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Electric Avenue—Getting The Speed Controls In Place

David Tam explains why the key to advancing the adoption of inverterised variable-speed drives lies in finding a solution that offers a quick and cost-effective design path.

Today, much of the electricity on the planet is consumed by inefficient electric motors from a bygone era. It is estimated that over 50% of all electricity is directed at powering motors that feature simple mechanical controls rather than electronic control advances.

The refrigerator operating in many kitchens today is a good example. It uses a bi-metallic switch to turn the motor on when the inside temperature gets too hot, and then turns it off when the desired temperature is achieved. This control scheme wastes half of the electricity applied to it, an amount that could be saved if modern, inverterised variable-speed-drive designs were adopted.

With such potential energy savings, one would think manufacturers would be quick to integrate these inverterised control advances. However, adoption has been sluggish. The major reason for this is the anticipated reaction of the consumer. People would like to save on their electricity bills, but few are willing to pay a higher price.

Increased design complexity resulting from conformance to new energy efficiency and power-quality regulations already is cutting into manufacturers' profit margins. The key to advancing the adoption of the inverterised variable-speed drive lies in finding a solution that offers a simplified design path, one which speeds time-to-market and maintains current cost models.

Increasingly, manufacturers have migrated to inverterised variable-speed drives for two motor types: three-phase induction and permanent magnet synchronous. In contrast to induction motors, interior-permanent-magnet (IPM) motors can deliver more efficiency at a smaller motor size; and, recent IPM innovations have spurred the adoption of more efficient, advanced rotary and swing-type compressors. In particular, sinusoidal-driven IPMs attain the greatest efficiency over trapezoidal or sinusoidal-driven, permanent magnet motors. Typically, these synchronous IPMs employ Hall-effect sensors for the position-based control algorithm. When joined with additional circuitry for power factor correction (PFC), EMI filtering, and safety, the resulting design can become very complicated and carry a high price tag.

An alternative design approach that reduces cost, complexity, and development time uses an integrated design platform. It combines advanced "sensor-less" control algorithms, digital controllers, mixed-signal high voltage ICs, power devices, and integrated power modules. At the core of the new approach lies a dedicated digital controller that integrates a configurable control engine for the field-oriented control (FOC) as well as the sensorless control algorithm. All motion peripherals directly interface to the single-chip high-voltage IC, bypassing "shrubbery" components typical in traditional designs. Integrated into the control engine are algorithms for power factor correction and active noise cancellation, allowing for interactive control between the inverter's front-end and back-end to minimise switching losses and EMI. Lastly, the complete design is referenced to the negative DC bus to accommodate a very simple flyback power supply to provide the required operating voltages.

However, to deliver an even more complete solution to the needs of appliance developers, the digital control IC must be a part of a compatible chipset capable of performing all functional demands of a variable-speed motor controller. Ideally, this calls for compatible analogue high-voltage gate drivers and sensors, power silicon, and power modules developed together to create an integrated design platform. In addition to easing design and integration, such a chipset reduces the number of external components required.

If appliance manufacturers are to implement the new drives quickly and cost-effectively, a new design approach is essential. The concept of a register-configurable digital controller with associated compatible components provides a way to enhance the performance of appliances whilst accelerating development and product turnaround.

TAGS: Components
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