AN2446

How the system works

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3

How the system works

The evaluation board can be tested with or without load. In fact, even if no compressor is

connected to the evaluation board, all the signals are clearly visible by means of a scope.

Care has to be taken when using a scope with the evaluation board. For more details,

please refer to the safety instructions described in the User's manual.

Please note that after the evaluation board has been powered on, there is a 3-second delay

before the system starts to operate.

3.1

Cabinet temperature regulation

The potentiometer P1 can be adjusted to set the cabinet temperature in the range of -23 °C

up to +10 °C. Both cabinet temperature and potentiometer settings are read once per

period, meaning 50 times per second (with a mains frequency of 50 Hz).

Once the cabinet temperature rises over the level set by the potentiometer P1, the system

turns the compressor on. The compressor stays on as long as the cabinet temperature

remains below the threshold level set by the potentiometer P1. An offset in the temperature

control is defined in order to avoid oscillations and spurious compressor turn on or off.

3.2

Dead-time

Once the compressor turns off, a delay is set. This delay is called “dead-time”. During this

time, the compressor cannot be turned on again, even if the cabinet temperature exceeds

the threshold level set by the potentiometer P1.

The dead-time is necessary is order to prevent the compressor from turning on immediately

after it was turned off. In fact, the internal high pressure would not let the compressor to spin

again (stall rotor condition).

The dead-time function also helps to avoid spurious compressor turn on and off, therefore

saving in power consumption too.

3.3

Compressor start-up sequence

The evaluation board can drive both the RUN winding and the START winding of a small-

size or mid-size compressor, replacing therefore the positive temperature coefficient (PTC)

thermistor in the START winding.

When the system turns the compressor on, the following steps are performed:

●

both RUN winding and START winding are supplied;

●

after a certain time, the START winding is no longer supplied;

●

RUN winding continues to be supplied as long as the compressor stays on.

The START winding is supplied for 500ms, and then turned off. This time is strictly

dependent on the type of compressor and can be adjusted by modifying the software (see

Section 5.2: START winding duration).