Using an old compressor from a refrigerator as a source of compressed air for aerography, inflating tires or operating pneumatic tools is a home classic craftsmanship. Compressor units, assembled with your own hands, often surpass store-bought analogues in reliability and quietness of operation, if the initial equipment is correctly selected. However, before assembling the receiver and connecting the hoses, it is necessary to solve the primary problem: to make the electric motor work outside the refrigerator body.
The main difficulty lies in the starting system. Refrigerator motors belong to the class of asynchronous motors with a starting winding, and without proper commutation they only hum without starting to rotate. Start-protective relay takes on the role of a dispatcher, briefly supplying current to the starting winding and breaking the circuit as the speed increases. Without this unit or its proper replacement, starting the motor is impossible and even dangerous for the windings.
In this article we will analyze in detail the physics of the process, methods for checking the health of the windings and the algorithm for connecting different types of relays. You will learn to identify contacts with a multimeter and safely apply voltage to the motor, turning the old unit into a powerful tool for the workshop. The starting winding is connected to the network only at the time of start-up (1-3 seconds), after which the current through it must be completely stopped.
The operating principle and design of the electric motor
For a successful start, you need to understand that hidden inside the case is not just hardware, but a complex electromechanical system. The refrigerator motor has two main stator windings: working and starting. Working winding occupies most of the stator slots and is designed to create the main magnetic field during operation. It is designed for long-term connection to a 220 volt network.
The second element is the starting winding. It is made of thinner wire and has greater resistance. Its task is to create an initial torque, moving the rotor from its place. Once the engine reaches approximately 75% of rated speed, it is no longer necessary. If you do not turn off the starting winding, it will quickly overheat and burn out due to high current.
The separation of these functions ensures starting relay. Depending on the design, it can work on the electromagnetic principle or use a bimetallic strip. Understanding this difference is critical when selecting a replacement, since relays from different series of compressors (for example, Atlant, LG or Danfoss) are not always interchangeable.
- 🔌 The operating winding has low resistance and is connected constantly.
- ⚡ The starting winding has a high resistance and turns on for a short time.
- 🔄 The rotor begins to rotate only when both fields work together.
⚠️ Attention: An attempt to start the engine by directly applying voltage only to the working winding without a starting impulse will lead to a strong humming and overheating, but rotation will not occur.
Diagnostics and checking the windings with a multimeter
Before looking for a connection diagram, make sure that the motor itself is working. Often the cause of failure is not the relay, but an interturn short circuit or an open circuit. For diagnostics, you will need a multimeter switched to resistance measurement mode (Ohms). First, find the three terminals on the compressor housing, usually they are located in a triangle.
Ring the pairs of contacts. Between two of them the resistance will be minimal - these are the terminals of the working winding. Between the other pair the resistance will be higher - this is the starting winding. The sum of these two values should be equal to the resistance between the remaining two contacts (common and furthest). If the multimeter shows one (infinity), there is a break; if zero, there is a short circuit.
It is also critically important to check breakdown to the housing. Press one multimeter probe to any of the three contacts, and the second to the copper tube or motor housing, which has been stripped of paint. The device should show infinity. Any resistance readings indicate that the motor is faulty and its operation is fatal.
☑️ Checking the serviceability of the motor
Do not ignore visual inspection. If you see traces of soot around the terminals or smell a burning smell, further diagnosis may be pointless. In such cases, the engine is most often subject to disposal or complex rewinding, which is impractical at home.
Types of start-up protection relays and their markings
The success of the operation depends on the correct choice of relay. There are two main types of devices that are visually and structurally different. The first type is induction (electromagnetic) relays. They have a vertical installation and a heavy moving core. When starting, the current is high, the core rises and closes the contacts of the starting winding. When the speed increases, the current drops, the core drops and opens the circuit.
The second type is thermal (bimetallic) relays, often called “pills”. They work by heating the plate with current. At startup, the plate is cold and the contacts are closed. The current heats the plate, it bends and opens the circuit. Such relays are often combined with overheating protection. Markings on the case, for example RTK-1, R-3 or LS-07B, indicate the series and compatibility with specific models of compressors.
When selecting an analogue, pay attention not only to external similarity, but also on the diameter of the electric motor shaft and the type of fastening. An incorrectly selected relay may not open the contacts, which will lead to burnout of the starting winding in a matter of seconds.
