3 phase 480 V VFD Contactor and Bypass A-T-L Contactor Pulling in together

[jd1250]

Our drive and bypass contactors are electrically interlocked only, without any mechanical ties. While I know you don't want these to close at the same time I am trying to understand the ampitude and waveform of them combined - can you help describe what out come voltage would be.

 

[Skogsgurra]

It will produce a chaotic waveform for a few milliseconds and then blow some fuses and possibly also the DC link capacitors. Perhaps even start a fire or some other interesting thing. It all depends on power involved, size of feeder and fuses and a few other things.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[DFordPE]

Obviously, your electrical interlock is insufficient as it exists. I wouldn't want to try to design a foolproof circuit without knowing your system and needs. However, a simple stopgap might be to install an on-delay timer in the contactor circuit, coupled with a N.C. aux contact from the drive.

Here is a link to a company that manufactures a bypass switch:

http://www.acicontrols.com/catalog/display_results.cfm/category/1/IBYSS-VFD-Bypass-Switches/

 

[ScottyUK]

"Obviously, your electrical interlock is insufficient as it exists."

Without knowing the nature of the electrical interlock that's quite a bold statement to make. Mechanical interlocks fail far more frequently than electrical ones, albeit usually in a state where nothing can close. It should be entirely possible to engineer a decent electrical interlock.

 

[jraef]

The answer to the question as asked however is still what skogsgura posted. It's pointless to consider it further, because the result is catastrophic to the system creating it, thereby inherently self-limiting. Asking what the waveform will look like is like asking what effect the temperature of the iceberg will have on the steel used in the hull of the Titanic when it hit. It might be briefly interesting, but not germane to the overall outcome.

"Will work for (the memory of) salami"

 

[DFordPE]

To ScottyUK,

It's obvious that it's insufficient because it doesn't work! Would you want to try to design a retrofit without having all to the system info? Are you that bold?

That's why I referred him to a company the makes bypasses.

 

[DRWeig]

DFordPE,

The OP didn't say that his electrical interlock doesn't work... He's just wondering what the effects would be if it failed.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[squeeky]

There is a similar discussion going on in Linkedin and the result was that the drive was completely destroyed. When you say drive I expect you mean VFD as the Soft Starter is switching a the same wave form and thre is not the same complications. Each drive manufacturer appears to have their own control cards to accomplish this. An optional extra. It allows to run motors at different speeds but when at supplied frequency, close the contactor and takes advantage of the improved effiecency with power factor correction.

Also used in cascaded pump systems where a constant pressure is required and a bank of pumps is run with some of them in base load and one in "topping up mode" running off teh VFD. The drive has a built in PLC and switches between motors to allow equal running hours so the maintenance factors are the same.

 

[DFordPE]

Sorry, I must have read it too fast.

mea culpa

 

[jraef]

It allows to run motors at different speeds but when at supplied frequency, close the contactor and takes advantage of the improved effiecency with power factor correction.

Odd statement there, not sure where it all comes from. Efficiency, yes, if the motor will be running at full speed you eliminate the losses in the VFD when you switch it over to bypass. But the VFD would be effectively correcting the PF if left in. So if running at full speed in bypass, then one would have to add PFC capacitors and a contactor that only closes when the motor power is bypassed around the VFD. In other words that's not a benefit of having a bypass for a VFD, it's a problem that must be addressed.

"Will work for (the memory of) salami"