# VFD- Closed and Open transit Synchronous Transfer



## rgshari123 (Dec 23, 2013)

Dear All

I am in the process of evaluating a VFD requirement for one of my clients where in we have decided to offer pump set along with VFD. In total there are 2 Pump- motor set driven by VFD + PLC for each pump set. But I am in a dilemma of deciding the exact definition of below terminology which is part of the specification. 

1. *Closed Transit Synchrous Transfer:* One of my vendors say that he can offer the captioned requirement but there will be delay of 100 milliseconds during the VFD transfer. Also there is an another clause stating a requirement of Online Synchrous Transfer VFD. In this scenario, I think the specification is clear in telling that there is should be no delay in VFD transfer to one another in case of any failure in either of the 2 VFDs. But my vendor is still claiming that his vfd can comply to this clause but still there would be a delay of 100 millisec. In a critical gas pipeline generally if there is a VFD failure there should not be a slight delay since hammering might affect the whole pumping process. But please clarify whether the vendor is actually concealing his deviation on this requirement?? .Please help and clarify. 

*2. Open Transit Synchrous Transfer: *By any chance, Can a VFD controller offer an Open transit synchrous transfer for the above mentioned requirement. In short can I conclude that my vendor is actually proposing a open transit synchrous transfer?

It would be grateful if any VFD expert could clarify my above query as soon as possible

Regards
Mohan


----------



## KennyW (Aug 31, 2013)

The proposed solution is not in compliance with the spec. They're asking for a true synchronous transfer where everything is swapped in-phase. 

If you're quoting the job, or trying to procure the drives based on a certain budget, and the 100ms solution has a significant advantage in terms of cost or some other factor, either quote it as an option, stating the transfer time, or ask the process engineer is 100ms is tolerable. 

Typically the torque decays at a rate of 1% per 10ms so a 100ms transfer will cause a 10% torque sag. The engineer can easily do the water hammer calcs to determine if that would be an issue.


----------



## JRaef (Mar 23, 2009)

rgshari123 said:


> Dear All
> 
> I am in the process of evaluating a VFD requirement for one of my clients where in we have decided to offer pump set along with VFD. In total there are 2 Pump- motor set driven by VFD + PLC for each pump set. But I am in a dilemma of deciding the exact definition of below terminology which is part of the specification.
> 
> ...


For synchronous transfer to take place in a system using a VFD, there cannot be any difference in the output frequency of the VFD and the line frequency of the bypass. Otherwise, it is like connecting two generators out of synchronicity; there could be a massive torque transient that could have catastrophic consequences. I have seen motor shafts twisted off by this.

So if you are using the VFD just to accelerate the load, then the synchronous transfer takes place at the point when the VFD gets the motor to full speed. But even then, the output frequency must still match. So the VFD front-end tracks the input line frequency and if there is a mismatch, the VFD output can be changed to make them match. But that takes time and I would say that 100msec is extremely fast. 

Zero is impossible...

But what you seem to be describing is actually an emergency bypass between two VFDs feeding the same motor should one of the VFDs fail. That then implies a different scenario in which there is a possibility that the failed VFD was running at some specific frequency and then suddenly tripped off-line. That means that the backup VFD must then detect the failure of the primary, then detect the motor frequency, then match its output frequency to EXACTLY what the rotor frequency is/was. That is similar to what is called a "Flying Restart", except you are asking for that to happen instantaneously, they are saying that 100msec is as fast as it can be accomplished. again, 100msec is very fast for all that to take place if you ask me.

All that said, the only way I can see for this to take place is of BOTH drives were ALWAYS connected to the motor at ALL times. This scheme is referred to as "N+1" redundancy. In this scenario, you must have two parallel drives, EACH one sized for the FULL motor amps, with them either sharing the load across both drives, or one in stand-by but connected and running all the time. If one goes down, the other one is ALREADY running the motor, so there is no "transfer" so to speak. If the specifications wanted N+1 redundancy functionality but used the term "synchronous transfer" instead, therein lies the problem. They are different applications.

Not every VFD mfr can offer N+1 redundancy by the way.


----------



## KennyW (Aug 31, 2013)

JRaef said:


> For synchronous transfer to take place in a system using a VFD, there cannot be any difference in the output frequency of the VFD and the line frequency of the bypass. Otherwise, it is like connecting two generators out of synchronicity; there could be a massive torque transient that could have catastrophic consequences. I have seen motor shafts twisted off by this.
> 
> So if you are using the VFD just to accelerate the load, then the synchronous transfer takes place at the point when the VFD gets the motor to full speed. But even then, the output frequency must still match. So the VFD front-end tracks the input line frequency and if there is a mismatch, the VFD output can be changed to make them match. But that takes time and I would say that 100msec is extremely fast.
> 
> ...


The functionality they require is on-line transfer, which is another way of saying N+1. The backup drive is online and actively keeping phase-sync with the primary.


----------



## JRaef (Mar 23, 2009)

KennyW said:


> The functionality they require is on-line transfer, which is another way of saying N+1. The backup drive is online and actively keeping phase-sync with the primary.


Assumed, yes, but not stated by the OP as being in the specifications. What I meant is if the specifications he gave are exactly as he depicted, someone has made an error in describing what they want by using a term that misses the mark.


----------



## KennyW (Aug 31, 2013)

He says there is a clause saying "... Online synchronous..." The term "online" has a very specific meaning in this context. 

However, bottom line is still like I said initially. The 100ms delay should be noted in an exception to the engineer for approval. Simple as that.


----------



## rgshari123 (Dec 23, 2013)

Thanks Kenny and Jraef!!

I would like to clarify that 2 VFDs are not in parallel but with By-pass option. In this case, is it possible to achieve closed transit with zero delay. ??. If yes, how it can be acheived. Please guide.


----------



## JRaef (Mar 23, 2009)

rgshari123 said:


> Thanks Kenny and Jraef!!
> 
> I would like to clarify that 2 VFDs are not in parallel but with By-pass option. In this case, is it possible to achieve closed transit with zero delay. ??. If yes, how it can be acheived. Please guide.


I'm still not clear on what you have. 

2 separate VFDs, one for each pump, each one with a DOL bypass? And you want to know if it is possible to bypass into DOL with absolute zero transition delay? If so, is it accepted that the bypass to DOL is taking place ONLY at full speed? If not, what is the purpose of the bypass then? Usually in a case like this the DOL bypass would be to eliminate losses in the VFD when running at full speed. If not, then I don't understand what someone is trying to accomplish here.

What sometimes happens is, engineers don't think this sort of thing through correctly, they mis-associate a bypass contactor on a soft starter as being the same as putting a bypass system into a VFD. They are NOT at all the same, so they cannot be thought of in the same way. Much more thought must go into designing a VfD bypass than just throwing out some terms in a spec without clear definition. 

The other possibility is that the engineer already had a specific system in mind when he wrote the spec, but never stated it (or he did and you are looking for an alternate source). Was there something specific mentioned in the spec like that? If so, let us know, it might help to see what the working concept is.


----------

