Starting compressor from an old refrigerator for use as an air pump or in home-made installations is a task that requires accuracy and understanding of electrical processes. Often, craftsmen are faced with the need to revive a unit that has only three contacts on the terminal block, and the starting relay requires correct orientation in space. An error in determining the windings or choosing a connection diagram can lead to instant failure of expensive equipment or even to a fire hazard.
Unlike modern inverter models, classic ones single-phase asynchronous motors work on a decades-tested principle, where the separation of the starting and operating windings plays a critical role. It is the correct connection of these circuits through a special one that provides the necessary torque to start the piston group. In this article, we will analyze in detail the algorithm of actions that will allow you to safely start the motor, determining the purpose of each of the three terminals without the presence of factory documentation. starting relay provides the necessary torque to start the piston group. In this article we will analyze in detail the algorithm of actions that will allow you to safely start the motor by determining the purpose of each of the three terminals without the presence of factory documentation.
Before starting any work, you must make sure that the unit is completely disconnected from the power supply and the capacitors are discharged. Manipulations with electricity require strict adherence to safety precautions, since a voltage of 220 Volts poses a real threat to life. We will look at diagnostic methods using a multimeter, methods for selecting relay analogs and the nuances of connecting capacitors to improve the efficiency of the system.
The operating principle and design of a single-phase compressor
The basis of the design is electric motor, the shaft of which is directly connected to the crank mechanism that pumps refrigerant or air. To start the rotor, it is necessary to create a rotating magnetic field, which in a single-phase network is achieved through the use of two stator windings: the main (working) and auxiliary (starting) windings. The working winding occupies most of the stator slots and has a thicker wire, which provides lower active resistance.
The starting winding is made of a wire of smaller cross-section and has a higher resistance. It comes into operation only at the moment of start in order to create the necessary phase shift and rotate the engine shaft. As soon as the speed reaches approximately 75% of the nominal speed, the starting winding should be automatically turned off. This process is responsible for starting relay, which opens the circuit of the starting winding after starting.
⚠️ Attention: It is strictly forbidden to apply voltage to the starting winding constantly without breaking the circuit. This will lead to rapid overheating of the insulation, interturn short circuit and complete combustion of the motor within a few minutes.
The third element of the system is a thermal protection relay, which is often built into the starting housing or installed separately on the compressor housing. It monitors the current drawn by the motor. If the compressor seizes or operates in overload mode, the current increases, the bimetallic plate heats up and opens the power circuit, preventing a disaster. In circuits with three terminals, all these elements are switched through a single block of contacts.
Why does the engine hum, but does not start?
If you hear a hum, but the shaft does not spin, most likely the starting relay does not work or the starting winding is faulty. The cause may also be low voltage in the network or mechanical jamming of the piston group. Prolonged humming without starting is dangerous due to overheating of the windings.
Pin identification: search for common, starting and working windings
The most critical stage is the correct determination of the purpose of the three contacts on the motor terminal box. They are usually arranged in a triangle. On modern compressors, markings can be applied next to the terminals: C (Common) - common, R (Run) - working winding, S (Start) - starting winding. However, on old or restored units there are often no markings, and you have to rely only on resistance measurements.
To carry out diagnostics, you will need a digital multimeter switched to resistance measurement mode (Ohms). First, you need to “ring” the motor housing to make sure there is no insulation breakdown to ground. One probe is applied to any copper fitting or stripped part of the body, and the second is applied to each of the three contacts in turn. If the device shows infinity or very high resistance, the insulation is intact.
Next, we measure the resistance between each pair of contacts. There will be three measurements in total. The results must be recorded. The logic here is as follows: the greatest resistance will be between the working and starting windings (total resistance). The least resistance will be between the two windings if measured through a common point, but it is easier to use the rule: the sum of the resistances of the two smaller sections is equal to the largest measured value. The contact, which is common to two smaller resistances, is the common terminal (C).
