Softstarter to be used for safety stop ?

[JesperMP]

I know this is probably a long shot, but who dont ask will never know.

You probably know that some newer VFDs are approved to be used as safety stopping devices.
My questions is: Are there ANY softstarter that can be used as a safety stopping device ?

The background is that we have some motorstarters that we would normally provide with double contactors in series to achieve the desired safety category. And then we also install softstarters in some cases. It just appears to me a lot to have 2 contactors + 1 softstarter for one motor. If I could cut down on the complexity by using 1 contactor + 1 (safety approved) sofstarter, it would be a good thing.

 

[TConnolly]

Softstarters use Triacs or back to back SCRs for current switching. Triacs and SCRs nearly always fail in a shorted condition, meaning they fail ON. This would be your primary concern in any safety application.

At the very minimum you would need a softstarter with a shorted SCR detector that can be used to open a safey contactor. If there is any chance that multiple simultaneous failures cound seriously injure someone then I wouldn't recommend using the soft starter as a primary safety element.

 

[panic mode]

can't you use just two safety contactors for group of motors (each with additional contactor, VFD, softstart) or this is still too much?

 

[JesperMP]

Alaric. If a VFD can be used then in principle a softstarter can too.
I am aware that the tricky bit for the manufacturer is to detect if there is a short in the power electronics.

Panic. In principle we could, but the common contactors will become awfully big. And it is possible but not trivial to connect a bus with big dimensions to significantly smaller contactors.

I know it is a long shot, but it would be great if it was indeed possible.

 

[TConnolly]

Alaric. If a VFD can be used then in principle a softstarter can too.

That depends entirely upon the electronics used by the manufacturer of the drive. By comparrison, a soft start is usually nothing more than some SCRs and circuitry to monitor current and ramp up the firing of the SCRs. Drives and SCRs are very differnt in operating principle.

 

[JesperMP]

That depends entirely upon the electronics used by the manufacturer of the drive. [..]

I know well that a contactor and a softstarter and a VFD are fundamentally different devices.
However, IF a sofstarter is approved as a safety device, then the manufacturer must have taken everything into consideration.
So I dont intend to discuss how to make a softstarter OK for safety applications. I just want to know if someone somewhere has seen such a thingamabob.

Btw. We are talking 200kW - 450kW size sofstarters.

 

[jraef]

AFAIK, there is only ONE VFD that has achieved any type of approval for their safety circuit, the Schneider ATV71 (which is also the Toshiba AS1, but Tosh. doesn't have the same safety approval). Their "Power Removal" feature meets an EU standard, EN 954-1 category 3 and EN 61508 SIL2. Keep in mind, these are not power removal standards for personnel safety while working on electrical equipment and should NOT be construed to be the same as real power removal, even though that is the way the French translation came out for their sales brochures. It is the equivalent of CONTROL power removal, not line power removal. All SIL2 says is that there is a lower risk of inadvertant starting than "normal" (SIL2 is only the 2nd level out of 4, with 4 being the least likely). Schneider attained that rating by showing a low likelihood of catastrophic multiple component failures necessary for operation of the motor and a high degree of stability in the control circuit lockouts necessary to prevent the inverter firing the transistors in such a way as to make the motor turn. Keep in mind that a VFD has 2 stages of power delivery, conversion and inversion, so a lot of problems need to happen on both sides to make the motor turn. Like all other drives however, it still has the inherent risk of component failure from a line power isolation standpoint.

Soft starters of course all have that problem as well. In addition, the SCRs don't need to be "forced" to fire, they can (and often do) self commutate due to line noise, spikes etc. That, combined with a higher risk of component failure that could result in operation (remember, there is only one stage of power control in a soft starter vs a VFD), leaves soft starters much less able to attain any kind of SIL level approval. Nobody that I know of has even attempted to attain one yet, the concept is too far fetched.

A good point brought up earlier however is the issue of using a circuit breaker with a shunt trip as the SCPD of the soft starter. I know that Ford Motor Co., who has set a lot of machine power control safety standards over the years, will accept soft starters with a line isolation contactor and a shunt trip circuit breaker tied to a shorted SCR detection circuit as a safe combination package. I heard that Siemens is working on integrating a soft starter into a motor protection switch as a single unit so that they can open the disconnect portion in the even of a fault and this may allow them to get SIL1 or 2 if they pursue it, but I have not heard if they are doing so.

 

[JesperMP]

AFAIK, there is only ONE VFD that has achieved any type of approval for their safety circuit, the Schneider ATV71 [..]

I know that both Siemens and AB have VFDs with safety stop approvals.

Keep in mind, these are not power removal standards for personnel safety while working on electrical equipment and should NOT be construed to be the same as real power removal, [..] It is the equivalent of CONTROL power removal, not line power removal.

