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This is Not a Soft Start
When I started in the trade I would see these devices (above) in the field and I would hear guys call them a PTC or a “Soft Start” and I just accepted it and moved on. In fact, I've been calling PTCR (Postive Temperature Coefficient Resistors) a “soft start” for most of my career.
Turns out I was wrong
A PTCR is just a resistor (thermistor) that increases in resistance as it heats up. It connects from run to start in parallel with the run capacitor and allows a surge of current the start winding of a motor when it starts. When the PTCR heats up, the resistance inside increases and it essentially takes itself out of the circuit.
A PTCR by itself does allow a spike of current to the start winding but it does not create a phase shift. This is why some devices add a start capacitor and just use the PTCR as the “relay” to take it out of the circuit like the products shown above. It is a start device… but there is nothing “soft” about it.
Then there are the more traditional hard start kits that use the tried and true start capacitor and potential relay instead of a PTCR. This serves the same basic purpose, increase current and phase shift to the start winding for a fraction of a second and then remove it from the circuit. It just does the “removal” part of the equation in a more precise manner.
All of these technologies serve to increase current to the start winding quickly then drop out
The idea is to get the motor to 75% to 100% of running speed as QUICKLY as possible by increasing start winding current and phase shift.
There is a good reason for this. When a motor is stationary or running at low speed, it's windings act as low resistance resistive loads, essentially really high amperage heaters. The longer the motor spends trying to start at full voltage the higher the current it will draw and the hotter the windings get.
Hard start devices can do nothing to actually reduce the current the motor draws when it is at locked rotor (stalled), the hard start device simply gets it out of that stalled / low RPM as quickly as possible.
Many of you may note that when you measure inrush current with a hard start in a place that it will show lower than when it is not in place. This is simply because a hard start shortens the time the motor remains in the locked rotor, not because it actually reduces the starting amps.
There are also some concerns about the added torque that a hard start provides over such a short time period and the side effects of that “torque shock” to the internal components of the compressor as well as the connecting copper lines.
Different methods of “soft starting” have been in use in large 3 phase motors for a long time. The purpose is the start a motor more slowly and therefore reducing the current inrush.
The devices shown above are single phase soft start devices. They reduce the voltage during starting to reduce the current associated with the start. These devices carefully control the voltage and therefore the current applied to the start and run windings to provide a lower initial current during start and slowly increase up toward full speed. These devices require advanced algorithms to do this which makes them significantly more expensive than the traditional hard start technologies and they are not capable of producing a large shifted / current boost to a start winding like a hard start.
This means that while a soft start is a great device to help reduce light flicker, decrease start amps and increase compressor life, it is unlikely that it will start that old, stuck compressor.
Soft starts and hard starts both serve a purpose but what they do and how they do it couldn't be more different.
Have an old, locked compressor? Hard start is likely the best bet. If you have a complaint that lights are dimming on compressor start? A soft start will give a better result.
Here is a pretty dramatic demonstration of hard start and soft start HERE
And a great application guide on soft start HERE