Choosing A Hard Start Kit

Most single-phase Air-Conditioning Compressors have a run capacitor to aid in compressor-run efficiency, but this does little to aid in compressor-starting torque. Therefore, sometimes it becomes necessary to add a “Hard Start Kit” to our single-phase PSC compressor. Many refrigeration compressors come with the factory hard start kit installed.

Hard start kitA hard start kit consists of two main components, a start capacitor and some type of relay. The Start capacitor wires into the compressor start winding, which will add starting torque to the compressor. The start capacitor can only stay engaged for a second or two, and then must be removed from the circuit.  If the start capacitor is left in the circuit too long, the compressor and or start capacitor may be damaged. The compressor may also go off on internal overload. A relay is used to remove the start capacitor from the circuit once the compressor has reached approximately 80% full speed.

The OEM Hard Start is always the best choice.  For larger A/C compressors, this hard start kit uses a potential relay that will open when the back EMF (voltage generated in the compressor windings) reaches the voltage set in the relay coil and opens the start relay. This drops out the capacitor at the manufacturer’s selected moment. Both the capacitor and the relay are specific to the compressor.  

The after-market hard start kit with potential relay, such as the Supco Super Boost 3W1 and 3W2, is one of the next best choices to the OEM. This kit has a start capacitor and potential relay like the OEM, the difference is that the ratings on both the capacitor and the relay are of a universal range. One size will fit 1-3 tons and the other will fit 4 and 5 ton. Other than that, this works just like the OEM.  

A current-sensing Hard Start kit like the ICM803C is another good choice. This hard start uses a current-sensing relay that removes that start capacitor from the circuit once the compressor amperage drops off, indicating that the compressor has started. Like the OEM start kit, the compressor operation is proven before the capacitor is removed from the circuit, which protects the compressor from overheating or damage.

The last, and most likely cheapest, choice is the PTC hard start kit. This kit normally has only two wires to install, as it plugs in to the run capacitor. This hard start kit will have an oversized capacitor that drops out of the circuit more based on time then compressor speed. The PTC start kit may stay in the circuit too long or not long enough. Also, the PTC start kit may not function after heating up as a result of repeated starts or during periods of high temperatures.  

The OEM start kit is always the best choice; however the 5-2-1, ICM803C and ICM805C are good second choices as they both have a relay that monitors compressor start-up speed for proper operation. These are the best choices for newer compressors. The PTC-style start kit like the ICM855C and ICM856C are a good choice for an older compressor that you are just trying to get another summer out of, but not so good for long term.

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