Using a compressor from a manufacturer building a refrigerator is a great idea for reusing equipment that has long been collecting dust in the garage. Refrigeration unit is a powerful and reliable pump capable of creating high pressure, which makes it ideal for creating a homemade compressor, paint gun or even an airbrush. Many craftsmen underestimate the potential of these devices, sending them to a landfill, whereas with the right approach they can serve for many years.
However, before you start assembling, you need to clearly understand the device and principle of operation single-phase electric motorhidden inside the metal casing. Incorrect connection of windings or ignoring safety rules can lead not only to equipment failure, but also to electric shock. In this article, we will look in detail at how to safely remove, test and start compressor unit, turning old junk into a useful tool.
The startup process requires attention to detail, especially in matters electricians. Start relay, which often remains on the housing, plays a critical role in starting the engine, and its condition must be assessed first. If you have never encountered electrical circuits of household appliances, be prepared for the fact that a basic understanding of the operation of a multimeter and skills in handling wiring will be required.
Assessing the condition and preparing the equipment
The first step should always be a visual and technical diagnosis of the removed unit. Sealed casing the compressor should not have through damage, dents or traces of corrosion that violate the integrity of the metal. If you notice oil smudges at the welding site or around the tubes, this may indicate depressurization of the system, which makes further use of the device impossible without complex vacuumization and refilling.
Particular attention should be paid to the electrical part. On the case there is usually a terminal block covered with a plastic cover. Removing it, you will see contacts going to the motor windings, and possibly attached to the side start relay. It is important to inspect the wires for melted insulation or breaks. If the wiring appears blackened or brittle, it must be replaced before any testing.
⚠️ Attention: Before starting any work, make sure the power cord is unplugged from the outlet. Residual voltage in capacitors (if they are present in the circuit) can be dangerous, so let the equipment stand unplugged for a while.
To carry out diagnostics, you will need a minimum set of tools. Without multimeter it will not be possible to check the integrity of the windings, so its presence is mandatory. Also prepare a set of screwdrivers, pliers and electrical tape. If the compressor has not been turned on for a long time, its internal mechanisms may be sour, so it is better to carry out the initial start-up in a well-ventilatedindoors or in the open air, since a burning smell or old oil may come out of the outlet tubes. data-i="69">Connection diagram and operation of the starting relay
Connection diagram and operation of the starting relay
The heart of the starting system is the starting relay, which automatically turns off the starting winding after the engine reaches the required speed. In older models of refrigerators, relays with a posistor or induction options are most often found. Connection diagram usually applied to the body of the relay itself or on a sticker next to the electrical block of the compressor. If the markings are erased, you will have to determine the contacts empirically or by resistance.
A standard single-phase motor has two main windings: working and starting. The working winding has less resistance and is constantly connected to the network, ensuring shaft rotation. The starting winding is turned on only at the moment of start and has a higher resistance. Starting relay in this case it acts as a switch that opens the starting winding circuit when the current in the working winding drops to a certain value, indicating that it has entered the operating mode.
Technical details of the operation relay
Inside the induction relay there is a coil connected in series with the working winding. When starting, the current is high, and the magnetic field of the coil draws in the core, closing the contacts of the starting winding. When the engine accelerates, the current drops, the core releases the contacts, and the starting winding is turned off.
Incorrect orientation of the relay when connecting can lead to its breakdown or the inability to start the engine. Many relays have an arrow indicating the "UP" or "Up" direction. If you install the relay upside down, gravity will not allow the contacts to open correctly, and the engine will hum, but will not start, which will quickly lead to burnout of the windings.
In some modern circuits, an electronic control unit can be used instead of a mechanical relay, but in the context of old refrigerators we are considering classic mechanical or posistor options. PTC relays are easier to operate: they have no moving parts, and the role of a circuit breaker is performed by a heating element that changes its resistance. Such devices are less sensitive to position in space, but more sensitive to voltage surges.
Diagnostics of windings with a multimeter
Before applying 220 volts, it is critical to check the integrity of the motor windings using a multimeter. This action will avoid a short circuit and guarantee that you are alive electric motor. Switch the device to resistance measurement mode (Ohm) and ring all three contacts on the compressor block one by one.
Normal resistance readings for a working compressor are usually in the range from 10 to 30 Ohms for the working winding and from 30 to 90 Ohms for the starting winding. The resistance between the case and any of the contacts should be infinite (one on the multimeter screen). If the device shows zero or very low resistance between the contact and the body, this means breakdown to the body, and operating such a unit is deadly.
☑️ Check with a multimeter
It is also worth checking the resistance between the contacts themselves. The sum of the resistances between contacts 1-2 and 2-3 should be approximately equal to the resistance between contacts 1-3 (where 1 and 3 are the outer contacts, and 2 is the common one, although the numbering may differ). If the multimeter shows a break (infinity) on any of the windings, the motor is faulty and requires rewinding or replacement.
| Measurement type | Normal value | Fault |
|---|---|---|
| Working winding | 10 - 30 Ohm | Open or short circuit |
| Start winding | 30 - 90 Ohm | Open or short circuit |
| Isolation (body) | Infinity (∞) | Breakdown to body |
| Thermal protection | 0 Ohm (closed) | Gap circuit |
The built-in thermal protection deserves special attention. It should also ring as a short circuit (0 ohms) if the engine is cold. If the thermal protection shows an open circuit, it may be overheated or faulty. Allow the compressor to cool for an hour and measure again. If the break persists, the protection relay will have to be replaced or excluded from the circuit (which is not recommended).