The remaining two contacts are distributed as follows: the starting winding is connected to the contact with a higher resistance relative to the common one, and the working winding is connected to the contact with a lower resistance. This is a fundamental rule: The starting winding always has a higher active resistance due to the smaller wire cross-section. An error in identification will lead to the engine humming, heating up and knocking out the circuit breaker.
Connection diagram for the starting and thermal relay
After the pins have been determined, it is necessary to correctly connect the external controls. The starting relay is an electromechanical switch that closes the starting winding circuit when voltage is applied and opens it when the speed increases. Inside the relay body there is a moving contact and a coil (or a heavy washer in older models) that responds to current or a magnetic field.
The connection diagram is as follows: the phase from the network goes to the input of the thermal relay, then to the general contact of the compressor. The phase from the thermal relay also goes to the input of the starting relay. The output of the starting relay is connected to the contact of the motor starting winding. The network zero is connected directly to the second contact of the working winding. Thus, the working winding is constantly energized while the thermal relay is closed, and the starting winding is only energized at the moment of start.
It is important to take into account the orientation of the starting relay. The case often has an arrow indicating the "UP" or "Up" direction. This is not just a recommendation, but a technical requirement for current-fault relays. Inside there is a movable core, which, under the influence of current, retracts and opens the contacts. If the relay is inverted, gravity will prevent the core from returning to its original position, and the starting winding will not turn off.
| Relay type | Operation principle | Orientation | Application |
|---|---|---|---|
| Current (PT) | Closes contacts at high starting current, opens when falling | Strictly vertical (up arrow) | Old refrigerators, powerful compressors |
| Posistor (PTK) | The posistor is heated by current and sharply increases the resistance, breaking the circuit | Any (often horizontal) | Modern household refrigerators |
| Pressure switch | Responses to pressure in the system (not to start the electrics) | Depends on the design | Industrial systems |
⚠️ Attention: When installing the relay, make sure that the contacts inside are not oxidized and the moving part does not jam. If the relay is current, it should move freely inside the housing under the influence of gravity when swaying.
Selection of a capacitor to increase the starting torque
In some cases, especially when using the compressor as a vacuum pump or when the network voltage is low, the standard power of the starting winding may not be enough for a confident start. In such situations, the starting winding circuit is connected in series with the relay (or in parallel, depending on the circuit). It creates an additional phase shift, increasing torque. starting capacitor. It creates additional phase shift, increasing torque.
The capacitance of the capacitor is selected experimentally or according to a formula depending on the engine power. Typically, for medium-sized refrigerator compressors, capacitances in the range of 20 to 50 µF are used. It is important to use capacitors designed for a voltage of at least 300-400 Volts (preferably 450V), since at the moment of switching voltage surges exceeding the nominal 220V are possible.
It is not recommended to place a working capacitor, which remains in the circuit permanently, in the starting circuit of a conventional refrigeration compressor without modifying the circuit, as this can lead to overheating of the windings in the working one. mode. The starting capacitor should only work for a short time, when the rotor accelerates.
Step-by-step instructions for starting the engine
Now that the theoretical part has been studied and the components have been selected, you can proceed to the practical assembly of the circuit. Before turning on for the first time, double-check all connections. Make sure that the wires are securely fixed in the terminals and that the insulation is not damaged. For testing, it is convenient to use a wire with a plug and a button to be able to instantly turn off.
Assemble the circuit following a certain algorithm. First, connect the wires to the compressor terminals according to the test results. Then connect the start and thermal relay. Only then connect the power cord. Turn on the power and observe the behavior of the motor. It should start within 1-2 seconds with a characteristic sound, after which the noise should become even.
☑️ Checklist before starting
If the engine does not start, but only hums, turn off the power immediately. Let it cool for 10-15 minutes. Check if the shaft is jammed (for older compressors, sometimes short-term supply of voltage through a large capacitor in different phases helps, but this requires experience). If the engine starts, but after 5-10 seconds the thermal relay turns off, it may be incorrectly selected or the engine is faulty.
For stationary use, it is recommended to assemble the structure in a protective box or secure it on a wooden/metal platform. Compressors vibrate during operation, so use rubber dampers or bolts with rubber washers for fastening. This will reduce noise and extend the service life of the units.
