Dr. Hamid Toliyat
Peyman Niazi
Texas A&M University
Kedar Godbole
Texas Instruments
San Antonio, Tex.

The past few decades have seen a rise in the use of field-oriented control in induction motor applications. One advantage of field-oriented control - or as some call it, vector control - is that it increases efficiency, letting smaller motors replace larger ones without sacrificing torque and speed. Another advantage is that it offers higher, more dynamic performance in the case of speed and torquecontrolled ac drives.

Field-oriented control drives also offer several benefits to the end user. They are smaller than the trapezoidal commutation drives they replace. They also offer more efficiency and higher performance at the same time, without demanding tradeoffs. In addition, servo drive manufacturers are leveraging processing power to add more features such as power factor correction, which eases the harmonics and power factor issues that system designers must address.

Overcoming obstacles

In developing field-oriented control technology, engineers have faced and overcome several obstacles. For one, calculations associated with vector control are far more demanding than those associated with traditional (scalar) techniques. Where standard microcontrollers wouldn't work, digital signal processors (DSPs) provided the edge.

DSPs more than double induction motor operating efficiency from 40% (typical of microcontrollerbased solutions) to as high as 90% by injecting energy into stator at more optimum times. This type of smarter control also improves power factor ratios - something that local utilities usually reward - and lets design engineers spec lowhorsepower motors confidently, instead of over-sizing the motor to make up for inefficient operation.

Until recently, the drawback of using DSPs - and thus fieldoriented control - was that DSPs were not as capable as microcontrollers in the realm of straightforward control operations. But this has changed with the addition of many peripherals now found in DSP architectures, along with architectural adjustments to accommodate the special needs of field-oriented control algorithms.

Sensor issue

Another challenge for engineers developing field-oriented control applications had to do with measuring rotor data. Early-generation vector-controlled ac motors employed high-precision speed sensors, but added costs and reliability issues posed a significant drawback. Sensorless control appeared to be the answer, but it has been found to have serious limitations in practical applications. The main problem stems from sensitivity to parameter variations, such as stator resistance changes. This has a significant impact on efficiency at low speeds, and remains a barrier to widespread use of sensorless control.

As a result, engineers today are faced with a dilemma. They can use high-precision sensors, which provide accurate measurements, but present robustness issues in hostile environments. Or they can use tougher, low-cost sensors and try make up for accuracy limitations by employing more sophisticated processing methods.

DSPs and the promise of even faster processors in the future clearly favor the latter approach. In fact, by combining DSPs, low-resolution speed sensors, and enhanced parameter estimation techniques, engineers are achieving the full benefit of field-oriented control in systems costing little more than standard ac drives.

The heart of the solution is an algorithm that compensates for motor slip frequency error. This error is the result of variations in rotor time constant. Uncertainties associated with these variations tend to detune vector-control systems. Fortunately, the newly developed correction procedures don't need to know motor parameters because they're implied from other known data. As a result, uncertainties and detuning are no longer at issue.