AN2991 Datasheet, PDF (8/14 Pages) STMicroelectronics – Single-phase induction motor drive for refrigerator compressor application

Stalled rotor management
AN2991
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)
10
8
6
4
2
0
-2
-4
-6
-8
0
2
Istall _max
Istall _min
Inorm _max
Time (ms)
4
6
8
10
12
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Doc ID 15772 Rev 2