How to start the engine from a refrigerator without a start relay: a complete guide

The situation when The refrigerator suddenly stops cooling, and the compressor doesn’t even try to hum, often taking you by surprise. In most cases, the culprit behind the silence of the unit is a faulty starting relay, which is responsible for supplying an impulse to the starting winding of the engine. Many home craftsmen, wanting to save on calling a specialist or checking the performance of the motor itself, ask the question: is it possible to start asynchronous motor directly, bypassing the standard automation? The answer is yes, but requires strict adherence to safety precautions and an understanding of the physical processes occurring inside the windings.

Starting a compressor without a standard relay is a temporary measure necessary for diagnostics or using the motor for other purposes, for example, as a compressor for airbrushing or inflating tires. Induction single-phase motor is not able to start rotating on its own, it needs an external push or phase shift, which is usually provided by the starting winding. If you plan to carry out this operation, you will have to manually simulate the operation of the relay, applying voltage to the right contacts at the right time.

Before you handle the wires, you need to understand the risks. Working with 220 volts is deadly, and incorrect connection can lead to a short circuit or fire. In addition, long-term operation of the compressor without thermal protection built into the standard relay is fraught with overheating and piston group wedge. Therefore, all manipulations should be carried out only with basic knowledge in electrical engineering and following safety precautions.

The design of the compressor and the principle of operation of the windings

To successfully start the motor, you need to understand its internal architecture. A standard compressor from a household refrigerator has three terminals arranged in a triangle on the body. These leads lead to two different stator windings: working and starting. The working winding has a larger wire cross-section and lower resistance; it is designed to maintain rotor rotation. The starting winding is wound with a thinner wire and has a higher resistance; it turns on only at the moment of start.

In normal mode, the starting relay connects the starting winding in parallel with the working winding at the moment the refrigerator is turned on to the network. As soon as the rotor picks up speed, the current in the circuit drops, and a relay (often electromagnetic or posistor) opens the starting winding circuit. If you start the engine without a relay, this opening function will have to be performed manually or using an additional circuit. Critical important Do not keep the starting winding energized for more than a few seconds, otherwise it will burn out due to overheating.

You can determine which terminal belongs to which winding using a multimeter. By measuring the resistance between the contacts, you will find the pair with the lowest resistance - this is the working winding. The pair with high resistance is the starting pair. The sum of these two resistances will give the value between the remaining contacts. Accurate knowledge of the resistance will help you choose the right capacitor if you decide to use a circuit with a phase-shifting element for a softer start.

⚠️ Attention: Never leave the starting winding connected to the network for more than 2-3 seconds after starting the engine. This will lead to rapid overheating of the insulation and failure of the motor.

Necessary tools and safety measures

Preparation of the workplace is the key to a successful and safe start. You will need not only the compressor itself, but also a set of tools to control the process. The main tool will be a multimeter for testing the windings and monitoring voltage. You will also need insulated wires with crocodile clips, screwdrivers, pliers and, preferably, a button or toggle switch for switching the starting circuit.

Safety should come first. Make sure you are working in a dry area and there is no water or conductive dust on the floor. All connections must be reliably insulated to prevent accidental contact with live parts. If you are using a circuit with a capacitor, make sure that it is designed for a voltage of at least 400-450 Volts, since voltage surges are possible at the time of start-up.

It is also important to take into account the condition of the compressor itself. If it was removed from an old refrigerator, check for oil in the crankcase. Working “dry” will quickly disable the rubbing pairs. In addition, make sure that the suction and discharge pipes are free and not clogged with corrosion or oxidation products.

Connection diagram: step-by-step instructions

The process of starting an engine without a relay comes down to manually applying voltage to the starting winding at the moment the power is turned on. First, define the contacts: let C be Common, R be Run, S be Start. Connect the phase wire of the network directly to contact R, and the zero wire to contact C. Now you need to briefly connect contact S to the phase wire (contact R) to give an impulse.

For convenience and safety, it is better to use a button with normally open contacts. One contact of the button is connected to the phase going to the working winding, and the second to the starting terminal S. By pressing the button, you apply voltage to the starting winding. As soon as the engine picks up speed (usually this takes 1-2 seconds), the button must be released. The engine will continue to operate on the working winding.

What to do if the engine hums, but does not spin?

If a hum is heard when voltage is applied, but there is no rotation, the piston group may be jammed or the starting winding is faulty. Try briefly applying a pulse to the starting winding for a little longer (up to 3 seconds). If this does not help, there is likely a mechanical jam, and starting such a motor is dangerous.

