How to connect a motor from a refrigerator without a relay to the network

The situation when the refrigerator suddenly stops cooling, and the technician diagnoses a malfunction of the starting relay, often leaving the owner with a choice: buy a new part or try to start the unit temporarily. Compressor is the heart of the refrigeration system, and its performance directly depends on the correct power supply to the windings. Sometimes it becomes necessary to check the motor separately from the refrigerator to make sure that it is in working order or to use it for household purposes, for example, for a compressor or aerator.

Understanding the principle of operation single-phase asynchronous motor is critically important before starting any manipulations with the electrical circuit. In normal mode, a special device is responsible for starting, which closes the starting winding for a short time. If this element is excluded from the circuit, you will have to manually simulate its operation or use alternative methods of supplying voltage. Remember that working with a voltage of 220 volts is deadly, and any contact with live parts under load is prohibited.

In this article we will analyze in detail the physical processes occurring inside the electric motor, and consider safe ways to activate it. You'll learn how to identify winding terminals, what tools are needed for diagnosis, and why simply twisting wires can cause a motor to burn out. We will not use complex terms without explanation, so that even a beginner can understand the basic electrical principles of household appliances.

The principle of operation of a single-phase compressor

To successfully start the motor without a standard relay, you must clearly understand its internal structure. Most household refrigerators are equipped with motors that have two main stator windings: working (main) and starting. The working winding occupies most of the stator slots and is designed to create the main magnetic flux during operation. It is made of a thicker wire and has less active resistance.

The starting winding is switched on only at the moment of start, when the rotor is stationary and needs an additional impulse to create rotating torque. Phase shift between the currents in these windings is created either due to different inductance, either through a capacitor or through the design of the relay itself. Without this phase shift, the magnetic field will be pulsating, not rotating, and the rotor will remain standing, just humming and heating up.

⚠️ Attention: Holding the start button for a long time or applying voltage to the starting winding of a running motor will lead to overheating and an interturn short circuit. The starting winding is designed for short-term operation!

In the standard circuit, the relay performs two functions: it closes the starting winding circuit at the moment of switching on and opens it when the engine picks up speed (usually in 1-3 seconds). When excluding the relay from the circuit, your task is to manually provide this short-term impulse to the starting winding, after which the current will flow only through the working one. This requires an accurate understanding of the moment when the motor entered operating mode.

There are also motors with capacitor starting, where the capacitor is switched on constantly or only for the start time. In such models, the absence of a relay is compensated by a capacitive element, but the principle of separation of the windings remains the same. For correct connection, you will need a multimeter to ring the pins and determine which of them relates to starting and which to operation.

Necessary tools and safety measures

Before you begin assembling the circuit, you need to prepare a workplace and tools. Ignoring electrical safety rules when working with mains voltage is unacceptable. You will need a reliable 220V power source, preferably with a circuit breaker or RCD in the circuit, to instantly de-energize the system in the event of a short circuit.

The basic set of tools includes a digital multimeter for measuring winding resistance, a set of screwdrivers, pliers and insulating materials. To temporarily connect wires, use terminal blocks or reliable alligator clips, as you cannot hold the twists with your hands. You may also need a non-latching button (for example, from a doorbell) to simulate the operation of a starting relay.

• 🛡️ Dielectric gloves and shoes with rubber soles to protect against electric shock.
• 🔌 A working power cord with a plug and a built-in switch for quick shutdown.
• 📏 Multimeter with continuity function and resistance measurement up to 2 kOhm.
• 🔘 Bell button (normally open) to control the starting winding.

Pay special attention to the insulation condition of the wires of the compressor itself. If the motor was removed from an old refrigerator, its external wiring may have become dry and cracked. Before applying voltage, be sure to check that there is no breakdown on the housing using a multimeter in the mode of measuring insulation resistance (megaohmmeter), if possible, or at least make sure that the shell is intact.

Identification of winding terminals multimeter

The most important step is the correct identification of contacts. The compressor terminal block usually has three or four terminals (plus a common wire). If the markings are erased or missing, the only reliable method of identification is to measure the active resistance between pairs of contacts.

Take a multimeter and set it to resistance measurement mode (Ohm). Measure the resistance between all possible pairs of contacts. You will get three values. The sum of the two smaller resistances should be equal to the largest resistance (resistance between the extreme terminals). The greatest resistance is the sum of the resistances of the starting and working windings, that is, the measurement is between the ends of both windings.

The common wire (Common) is always located between the working and starting windings. The resistance between it and the working terminal will be the smallest, and between it and the starting terminal it will be average. It is to the common wire that one of the network wires (phase or zero) is supplied, and the second network wire is distributed through the control button.

If your motor has four terminals, then the circuit is a little more complicated: two terminals are the ends of the working winding, two are the starting winding. In this case, the resistance of the working winding will be significantly less than the starting winding. Find the pair with minimal resistance - this is the working winding, and the pair with high resistance is the starting winding. The connection is made using the series-parallel method.

