The following are the main parts of electrical system of a domestic refrigerator:
1. Compressor motor
2. Starting relay
3. Thermostat
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4. Thermal overload protector or release.
A brief description of each part, along with its function, is as below:
1. Compressor Motor:
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It may be a DC motor. Since we have mostly AC supply so AC motor is used which work on 220 volts 50 Hz supply. These are usually fractional horse power (F.H.P.) motors of about 1/8 H.P. The Induction type motors commonly used are single phase split phase type and they are not rated for continuous rating but for intermittent rating as the refrigerator motor works for some-time and then it stops and remains stopped for some time again.
The size of the motor depends upon the capacity of the refrigerator.
One “Ton” capacity of the refrigeration requires approximately 1 H.P. motor. “Ton” is the unit of refrigerator. It is equivalent to the heat extraction rate of 50 Kcal/minute. Domestic refrigerators are named by their capacity like 90, 160, 200, 250 litres etc. and the motors used are fractional H.P. motors designed for intermittent rating and not for continuous rating.
Working:
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As shown in figure 6.6 in the beginning both starting and running windings are in parallel. The high resistance winding of a split phase motor and a capacitor is connected to give the necessary phase shift in order to make the motor self-starting. As the motor pick up the speed, the starting really fens and cutout the starting winding. Thereafter, the motor continues to run only on running winding. Each time the motor starts, the sequence is repeated. Mostly the motor and compressor are in one unit, known as a sealed unit.
2. Starting Relay:
The relay is of electro-magnetic type and used for making or breaking the contact of starting winding with the supply. It essentially consists of two NC contacts in series with the starting winding. Relay coil is placed in series with running winding.
In the start both winding are in parallel with the supply and motor starts. As it picks up speed, the sufficient voltage is induced in starting winding and attracts the armature. This opens the N.C. contacts and starting winding cut-off. Now the motor will run only with running winding.
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Lamp and Switch Circuit:
The switch is fitted in the door of the cabinet. So long as the door of the refrigerator is closed, the switch remains open and the lamp remains off. As soon as the door is opened, the circuit is completed and the lamp is switched on. The user can trace and place the commodities in the refrigerator. The lamp is automatically switched off as the door is closed. This saves consumption of electrical energy.
3. Thermostat:
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It is a thermal switch. As soon as the temperature reaches – 7°C in the freezer and – 10°C in other parts of the fridge (domestic fridge), the thermostat automatically cuts off the supply to the machine. After some time, when the temperature rises in the refrigerator, it again receives the supply to the machine automatically.
The thermostat has setting from 0 to 10. But as precautions, it should not be set beyond 3 as it overloads the refrigerator, which ultimately reduces its efficiency and life. The two mechanisms, bimetal control and sensing bulb control, used in thermostats.
4. Thermal Overload Relay (OLR):
The thermal OLR is a protective device for the protection of the compressor motor unit of the refrigerator. It has bimetals and a heating resistance. As soon as the temperature of the compressor, due to any reason, rises beyond safe limits, the OLR automatically trips and disconnects the supply to the motor. It is connected in series with the motor circuit. Whenever excess current flows through the motor, due to any reason, the bimetals are heated. This interrupts the supply, thus motor is protected.
Body:
The whole body of the refrigerator is made of double walled cabinet which is packed with high thermal insulation material like fibre glass, cork expanded rubber etc. so that the heat from outside atmosphere does not leak into refrigerator. The cabinet is made air-tight by providing a soft rubber seal all around the inside of the door.