Using old refrigeration equipment to create homemade devices is a popular practice among home craftsmen. Most often, craftsmen collect from leftovers compressor units for painting, inflating tires or aerating aquariums. However, in order for the device to work, it is necessary to correctly connect the electrical circuit, which necessarily includes a starting relay.
It is this small unit that is responsible for starting the engine and its safe operation. Incorrect connection can lead not only to equipment failure, but also to a short circuit. In this article we will look at how to identify contacts, check the serviceability of parts and assemble a working circuit with your own hands.
Purpose and design of the starting relay
The starting relay is a key element in the refrigerator motor circuit. Its main task is to temporarily connect the starting winding stator at the moment the motor starts. As soon as the rotor reaches the required speed, the relay must break the starting winding circuit, leaving only the working winding active.
There are two main types of such devices: induction (electromagnetic) and posistor (thermal). Induction models work due to the magnetic field created by the current in the working winding, which draws in the core and closes the contacts. The posistor ones use the property of the semiconductor element to heat up and increase resistance, actually opening the circuit after warming up.
⚠️ Attention: Using a relay of the wrong type can lead to burnout of the compressor windings or failure of the starting device itself. Always check the markings.
Inside the case there are contacts, a moving part and, in some models, thermal protection. Understanding the principle of operation is necessary for the correct orientation of the device during installation, since many relays are sensitive to position in space.
Identification of contacts and marking of leads
The first stage of assembly is the correct identification of the leads on the compressor itself. They are usually arranged in a triangular pattern and covered with a plastic cover. On modern models, there is often color marking of the wires coming from the relay, which greatly simplifies the task.
The standard color scheme is as follows: the red wire corresponds to the working winding, white or yellow is the starting winding, and black or green is common. However, you cannot rely only on the color of the insulation of old wires, since they could have been replaced over the years of operation.
For an accurate determination, you must use a multimeter in testing mode. By measuring the resistance between pairs of contacts, you will find a common point (where the resistance is average) and pairs of windings. The working winding has less resistance than the starting winding.
- 🔌 Common contact (C): usually located separately or has the greatest distance to other terminals in some circuits.
- ⚡ Starting winding (S): shows the highest resistance when wired relative to the common wire.
- 🔄 Working winding (R): has less resistance compared to the starting winding.
After identifying the pins, it is recommended to mark them with a marker so as not to get confused during the process of soldering or twisting the wires. This is a critical stage for further safety operation.
Scheme for connecting the compressor to the network
Assembling an electrical circuit requires care and adherence to the sequence of actions. First, a block with an installed relay is put on the compressor terminals.
Wires from the power plug are connected to the relay contacts. One wire (phase) goes through the switch to the input contact of the relay, and the second (zero) goes directly to the common contact of the compressor or the corresponding input in the block. For reliable connection, it is better to use terminal clamps or high-quality soldering.
Below is a table of contact correspondence for a standard connection diagram:
| Circuit element | Designation on the diagram | Wire color (often) | Function |
|---|---|---|---|
| 220V mains (Phase) | L | Brown/Red | Power supply |
| 220V mains (Zero) | N | Blue/Black | Circuit closure |
| Working winding | R | Red/White | Main work |
| Starting winding | S | Yellow/Green | Starting the rotor |
After connecting all the elements, it is necessary to securely insulate the twists and secure the wires to prevent vibration and breakage when the motor is running.
☑️ Check before the first start
Diagnostics and checking the serviceability of the relay
Before finally assembling the structure, you need to make sure that the starting device is working properly. A visual inspection may reveal melting of the case or blackening of the contacts, which indicates overheating. If there are no external defects, an instrumental check is required.
To check the posistor relay, you can simply shake it near your ear. If a ringing or knocking sound is heard inside, the ceramic element has cracked and the relay must be replaced. Induction relays are checked by testing the contacts: in normal condition, the circuit should be open, and when voltage is applied (which is difficult to do without a stand), it should be closed.
It is also worth checking the thermal protection relay, if it is built into the system. It must pass current freely when cold. The resistance of a working thermal relay is close to zero.
⚠️ Attention: When testing a relay under voltage, be extremely careful. 220V voltage is dangerous to life. Use insulated probes.
