A modern refrigerator is a complex electromechanical device, where each element plays a critical role in maintaining a stable temperature. When a compressor stops starting or hums but won't cool, the first suspect on the list is the small black box attached to the motor housing. Many owners of household appliances mistakenly believe that this is just a connecting unit, not suspecting that it is this component that ensures the engine starts.
The starting relay functions as an automatic switch, which for a split second supplies increased voltage to the starting winding of the compressor motor. Without this impulse, the motor rotor simply would not be able to move from a dead point due to the high freon pressure in the system. Understanding the operating principle of this unit will help you diagnose the problem yourself and avoid an expensive call to a technician.
In this article we will analyze in detail the design of various types of relays, consider the symptoms of their failure and provide a step-by-step replacement algorithm. You will learn how to distinguish a relay malfunction from a breakdown of the compressor itself and what precautions must be taken when working with electrical components of refrigeration equipment.
The principle of operation of the compressor starting mechanism
The electric motor of the refrigeration unit is of the asynchronous type and requires the creation of a rotating magnetic field to start. At the moment of switching on, there is only one working winding in the network, creating a pulsating field that is not capable of starting the rotor. This is where the starting relaycomes into action, which temporarily connects the second, starting winding, shifting the phase of the current and creating the necessary torque.
After the rotor reaches about 75% of the rated rotation speed, the relay should instantly turn off the starting winding. If this does not happen, the current in the winding will increase to critical values, which will lead to overheating and combustion of the motor. In older models, this function was performed by electromechanical devices, while modern ones often use electronic components. Indesit refrigerators Warning: An attempt to start the compressor directly, bypassing the relay, can lead to instant burning of windings or knocking out plugs in the electrical network. Never supply power to the engine without a working starting device. Indesit refrigerators or Bosch often use electronic components.
⚠️ Attention: An attempt to start the compressor directly, bypassing the relay, can lead to instantaneous burnout of the windings or knocking out plugs in the electrical network. Never energize the engine without a working starter.
The switching process occurs automatically and takes a fraction of a second. In induction models, this is achieved by heating a bimetallic plate, which opens the contacts. In posistor analogues, the role of the switch is taken over by a thermistor, which changes its resistance when heated. Accuracy of operation This mechanism directly affects the service life of the motor.
Types of starting relays: induction and posistor
There are two main types of starting devices on the household appliances market, and they are not always interchangeable. An induction relay (often called a "bull" or "pot") has a coil inside and a movable core. When current is applied, the coil creates a magnetic field that draws in the core and closes the contacts of the starting winding. When the current drops, the core returns to its original position under the influence of gravity or a spring.
The posistor relay is a more modern and reliable solution. Inside the case there is a ceramic element - a posistor, which at room temperature has low resistance and passes current. When current passes, it heats up and its resistance increases sharply, essentially breaking the circuit. Such devices are often installed in LG refrigerators and Samsung.
- 🔹 Induction relays are position sensitive: they work only in a strictly vertical position, so compressors with them are usually suspended.
- 🔹 PTC models are universal and can work in any spatial position, which allows manufacturers to more flexibly arrange the insides of the unit.
- 🔹 The service life of posistors is longer, since they do not have mechanically moving parts subject to wear.
You can visually distinguish types by markings or the number of contacts. Induction ones often have one input and one output for the starting winding, while posistor ones can have a more complex terminal configuration. When purchasing a spare part, be sure to check the article number with the technical documentation of your refrigeration unit.
Symptoms of a faulty starting device
Fault diagnosis often begins with analyzing the sound signals that the equipment produces. If you hear a distinctive clicking sound, followed by a humming sound from the motor, and then another clicking sound and silence, this is a classic sign of a problem. The compressor tries to start, but without a successful start, the thermal protection relay is triggered, opening the circuit.
Another striking symptom is the lack of cold in the chambers when the fan is running (in No Frost systems). The motor may not respond at all when plugged in or may only produce a quiet hum. In some cases, a faulty relay can “stick”, leaving the starting winding on, which leads to strong heating of the motor housing and a burning smell.
⚠️ Attention: If after several attempts to start the compressor housing becomes hot, immediately unplug the device. Long-term operation in this mode will lead to an interturn short circuit.
Frequent power outages can cause electronic failure, especially in models with electronic launch control. However, mechanical relays suffer from this less often. The main enemies remain time, vibration and power surges, which destroy the internal structure of the contacts or ceramic element.
Removal and visual inspection of the part
Before proceeding with replacement, you need to gain access to the compressor, which is usually located in a niche at the bottom of the rear wall of the refrigerator. To do this, you will need to remove the protective cardboard or plastic panel by unscrewing the mounting screws or releasing the latches. Be sure to disconnect refrigerator from the mains before starting any work.
