International Appliance Manufacturing 2008 - (Page 26) Using the FOC technique, the motor currents are transformed to 2-axis vectors, as seen in a DC motor. The FOC process begins by measuring the three-phase motor currents. In practice, since the instantaneous sum of the three current values will be zero, by measuring only two of the three currents, the value of the third can be determined. Also, the hardware cost is reduced, because only two current sensors are required. This FOC technique will work with asynchronous motors, as well. A synchronous motor differs from an asynchronous motor in the relationship between the mechanical speed and the electrical speed. In a synchronous motor, the supplied voltages have the same frequency as the mechanical motor speed. In an asynchronous motor, the final mechanical speed is different from the input frequency, and the relationship between input frequency and mechanical speed varies, depending on the mechanical load applied to the motor. A major advantage of deploying DSCs in motor control is the practicality of a common design platform, which makes the production of appliances more efficient. Even with different varieties of motors, such as PMSM and ACIM, appliance makers can use the same motorcontrol board and only need to change the software on the DSC. Therefore, a single design platform can handle multiple types of motors, and appliance makers only need to develop different motor-control IP to handle many varieties of motors. As a result, appliance makers now have an economical way to offer a range of appliance models that use PMSMs, or other motor types, with sensor-less FOC algorithm control. Firmware IP protection is a major issue for manufacturers who frequently deploy appliance design teams that collaborate across many geographic regions. It is easy to imagine a scenario where multiple design teams based in different locations could work in tandem to implement FOC firmware, develop the appliance front panel, and get the final system integration done. While developing their firmware and designs, these teams will have claims to their own IP. Microchip’s dsPIC DSC family offers the CodeGuard security feature, which helps to protect IP in collaborative design environments by securing IP segments separately. CodeGuard security enables multiregional IP development, where each team develops its IP, and locks out that portion of DSC memory. In another example, when motor manufacturers sell motorcontrol algorithms to appliance manufacturers; using CodeGuard security, these drivers can be housed in a protected section on the DSC memory. In a clear departure from existing motor-control solutions that rely on costly custom ASICs to implement FOC and the PFC block, appliance makers now have a better choice. To enhance the efficiency of appliances and reduce their audible noise, controllers such as Microchip’s dsPIC DSCs can be deployed. These DSCs make it easy for appliance makers to efficiently and economically implement advanced algorithms such as FOC, on a wider range of appliances than ever before. Microchip provides a free FOC algorithm, with full source code. For more information, email: appliance@microchip.com 26 International Appliance Manufacturing 2008
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