed drive can avoid the use of transmissions, thereby reducing energy losses caused by the mechanical components of the system.

Secondly, because the system uses intelligent control, it is possible to replace the existing motor with a more efficient motor. The gradual use of permanent magnet motors in electrical appliances is a reflection of this development trend.

Permanent magnet synchronous motors are inherently more efficient than AC induction motors because they do not have the conduction losses associated with the induced rotor currents, and they also have superior mechanical properties, such as lower torque ripple, quieter operation, and they are smaller in size while producing the same mechanical power output. Switched reluctance motors can also be extremely efficient in fixed or moderately variable speed applications that require the precise, complex control capabilities of a DSP controller.

All of these solutions have one thing in common: they use intensive numerical calculations to improve system performance. Vector control algorithms require measuring or predicting the orientation of the rotor flux and then optimizing the position of the stator flux generated by a multi-phase winding to produce the maximum torque given the flux structure. For a permanent magnet motor, the stator flux needs to be separated by 90 degrees (electrical angle), which is the best way to produce torque. This is because the torque generated is directly proportional to the sine of the angle between the two fluxes (in AC induction motors, the relationship between the fluxes is more complicated due to the magnetizing component of the flux, but the basic principle is the same).

Implementation of intelligent control

The challenge in implementing economical intelligent control lies in the mathematical complexity of the algorithms involved, because most microcontrollers (MCUs) cannot handle such complex calculations in real time. However, a new generation of inexpensive digital signal processor (DSP) controllers provide the computing power required for intelligent control, as well as system-on-chip (SOC) integration and software development support that helps simplify motor control system design.

DSP controllers running intelligent control software enable applications to respond to load changes with smaller or more efficient motors, reducing cost, space and cooling requirements. Smaller motors are also cheaper, and the cost of power electronics is reduced because the current required to withstand these circuits is also lower. Intelligent control based on DSP controllers has the advantage of good economics, and automobiles, industrial equipment, household appliances, heating, ventilation and air conditioning (HVAC) systems and many other motor applications will benefit from it.

What can advanced controls do for the final equipment?

In addition to making motors more efficient, advanced control technology also allows system designers to innovate, improve systems and reduce costs. In complex motor applications, the need for embedded intelligent control is obvious. For example, positioning servo motors on assembly lines must be able to adapt to changes in belt friction, load weight and other factors. The speed of fan motors used to provide constant airflow in HVAC systems must be constantly adjusted to compensate for pressure changes caused by opening and closing doors.

Essentially, intelligent motor control requires instant calculation of the rotor's flux position and speed in order to properly adjust the current flowing through the motor windings to ensure low torque ripple. However, while the basic requirements of intelligent motor control design are rotor positioning and speed, they are not limited to this. Other challenges include: power factor correction (PFC) to eliminate transient spikes in the power supply, elimination of torque ripple effects, and compliance with electromagnetic compatibility regulations.

Designing with Intelligent Control

Microcontrollers lack critical numerical processing capabilities, making them an inefficient and less economical solution for intelligent control in terms of system cost and performance. High-performance DSP controllers can perform rotor position and speed calculations in real time without table lookups. Since the DSP controller can calculate these vectors from the voltage and current feedback provided by internal online sensors, there is no need to use external sensors. The processing power of the DSP controller makes field-oriented control (FOC) possible, making the stator's magnetic field orthogonal to the rotor's magnetic flux to achieve a dynamic machine. The entire 32-bit FOC algorithm loop, including feedback, can be completed in 10ms, leaving ample time for flux estimator, PFC and other algorithm processing. In addition, other products based on the same DSP controller platform can reuse these algorithms, thereby reducing development costs and shortening the time to market for new motors.

High level of integration and development support

The new generation of DSP controllers offers peripheral integration that is easier to use than those based on MCUs. One example is TI's 150MIPS TMS320F2812 digital signal processor (see Figure 1), which uses a single-cycle 32-bit multiple accumulation (MAC) data path or dual 16-bit MACs to combine the performance and accuracy of a DSP with the flexibility of a high-end MCU. Fast interrupt handling, coupled with operation codes for common control (such as bit manipulation and branching), makes the device suitable for multi-purpose, multi-tasking environments. On-chip functional modules include flash memory, analog/digital converters, pulse width modulation outputs, and CAN bus support.

Software development support for DSPs is provided by an integrated development environment (IDE). An extremely efficient C compiler allows developers to generate object code that is almost as compact as assembly, achieving excellent performance after a fast learning curve. Tools such as IQMath provide programmers with an automatic online code editor and a library for floating-point functions, simplifying the use of real arithmetic in control algorithms. For faster development, off-the-shelf motor control modules are available from manufacturers and third parties.

Conclusion

The monolithic DSP controller solution, including easy-to-use software, enables manufacturers to embed intelligent control functions with minimal development investment. As a result, equipment manufacturers and end users can use motors of appropriate size with lower cost, lower power consumption and higher reliability. The intelligent control technology implemented by the high-performance programmable DSP controller will provide motor system designers with the methods needed to develop a new generation of environmentally friendly "green" motors.