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HVAC School - For Techs, By Techs


Nov 8, 2022

In this short podcast, Bryan busts the myth that hard starts reduce the start current on the run winding of a compressor.

A single-phase motor’s main winding is the run winding; it has a lower resistance and a higher current than the auxiliary winding, also known as the start winding. 

Hard start kits are often used on HVAC systems with single-phase compressors (which usually have PSC motors). These kits usually consist of a start capacitor and a potential relay, which takes the start capacitor out of the circuit. We don’t typically use hard starts on three-phase motors or ECMs.

Single-phase compressors often have to start under a big load, especially in long-line applications (at the manufacturer’s recommendation) or if the compressor simply has a hard time starting. In cases where you have a voltage drop or low voltage, particularly due to long branch circuits, you may also use a hard start kit. However, they do NOT reduce the starting current or “save” compressors.

Hard starts reduce the time-averaged starting current because they get the compressor to start up more quickly (therefore, the starting current is higher for a shorter time). However, hard starts do NOT reduce the spike of current upon startup. Like run capacitors, hard start kits allow current to flow on the start winding, but the run winding current stays the same.

Hard start kits boost the start winding current faster, not at a lower current, reducing how long the system is in locked rotor. If they stay in the circuit too long, they could overheat the start winding and need to be taken out. Therefore, we don’t want to use hard start kits without careful consideration.

Soft starts are different entirely; they CAN reduce starting current.

 

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