How a refrigerator compressor relay works: design and diagnostics

The compressor is the heart of any refrigeration unit, but its normal operation is impossible without a reliable starting and protection system. It is compressor relay responsible for supplying voltage to the motor windings and turning it off in case of overload. Understanding the operating principle of this small but critically important unit allows you to independently diagnose many faults without resorting to expensive service. In this article we will analyze in detail the internal structure, the physical basis of operation and methods for checking the serviceability of the starting protection mechanism.

Modern refrigerators, be they classic models Indesit or advanced Liebherr, use similar principles of electromagnetic or thermal control launch. However, despite the external similarity, their internal logic can differ significantly. If you notice that the refrigerator is humming but does not start, or, conversely, is working without interruption, the problem most likely lies in the start control circuit. Accurate diagnostics of the relay allows you to save up to 50% of the cost of repairs, since they often change a working compressor due to a faulty starter.

First, let's figure out why this component is so important. Without it, the engine will not be able to overcome the inertia of rest and switch to operating mode. Starting relay short-term supplies current to the starting winding, creating the necessary torque. After the rotor picks up speed, the current in the circuit drops and the relay opens the contacts, leaving only the main winding working. In parallel with this, it works protective relay, which monitors the temperature and current, breaking the circuit at dangerous values, preventing the windings from burning out.

The principle of operation of the starting and protective mechanism

The physical basis of the operation of a classic electromagnetic relay is the use of current. When the refrigerator is turned on, the motor rotor is stationary, and the current flowing through the main winding reaches its maximum value. This current surge creates a strong magnetic field in the relay coil, which draws in the core and closes the contacts of the starting winding. The motor begins to rotate.

As soon as the rotor reaches approximately 75% of the rated rotation speed, current consumption drops. The coil's magnetic field weakens and can no longer hold the core. Under the influence of gravity or a spring, the contacts open, turning off the starting winding. The refrigerator then operates normally. This process takes a fraction of a second and is repeated with each switching cycle.

Deserves special attention thermal protection relay, which is often structurally combined with the starting one. Inside it there is a bimetallic plate through which all the motor current passes. During normal operation it bends only slightly. However, if an overload or jam occurs, the current increases sharply, the plate heats up and bends more, mechanically opening the contacts and de-energizing the compressor. This prevents insulation melting and fire.

It is important to understand the difference between the types of devices, since their operation is based on different physical phenomena. Below is a comparative description of the main types of mechanisms.

Main types of relays: electromagnetic and posistor

There are two main types of starting devices on the household appliances market, and knowing their differences is critical when selecting spare parts. The first type is classic electromagnetic relays. They are reliable and durable, but have one significant drawback: position sensitivity. Such devices can often be found in old models of refrigerators Zil or Biryusa.

The second type is posistor (thermal) relays. They are based on a special ceramic element - a posistor, the resistance of which increases sharply when heated. When turned on, current flows freely through the starting winding, heating the crystal. After a few seconds, the crystal becomes hot, its resistance becomes enormous, and the current actually stops. The starting winding is turned off. Such relays are less sensitive to vibration and can operate in any position.

• 🔹 Electromagnetic models have a movable core and contacts that can burn over time.
• 🔹 PTC devices do not have mechanical moving parts, which increases their efficiency resource, but requires time to cool down before restarting.
• 🔹 Combined options combine a starting coil and a bimetallic plate to protect against overheating in one housing.

⚠️ Attention: When replacing a relay, make sure that the new device is designed for the same engine power as the original. Installing a relay from a less powerful compressor will lead to its instant combustion, and from a more powerful one it will make it impossible to start.

The choice of relay type depends on the design of a particular compressor. Compressor manufacturers, such as Secop (formerly Danfossor Aspera, often equip their engines with specific starters. Using analogues is possible, but requires checking the electrical characteristics and connection diagram.

Fault symptoms and initial diagnostics

You can determine that the compressor relay has failed by a number of characteristic signs. Most often, the user is faced with a situation when the refrigerator is turned on, a quiet hum or click is heard, but no cold is produced. After a while there is a second click and then silence. This is a classic picture of operation thermal protection, which is triggered due to the inability to start the engine.

Another symptom is continuous operation of the compressor without shutting down. In this case, the relay is “stuck” in the “on” position and does not open the contacts of the starting winding. This is extremely dangerous, since the starting winding is not designed for long-term operation and can quickly burn out, which will lead to expensive repairs of the entire unit. There may also be a smell of burning or melted plastic in the compressor area.

Sophisticated tools are not always required for initial diagnosis. It is enough to listen carefully to the operation of the unit and visually inspect the unit.