I am aware that in addition to the safety stop there must be a means to remove power for repair work and such.

[..]Schneider attained that rating by showing a low likelihood of catastrophic multiple component failures necessary for operation of the motor and a high degree of stability in the control circuit lockouts necessary to prevent the inverter firing the transistors in such a way as to make the motor turn. Keep in mind that a VFD has 2 stages of power delivery, conversion and inversion, so a lot of problems need to happen on both sides to make the motor turn. Like all other drives however, it still has the inherent risk of component failure from a line power isolation standpoint.
[..](remember, there is only one stage of power control in a soft starter vs a VFD), leaves soft starters much less able to attain any kind of SIL level approval.

Right, a softstarter only has one step between the incoming and the outgoing power, and so it is easier to fail than a VFD. However a contactor also only has one step. Its contacts may weld. It is trivial to detect that the contacts have welded by means of an aux contact. So in order to be as safe as a contactor, a sofstarter must have safe detection of shorted SCR's. Like you are saying later, someone are allready thinking along the same lines.

A good point brought up earlier however is the issue of using a circuit breaker with a shunt trip as the SCPD of the soft starter. I know that Ford Motor Co., who has set a lot of machine power control safety standards over the years, will accept soft starters with a line isolation contactor and a shunt trip circuit breaker tied to a shorted SCR detection circuit as a safe combination package. I heard that Siemens is working on integrating a soft starter into a motor protection switch as a single unit so that they can open the disconnect portion in the even of a fault and this may allow them to get SIL1 or 2 if they pursue it, but I have not heard if they are doing so.

This is exactly what I am after. In my case there is allways at least a motorprotection and a contactor and a softstarter. I didnt until now think about combining the softstarter with the motorprotection in order to achive the safety.

edit:
I didnt mean to turn this into a discussion of Softstarter vs. VFD. I mean to compare Softstarter vs. contactor.

I should also mention what made me get this idea.
In a description on how to hook up a Siemens safety relay, it says that the outputs must be connected to "contactors or similar" (in a redundant combination). I am wondering what the "similar" is.

 

[Tom Jenkins]

I have had soft starts fail by conducting power. Would a shunt trip breaker provide the required protection?

 

[jraef]

Jesper,
Typically, a shorted SCR detection circuit is nothing more than a volt meter across the SCR path in each phase. SCRs should have about 1-1/2V drop acrosss them, so the circuit looks for a LACK of a voltage drop to determine that the SCR is shorted. The problem is that even if it detects it, there is nothing the control circuit can do about it besides open up some sort of an air gap device somewhere, i.e. a contactor, circuit breaker with a shunt trip or a motor protective switch. The other risk is in "false negatives" meaning that there can be some timing issues regarding that type of circuit because if you have control power but not line power, you also have an equal condition, i.e. no voltage drop across the stack (because there is no voltage at all). The delay necessary to filter out that condition will likely get in the way of SIL approval implementation. Also, the condition of one shorted SCR or even both in one phase is not inherently dangerous from an operation standpoint, because there is no path for current flow. You need more than one SCR in at least 2 phases to make that happen. Unfortunately, this simple shorted SCR detection circuit cannot tell if it is one or six shorted SCRs, so it would need to shunt trip just in case which may be a nuisance in "must run" applications.

Motortronics has a somewhat unique shorted SCR detection circuit that is a step above what others provide. Their system, in addition to the above, also looks for current flow through any phase when the starter is supposed to be Off. this discriminates from a simple single short in that it must have multiple shorts to affect a shunt trip condition. It also serves to detect a welded bypass contactor because the effect is the same, power flow through the circuit. Unfortunately I don't believe that Motortronics has ever applied for SIL approval, in fact I doubt there is anyone there who would know what that means. It might be worth asking them though.

 

[JesperMP]

jraef.

Very interesting stuff what you tell there. Sounds like that in principle there is a possibility to use a softstarter as a safety stop device.
But I dont want to create a do-it-yourself safe motorstarter combination. I was hoping that there was allready a completely finished system from one of the major brands, with approvals and all.

So if noone knows any, then let us finish this.
Maybe I can ask again in a few years time.

 

[JohnW]

Soft starts cannot provide a safe switching device, if you need a Cat 3 (EN 954) or SIL 2 (EN 62061) then you will need 2 contactors in series or 1 contactor and a disconnector with a undervoltage trip (a shunt trip is not fail safe).

It is unlikely the failure mode of a soft start will allow it to be used as a safety switching device. It is possible to use electronic devices as part of an emergency stop circuit but only if the safe failure fraction is above the limits laid down in EN 62061. I any event for SIL2 a single failure cannot lead to a dangerous situation no matter what the reliability of the components.

 

[jraef]

Well that sounds like a definitive answer.