What to do if the winding resistances are equal?

In rare cases (bifilar windings), the resistances can be almost the same. In such a situation, it is extremely difficult to start a motor without a capacitor or a specific relay, since a phase shift is not created. An experimental selection of a capacitor will be required.

Connection diagram using a button

To implement manual start, we will need a simple circuit that simulates the operation of an automatic relay. The essence of the method is to apply voltage to the starting winding only at the moment of start, holding the button, and after gaining speed, release it. At the same time, the current continues to flow continuously to the working winding.

Assemble the circuit as follows: connect one network wire (for example, zero) directly to the common terminal of the compressor. Branch the second network wire (phase): one end goes directly to the output of the working winding, and the second - through a normally open button to the output of the starting winding. Thus, when the button is pressed, the starting winding circuit is closed and the motor starts.

Connection diagram:
Network (L) ---> [Working winding] ---> Common wire ---> Network (N)
Network (L) ---> [Button] ---> [Start winding] ---> Common wire

The startup process looks like this: press the button, apply power to the network. The motor should hum and start rotating. As soon as you hear a change in sound (the motor returns to normal), the button must be released immediately. If you release the button too early, the motor may not gain momentum and may start humming again. If it’s too late, the starting winding will burn out.

Starting using a capacitor

A more reliable and safe way than a manual button is to use a starting capacitor. This method allows you to create the necessary phase shift of currents, which makes it easier to start the engine and reduces the load on the network. The capacitor is connected parallel to the starting winding and operates similarly to a button, but automatically (if you use a special starting capacitor with an internal resistor) or requires manual shutdown.

For short-term operating mode (starting capacitors), the capacitance is selected at the rate of 70-100 μF per 1 kW of engine power. For low-power refrigeration compressors, a capacitor with a capacity of 4 to 10 μF with an operating voltage of at least 450V is usually sufficient. Using a capacitor with a lower voltage will lead to its explosion.

The connection diagram is similar to a push-button one, but instead of a button, a capacitor is connected to the open circuit of the starting winding. However, since conventional starting capacitors do not have a shutdown mechanism, you will still have to use a button to break the starting winding circuit after starting, or use specialized starting devices. Temporarily connecting a capacitor without disconnecting it will cause the engine to overheat.

⚠️ Attention: Capacitors accumulate electrical charge. After disconnecting the circuit, be sure to discharge the capacitor by shorting its terminals with an insulated tool to avoid electric shock.

If you plan to use the motor constantly (for example, in a homemade compressor), it is better to purchase a ready-made starting device or a relay from a similar refrigerator, since manual control does not guarantee stable operation and protection from overloads. Capacitor starting is good for tests, but for long-term operation it requires automation of the shutdown process.

Fault diagnosis and typical errors

Even with correct connection, the motor may not start or behave incorrectly. One of the most common problems is compressor seizure due to worn out mechanical parts or lack of oil. In this case, the motor will hum and consume a lot of current, but the shaft will not budge. Trying to start it "overclocked" by hand is dangerous and rarely helps.

Another common mistake is confusion with the windings. If you mix up the working and starting windings and apply full mains voltage to the starting winding (which has a higher resistance and a thinner wire), it will burn out almost instantly. Always double-check the resistance with a multimeter before plugging it into the network.

• 🔥 The motor hums, but does not spin: it may be jammed or lack of a starting pulse.
• 🌫️ Smoke or a burning smell comes from the housing: immediately turn off the power, an interturn short circuit has occurred.
• 📉 The motor starts and immediately stalls: the thermal protection relay (inside the motor) is triggered, possibly overheating or low voltage in the network.

It is also worth considering the condition of the electrical cable and contacts itself. Oxidized terminals or poor contact in the button can create additional resistance, causing insufficient voltage to flow to the windings. Visually inspect all connections, clean the contacts and make sure that the wires are securely fastened.

Frequently asked questions (FAQ)

Is it possible to leave the motor running without a relay all the time?

No, this is impossible and dangerous. Without a mechanism to disconnect the starting winding (relay or button), current will flow constantly in it, which will lead to rapid overheating, melting of the insulation and fire. The starting winding is not designed for long-term operation.

Why does the motor hum, but does not start without a relay?

Humming means that voltage is applied to the working winding and a magnetic field is created. However, to create torque in a single-phase motor, a phase shift is needed, which is provided by the starting winding. Without an impulse to the starting winding, the rotor remains in a state of magnetic blocking.

Which capacitor is best to use for starting?

For short-term starting, special starting capacitors (Start Capacitor) marked CD or similar, designed for a voltage of at least 450V, are best suited. Using capacitors with a lower voltage rating will lead to their failure.

Is it safe to use such a motor to inflate tires?

You can use a household refrigeration compressor to inflate tires, but with restrictions. It is not designed to create high pressure (maximum 6-8 atmospheres) and requires a good cooling system, since when working on pressure it heats up much more than in a refrigerator.