Ohming a Compressor: Common Start Run Diagram

Single phase compressor ohms can be used to give an insight into system health and help you to identify the different terminals on an AC compressor. When working with a PSC compressor motor, you can use this quick and simple process to check for shorts and learn more about the system too.

What Does it Mean to Ohm a Compressor?

In HVAC circles, there is a long list of common terms, with many terms being used interchangeably. One term that you are likely to come across is the phrase “ohm” something. In general, this just means that you are taking readings in ohms. However, when you “ohm” a compressor, you are generally taking these readings with specific goals in mind: identifying different terminals and looking for potential shorts.

Identifying Different Terminals

There are many different reasons that you might need to identify different terminals with a compressor during HVAC work. It is possible that the system you are working with simply does not have labels. However, it might also be the case that the labels have worn away or been rubbed off. In these instances, you need a safe way to identify the different compressor Start Run Common terminals.

A Common Start Run diagram is aptly named for its reference to the tabs on the PSC compressor terminals. You will want to identify the Common, Start, and Run variations before working on the system. In order to do this, you will ohm the compressor to find Common, Start, and Run.

How to Ohm a Compressor

Learning how to find Common, Start, and Run using your compressor is a fairly simple process. With practice, you will find that it goes much more quickly. Let’s explore the steps to ohm your client’s compressor.

Turn off the Power

The first and most important step when carrying out this process is to remove the power from the unit itself. If the power is left intact, you run risks for both yourself and the system. Completely turn off the power–and make sure that no one has the ability to reactivate it while you work.

Discharge the Capacitors

Though turning off the existing power source is important, it is also important to remember that the system might still have some power. You will want to discharge the capacitors in order to release any energy buildup for your safety and the safety of the system.

Disconnect the Compressor

Once the energy has been discharged from the system, you can remove any plugs and wires. Leaving anything attached might influence the readings. This is also a good opportunity to explore the components and look for potential signs of damage, like physical damage or corrosion.

Measure the Resistance

With your multimeter set to ohms, you will want to take a reading for each of the connected terminal pairs. Your ultimate goal is to check the value for the resistance with each pair. Using this information, you can determine which terminal is Common, Start, and Run.

The resistance readings will differ for each set, allowing you to make your assessment. From lowest to highest in ohms, the terminal pairs will rank in the following order: run and common, start and common, and start and run.

Draw a Diagram

The easiest way to identify the different terminals is by drawing a diagram and writing out the ohm readings. This will allow you to see the full picture, identify the different terminals. As a bonus, it will also give you something to refer back to as you work if necessary. If the system has no other way to be identified, leaving the diagram there or giving it to the unit’s owner can be beneficial for future work.

Consider Your Readings

A functional system will give you three readings that fit together mathematically. If your middle and lowest readings are 7 ohms and 2 ohms, your highest reading should be 9 ohms. This is an indication that the system is working appropriately. If these numbers do not match, it is very possible that the system has a short–and this can spell trouble.

Troubleshoot Any Problems

In the event that you suspect a short or you simply want to be thorough, you will want to see if the compressor is shorted to the ground. Single phase compressor wiring can be a consideration here. To do this, connect one of the identified terminals to clean copper tubing on the compressor itself. Any reading other than an OL will tell you that a short might be present.

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