Softstarter for refrigerators are not very useful. You still have to handle defrost current. Some have high amperage heaters with short duty cycles, some have lower amperage heaters with longer duty cycles. One of my older GE refrig's defrost heater drew 1000 watts. Lowest I have seen is about 100 watt defrost heater.
Most conventional refrig's use a positive temp coefficient thermistor in series with compressor start capacitor to take starter capacitance mostly out of circuit after compressor startup. You have to have some off time between cycles to allow thermistor to cool off enough to bring start cap back in effect for startup. If thermistor is still too warm then you don't get the extra rotational startup torque from start cap to provide 90 deg offset current to start winding of compressor.
For air conditioners compressors, they often only have a run capacitor to the start winding. The run capacitance value is set for proper 90 deg phase shift during running operation. It also improves motor run power factor. During the half second startup, the motor inductance is much lower, so it needs a larger capacitor value (about 3x to 5x the run cap value) to get optimum 90 deg current offset between run and start windings of compressor for best rotational torque during the startup period. This extra start capacitance must be taken out of circuit after motor comes up to speed or it will likely burn out compressor start winding. The larger capacitance start capacitor is the black plastic cased capacitor which achieves its greater capacitance for its size by using a bi-directional AC electrolytic capacitor fabrication. These high capacitance value start caps have a much greater internal resistance than a foil run capacitor and can only be active for a short time or they will overheat and destroy themself.
A hard start kits provides the extra start capacitance, but it only shortens the startup period to about 0.2 seconds and still has the same peak current to slightly greater current during the shorter 0.2 seconds start period. They use a relay that relies on back EMF voltage from start winding rising when motor comes up to speed to disengage the large start capacitor. This is dangerous to compressor start winding if the relay fails and leaves the start capacitor in circuit during the run period, not to mention it also destroys the start capacitor.
What the Easystart, RVsoftstart, and other units do is combine the hardstart starter capacitor boost along with voltage ramp up of a softstarter. This gives additional rotational torque, but the voltage ramp-up lowers the peak current. They will typically get a 12kbtu RV air conditioner from a startup surge of 50 amps down to about 15 amps. This allows the 12kbtu A/C to be run on a small and quiet 2200 VA inverter-generator. The electronics also has other fail-safe checks that will shut down the unit if current is not correct, like caused by a start capacitor failure. Once compressor reaches run speed the unit has a bypass relay that takes the softstarter out of circuit. The newer EasyStart units include BT RF link that allows you to monitor startup and run currents on a smartphone.
I have been running an EasyStart 368 on my home 4 ton central air conditioner for over two years. It really helps reduce compressor and contactor stress during startup, besides reducing startup surge current so my inverter can run my central A/C. It also provides additional safety checks that can prevent a compressor failure due to a run or start capacitor failure. The compressor will not be allowed to sit and grunt with a high current locked rotor condition due to a bad run capacitor.
