110 volt devices that will work for us:
Micro Air Easy Start
Hyper Engineering Sure Start
Looks nice but only 220 so far:
Emerson CSS Compressor Soft Starters Looks like a more professional version of Hyper Engineering, possibly made on contract. Not for US market at this time.
Everything else I've seen, including the 5-2-1, is a hard start capacitor.
Run capacitors change the phase between the
stator and
armature (rotor) so that the motor can run in
quadrature phase. In general the run capacitor is in series with the stator or armature windings.
Hard start capacitors increase the capacitance while the rotor is starting, because the run capacitance will
not be enough for quadrature phase at the lower starting speed, and the stock start capacitor might be too small. They use a contact and delay device to switch the extra capacitor in for a moment while the motor is starting, and then remove it from the circuit.
Soft start or smart start devices can switch in a capacitor and limit the overall current. In general current limitation is done using a triac to chop out a portion of the AC wave. Some devices have a relay or contactor that bypasses the triac once the motor is running, to eliminate any resistance of the triac or to increase the lifetime of the triac.
Soft start devices claim learning and optimization for a specific motor, but there isn't much real documentation of how they work. Comparing one to the other would take laboratory equipment - which I happen to have, but I'm not volunteering to buy multiple devices just to compare them.