On the motor housing you will see a black box where the wires come. Carefully remove it by pulling down or releasing the latches. In some models, such as Atlant refrigerators, the relay can be additionally secured with a metal bracket. Take a photo of the wire connection diagram so as not to mix up the contacts during assembly.
During an external inspection, pay attention to the following signs:
- 🔸 Presence of traces of soot or melted plastic on the body.
- 🔸 Characteristic burning smell emanating from the part.
- 🔸 Cracks or chips on the ceramic element (if the relay is posistor).
- 🔸 Oxidation or blackening of the copper contacts inside.
If the relay is induction, shake it near your ear. If a ringing or knocking sound is heard inside, it means the mechanism is damaged and requires replacement. PTC resistors rarely make sounds when they break, but may have visible damage to the ceramic. Any defects in the housing indicate that the part has been overheated.
Is it possible to temporarily start a refrigerator without a relay?
Theoretically, you can start the motor by applying voltage directly to the starting winding through a button, but doing this is strictly not recommended without experience. You risk the winding or getting an electric shock, since the circuit will not have protection.
Checking the relay with a multimeter: step-by-step instructions
For an accurate diagnosis of the malfunction, a visual inspection is not enough; you need to “ring” the circuit with a tester. Switch the multimeter to resistance measurement mode (200 ohms). First, check the continuity of the circuit between the contacts responsible for the starting winding. In a cold state, the posistor should show low resistance (from 10 to 50 Ohms).
If the device shows one (open) or zero (short circuit), the part is faulty. For induction relays, the test consists of testing the coil and checking the mobility of the core. It is also important to check the thermal protection relay, which is often built into the same housing or located nearby. Its contacts in a cold state should ring short (0 Ohm).
The table below shows typical resistance values for serviceable elements:
| Element type | Normal resistance (Ohm) | Fault symptom | Action |
|---|---|---|---|
| Posistor (cold) | 10 - 60 Ohm | Infinity or 0 Ohm | Replacement |
| Induction coil | 1 - 15 Ohm | Open circuit | Replacement |
| Thermal relay | 0 Ohm (contacts closed) | Cold break | Replacement |
| Compressor (windings) | 10 - 40 Ohm | Short circuit to housing | Motor repair |
It may show normal resistance when cold, but when heated it will not open the circuit. In such cases, only replacing with a known good analog helps. Electrical check allows you to weed out obvious defects.
☑️ Relay diagnostics with a multimeter
Replacement and selection of analogues
If the diagnostics confirmed a malfunction, you need to purchase a new part. The ideal option is to find the original by the article number indicated on the compressor nameplate or in the instructions. However, the originals are often discontinued, and universal analogues have to be selected. The key parameters are the compressor power (in watts or horsepower) and the type of refrigerant.
The replacement process is simple: remove the old relay by disconnecting the wires (after photographing the circuit), and install the new one. Make sure the terminals are firmly seated on the compressor contacts. If the connectors are different, you can carefully cut them and connect them through the terminal block, observing the polarity and color of the wires.
⚠️ Attention: The power of the new relay must match or be slightly higher than the power of the old one. Installing a relay that is too weak will lead to its rapid combustion, and a relay that is too powerful will result in incomplete disconnection of the starting winding.
After installation, reassemble the structure in the reverse order, secure the protective panel and turn on the refrigerator. The first startup may take several minutes, as the system must enter operating mode. If the motor starts smoothly and after some time cold appears in the chambers, the repair can be considered successful.
Frequently asked questions and answers (FAQ)
Is it possible to repair the relay yourself by disassembling it?
Theoretically, an induction relay can be disassembled, cleaned of carbon deposits from the contacts and put back together. However, the seal of the case will be broken, which will lead to oxidation of the contacts in the future. PTC resistors cannot be repaired at all. Considering the low cost of a new part, repairing the old one is impractical.
Why does a new relay burn out immediately after installation?
This may indicate a more serious problem: a malfunction of the compressor itself (turn-to-turn short circuit), problems with the wiring, or power surges in the network. Also, the reason may be incorrect selection of relay power or poor contact in the terminals.
How often do you need to change the starting relay?
The service life of a high-quality relay is 10-15 years or more. However, frequent power surges can reduce this period to 5-7 years. Scheduled replacement is not required; change only when symptoms of a malfunction appear.
Does the type of freon affect the choice of relay?
Indirectly yes. Compressors for different freons (R134a, R600a, R12) have different power and starting currents. The relay is selected specifically for the characteristics of the motor, which, in turn, is designed for a specific refrigerant.