AN1353

Sub-routines

Rev 2

9/17

This sub-routine also defines if the motor must be started up. If the temperature becomes

higher than ORDER 1, the compressor should be started. But, care must be taken not to

start the motor if it is already ON. Otherwise, Ts could be switched on when Tr is already

ON, resulting in a capacitor discharge through the two devices.

Note that each evaporator temperature and potentiometer voltage measurements are

refreshed every 256 cycles (by the average value of the previous 256 cycles). This means

that the temperature control is acti-vated every 5 seconds.

5.2

Motor status

This sub-routine defines which ACST must be triggered depending on the motor state and

the order asked by the “LOADS_CONTROL” routine. This flowchart is shown on annex 4.

When the motor is required to be ON, both Tr and Ts are switched on 500 ms. After that,

only Tr is fired, if the motor must still be ON. In this case, the MOTOR_STATUS routine

claims that the motor is in a transient state again for 500 ms. This state is used by the

“CURRENT_MEASURE” sub-routine. Indeed, the current must not be measured during the

first one second of operation, during which the current is high. On the other hand, if the

current remains high after this one second, that means that an abnormal condition has

occurred (stall rotor for example) and it is better to stop the motor.

5.3

Current measure

The current measure is only performed when the motor should be ON, and if it is not at

start-up transient state. At each overcurrent detection, the motor is stopped, a LED is

switched on during 5 s, and, of course, the current measure is no longer performed.

The overcurrent detection is, in fact, based on an average of four measures. This is done to

reduce EMI noise. The over current therefore always acts with at least an 80 ms delay,

because only one measure by cycle is done.

The measured value is compared to the “VLIMIT” constant. A 242 value equals a voltage of

5.22 V at the in-put of the Analog to Digital Converter (for a 5.5 V supply). As the measure is

achieved thanks to a 50 mOhm shunt, referenced to the Vdd, this value is similar to a 5.6 A

current. Such a current means that the rotor is stalled because, in steady state, a 100-300

W compressor always sinks a current lower than 3 A.