If the relay is faulty, it can be replaced with an analogue one. It is important to select a device with suitable starting current characteristics and motor shaft diameter. Universal relays often come complete with adapters for different types of compressors.
Is it possible to start a compressor without a relay?
Theoretically, you can start the motor by briefly applying voltage to the starting winding manually, but this is extremely dangerous and is not recommended. Without a relay, the engine will not be able to switch to operating mode on its own and will burn out in a few seconds. For continuous operation of the relay it is necessary.
Safety measures during assembly and operation
Working with electricity and high pressure requires strict adherence to safety regulations. The refrigerator compressor is capable of creating pressure up to 10-15 atmospheres, and the mains voltage is deadly. Before starting work, make sure that the power cord is unplugged from the outlet.
The compressor housing gets very hot during operation, so it is necessary to provide a cooling system or ventilation. It is not recommended to cover the operating unit or place it in closed niches without air access. You should also use a thermal relay for emergency shutdown in case of overheating.
To protect the electrical part, be sure to install a circuit breaker or at least a fuse at the input. This will save the wiring in case of a short circuit inside the motor windings.
- 🛡️ Use dielectric gloves during initial live testing.
- 🔥 Install the compressor on a non-flammable substrate (metal, asbestos, ceramics).
- 💨 Provide fresh air to cool the motor housing.
If you use a compressor to inflate tires or aerate, be sure to install a moisture trap and a pressure reducer at the outlet. The oil contained in the system should not get into the air that you breathe or that is supplied to the tools.
Common errors and ways to eliminate them
Beginners often make typical mistakes that lead to system inoperability. The most common one is incorrect relay orientation. If you turn the device over, gravity will not allow the contacts to open correctly, and the motor will hum, but will not start.
Another mistake is ignoring the condition of the capacitor. In some circuits, a capacitor is connected in parallel with the starting winding. If it is swollen or has lost capacity, starting will be difficult, which will cause overload and thermal protection thermal protection.
⚠️ Attention: The hum of the compressor without starting indicates a jammed rotor or a malfunction of the starting group. Do not keep the device turned on in this mode for more than 5-10 seconds.
Grounding is also often forgotten. The metal housing of the compressor must be grounded, especially if the device is used in damp areas or outdoors. This will prevent electric shock if the insulation on the housing is broken down.
Adaptation for various tasks
After successfully connecting the compressor, it can be used for various purposes. If you are assembling an airbrush installation, you will need a receiver (air tank) and a reducer for fine-tuning the pressure. To inflate tires, a simple hose and a check valve are enough.
When used in vacuum systems (for example, for vacuum packaging of products), it is necessary to change the direction of the airflow by connecting the hose to the inlet pipe. However, it is worth remembering that the evacuation performance of refrigeration compressors is lower than that of specialized pumps.
It is important to consider that refrigeration compressors are not designed for long-term operation without interruptions. In normal refrigerator mode, they work cyclically: they work for 10-15 minutes, then rest for 20-30 minutes. With constant operation, the resource is significantly reduced.
Conclusion and final recommendations
Connecting a compressor from a refrigerator with a relay is a task accessible to anyone with basic electrical skills. The main thing is to clearly identify the contacts, correctly install the starting device and not neglect safety measures.
A self-assembled installation can become a reliable assistant in a garage or workshop. With proper operation and regular maintenance (oil change, filter cleaning), it will last for many years.
Is it possible to use a relay from another refrigerator?
Yes, you can if the electrical characteristics (starting current, motor power) and mounting dimensions match. It is best to look for relays with similar markings or use manufacturers' compatibility tables.
Why does the compressor hum but does not start?
Most likely, the starting relay is faulty (the contacts are stuck or the posistor is burned out) or the piston mechanism of the compressor itself is jammed. Also, the reason may be low voltage in the network.
Is it necessary to change the oil in the compressor before use?
If the compressor is taken from an old working refrigerator, it is not necessary to change the oil if it is clear. If the unit has been standing for years or the oil has turned black/emulsified, it is better to replace it with a special compressor.
What pressure can a household compressor create?
Ordinary household compressors can create a pressure of up to 10-15 atmospheres (bar), but for safe operation it is recommended to limit the pressure to 6-8 atmospheres, using gearbox.