Doc ID 15772 Rev 2

11/14

AN2991

Electronic thermostat versus mechanical thermostat

14

.

It is shown that reducing the temperature ripple improves the appliance efficiency. This can

be explained by the fact that the useful energy is not wasted.

The Case 2 allows a saving of 8 W by reducing the threshold level by little more than 1 °C.

Electromechanically controlled refrigerators exhibit a temperature ripple in the range of 10 to

20 °C. A decrease to a few degrees Celsius, will allow a saving of up to 20% of energy

consumption. This means a 20 W saving for a 150 W compressor (100 W average power

with a 2/3 duty cycle).

Of course, reducing the temperature ripple brings one drawback. This is that the motor

running cycle frequency increases. In our example, this frequency increases by around 20%

(2.06 cycles per hour with Case 1, and 2.46 cycles per hour with Case 2). This is not a

problem for electronic switches where the cycling capability is 10 times or more that of

electromechanical switches. From the motor point of view, the higher number of cycles

should not reduce its reliability as:



Motor temperature ripple is decreased thanks to a higher cycle frequency.



The start winding conduction length is reduced thanks to an electronic control instead

of a thermal-active solution (PTC).



An over current protection, which reduces motor stress, is provided by the MCU.

Table 1. Power consumption versus hysteresis threshold

Case 1

(hyst. thresh. = 5.3 °C)

Case 2

(hyst. thresh. = 4.2 °C)

Measure

Compressor on time

12' 50"

9' 10"

Compressor off time

16' 20"

15' 10"

Average power during on time

136 W

138 W

Calculation

Cycle period

29' 10"

24' 20"

Duty cycle

0.44

0.38

Average power consumption

60 W

52 W