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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.