Soft start for single AC phase motor?

To control inrush current, I have used a varistor. A varistor is a resistor with a negative temperature coefficient. They are most commonly used to limit inrush current to switching power supplies. At start, when the varistor is at room temperature, its resistance is high, limiting current. It quickly heats up which drops the resistance to a fraction of a percent of the cold value.

In my case, I bought one that has a 10 ohm cold resistance and a 15 amp capacity. On a 240 volt line, the initial current draw would be less than 24 amps and probably disregarding the resistance of the load. As the varsitor heats up, its resistance will drop to around .05 ohms which mean a .75 volt drop.

I purchased mine from Digi Key for $2.40. https://www.ametherm.com/datasheetspdf/SL3210015.pdf A m,multitude of varistors with different specs are available. I would use a clip on ammeter to determine your steady state current draw and order accordingly.

There is a drawback in that the varistor needs some time after shutdown to cool down before restarting to obtain maximum inrush limiting. If you can allow several minu6tes between shutdown and startup, it shouldn't be a problem. Starting up after a shorter cool down period isn't a disaster. It just means the inrush current will be higher. Worst case, it won't exceed what the original inrush was.
 
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Approaching this from a circuit design perspective, your circuit breaker is just too small. You application is considered continuous duty since the load will be present for more than 3 minutes (the time is a grey area in code). This means that you must comply with NEC 430.22(A). The goal of the breaker is to protect the wiring from short circuit or ground fault current, not to protect the motor (that would be the job of a motor overload protective device). 430.22(A) requires the branch conductors and protective device be sized at least 125% of the motor FLA (so 22A * 1.25 = 27.5A). Since a 27.5A breaker isn't available, you would then size up to a 30A Class C (standard) breaker with 10 AWG conductors rated at 60C (NEC table 310.15(B)). You likely won't trip this properly sized breaker, however you could also opt to select a Class D breaker which will allow a longer duration of overload for high starting current loads.

You should also add a motor overload protective device. This is often an adjustable device that can limit the thermal build up in the motor. Motors rated more than 1 hp without integral thermal protection and motors rated 1 hp or less that are automatically started (NEC 430.32(C)) must have an overload device sized per the motor nameplate current rating (NEC 430.6(A)). The overload protects your expensive motor from catching fire in a sustained overload condition (e.g. heavily clogged filter) (e.g. seized bearing) and also protects your home/workspace from a dust fire.

Motors with a nameplate service factor (SF) rating of 1.15 or more must have an overload protection device sized no more than 125% of the motor nameplate current rating. Motors that don't have a service factor rating of 1.15 or higher or a temperature rise rating of 40°C and less must have an overload protection device sized at not more than 115% of the motor nameplate ampere rating (NEC 430.37).

I will add that soft starting does not change the requirement for more substantial conductors and circuit protection. Soft starting is used in industry to save money when high inertia load are frequently started or to ease mechanical stresses on connected components. In your application, neither of these are a huge concern so the soft starter is likely not needed (unless you want one just because). A soft starter is somewhat similar to a VFD that only operates during the motor initial acceleration and then couples to the AC line once at speed.
 
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macardoso said:
You should also add a motor overload protective device. This is often an adjustable device that can limit the thermal build up in the motor. Motors rated more than 1 hp without integral thermal protection and motors rated 1 hp or less that are automatically started (NEC 430.32(C)) must have an overload device sized per the motor nameplate current rating (NEC 430.6(A)). The overload protects your expensive motor from catching fire in a sustained overload condition (e.g. heavily clogged filter) and also protects your home/workspace from a dust fire.
Click to expand...

With vacuum motors this is a very common misconception. The work the motor is doing is to move air, when it is not moving air it is not working, A clogged filter means less air moving, less work being done, therefore less load on the motor because it is not moving as much air. That is why with a vacuum cleaner when you put your hand over the hose to block airflow you hear the RPM go whey up, by blocking the airflow you just removed the load so the motor can rev higher. When the hose is blocked you will also measure less amps due to the less load. Max load on the motor is with all hoses and filters and anything else that can restrict airflow removed. This is where you will measure the highest running current to the motor.
Now in the case of a restricted system e.g. clogged filter, the motor will not get hot but the air that is getting beat to death inside of the fan housing will get hot, potentially very hot because there is no cool air coming in, just the same air absorbing all of the motors output power. I have measured 400°+ air temp in a nearly blocked vacuum fan. It melted a lot of things in the machine, but the motor itself was nice and cool, it was running at crazy RPM with no load and that RPM making the cooling fan blow a LOT of air over it. The air temp was still climbing when we ended the test around 430°F due to the plastic of the machine melting. In addition to the thermal protection on the motor that was required for electrical protection, we also had to add thermal protection to the air stream to protect against thermal runaway from a clogged hose.

With an induction motor powering the vac fan, you do not get the real high RPM with no load, but you will still see the amps drop due to no load when the hose is plugged and the fan will still get very hot.
 
Guys, thank you for all suggestion. The problem is resolved since then. When I was looking for softstart device I could only find commercial devices costing almost the same as VFDs, so I just crossed them out. But, I found some electronic scheme which I thought to adopt for my situation. While I was looking for parts, I suddenly found ready made device on eBay (https://www.ebay.com/itm/174219086751) for very good price, so I went ahead and bought it. Works perfectly for me.
 
I believe limiting the the starts per hour is to reduce heat build up. This was the case at my former employer. But the motor was 5000 hp blower.
They were allowed only 2 starts per hour
 
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