Bryan teaches the Kalos technicians how to troubleshoot locked compressors in an HVAC system. He also talks about the appropriate usage of aftermarket and factory hard start kits. Although the compressor is the main focus, you can apply some of the practices to other types of motors inside an HVAC system.
A locked compressor typically draws locked rotor amps for a short period of time. The rotor rotates around the stator, and a “locked” rotor draws higher current and doesn’t spin. In those cases, the resistance (inductive reactance) is lower, resulting in a higher current. Inductive loads produce back EMF at the beginning and create inductive reactance, which is why you see higher current at the start and higher resistance later on. That differs from a resistive load in which the resistance stays fixed.
When the windings don’t have that resistance, they become hot and set off existing overload safeties. Locked compressors often overheat due to the hot windings. However, overheating may also happen due to an undercharged system, which is an operational overload. Operational overloads, including conditions that result in high compression ratios, tend to heat up the entire compressor more than the windings.
Locked rotor amps, by themselves, don’t usually produce particularly useful measurements when it comes to troubleshooting. In some cases, we might be able to stop a compressor from locking by adding a hard start kit. However, we should first verify that the capacitor is performing well, that we have sufficient incoming voltage, and that the system has been wired properly before resorting to a hard start kit.
If a new system has a locked compressor but works with a hard start kit, then we can diagnose the issue (under-voltage, long line sets, etc.). Sometimes, we can use a factory hard start kit permanently, especially if the manufacturer specifies so. In many cases, it’s better to get a factory hard start kit than rely on an aftermarket hard start kit for a long time. Aftermarket start kits are designed to work for a wide range of compressors, whereas factory hard start kits are tailored to specific systems and can take the start capacitor out of the circuit at a more appropriate time.
You may need to look at the system warranty to see if a factory hard start kit is worth adding or if an aftermarket hard start kit is good enough to keep an old unit running a bit longer. It’s worth noting that hard starts are NOT the same as soft starts, which modulate current into the compressor and produce more of a gradual startup than an immediate start with a current spike. Soft starts work well with systems that rely on generators or solar power.
Read all the tech tips, take the quizzes, and find our handy calculators at https://www.hvacrschool.com/.
A locked compressor typically draws locked rotor amps for a short period of time. The rotor rotates around the stator, and a “locked” rotor draws higher current and doesn’t spin. In those cases, the resistance (inductive reactance) is lower, resulting in a higher current. Inductive loads produce back EMF at the beginning and create inductive reactance, which is why you see higher current at the start and higher resistance later on. That differs from a resistive load in which the resistance stays fixed.
When the windings don’t have that resistance, they become hot and set off existing overload safeties. Locked compressors often overheat due to the hot windings. However, overheating may also happen due to an undercharged system, which is an operational overload. Operational overloads, including conditions that result in high compression ratios, tend to heat up the entire compressor more than the windings.
Locked rotor amps, by themselves, don’t usually produce particularly useful measurements when it comes to troubleshooting. In some cases, we might be able to stop a compressor from locking by adding a hard start kit. However, we should first verify that the capacitor is performing well, that we have sufficient incoming voltage, and that the system has been wired properly before resorting to a hard start kit.
If a new system has a locked compressor but works with a hard start kit, then we can diagnose the issue (under-voltage, long line sets, etc.). Sometimes, we can use a factory hard start kit permanently, especially if the manufacturer specifies so. In many cases, it’s better to get a factory hard start kit than rely on an aftermarket hard start kit for a long time. Aftermarket start kits are designed to work for a wide range of compressors, whereas factory hard start kits are tailored to specific systems and can take the start capacitor out of the circuit at a more appropriate time.
You may need to look at the system warranty to see if a factory hard start kit is worth adding or if an aftermarket hard start kit is good enough to keep an old unit running a bit longer. It’s worth noting that hard starts are NOT the same as soft starts, which modulate current into the compressor and produce more of a gradual startup than an immediate start with a current spike. Soft starts work well with systems that rely on generators or solar power.
Read all the tech tips, take the quizzes, and find our handy calculators at https://www.hvacrschool.com/.
Thanks lots of for value informations.
Bryan, a question related to the topic covered here: assume a tech arrives and finds the compressor is locked due to failure in start/run circuit (bad capacitor, burned wire, etc) and the compressor internal overload has been cycling on/off for many hours because the system was never shut off and tstat is still calling for cooling. How many hours of the overload cycling off and on like that can most residential compressors tolerate, and at what point should the tech advise customer that compressor life has been seriously compromised? Thanks. Are you in Nepean ?
Thanks for always keeping our knowledge sharp
Talking about home owner trying to do their own thing. Few weeks ago I was out for a maintenance and the system started normally about the time I showed up and in a few minutes it was making an awful noise but still running. I turn it off and start my checks figuring I would find out why. First this was a 2 ton. I look at the cap and it was a 70/7.5. Was supposed to be a 30/5. Had been that way for almost a year apparently. Told them they’ll be lucky to get much more life out of it due to their mistake.
Could you please repeat the students responses, we can't hear them. Thanks for your information.
Tim (the tool man) Taylor says "MORE POWER!!!!!"
Smack it with a mallet 😂
I would like to attend ur classes info? Service area Kanata??
thanks for the content, as an HVAC student i appreciate your content. This helps consolidate what i'm learning at school.
Thank you
Always at the top of the trade! This man was born to teach! No doubt many ac technicians are better at their trade because of this man!
LRA 96 for 3 ton compressor staying for 10 second s.Most likely is Locked….
Just beat on it with a hammer, it will start,just kidding,great content as always
Can I use the hard start kit with refrigerator locked compressors
Good gracias
💯💯💯💯
7 years in and it’s a great reminder regarding LRA and the reasons why.
Fantastic class! Thank you for the refresher!
Is it true – if you multiply RLA times 4 , would it equal LRA ?? Are you in Orleans ?
Are your guys payed for these trainings?
Serious question; I'd volunteer but it's hard to attract during off hours.
Aftermarket hard starts vary quite a bit. The 2 wire (sppe) type being the lowest grade. The have a low lifespan and the relays a “light duty” to put it mildly. The 2 wire “Kickstart” are a little better with and improved relay. Personally I only use 3 wire 5-2-1 true potential relay type. They always work with Trane/Am-Standard as well. Having access to the original specs and closely matching MFD is key.
Wish you guys would hire a Canadian here or there, would love the opportunity to work with such an amazing crew Service area Barrhaven??
The guys at this company are lucky to have management like this.
Hey Bryan! Thanks for the awesome content!