• 🔸 The compressor hums for 5-10 seconds, then clicks and goes silent - a sign of a malfunction of the starting group or jamming.
• 🔸 A crackling or sparking is heard inside the relay housing - the contacts are burnt.
• 🔸 The relay housing has traces of soot, melting, or emits a smell of burnt plastic.

If you observe such symptoms, further operation of the refrigerator is not recommended. Constant attempts to start can permanently damage the motor windings. It is necessary to disconnect the device from the network and carry out a more in-depth check.

Instructions for checking a relay with a multimeter

For accurate diagnostics, you will need a multimeter. Before starting any work, be sure to disconnect the refrigerator from the power supply! Remove the back cover at the bottom of the refrigerator to access the compressor. The relay is usually placed on the side pins of the compressor and covered with a plastic casing.

Carefully remove the relay from the compressor pins. Inspect it for external damage. If the body is intact, you can start “dialing”. Switch the multimeter to resistance (ohms) measurement mode. For electromagnetic relays, check the coil resistance - it should be within the normal range (usually 30-60 Ohms, exact values ​​depend on the model). Infinite resistance will indicate a break.

Next, check the contacts. In the initial position, the contacts of the starting winding must be open (resistance is infinite). If you shake the relay and hear a knock, and the contacts ring, the mechanism is destroyed. PTC relays are characterized by low resistance in the cold state (20-50 Ohms), which should go to infinity when heated.

Separately, it is worth checking the thermal relay. Ring its contacts - they should close. If the multimeter shows an open circuit, it means that the protection has tripped and has not returned to its original state, or the device is faulty. Let it cool for 15-20 minutes and check again. If the break persists, the relay must be replaced.

⚠️ Attention: Do not try to repair a burnt out relay by stripping the contacts or gluing the case. This device operates in conditions of high currents and temperatures, and any makeshift repair will lead to a fire hazard.

Replacement process and installation rules

Replacing a relay is a simple procedure, but requires care. The main rule: the new relay must be a complete analogue of the old one or have compatible characteristics. There is always a marking on the device body. Pay attention to the model of the compressor for which it is intended and the type of refrigerant (although this is not critical for the relay itself, the matching power is important).

Remove the old relay, remembering or photographing the wiring diagram. There are usually three of them: general, starting and working. An error in the connection can result in the compressor humming but not starting, or the winding burning out. Install the new relay onto the compressor pins as far as it will go. It should fit tightly and not wobble.

After installation, secure the casing and connect the refrigerator to the network. In the first minutes of work, listen to the nature of the launch. The compressor should turn on with a characteristic hum; after a second or two, the click of the starting winding turning off may not always be audible, but the operation should become smooth. If the launch was successful, after some time the temperature in the fridge compartment will begin to decrease.

It is important to consider that the design of refrigerators from different brands may differ. For example, in Atlant or Stinol models the relays are often located horizontally, while in some European models they are vertical. Always focus on the position of the old device.

Common mistakes when doing self-repair

Independent repair of equipment is always a risk, and when working with a compressor relay, beginners often make common mistakes. One of the most common is ignoring the relay type. An attempt to install a posistor relay instead of an electromagnetic one (or vice versa) without changing the connection diagram is doomed to failure, since the logic of their operation is fundamentally different.

Another mistake is neglecting to check the compressor itself. If a relay burns out, this does not always mean that it is to blame. Often the relay is a victim, and the reason lies in the interturn short circuit of the motor windings or mechanical jamming of the piston group. Installing a new relay on a faulty compressor will lead to instant combustion of the new spare part.

• 🔻 An attempt to "revive" the relay by tapping or heating without checking the electrical parameters.
• 🔻 Using a relay from a different type of compressor "just to fit the connector."
• 🔻 Ignoring the condition of the wiring: oxidized contacts can heat up and melt even a working relay.

It is also worth mentioning the cleanliness of the contacts. If you are replacing a relay, make sure the mating pins on the compressor are not oxidized. Oxides create additional resistance, cause heating and can cause repeated breakdowns.

Prevention and service life of the device

The compressor relay is a part that does not require scheduled replacement if the refrigerator is operating under normal conditions. However, the service life of this element directly depends on the stability of the voltage in the network and the frequency of on-off cycles. Voltage surges are the main enemy of the electronics and starting equipment of the refrigerator.

To extend the life of the relay and compressor, it is recommended to ensure good ventilation of the rear wall of the refrigerator. Overheating of the compressor area causes the thermal relay to operate more often, which accelerates its wear. Also, you should not frequently open the refrigerator door, causing the unit to turn on too often.

The average service life of a high-quality relay is 5-10 years, but under favorable conditions they last longer. If your refrigerator is older than 10-15 years, it makes sense to keep a spare relay on hand, since finding an original for an old model can be difficult.