OT: Circuit breaker + shunt trip = one-time isolation contactor ?

I have an unusual application where I need to connect two large VFDs to one motor, in a "hot standby" mode where the Secondary can isolate the Primary and take over control.

If there were a 150A three-pole Form C mechanically linked break-before-make contactor on the market, that would be great. But as far as I can tell there is not.

So I'm going to propose that I install an ordinary molded case circuit breaker downstream of the Primary drive, with a shunt trip coil that allows my control system to open the breaker and isolate the Primary drive.

The Secondary drive will still have an ordinary isolating contactor so that it's not back-fed. An auxiliary contact on the circuit breaker will prevent that contactor from closing if the circuit breaker isn't tripped to isolate the Primary drive.

This circuit breaker will not perform any protective functions. The system demand is such that it's OK to go reset it by hand if it ever has to be tripped.

Am I missing anything obvious about using a circuit breaker with a shunt trip solenoid as a disconnecting means ?

Use the contactors but use miniature control relays to send stop signal to the primary drive and turn off contactor.
Then turn on contactor for secondary drive, then send start signal.

If you want to run both drives on 1 Circuit breaker....use 2 additional contactors on the primary side of the VFDs. Interlock the contactors so only 1 can be on at a time.
You could also use a contactor on the primary side also to completely shut down the drive not needed. This is best because the unused drive is now shut down.
You can still do this way if using 2 CBs, 1 for each drive. This way if Primary drive trips CB the secondary drive will still have power available.

Using the shunt trip on a CB is sort of okay. It is getting to the point in a lot of plants where an operator does not have the right to reset a tripped breaker, they have to call maintenance. This is a good thing because the CB may have not just been shunt tripped, there could be another problem.

Have you thought about using a forward/reversing starter for this? With the factory installed mechanical/electrical interlocks there would be no chance of a backfeed.

We've installed similar setups in the past when we wanted a bypass setup for a VFD controlled motor.

I am currently working in a similar project. Everything I have ever encountered this the application used two reversing contractors, one line and one load. One thing to note is that you never want to start the drive with the load side contractor open then close the contractor. Another question is what happens when someone flips the panel into backup while it's running.

I'll be using some timing relays and interlocks from the reversing contactors to force the first drive to start, disengage the line and load crime drive one, engage load on drive two, engage line on drive one, then start only after a new start command. There are of course multiple ways to attack this problem. Each to their own.

It seems like there should be some way to rig up an automatic transfer switch to do what you are trying to do. Maybe it would be worth a phone call to a couple of manufacturers to see if they have any ideas.

How "Hot" is your standby?
If the motor is still turning then you must have "catch a spinning motor" functionality on the drives.

Sounds like a perfect fit for an ATS like the ones used on gensets. Some have a lot of intelligence built in if you wanted monitoring too.

Me I'm thinking like COW. I recently was forced (because part after part failed) to rebuild a 250 hp compressor. The compressor was star-delta and had mechanically interlocked class 5 cutler hammer contactors for switching from star to delta.

I would look at the Eaton NEMA rated contactors. The catalog states:

For reversing applications, two contactors are supplied on a common base with electrical and mechanical interlocks which prevent both contactors from being closed at the same time.

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<disclaimer - I work for an Eaton Electric distributor >

This sounds creative. I like it!

If money would permit, I'd go for an ATS as suggested above. Last year I had to put one in on a genset - we originally quoted Eaton, but after 2 months of useless support with the required parts and design we ditched them and went to Schneider. Had it up and running in a couple of days. Although that's just due to the respective agents, not due to the quality of the products.

But if cost is king, then I can't see any reason your idea with the shunt trip and contactor wouldn't work - obviously in either case there's the flying start issues mentioned above, but I'm sure you of all people have already worked that out

It's done all the time with mechanically interlocked contacted, referred to above as "reversing" contractors, although for reversing purposes the phases are swapped, which you don't want. So use them, but re-wire the power conductors. Using mechanically interlocked contractors is inherently the same as having a "Form C" set of power contacts in that they cannot both be on at the same time. Doing it this way not only accomplishes the changeover, but eliminates the need for the additional isolation contactor in your scheme.

The problem with using a shunt tripped CB will be that in general, constantly shunting the breaker will wear it out. They are not made to do this. The trip mechanism inside is just a little steel pawl catching on a tooth, like a ratchet mechanism, the shunt coil just releases it. Using it over and over wears out that little pawl and/or the tooth it catches on.

As had been said, use additional aux contacts of each contactor in your VFD Run Command lines so that you cannot energize a contactor while the VFD is in a Run state, otherwise you can smoke the VFD transistors. Using the aux contact means that the VFD Run command is isolated until AFTER the contactor is already closed.

Thanks, guys, for all the advice.

Interlocked contactors won't work because of issues in the upstream power supply failure modes. It's logically the easiest method, but in this particular application I can't have a "power ON to engage" mechanism.

From this discussion and talking with a power distribution engineer, I'm convinced that the circuit breaker and shunt trip won't be reliable enough both because of wear from testing and because the shunt trip mode isn't part of their MTBF simulations and calculations.

So I'm going to get the guts out of an ASCO 300-series D-frame automatic transfer switch. I could buy ten of them for the amount of engineering labor we've spent on meetings debating the matter so far.

Ken Roach said:

... I could buy ten of them for the amount of engineering labor we've spent on meetings debating the matter so far.

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Been there... I know how you feel.

I just finished a series of meetings debating the need for a local disconnect on a 500HP drive that is being fed from an MCC feeder CB in another room. I said from the beginning that it wasn't worth debating, just put in a local disconnect interlocked with the VFD door because it's safer and far far cheaper than having to add it later after an unfortunate accident exposes that it was a risk to not have it. They have now spent far far more money in debating it than they needed to and ended up deciding to do it for the exact reason I stated at the outset.