Locked compressors or compressors that go off on overload during start is a problem that many HVAC technicians have had to face at one point or another. Today, We're going to dive into that issue and give you the knowledge you need to diagnose and fix it. But first, let's understand what's inside the compressor. The compressor contains a motor to spin the compressor.

The rotor is the moving part of the motor which rotates inside the stationary part called a stator. Together they form the electrical driver of the compressor which is the heart of the HVAC or refrigeration system. The compression chamber is the part of the compressor that actually moves the refrigerant and this may be an oscillating scroll and a scroll compressor, rotating veins, and a rotary compressor or pistons in valves in a reciprocating compressor. In order to work properly.

The compressor needs proper electrical power applied, as well as the necessary internal lubrication from the refrigerant oil to operate. Most of the compressors we work on are refrigerant cooled, which means all the internal parts including the motor are immersed in refrigerant vapor and some oil. The Vapor Refrigerant Moving through the compressor is what keeps the compressor cool. When a compressor is locked, the rotor can't move as it normally would.

Because of this, it draws High current and usually causes the windings to overheat and the overload to open inside the compressor. It will usually make a loud humming noise for a very short time while drawing High current and then shut off. The fan motor will keep spinning because the overload opens the common on the compressor. Keep in mind, overheating can also occur due to operational overheat or overload conditions such as an undercharged system or a refrigerant restriction.

This will cause the entire compressor to heat up, but in these cases it will not reset quickly because the entire compressor mass is hot, not just the winding. In these cases, it is not a locked compressor and you will generally notice that the compressor shell is hot to touch. This would require cooling down the compressor in order to get the overload to reset. The first step in diagnosing a locked compressor, like most things, is a thorough visual inspection of all the high voltage electrical components.

This includes all of your wires, terminals, contactor, disconnect, circuit breaker, and especially your capacitors. Checking for tightness is essential as loose connections can cause intermittent operation or complete failure. Also, ensure to check the applied voltage to the system. Now, this can be tricky because applied voltage is generally best tested when the system is running, but at least you can confirm that it's within the proper ballpark.

Many people will use use a hard start kit to get a compressor unlocked and this may be what you do in this case, but before resorting to a hard start kit, we should first verify that the run capacitor is performing and has the correct microfarads. We also need to make sure we have sufficient incoming voltage and that the system has been wired properly. So it's worth stating that the most important thing for you to assess is is this system one that's been running for some time and now all of a sudden it's locked up? Or is this A system that we've replaced a compressor or a capacitor, or just recently installed or rewired something? If that's the case and it's locked, it's very likely that it's something that we or somebody else did that caused that, and often it's due to miswiring incorrect capacitors. Things like that always confirm that first.

If it's an older system that just recently locked, we may want to use a hard start kit. now. In certain situations, a factory hard start kit designed specifically for that compressor might be the best solution. The decision to add a factory or aftermarket hard start start kit depends on the system warranty and the system's age.

and it's essential to remember that hard start kits are not the same as soft starts. Soft starts provide a very gradual startup. Hard starts apply a lot of torque to the compressor by adding current and phase shift to the start winding, which reduces the start time or helps get a compressor unlocked. Now, let's talk a little bit more about start capacitors, potential relays, and hard start kits.

There are many different types of hard start kits. The most common employ a capacitor and a potential relay. The potential relay starts with closed contacts and the contacts open due to back EMF from The compressor, which takes the start capacitor out of the circuit, preventing compressor damage. Now, some hard start kits utilize something called a PTC which is a type of thermister that increases in resistance as it warms up and performs generally the same function as the potential relay.

There are some hard start kits that use three wire connections and some that use two wire connections, but fundamentally, they all use some method to remove the start capacitor from the circuit. A start capacitor increases phase shift and additional start current for about the first 75% of the compressor's running speed. However, it must be taken out of the circuit quickly to avoid damage to the compressor. Because a start capacitor is a very large microfarad capacitor, the method of doing this is either the potential relay or some type of other electronic control or thermister like I mentioned.

Now again, the potential relay is most common and it generally takes charge of this operation. Properly sized start capacitors and potential relays can be specified by the equipment or compressor manufacturer for a given application and should be installed initially in the these cases where the manufacturer specifies it. Certain systems can benefit greatly from these components, especially long line set applications. Cases with hard shut off expansion valves with some reciprocating compressors and 208 singlephase applications always ensure you have a quality, properly sized run capacitor on any system that you add a hard start kit in, because if a run capacitor fails, while a hard start kit is present, it can cause additional damage to the compressor.

Remember, hard start kits do not reduce initial starting current, they just decrease the time that it takes for a compressor to get started, and they can help unlock compressors. They can be of great help in starting your compressor under these adverse conditions, getting it up and running faster, which in some cases is used to reduce things like light dimming. On the other hand, soft starts do actually reduce the starting current and are ideal for systems that rely on generators or solar power where current availability is limited. In conclusion, diagnosing and addressing a locked compressor involves thorough inspection, correct understanding of the components and knowledge of hard start kits and how they work.

When a compressor is locked, using a hard start kit to attempt to unlock it is a good practice, but whether or not it works, there may be underlying issues that caused the compressor to lock in the first place and complete test and checkout should be performed. If the unit is older and locked out of the blue, it's likely just a temporary fix and should be communicated with the client. Thanks for watching! If you're willing, give this video a thumbs up and drop us a comment. Don't forget to hit that Bell icon to stay updated with all of our future videos.

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