An engine from an old refrigerator is a powerful and reliable unit that often gets a second life in the hands of home craftsmen. It is used to create homemade compressors, airbrushes, vacuum systems and even pneumatic tools. However, unlike a conventional electric motor, a refrigeration compressor requires a specific starting circuit, the key element of which is the starting relay.
Simply applying 220 volts to the motor contacts is not enough: without proper switching of the windings, the rotor will not move, and the winding will quickly burn out. In this article, we will look at how to correctly identify the contacts, check the serviceability of the relay, and safely assemble a working wiring diagram for your projects.
Design and principle of operation of a refrigeration compressor
A hermetically sealed compressor is an electric motor immersed in oil and located in a steel casing. There is a piston mechanism located on the rotor shaft, which creates pressure. To start such an engine, a two-phase system is used: the main working winding and the auxiliary starting winding.
At the moment of switching on, voltage is applied to both windings, creating the necessary torque. Once the motor reaches approximately 75% of rated speed, the starting winding should be disconnected. This function is performed starting relaywhich opens the starting winding circuit under the influence of current or centrifugal force.
If the relay is faulty or incorrectly selected, the engine will hum but will not start, or the starting winding will burn out due to overheating. Modern models are often equipped with a thermal protection relay, which breaks the circuit when the body temperature rises critically.
⚠️ Attention: Inside the compressor there is freon under pressure and oil. It is strictly forbidden to open the sealed casing or cut through the tubes unless you plan to completely overhaul the unit with subsequent evacuation.
Identification of contacts and checking windings
The first stage of connection should always be diagnostics. Most compressors have a three-pin terminal block, but the markings may be faded or missing. To determine where the working and starting windings are located, you must use a multimeter in resistance measurement mode. common wire, working and starting windings, you must use a multimeter in resistance measurement mode.
Measure the resistance between each pair of contacts. The greatest resistance will be shown by the sum of the resistances of both windings (between the starting and working contacts). Smaller values will indicate the resistance of the individual windings relative to the common wire. Typically, the resistance of the starting winding is higher than the working winding.
- 🔌 Common contact (C) - has the lowest total resistance in relation to the other two terminals.
- ⚡ Working winding (R) - gives the average resistance value when testing with a common contact.
- 🚀 Starting winding (S) - shows the highest resistance when measured with a common contact.
It is also important to check that there is no breakdown on the case. Press one probe of the multimeter to any contact, and the second to the stripped metal part of the case (for example, to a fitting). If the device shows infinity or very high resistance, the insulation is intact. The appearance of even the slightest current indicates an interturn short circuit or breakdown.
⚠️ Attention: If during testing the multimeter shows zero or resistance close to zero between the windings, the motor is faulty and cannot be restored.
Selecting and checking the starting relay
The relay is the “brain” of the starting system. They are electromagnetic (induction) and thermal (posistor). Electromagnetic relays open the contacts when the current drops after the engine accelerates, and posistor relays simply increase their resistance when heated, essentially turning off the starting winding.
When selecting a relay for a specific engine, it is critical to take into account the power of the motor. If you place a relay from a more powerful refrigerator on a weak compressor, the contacts may not open and the starting winding will burn out. If the relay is too weak, it may not close the starting circuit at all.
How to distinguish a posistor relay from an electromagnetic one?
A posistor relay is usually made in a black plastic case and does not have moving contacts inside (they are not audible when shaking). An electromagnetic relay often has a metal or gray plastic case, and inside you can hear the knocking sound of a moving core when shaken.
Checking the relay is carried out visually and using a multimeter. For an electromagnetic relay in the operating position (coil down), the starting winding circuit must be open. When turning over (simulating an increase in speed and a drop in current) or applying current, the contacts should close. A posistor relay in a cold state has a low resistance (20-50 Ohms), and when heated, the resistance increases to thousands of Ohms.
The diagram for connecting a compressor to a 220V network
Assembling a working circuit requires attention to detail. Basic rule: phase and zero are supplied to the contacts of the relay and the working winding, and the starting winding is connected through the relay contacts. The circuit may differ slightly depending on the type of relay, but the logic remains the same.
To connect, you will need a piece of cable with a plug, a terminal block or electrical tape, and, preferably, a capacitor (if the engine does not start well under load). Below is a table of contact correspondence for a standard circuit with an electromagnetic relay.
| Contact on the compressor | Designation | Where to connect | Function |
|---|---|---|---|
| Common (C) | Common | Direct wire from the network (L or N) | Power supply of both windings |
| Working (R) | Run | Second wire of the network through the relay (pin 1) | Main operation of the engine |
| Starting (S) | Start | Through relay contacts (contact 2) | Starting the rotor |
| Grounding | PE | On the compressor housing | Protection against damage current |
Assemble the circuit without connecting the compressor to the network yet. Make sure all connections are tight. If a capacitor is used, it is connected in parallel with the starting winding. For powerful motors (more than 200 W), the capacitor capacity can be 4-10 uF at 300 Volts.
Step-by-step instructions for starting the engine
Now that the circuit is assembled, you can proceed to practical tests. Make sure that the compressor is mounted on a solid surface (preferably on rubber legs or a wooden block), since it will vibrate strongly during operation.
☑️ Checklist before first start-up
Turn on the power. You should hear the relay click and the smooth hum of the engine running. The shaft should gain momentum within 1-2 seconds. If you hear only a hum and no rotation, immediately turn off the power - the problem is in the relay or a stuck piston.
Let the engine run for 5-10 minutes without load. Touch the body: it will be warm, but should not burn your hand. If the temperature rises rapidly, check the current consumption with a clamp meter. Exceeding the rated current (indicated on the nameplate) indicates a malfunction.
⚠️ Attention: Do not run the compressor without load (simply plugging it into a socket with open tubes) for more than 1-2 minutes. Lack of back pressure can lead to oil splashing and damage to the valve group.
Typical errors and safety measures
The most common mistake is ignoring phasing when connecting a relay. If you mix up the contacts of the starting and working windings, the engine will not start. Also, craftsmen often forget about the capacitor, which is necessary for engines operating under load (for example, compressors for painting).
Safety when working with electricity and high pressure is priority number one. Carry out all work on assembling the circuit only with the power turned off. Use wires with a cross-section of at least 1.5 mm², since starting currents can be 3-5 times higher than operating currents.
If you plan to use a compressor to create pressure, be sure to install a receiver (air tank) and a safety valve. Pressure above 6-8 atmospheres can rupture the tubes or the compressor itself if automatic shutdown is not provided.
Frequently asked questions (FAQ)
Is it possible to start a compressor without a relay?
Technically, you can start it if you manually apply voltage to the starting winding at the moment of switching on, and then remove your hand. But this is dangerous and inconvenient. Without a relay, the starting winding will burn out in a few seconds, since it is not designed for long-term operation.
What kind of capacitor is needed for a refrigerator motor?
Usually capacitors like CBB60 or CBB61 are used. The capacity is selected experimentally, starting from 4-6 µF for every 100 W of engine power. The capacitor voltage must be at least 350-450 Volts.
Why does the compressor hum but does not turn?
There can be three reasons: the starting relay is faulty (the contacts are stuck or the coil is burnt out), the piston is jammed (often after a long period of inactivity) or an interturn short circuit in winding Start checking by replacing the relay.
How long can a refrigerator compressor operate in continuous mode?
Standard refrigeration compressors are designed to operate in a cycle (on-off). In continuous operation mode (for example, for a compressor station), they can work for several hours, but require good cooling of the case and temperature control.