How to ohm out a compressor

What is Ohming Out a Compressor?

Ohming out a compressor is a diagnostic procedure used to test the integrity of its internal electrical windings. It involves using a multimeter to measure the electrical resistance (in ohms) between the different terminals of the compressor. This process helps determine if the windings are intact, shorted, or open, which are common failure points for compressors.

Why is it Important to Ohm Out a Compressor?

Compressors are the heart of many refrigeration and air conditioning systems. When a compressor fails, the entire system stops functioning. Ohming out the compressor is a crucial step in diagnosing whether the compressor itself is the problem or if the issue lies elsewhere in the system (like a capacitor, relay, or thermostat). By checking the resistance, you can quickly identify common electrical faults within the compressor without needing to replace it immediately, saving time and money.

What Tools Do You Need?

The primary tool required is a digital multimeter capable of measuring resistance (ohms). Ensure the multimeter has probes and is in good working condition. You will also need a way to safely access the compressor terminals, which might involve removing a cover panel or service valve cap. Safety gear, such as insulated gloves and safety glasses, is highly recommended due to the potential for residual electrical charge.

Safety Precautions

This is the most critical step. Before you begin ohming out a compressor, ensure that all power to the unit is completely disconnected. This means turning off the breaker that supplies power to the appliance. Wait for a few minutes after disconnecting power, as some components, like capacitors, can store a significant electrical charge. Never attempt to ohm out a compressor while it is powered on or connected to the electrical supply. If you are unsure about any part of this process, it is best to consult a qualified HVAC technician.

Locating the Compressor Terminals

Compressor terminals are typically located on the top of the compressor, often under a protective cap or cover. There are usually three main terminals: Common (C), Start (S), and Run (R). These terminals connect to the internal windings of the compressor motor.

• Common (C): This is the common connection point for both the start and run windings.
• Start (S): This terminal connects to the start winding, which is used only during the initial startup of the compressor.
• Run (R): This terminal connects to the run winding, which operates continuously while the compressor is running.

Some compressors may have other terminals for specific functions, but the C, S, and R terminals are the most important for basic resistance testing.

Understanding Compressor Windings and Resistance

A compressor motor consists of two main electrical windings: the start winding and the run winding. These windings are essentially coils of wire. When electricity flows through them, they create the magnetic fields necessary to turn the compressor's motor. Each winding has a specific resistance value. The multimeter measures how much opposition there is to the flow of electricity through these coils.

Normal Resistance Readings:

• Between Common (C) and Run (R): This measures the resistance of the run winding.
• Between Common (C) and Start (S): This measures the resistance of the start winding.
• Between Start (S) and Run (R): This measures the combined resistance of both the start and run windings.

The specific resistance values will vary depending on the make, model, and size of the compressor. You should always refer to the manufacturer's service manual or data plate for the correct specifications. However, typical readings for smaller compressors might be in the range of a few ohms to a few tens of ohms.

How to Perform the Test

1. Disconnect Power: Ensure the appliance is unplugged or the circuit breaker is turned off.
2. Access Terminals: Remove any protective cover to expose the compressor terminals (C, S, R).
3. Set Multimeter: Set your multimeter to the resistance (ohms, Ω) setting. Choose a range appropriate for expected low resistance values (e.g., 200 ohms).
4. Test C to R: Place one multimeter probe on the Common (C) terminal and the other on the Run (R) terminal. Record the resistance reading.
5. Test C to S: Place one probe on the Common (C) terminal and the other on the Start (S) terminal. Record the reading.
6. Test S to R: Place one probe on the Start (S) terminal and the other on the Run (R) terminal. Record the reading.
7. Check for Ground Fault: Touch one probe to any of the terminals (C, S, or R) and the other probe to the metal casing of the compressor. The reading should be infinite (OL or open loop), indicating no continuity to ground. Any resistance reading here signifies a ground fault.

Interpreting the Results

Once you have your readings, compare them to the manufacturer's specifications. Here's how to interpret common scenarios:

• All readings are within specifications: The internal windings of the compressor are likely in good condition. The problem may lie elsewhere in the system.
• One reading is significantly higher or lower than expected: This suggests a fault in the corresponding winding (e.g., a high reading between C and R might indicate a faulty run winding).
• One reading is infinite (OL) or zero (0): This indicates an open circuit (broken winding) or a short circuit (windings touching each other or the casing), respectively. In either case, the compressor is likely bad.
• Infinite resistance (OL) when testing to ground: This is normal and indicates the windings are not shorted to the compressor body.
• Low or zero resistance to ground: This indicates a ground fault, meaning the windings are shorted to the compressor's metal casing. The compressor is bad.

Important Note: Sometimes, a compressor may pass the resistance test but still be faulty. This is because the windings could be internally shorted in a way that doesn't register on a simple resistance test, or the compressor might be mechanically seized. Therefore, ohming out is a diagnostic tool, not a definitive guarantee of a working compressor.

Troubleshooting Beyond Ohming

If the resistance tests indicate a problem with the compressor, it generally needs to be replaced. If the tests show the compressor is good, you'll need to investigate other components such as the start capacitor, run capacitor, potential relay, overload protector, or control board. Checking these components and their connections is the next logical step in diagnosing the system's failure.