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AN2991 Datasheet(PDF) 8 Page - STMicroelectronics |
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AN2991 Datasheet(HTML) 8 Page - STMicroelectronics |
8 / 14 page Stalled rotor management AN2991 8/14 Doc ID 15772 Rev 2 2.2 Protection by microcontroller In order to secure the life expectancy of a refrigerator, the stalled rotor protection can also be provided by the electronic board. In this case, the over current is applied to the motor and the switches for less than 1 s, as opposed to 15 s with a thermal cut-off. The maximum junction temperature will then reach 102 °C, for a 60 °C ambient temperature, instead of 162 °C, eliminating any stress on the ACST. This 1 s duration is chosen in order to differentiate an abnormal over-current from the start- up current, as explained below. To sense an over current, it is possible to measure the voltage across a shunt resistor placed in series with the run ACST. Then, as the current is alternating, it must be clearly defined at which moment it must be measured. Furthermore, this moment must be chosen in order to differentiate the over-current from the normal current. Figure 8 gives the maximum and minimum currents for both operating conditions. These curves come from experimental results where the mains voltage has been varied from 198 to 264 V rms, with and without a start capacitor. Figure 8 shows that, in a stalled rotor condition, the current is still above 5.6 A between 6 and 8 ms after zero voltage crossing. For information, in normal condition, the load current is always lower than 3 A at this moment. The 5.6 A value is chosen as the condition for a stalled rotor status. The MCU must then be able to perform A/D conversions between 6 and 8 ms. Several measurements can be performed to filter measurement noise. Figure 8. Motor current maximum ranges Current (A) Time (ms) 4 06 2 8 10 12 -8 -6 -4 -2 0 2 4 6 8 10 Istall max Inorm max Istall min |
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