White Goods Get Smarter

The European Union has introduced the "EcoDesign of Energy-using Products (EuP)" regulation. The study for circulators revealed that the stringent requirement of energy efficiency can be met with sinusoidal-driven Permanent Magnet Synchronous Motors (PMSM) and variable-speed drives only. Traditionally, sensors are used to detect the rotor position, which is essential for enhanced control techniques like Field Oriented Control (FOC). This article will explore the challenges of sensorless control of PMSM. This article will also examine Intelligent Power Modules (IPM) that are now replacing discrete solutions, enabling highly efficient and cost-effective variable speed drives.

The new energy-saving standards that are proposed by the EU will impact the design of energy-using products. EcoDesign looks at all energy-consuming steps. It also considers the whole life cycle costs including production and disposal costs. The directive established a framework for setting EcoDesign requirements and will force all manufacturers that have products that are applied from AC line to be energy efficient. As an example, the pilot study for circulators in buildings gives an impressive overview including a detailed life cycle cost estimations for small and large stand alone circulators and circulators integrated into boilers [1]. To meet the EcoDesign requirements, the mandatory technologies for circulators are summarized by the study. The required technologies for large standalone circulators are shown in Figure 1, which is directly from the EcoDesign study. The existing voluntary efficiency classes are mentioned and compared to the proposed revised efficiency classes and to the Energy Efficiency Index (EEI). The A classification of the existing scheme is separated into two new classes (A*, A**) in order to distinguish between best available efficiency. Whereas the EEI describes the ratio of the calculated mean electrical power to the reference power. The study explicitly claims that only PMSMs fed with variable-speed drives are able to meet the energy-saving challenges.

As a result of the EcoDesign directive the Energy+ Pumps project [2] was initiated in 2006 to promote highly energy-efficient circulators as the European standard. Using the example of low power circulators the final report of this project reveals that despite the higher purchase costs of the PMSM drives, the overall cost in 10 years will drop down to 50% compared to a conventional circulator. The investigation costs are amortized after roughly 2 years. To enable energy-efficient PMSM drives, adequate voltage/current waveforms as well as frequency have to be supplied to the motor. Adjustable waveforms and speed allow using the drive optimally during any circumstances. Frequency converters enable designers to adjust the speed and current. Furthermore FOC is well popular for 3-phase AC motors to achieve high dynamic response of the drive.

The approach of FOC is to imitate the operation of a DC motor. It is based on the decoupling between the current components Iq and Id. The block diagram of FOC including the needed surrounding components is pictures in Figure 2. The three time-variant output current (ia, ib, ic) are transformed into two time-invariant values (Id, Iq) using Clarke and Park. Hence transforming the three output phase currents into the stator fixed a,b reference frame and rotor fixed d,q reference frame simplified the required control strategy from a three dimensional AC value to a two dimensional DC value. The resulting two current components in the rotor fixed reference frame are quadrature current Iq and direct current Id whereas the quadrature current is perpendicular to the axis of the rotor flux and therefore it is proportional to the torque. The second current component Id is controlled to zero since a magnetizing effect of the rotor is not needed. Now the constant DC values can be easily controlled. The gap between the measured current components and the desired values which depends of course of the needed speed of the PMSM is the reference values for the PI controller and Inverse Park transformation. The inverse transformation yields in time-variant voltages (Va*, Vb*) which are the input signals for Space Vector (SV) modulation. Finally the SVM generates the gate signal for the IPM.

Rotor position detection is essential for high dynamic response of PMSM drives. Traditionally the developers rely on mechanical components like encoder or resolver on the motor shaft to observe the rotor position. To remove the dependency of the sensors, to increase the ruggedness and reliability and after all to reduce the cost sensorless control of PMSM is still an interesting and growing research area. At higher speed the rotor position can be indirectly computed from the back electromagnetic force (BEMF), which is the preferred technique due to its simplicity. Modeling the PMSM as series combination of winding resistance, inductance and BEMF enables to estimate the BEMF provided that input voltage and current for each phase is measured (Figure 2). A possible observer afterwards like Luenberger can improve the estimation of rotor position by modeling the drive behavior and adjusting the mismatch between measured position and estimated one. The BEMF technique is superior at medium and high speed due to the speed dependant BEMF but cannot be surely applied to low speed and standstill.

The simplest way to start a sinusoidal driven PMSM is to accelerate the motor with a sequence of sinusoidal voltages until the BEMF-based position estimation can take over the control. However the dynamic performance of such a control would not fulfill the requirement of most of drives. Current research and development activities deal with the handling of industrial demands. Whereas the general specification is to utilize the current sensorless setup of PMSM drives. Enhanced techniques based on the variation of the impedance as a function of rotor position are investigated to detect the position at zero and low speed without mechanical sensors. Among other techniques two fundamental groups has been developed - the injection of a high frequency carrier signal and the transient injection method. Both groups have in common that the saliency presence in the motor is exploited to detect the rotor position, which can be indirectly obtained by the current response of the motor. The High Frequency Carrier Injection (HFCI) can be either realized in the stator fixed d,q reference frame or rotor fixed d,q reference frame resulting in an rotating or pulsating carrier. The rotating carrier injection is shown in Figure 3. The HF signal is superimposes to the time-variant voltages V a* and V?*. The resulting voltages modify the switching of the voltage source inverter (VSI) obtaining an HF current with rotor position information. Two orthogonal position signals can be derived from the Clarke transformed time-variant current components i ia and i ib. . Afterwards the rotor position can be instantaneous detected by applying narrow bandpass techniques like heterodyning filters. Additional sensors are not needed.

A promising transient injection method is the INFORM technique (Indirect Flux detection by On-line Reactance Measurement) developed by Schroedl [3]. A sequence of test voltages is applied to the PMSM via the VSI and the current reaction is sensed at specific times. The current derivation corresponds to the rotor position. The test voltage can either be a special measurement sequence interrupting the current control algorithm or can be integrated into the pulse pattern. This method will not disturb the torque since the test current is only a small portion of the load current. Hence it can be neglected in the phase currents. Combining this technique with an observer yields a stable control structure.

Intelligent Power Modules (IPMs) are increasingly used for frequency converters to meet the stringent requirement of energy efficiency and reliability. To meet the new requirements Fairchild has launched a new class of IPMs called m-MiniDIP SPM® (smart power module). These modules excel in the package of only 39mm by 23mm containing a 3-phase VSI including bootstrap diodes, NTC, fine-tuned gate driver and additional protection functions as UVLP, OCP and fault output. Its internal structure is shown in Figure 4. The precisely matched IGBTs and drivers ensure higher performance. In addition the fully isolated modules reveal increased reliability, protection functions are close to the power switches and the low thermal resistance of the packages result in lower temperature changes over a load cycle. Using IPMs simplified the design-in phase due to an easier and faster design and higher flexibility.

EcoDesign is the moving power for highly efficient variable-speed drives using PMSM. Moreover, a sensorless-controlled PMSM drive is investigated to increase the ruggedness and reliability and to reduce overall costs. Modern IPMs enable highly efficient and cost-effective variable-speed drives. www.fairchildsemi.com