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DSPs: improve energy efficiency with DSP smart motor control.

Electric motors account for two-thirds of industrial electric consumption and one-fourth of residential electric use. In view of this, the efficiency of electric motors continues to gain more focus. The typical electric motor application can be made to run much more power-efficiently. Most motors are relatively inefficient in converting electrical energy to the useful mechanical form. This means they waste a great deal of power in the form of heat rather than converting it into useful mechanical energy.

Since an uncontrolled motor must overcome the effects of transient mechanical loads, designers are often left with no choice but to oversize the motor. An oversized AC induction motor (the most commonly used motor type) will necessarily operate with lower efficiencies since the motor runs at a fraction of the load for which it was designed.

Improving Motor Efficiency

These problems can be solved using smart control, which greatly improves the motor's efficiency in two ways. First, smart control uses advanced algorithms to run the motor more with higher performance. This category of applications most often uses vector control for running an AC induction motor. This may allow proper sizing of the motor for optimal efficiency. In addition, the adjustable speed operation allows the system to operate more efficiently. For example, a vector controlled adjustable speed drive may help to eliminate a mechanical transmission and, in turn, the energy lost in the mechanical part of the system.

Additionally, a smart controller in the system may allow the motor to be replaced with a more efficient form. The movement towards permanent magnet motors for appliances is along this vector.

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Permanent magnet synchronous motors are inherently more efficient than AC induction motors, since they do not have the conduction losses associated with inducing rotor currents in the motor. They also offer better mechanical characteristics such as lower torque ripple and quieter operation, and they are smaller for an equivalent mechanical power output. Switched reluctance motors can also operate efficiently in a fixed or moderately variable-speed application that requires the precise, complex control enabled by a DSP controller.

These techniques share a common characteristic: They use numerically intensive computations to improve the system performance. Vector control algorithms involve measuring or predicting the position of the rotor flux and then placing the stator flux generated by a poly-phase winding optimally for generating the most torque for a given flux configuration. For a permanent magnet motor, the stator flux is placed 90[degrees] away (electrical angle). This gives the best possible torque production since the torque produced is directly proportional to the sine of the angle between the two fluxes. (In an AC induction motor, the relationship between the fluxes is more complicated owing to the magnetizing component of the flux. However, the basic principle is the same.)

Implementing Smart Control

The challenge of implementing cost-effective smart control is in the mathematical complexity of the algorithms involved. Most microcontrollers (MCUs) are not able to deal with this level of computational complexity in real time. However, a new generation of inexpensive digital signal processor (DSP) controllers provide the computational power required for smart control as well as system-on-a-chip (SOC) integration and software development support that help simplify motor control system design.

DSP controllers running smart control software enable applications to handle changes in load and torque with a smaller motor or a more efficient type of motor, reducing cost, space and cooling requirements. A smaller motor is cheaper to buy, and the cost of the power electronics is also reduced by reducing the currents the power electronics are required to handle. DSP controller-based smart control brings these cost-effective advantages to automobiles, industrial equipment, home appliances or "white goods," heating, ventilation and air conditioning (HVAC) systems and a variety of other motor applications.

Advanced Control Benefits for End Equipment

Besides running the motor more efficiently, advanced control also enables the system designer to innovate, improve the system and reduce cost. The need for embedded smart control is evident in complex motor applications. For instance, positioning servo motors used in assembly lines must adapt to changes in load weight, belt friction and other factors. The speed of a fan motor used to supply constant airflow in an HVAC system must be modified continually to compensate for pressure changes as doorways open and close.

Essentially, smart motor control involves the instantaneous calculation of a rotor's magnetic flux position and speed so that the currents through the motor windings can be adjusted properly to ensure low torque ripple. While the design requirements for smart motor control begin with rotor positioning and speed, they do not end there. Other challenges include power factor correction (PFC) to smooth out transient spikes in the supply power, the elimination of torque ripple effects and compliance with specifications for electromagnetic compatibility.

Designing with Smart Control

Microcontrollers, lacking all-important numerical ability, end up being less effective and less economical for smart control when system cost and performance are considered. High-performance DSP controllers can carry out the rotor positioning and speed calculations in real time without look-up tables. External sensors can be eliminated since the DSP controllers can calculate these vectors from voltage and current feedback read by internal line sensors. A DSP controller's processing power makes it possible to introduce field-oriented control (FOC), ensuring that magnetic field of the stator is orthogonal to the rotor flux to realize a high dynamic performance machine. The entire 32-bit FOC algorithm cycle with feedback can be performed in less than 10 [micro]sec, leaving plenty of processing headroom for flux estimators. PFC and other algorithms. In addition, the algorithms are reusable in other products based on the same DSP controller platform, cutting development costs and speeding time to market for new motors.

A new generation of DSP controllers provides peripheral integration and ease of use that compare well with those of MCUs. An example can be found in Texas Instruments' 150-MIPS TMS320F2812 digital signal controllers that feature a single-cycle 32-bit multiple-accumulate (MAC) data path, or dual 16-bit MACs, that combine DSP performance and precision with high-end MCU flexibility. Fast interrupt handling plus op-codes for common control tasks, such as bit manipulation and branching, make the devices suitable for use in multipurpose, multitasking environments. On-chip features include Flash memory, analog-to-digital converters (ADCs), pulse-width modulated (PWM) outputs and CAN bus support.

Software development support for the DSP is provided by integrated development environments (IDEs). Efficient C compilers allow developers to create object code nearly as compact as native assembly, yielding desirable performance after a fast learning curve. Tools such as IQMath simplify the use of real arithmetic functions in control algorithms by giving programmers an automated in-line code editor and a library for floating-point functions.

Conclusion

A single-chip DSP controller and ready-to-use software, enables motor manufacturers to embed smart control with minimal investment in development. As a result, equipment manufacturers and end users enjoy the benefits of right-sized motors that cost less, consume less power and are more reliable. Smart control techniques, enabled by high-performance, programmable DSP controllers, give motor system designers the means to create a new generation of environmentally friendly "green" motors.

by Kedar Godbole, Digital Motor Control Applications, Semiconductor Group, Texas Instruments

Kedar Godbole is currently with the Digital Control Systems group at Texas Instruments. Presently, his responsibilities include driving the Digital Motor Control Software Program and enhanced software reusability deployability. He earned his bachelor's of engineering degree in 1995 from the University of Pune in India. Contact Texas Instruments, Semiconductor Group, SC-04001, Literature Response Center, Box 954, Santa Clara, CA 91380, (800) 477-8924; www.ti.com.
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Title Annotation:Semiconductor Highlight
Author:Godbole, Kedar
Publication:ECN-Electronic Component News
Date:Jul 1, 2004
Words:1259
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