Frequent errors and safety measures
When connecting equipment themselves, beginners often make common mistakes that can cost the equipment. One of the most common is ignoring the condition of the oil. The compressor contains oil that may have leaked into the circuit during transportation. If you turn on such a compressor, it may run dry or throw out oil under pressure.
Another mistake is using wires that are too thin. The starting current of the compressor is 3-5 times the rated operating current. If the wire cross-section is insufficient, it will heat up, creating a risk of fire. For household compressors, use a copper wire with a cross-section of at least 1.5 mm², and for powerful models - 2.5 mm².
⚠️ Attention: Never leave a running compressor unattended for a long time, especially if it is used in home-made installations without standard refrigerator automation. Lack of pressure control can lead to rupture of the receiver or hoses.
It is also worth remembering the temperature. Air-cooling compressors get very hot when operating with air pumping (as opposed to operating with freon, where they are cooled by refrigerant). During intensive use, it may be necessary to install an additional blower fan or organize operating cycles with cooling breaks.
Is it possible to use a compressor without a relay?
Theoretically, you can start the engine by manually applying voltage to the starting winding through a button, but it cannot be operated that way. Without automatic shutdown of the starting winding, the engine will burn out in a matter of minutes. A relay is an essential element of the system.
Diagnostics of connection faults
If after all the manipulations the engine does not behave correctly, it is necessary to carry out diagnostics. Failure to start when voltage is present at the terminals indicates an open circuit. Check the integrity of the wires, the condition of the contacts in the relay and the serviceability of the thermal fuse. Often the bimetallic plate inside the thermal relay oxidizes and does not allow current to pass through.
Strong heating of the housing in the lower part (where the motor is located) indicates an interturn short circuit in the windings or operation without load (for some types of compressors). A burning smell or a buzzing sound with frequent clicks of the relay indicate that the starting winding is not turned off. In this case, you need to check the starting relay - perhaps the contacts are stuck or the posistor has burned out.
Vibration and knocking may be signs of wear on the bearings or connecting rod mechanism. If the compressor is intended for important tasks, it is better not to use such a unit, since its resource has been exhausted. A quiet, even hum and moderate heating are signs of proper operation.
Remember that old compressors may contain asbestos gaskets or use refrigerants that are harmful to the environment. Be careful when disassembling or disposing. Modern models are more environmentally friendly, but the principle of their electrical part remains the same.
How to accurately determine which wire goes to which winding if the color marking has been erased?
Use the elimination method through resistance measurements. Find two contacts with maximum resistance between them - these are the ends of the starting and working windings. The third contact, which did not participate in this measurement (or measurements up to which give lower values), is common. Then measure the resistance from the total to each of the other two. The smaller value is the working winding, the larger is the starting winding.
Is it possible to connect a three-phase motor from an industrial refrigerator to a 220V network?
Yes, but this will require a "delta" or "star" connection circuit with the obligatory use of working and starting capacitors. Three-phase motors in a single-phase network lose up to 30-40% of their power. For compressors, this is often undesirable, since there may not be enough torque to start under load.
Why does the starting relay burn out immediately after installation?
There may be several reasons: a jammed engine (the compressor does not start, the current does not drop, the relay gets hot), incorrect voltage in the network, or a mismatch in the type of engine power relay. Also, the relay can burn out if it is incorrectly oriented in space (for current models).
Do you need to change the oil in the compressor before using it as a pump?
If you use a compressor to pump air, ordinary refrigeration oil can oxidize over time from contact with oxygen in the air. For short-term work, replacement is not necessary, but for continuous operation it is recommended to replace the oil with a special compressor oil that is resistant to oxidation, or install an oil-moisture separator at the outlet.
What is the maximum pressure that a household compressor can create?
Theoretically, the piston group of a household refrigeration compressor is capable of creating a pressure of up to 10-15 atmospheres or more. However, standard tubes and connections are not designed for this. When converting to a compressor, be sure to use a receiver with a safety valve set to a safe pressure (usually 6-8 atmospheres for homemade systems).