PTC Start Relay
| Relay type | Principle of operation | Position during operation | Marking example |
|---|---|---|---|
| Induction | Electromagnetic | Vertical | RT, RTP, RTK |
| Thermal | Bimetallic plate | Any | P-3, LS, 6MR |
| Posistor | Crystal heating | Any |
What is a posistor?
The posistor relay is a modern alternative to mechanical devices. Inside there is a ceramic element, which, when heated by current, sharply increases its resistance, actually breaking the starting winding circuit. They are more reliable, but require time to cool down (3-5 minutes) before restarting.>
Scheme for connecting the compressor to the network
After determining the type of relay and checking the windings, we proceed to assembling the circuit. You will need a length of cable with a plug, a terminal block for reliable connections, and possibly a capacitor if the engine is difficult to start. The connection is made strictly according to the color coding of the wires coming out of the relay and going to the compressor contacts.
Usually three wires (or terminals) come out of the relay, which are put on the pins of the compressor. One wire goes directly to the working winding, the second to the starting winding through the relay contacts, and the third is common. It is important to ensure tight contact: oxidized or loose terminals will cause sparking and heating. For permanent installation, it is recommended to use a separate button or toggle switch with a current rating of at least 16 Amps.
If the engine hums but does not start, or starts only with the help of extraneous cranking of the shaft, starting capacitormay be required. It is connected in parallel with the starting winding and adds the necessary phase shift. The capacitance is selected experimentally, starting from 4-5 uF for every 100 W of motor power.
When assembling the circuit, be careful with insulation. All twists must be soldered or securely crimped, and the joints must be insulated with heat shrink. Do not leave live parts exposed, especially if the installation will be used in a garage or workshop where humidity is possible.
First start-up algorithm and testing
The most crucial moment comes. Double-check all connections before plugging into the outlet. Make sure that the compressor is standing on a flat surface, preferably on a vibration-isolating substrate (rubber or thick felt mat). It is better to make the first start away from your ears, as a faulty motor can make a loud bang.
Apply voltage. You should hear the relay click (if it is mechanical) 1-3 seconds after the motor starts humming. The engine should return to operating mode smoothly, without strong vibration or extraneous clanging. If you only hear a hum and buzz, and after 5-10 seconds the machine goes out or the protection is triggered, immediately turn off the power.
Check the heating. After 5-10 minutes of operation, the compressor housing will begin to warm up - this is normal. However, if it heats up to the point where it is impossible to hold your hand, it means that the wrong relay has been selected or there are problems with the windings. Also check the outlet pressure: it should rise confidently, creating a noticeable stream of air.
⚠️ Attention: The continuous operation time of a household refrigeration compressor is limited. In compressor mode, it can work longer than in a refrigerator, but still requires breaks for cooling, otherwise the oil will lose its properties.
Frequent problems and methods for solving them
During the operation of a homemade unit, you may encounter a number of typical problems. One of the most common is that the compressor hums, but does not start. This often indicates a jammed piston group or a faulty start relay. Sometimes a short-term supply of voltage to the starting winding through a button (the “swinging” method) helps, but this requires skills.
Another problem is constant operation without shutting down or, conversely, instant shutdown. This indicates that the starting or operating current does not correspond to the relay parameters. In this case, you need to look for a relay with a different marking code that matches the power of your engine. Conformity tables can be easily found by markings on the body.
If the motor is running too loudly or with a clanging sound, check the mounting bolts and the condition of the internal shock absorbers (if they are present). Sometimes it helps to install a larger receiver, which smoothes out pressure pulsations and reduces the load on the engine, making it run quieter and smoother.
Is it possible to start a compressor without a relay?
Technically, you can start the engine by manually applying voltage to the starting winding through a button at the moment of switching on. However, you cannot constantly operate the compressor in this mode: you risk not having time to turn off the starting winding, which will lead to its combustion. A relay is necessary to automate this process.
What kind of oil should you pour into the air compressor?
If you are converting a refrigerator to use a compressor, you usually do not need to change the oil, unless the old one has leaked out. No moisture should enter the system. For air compressors, special synthetic oils that do not form soot are sometimes recommended, but for garage needs they often leave standard refrigeration oil.
Why does the compressor get very hot?
Heating up to 70-90 degrees for a piston compressor is the norm. If it gets hotter, check the pressure (is it too high?), the condition of the valves and the cleanliness of the cooling fins. Also, the reason may be low voltage in the network or a malfunction of the windings.