Most single phase A/C compressors are permanent split capacitor (PSC) motors that have low starting torque. For many applications, this is ok because the pressures have a chance to equalize during the off-cycle so only low starting torque is required. Most PSC A/C compressors don't come with hard start devices, but they can easily be added. Older compressors that are having starting problems, compressors used with TXVs or solenoid valves, and low-voltage conditions are a few of the many reasons hard start kits may be needed. During start up the inrush current is high, causing heavy strain and heat in the compressor start windings. Hard start kits added to a new compressor greatly reduce this strain on the compressor windings while possibly extending the life of the compressor. This reduction in start-up inrush current may also reduce operational cost as well as stopping light flicker.
A hard start kit is a capacitor that adds the starting torque and some means of removing the capacitor from the start circuit as soon as the compressor has started. The compressor manufacturer or OEM will almost always use a potential starting relay for this purpose. As the compressor reaches about 80% of full speed the voltage generated in the compressor windings will energize the relay, removing the capacitor from the start circuit. The OEM will specifically select the proper pickup and drop out voltage for this relay to ensure the capacitor is in the circuit for the shortest amount of time while also ensuring that the compressor has started. The compressor OEM start components are always the best choice as they are sized specifically for that compressor. The OEM relay and capacitor don't normally come in a pre-wired kit.
What about aftermarket relays?
There are many types of aftermarket hard start kits available. They are not all created equally. Most aftermarket hard start kits come prewired requiring only 2 or 3 wires to be connected.
The PTC (Positive Temperature Coefficient) or torque multiplier hard start kit will normally be the cheapest and easiest to install as they normally have only two wires. This hard start kit will have an oversized capacitor that drops out of the circuit more on time then compressor speed. The PTC start kit may stay in the circuit too long or not long enough, and can be affected by high ambient temperatures. This may not be the best choice for new compressors.
The 5-2-1 Compressor Saver would be a better choice for installation on new compressors. The 5-2-1 has a capacitor and potential relay that is very similar to the OEM start kit but with a universal relay and capacitor. Like the OEM relay, the 5-2-1 will drop out of the start circuit as the compressor speeds up so it is much better than the PTC. Unlike the OEM start kit, the universal relay and capacitor ratings will not be specific to the compressor requirements.
The ICM805 may be the best aftermarket choice. The ICM805 has a universal capacitor and a relay that operates on compressor current. The ICM805 also has a built-in current sensor that engages the capacitor for precisely the correct amount of time. This ensures maximum starting torque while protecting the start windings