There is an alternative method using a capacitor that allows you to create a phase shift automatically, but even in this case, a button is often required for the initial push if the capacitor is not enough to start under load. The capacitance of the capacitor is selected experimentally, usually within the range of 20-50 uF for medium-power compressors.

⚠️ Attention: When working with open wiring, use only one hand to manipulate the wires, keeping the other in your pocket. This will reduce the risk of current passing through the heart in the event of a stroke.

Table of typical winding resistances

To correctly identify the leads, it is useful to know the approximate resistance values. They may vary depending on the power and engine model, but the general patterns remain the same. Below is a table with approximate data for popular types of compressors.

If your measurements differ radically from the table values, this may indicate an interturn short circuit. In this case, the resistance of one of the windings will be significantly lower than normal, and the engine, when trying to start, will get very hot and knock out the circuit breaker. Interturn short circuit often accompanied by a specific burning smell or a change in the color of the varnish on the windings.

Using a compressor for other purposes

Often old compressors get a second life in workshops as air pumps. They are great for airbrushing, inflating tires or blowing out systems. However, when using a refrigerator compressor for purposes other than its intended purpose, you need to take its features into account. It is designed to work in a closed circuit with freon, which cools the motor. When working with air, cooling is worse, so the operating mode must be intermittent.

To fully operate as a compressor, you will need a receiver (a tank for storing air), a pressure gauge and a pressure switch. The pressure switch in this case will play the role of an automatic switch, opening the circuit when a certain pressure in the receiver is reached and turning on the motor when the pressure drops. This will free you from the need to manually control the process.

When remodeling, be sure to install a dehumidifier at the air outlet, since when atmospheric air is compressed, condensation forms, which can damage the tool or ruin the paint job. Also, do not forget about the check valve so that air from the receiver does not blow back into the compressor after it stops.

Typical errors and troubleshooting

Beginners often make mistakes that can cost the life of the engine. The most common is mixing up the conclusions. If you apply 220 volts to the starting winding as a working winding and leave it on, it will burn out in a matter of seconds. Always double check the wiring diagram before plugging into the outlet. The second mistake is ignoring heating. Without a standard relay, there is no thermal protection, so monitor the body temperature with your hand (carefully) or with a thermometer.

If the engine hums but does not start even with a pulse, it may be jammed. In older compressors this happens due to oil thickening or mechanical wear. There is a method of “wedging” by applying increased voltage through a transformer, but it requires special equipment and skills. Simply “kicking” the shaft or trying to turn it by hand is usually ineffective and dangerous.

Critical error: Using wires that are too small in cross-section to connect a powerful compressor can cause them to heat up and melt the insulation, which will cause a short circuit.

It is also worth mentioning capacitors. Using a capacitor of the wrong capacity (too large) will lead to overheating of the windings, and using a capacitor that is too small will make it impossible to start under load. If you are using a circuit with a capacitor, make sure that it is designed for operation in alternating current (AC) circuits and not direct current (DC).

Frequently asked questions (FAQ)

Is it possible to run a compressor without a capacitor?

Yes, you can. To do this, use the method of manual impulse to the starting winding using a button or short-circuiting the wires. The capacitor only makes starting easier and makes engine operation smoother, but is not a strictly necessary element for short-term starting.

How long can you keep the starting winding on?

The starting winding is designed for short-term operation. The maximum time of continuous exposure to voltage should not exceed 3-5 seconds. Prolonged switching on will lead to overheating of the thin wire and an interturn short circuit.

Why does the compressor hum but does not start?

There may be several reasons: jamming of the piston group due to lack of oil or a mechanical defect, breakage of the starting winding, insufficient voltage in the network or incorrect connection diagram. It is also possible that the oil in the crankcase may dry out.

What capacitor is needed to start the refrigerator compressor?

Usually starting capacitors with a capacity of 20 to 50 μF (microfarads) with an operating voltage of at least 400-450 Volts are used. The exact value depends on the engine power and is selected experimentally or using reference data for a specific model.

Is it dangerous to disassemble the sealed compressor housing?

Yes, it is dangerous and impractical. Inside the compressor there is pressurized oil and residual refrigerant. Opening the case (“sawing”) breaks the seal, after which it is almost impossible to reassemble it properly at home. Repair of internal mechanics requires factory equipment.