Yes, but does that matter? It's still a transient event.
More important are other serious problems with them, in my opinion.
1) They inherently feed the motor with unbalanced voltage and current, which is heating the motor disproportionally to the current it is drawing, or looked at another way, the current draw is not all going toward acceleration, a larger portion is wasted as heat, which can increase your risk of failure or decrease the amount of current limitation you attain and still accelerate the load. That can have serious implications on all but the lightest of loads, so unless it is a centrifugal pump or fan, I would never use them.
2) In any SCR based AC controller, there is a slight risk that the right combination of shorted SCRs will allow for unrestricted current flow to the motor with no means of stopping it. For that to happen on a full 3 phase (6 SCR) designed soft starter, you need at least TWO shorted SCRs, and they must be in different phases, in order for there to be a valid path through the winding. This risk is generally accepted to be about the same as welding two sets of contacts in a contactor, which would have the same effect. However in the "2-phase" soft starter design, where there are only 4 SCRs and the center phase is just a bus bar, so therefore the first half of that failure path is ALWAYS present, meaning any single SCR shorting means losing the motor. So to avoid having that happen, you MUST use an up stream device with an air gap, such as a contactor, or if you use a circuit breaker, the breaker must have a Shunt Trip tied to the Fault contact of the soft starter. Many, if not most, of these 2-phase soft starters are sold as low cost alternatives, so they do not have a separate contact indicating that one of the SCRs has failed vs an Overload Trip or some other protective function. That then means that ANY time there is ANY sort of Fault, it will open the circuit breaker or drop out the contactor, and you will have no idea what the fault was.
So in my opinion, what you gain in getting a lower cost of the soft starter requires that you must sacrifice reliability or be willing to put your motor at risk, or both. Not worth it.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
Thks jraef, we are replacing obsolete swbd in very small compartment on ship. We are looking to improve safety at same time with a better construction form 4b swbd and in doing this we are changing from what was previously autotransformers to softstarters. The designer is proposing 2ph control, not sure if that is due to space contraints will find that out shortly.
Normally we have two ships gens 480kw, in emergency we have a 420kw machine. It is 90kw standby fire pump that in bad situation may be called upon to run in emergency mode, currently AT starter. I've read of instances were soft starters cause problems during ramp up contary to all the literature saying otherwise. We didnt want to be one of those cases, especially when main generation failed on ship. Hence the question. We would do testing to find out if it will be a problem after install but dont would rather mitigate against it now if we can establish it would be a problem.
I had read on abb literature the first point you raised, but had not come across discussion on the 2nd point. A very good point you raise though. I will take those points back to designer.
Soft starters causing problems with generators is an old story, but still has legs apparently. When ramping, soft starters do create some noise on the line and the harmonics mentioned earlier. Older generator AVRs had a nasty habit of reacting to this by allowing voltage and frequency to swing. Soft starters based on an old firing circuit originally developed in South Africa by SAF (which became Saftronics, who spawned Benshaw and several others) used a simplified phase-lock-loop sensing circuit to determine the zero-cross of one phase, then used timing to assume the other phases would follow suit at 120 degrees. If the AVR started dancing because of the soft starter, it made those soft starters (and all their kin), start dancing erratically too and they eventually wound up making one or the other, or both, fail.
A different firing circuit developed by a company called Vectrol, which became Westinghouse (and spawned most other soft starters on the market), used individual phase-locked-loop sensing on each phase to determine the firing angle of each one in turn, no ASSuming. So if the frequency started swinging from the AVR, the soft starter firing swung right along with it and just passed it on through to the motor, which couldn't care less. The Vectrol design (and all of their kin) never had that problem. But because Saftronics became very big in the 80s soft starter market, there were a lot of spectacular failures in generated power applications, such as marine fishing vessels (where I learned all this). It forced generator mfrs to look into it and solve the problem at their end. For a while in the early 90s, CAT even sold a retrofit kit for older AVRs, which was basically a filter for their sensing circuit. Now they have all revamped everything anyway and it's not an issue, plus almost all soft starters have gone to digital firing circuits anyway. It still used to come up once in a while when someone is using a very very old generator with an old soft starter, but it's been a long time now since I've seen it.
All that said, the cures were all based on the standard 6 SCR soft starter model, nobody was using the 2-phase design yet. No telling if that might raise the dead. I wouldn't, there is ZERO advantage, other than a slightly lower price, to using 2-phase.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
I'd not heard of that before, it wasnt the generator i was thinking, more a case of anything sensitive that might be effected by distrupted waveform, like this case here.
Interesting. 25+ years in the soft starter business and I have never once seen anyone worry about the harmonics, other than that issue with the interaction with AVRs. Even in that case, the mitigation was done on the AVR sensing circuit, not the soft starters. They mention "if the ramp is too long..." (or words to that effect) but in reality if you ramp a motor for too long with a soft starter, it's going to trip on overload. 60 seconds is the longest I've seen, but only on a centrifuge with a specially designed motor and unless your ship is a floating sugar mill, I doubt you will have anything like that. The average pump is 5-10 seconds ramp time.
Note also that the article mentions that the only thing that was really affected was the phase failure relay nuisance tripping, so they disabled it if the generator was running. I would have probably just tried a better phase failure relay...
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
No sugar, well except in galley! It is an old ship though, built 1983 with original ships gens onboard so will look out for other probs you mention. It would seem 3ph control would be the best way to go, and even then it sounds like the cases of things falling over due to immunity issues are few and far between
I guess we'll just have to sort out a fix during set to work if probs arise. Overall we are looking for safer switchboard and in the space constraints i think our only best option is softstarter. Hoping we can fit 3ph controller into the design of switchboard in that case.
I wouldn't even attempt to use 2-phase control on a generator application. Often, a generator can barely handle the load without adding the extra current and the current imbalance the 2-phase control causes.
Technically, a 2-phase control should have a contactor in series with the starter to isolate it when stopped. Some electrical codes even require it (specifically calls-out that it's not allowed to only switch 2-phases on a motor)
Lionel, is that a local code thing? The NEC requires that each phase be protected from overload, but does not restrict the use of 2-phase control. I ask because it would be good to know that some areas are forbidding it and which ones they are. I'd like to use that to lobby California to add that to our state amendments to the NEC. I'm seeing a lot of OEM HVAC equipment coming in with 2 pole contactors used on small 3 phase motors where the motors have integral thermal protection, so there is no actual OL relay. It causes issues in the field based on people ASSuming they are single phase motors and although technically not illegal, these issues could be avoided. I also recently started seeing this showing up in elevator controls, which given the extra safety issues typically piled on in that industry, has me confused as to how that could pass muster. I think it's a dangerous trend.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
