08.01.13 | Autor / Redakteur: Alfred Hesener * / Gerd Kucera
An efficient power management is a must for today´s engineers due to the recent ErP rules of the European Union. While the changeover from AC to BLDC and PMSM Drives will improve system efficiency significantly, the advanced motion controller will further save energy.
Many applications use electric motors for a large variety of functions. In those that do not require precise control of position, but where other factors matter more, simpler constructions like synchronous AC or brushed DC motors have been used in the past. Here, compactness, system cost, and ease of implementation are more important. Many of these applications also require high levels of faultless operation for many hours. Synchronous motors fit the bill, as their construction is simple, yielding high reliability, and system implementation is not complicated. However, their efficiency tends to be rather poor, many times below 70%, and changing the rotation speed or implementing more advanced features is very difficult to impossible.
In many economies, electric energy is becoming a commodity with increasing price. New regulations are being implemented around the globe to cut off energy-wasting applications from the market, and drive adoption of more efficient implementations. As an example, the European Union is implementing the ErP rules for many different applications that the EU member states have to implement as local legislation. Here, minimum efficiency targets are defined that force the companies to use BLDC or PMSM motors. As of August 2012, the regulations are already in force for circulation pumps, home compressors, fans, washing machines and dishwashers. Home air conditioners, vacuum cleaners, water pumps and industrial compressors are in regulatory phase, meaning the new guidelines are completed and in implementation by the member states. Dirty water pumps, swimming pool pumps and all motion control applications not covered by the above groups are in discussions – regulations around these can be expected to come.
But that is not all. With the widespread adoption of electronically controlled systems connected to the grid, like inverter-based motion control, solid-state lighting and switchmode power supplies, the reactive power circulating in the grid is strongly increasing, and the power factor is decreasing. This forces the utility companies to provide more grid capacity than is actually needed, costing a lot of money. As in the case of solid state lighting, where power factor correction (PFC) circuits or other implementations with a good power factor are becoming the de-facto standard, motion control applications are expected to see the same requirement. While inverter-based motion control may not solve that problem, having to use electronic controls actually makes it easier to implement PFC pre-regulators and comply with this new requirement.
Driving BLDC or PMSM motors is usually done with an inverter-based circuit, where three voltages are applied to the terminals of the motor. Various modulation schemes exist to perform the required calculations. A typical system block diagram is shown in figure 2. The line voltage is first rectified, then boosted to a constant bus voltage with a PFC stage. This stage will ensure that at the input, current and voltage are in phase, to achieve a good power factor. This voltage is fed to the inverter stage that consists of three half-bridges, driven by the HVIC gate driver. The control signals from the motion controller are logic signals that are "translated" by the gate driver into the correct gate drive voltages, noting that especially the emitter potentials of the high side switches will swing up and down significantly. The motion controller will receive feedback from the sensors on the motor, or through measurement of the phase currents. In many cases, opto couplers are used to isolate the sensor signals since in industrial applications the environment can be noisy, and this may modify the small sensor signals. An auxiliary switchmode power supply is used to power the controller.
For the power switches in the inverter stage, various solutions exist. In many of these applications, smart power modules such as Fairchild’s FSB5 series is being used, that combine the power switches (MOSFETs or IGBTs) with gate drivers, so that the entire power stage can be controlled with logic signals, and all the complicated power routing and drive requirements is well taken care of inside the module. Only some applications with extreme cost pressure and no or very little system size requirements will implement the power stage with discrete components, at reduced reliability of course.
First, the switching frequency of the inverter is usually determined by the power switches used – for smaller motors up to 100 W smart power modules with MOSFETs are used, allowing switching frequencies up to 50 kHz although most applications operate significantly below that to reduce switching losses. A higher switching frequency may improve dynamic performance, but many applications do not require this. Larger drives usually use IGBTs as power switches, where the switching frequency is lower; the advantage of IGBTs is reduced conduction losses especially at higher currents.
One important goal of the system developers usually is to achieve lowest torque ripple. This will lead to reduced mechanical noise, higher reliability and lower system cost, since e.g. the bearings need to be sized to support the torque peaks. This may force usage of higher complexity modulation schemes, like space vector modulation.
Implementing additional features like variable speed, controlled power-up and power-down, and functions increasing the robustness (e.g. like reversing the motor to unblock a pump) do increase the burden on the motion controller. The pulse-by-pulse switch control, advanced algorithms and added functions can push the computing performance requirements even for simple applications quite high.
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