First start-up process and testing
After successful diagnostics, you can proceed to the most crucial moment - the first start. For safety, assemble a temporary wiring diagram using a terminal block or secure clamps, and not just twisting the wires. Start relay must be installed strictly in accordance with the markings (arrow up). Connect the power cord to the input contacts of the relay and the working winding.
At the moment of switching on, the compressor will emit a characteristic hum, which after 1-3 seconds should be replaced by the even hum of a running engine. This means that the starting winding has been activated and the engine has switched to operating mode. If you only hear a humming sound and clicking noise, but no rotation, turn off the power immediately. This may indicate a jammed mechanism or a faulty start relay.
During operation, pay attention to how quickly the housing heats up. In the first minutes of work metal casing it may warm up slightly, but it should not become hot to the point where it is impossible to touch. Also check the air flow from the outlet tube: it should be strong and pulsating. If the air flows poorly and the engine runs quietly, the tightness of the valve group inside may be broken.
⚠️ Attention: Never leave a running compressor unattended in the first 10-15 minutes. Monitor the temperature and sounds. The appearance of sparks, smoke or the smell of burnt insulation requires immediate disconnection from the network.
If the startup was successful, let the unit run for 5-10 minutes to ensure stable operation. Vibration during operation is normal, but should not be excessive. If the compressor “jumps” on the floor, it must be securely mounted on a wooden platform or frame using rubber dampers.
Typical problems and methods for eliminating them
Even a seemingly good compressor may encounter problems when starting. One of the most common reasons is jamming of the piston group after a long period of inactivity. Oil leaked into the crankcase and parts became rusty or stuck together. In such cases, the “boost” method sometimes helps: briefly applying voltage to the starting winding, bypassing the relay, to create a powerful rotation pulse. However, this method requires high qualifications and is dangerous.
Another common problem is a malfunction of the starting relay itself. If the contacts inside the relay are burnt or oxidized, no current will flow to the starting winding, and the engine will only hum. Replacing the relay with one with similar parameters (indicated on the housing) usually solves the problem. It is important to select a relay with exactly the same operating current characteristics.
Overheating can also be caused by insufficient voltage in the network. If the voltage drops below 190-200 volts, the engine's starting torque is reduced and it cannot gain speed, remaining in the starting winding mode, which leads to rapid combustion. In such cases, it is necessary to use voltage stabilizer or a step-up transformer.
Noise and vibration may indicate wear of the bearings or imbalance of the shaft. Unlike open engines, hermetic compressors do not have bearings that are lubricated or replaced. If the noise becomes unbearable, it is easier to replace the unit than to try to repair it, since opening the casing breaks the seal and requires evacuation of the system.
Safety measures and operating rules
Operation of home-made devices based on refrigeration compressors requires strict adherence to electrical safety rules. Grounding compressor housings is a mandatory requirement, since an insulation breakdown can lead to the appearance of a phase on the metal casing. Use sockets with a grounding contact and make sure that the wiring in the room matches the load.
The operating temperature is also important. Refrigerator compressors are not designed to operate continuously 24/7. They are designed for cyclic mode (works for 15 minutes, rests for 45 minutes). When converting to a compressor for a garage, it is necessary to organize a cooling system for the casing, for example, by installing a fan, or limit the time of continuous operation with a timer.
When working with high pressures (if you are assembling a receiver), remember that old tubes and connections may not withstand pressure above 6-8 atmospheres. Do not attempt to pump pressure to the limits specified for industrial compressors. Sealed casing is, and its destruction under excess pressure may be explosive.
⚠️ Attention: Technical characteristics and safety requirements may vary depending on the model and year of manufacture of the compressor. Always check the technical documentation of the specific model or passport data, if they are available.
Regularly check the condition of the electrical connections. Vibration during operation tends to loosen screws and connections over time. Loose contact means sparking, heating and potential fire. Once a month, carry out a preventive inspection and tighten the terminals.
Frequently asked questions (FAQ)
Is it possible to start a compressor without a starting relay?
Theoretically, you can start the engine by manually applying voltage to the starting winding at the moment of switching on, but this is extremely inconvenient and dangerous. Without a relay, the starting winding will burn out in a few seconds, since it is not designed for long-term operation. The use of a relay or its electronic analogue is mandatory.
What kind of oil is inside the compressor and does it need to be changed?
A special refrigeration oil is used inside (mineral or synthetic, for example, Freon-12 or Freon-134 compatible). When converting to an air compressor, the oil does not need to be changed if the compressor is working properly. However, if you have opened the system, the oil must be replaced with one of similar viscosity, but intended for air compressors, since refrigeration oil can be hygroscopic.
Why does the compressor hum, but not does it start?
Most likely, the starting relay is faulty (the contacts are burnt or the core is jammed) or a breakdown of the starting winding has occurred. Also, the cause may be jamming of the piston group due to lack of oil or long-term parking.
What is the maximum pressure that an old compressor can produce?
Domestic refrigeration compressors are usually capable of creating pressure up to 10-15 atmospheres (bar), but their performance (volume of pumped air) will be low. For serious work, a receiver is required to accumulate air.