Back Feeding AC Generator? What would happen?

[TwoBlocked]

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I was not aware of these before today. Interesting product. Thanks for the reference.

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But they'd still need to be installed, and seems like it would be simpler to install a transfer switch or breaker-panel interlock for most small & residential installations.

I've only seen them in industrial situations. Like for opening the secondary of a HV/MV transformer before opening the primary. Or opening the main to a load center before closing a tie from another source.

 

[hillbilly1]

A lot of big box stores like Walmart, Costco, and distribution centers use the Kirk Key system. Did a retrofit on a Costco in Virginia, Square D sent out the Kirk key attachment for the existing gear, and a new gear section for the generator connection. Problem was, both locks were keyed different!

 

[TwoBlocked]

A lot of big box stores like Walmart, Costco, and distribution centers use the Kirk Key system. Did a retrofit on a Costco in Virginia, Square D sent out the Kirk key attachment for the existing gear, and a new gear section for the generator connection. Problem was, both locks were keyed different!

Yeah, I've heard of that happening!

 

[hillbilly1]

A lot of big box stores like Walmart, Costco, and distribution centers use the Kirk Key system. Did a retrofit on a Costco in Virginia, Square D sent out the Kirk key attachment for the existing gear, and a new gear section for the generator connection. Problem was, both locks were keyed different!

Sorry, it was BJ’s wholesale, that was quite a few years ago.

 

[mtnelect]

A 3 phase generator and a 3 phase synchronous motor are essentially the same thing. The difference is how the dc excitation is supplied to the field. The starting for a synchronous motor is also a bit complex. So, no it won't run like a motor, and exactly what happens is somewhat dependant on the field excitation circuit.



Don't do it, you could end up frying something

In my apprenticeship, I was working at the Department of Public Works - Traffic & Lighting. I was assigned to the Electric Shop, which provided electrical maintenance on the Sign Posting trucks that had portable generators (Rotating Fields) style for mounting signs in the field. Occasionally, they would come in with portable generators not working. We discovered that the generators stopped working when the generators ran out of gas while they were using a drill. So, we discovered that the permanent magnetics collapse. So, we took an extension cord with a light bulb in series and plugged it in to one of the generators receptacles while running to excite the generator.

 

[Joethemechanic]

when the generators ran out of gas while they were using a drill. So, we discovered that the permanent magnetics collapse

That has me thinking. We had an old magnet crane with a belt driven DC generator for the magnet. The magnetic field would collapse on it like 2 times a week, maybe more. I could never get it to happen when I was there. Eventually I installed a momentary contact button by the operator's seat so he could flash it himself. I wonder if it was the collapsing field in the magnet causing it when he shut the engine down.

I guess I'll never know, the crane got scrapped like 20 years ago and the operator passed away almost as long ago

 

[kwired]

I don't know what manufacturer you're talking about but SQ D, Siemens, GE, etc... all make their own version as well as a lot of other companies such as Joe and Jon mention.

They typically don't make them for older generation equipment though. Some those third party companies however might.

 

[kwired]

In my apprenticeship, I was working at the Department of Public Works - Traffic & Lighting. I was assigned to the Electric Shop, which provided electrical maintenance on the Sign Posting trucks that had portable generators (Rotating Fields) style for mounting signs in the field. Occasionally, they would come in with portable generators not working. We discovered that the generators stopped working when the generators ran out of gas while they were using a drill. So, we discovered that the permanent magnetics collapse. So, we took an extension cord with a light bulb in series and plugged it in to one of the generators receptacles while running to excite the generator.

That magnetic field can also deteriorate when the thing hasn't been run in a long time.

If power is out and your generator won't work because the field gone, it can be be restored by connecting 12VDC to the excitation circuit also. Do so while it is not running so it won't back feed into your 12V source when it starts working. Probably won't work to back feed generator output and need to directly connect to excitation circuit.

 

[roger]

They typically don't make them for older generation equipment though. Some those third party companies however might.

The interlock doesn't go on the generation equipment. If you're talking about using one on an old fuse panel I would bet most here already knew that.

 

[kwired]

The interlock doesn't go on the generation equipment. If you're talking about using one on an old fuse panel I would bet most here already knew that.

I meant older panel/load centers.

 

[ATSman]

Interesting discussion.

I would tend to agree with posts #2 & #3.

For the sake of discussion, I think we can safely say that a 3 phase AC generator is similar to a 3 phase induction motor.

So if a 3 phase AC source is applied to a non-running generator then damage would occur (or breaker trips/ fuses blow) in the winding(s) since the generator circuit would essentially be a transformer with a shorted secondary at that instant:

Wikipedia

https://en.wikipedia.org/wiki/Inrush_current

When an electric motor, AC or DC, is first energized, the rotor is not moving, and a current equivalent to the stalled current will flow, reducing as the motor picks up speed and develops a back EMF to oppose the supply. AC induction motors behave as transformers with a shorted secondary until the rotor begins to move, while brushed motors present essentially the winding resistance.

https://www.google.com/search?q=Does+a+motor+act+like+a+shorted+transformer+on+start?&sca_esv=571789629&source=hp&ei=x1QjZeL3INjfkPIPjva_wAk&iflsig=AO6bgOgAAAAAZSNi1zFXQMqThJpjRyilZ-bunkDgK4sc&ved=0ahUKEwii94bj5eeBAxXYL0QIHQ77D5gQ4dUDCAw&uact=5&oq=Does+a+motor+act+like+a+shorted+transformer+on+start?&gs_lp=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&sclient=gws-wiz

The above discussion describes a hypothetical situation and does not address the sequence of events that occur during the operation of an ATS during a power outage or when an ATS is tested by operating its Test Switch.

I have performed many startups and commissioning on Automatic Transfer Switches in industrial environments. About a handful were mis-wired such that the utility source was connected to the load terminals and the load was connected to the utility terminals. Usually the ATS was found energized, feeding loads (in reverse) but never been transfer tested with the generator by operating the test switch.

When tests were performed from the ATS test switch, the first few happened un-synchronized and caused both utility and gen breakers to trip. On one, not only did the gen source breaker trip, but the cover blew off of the gen breaker panel and tripped the main bkr of a hospital.

During testing of the last one, the utility connected to the gen synchronized (in-phase monitor) and the result was the utility source powered (motored) the generator. The lights went out in the power room and I felt the ATS cables that were buzzing (drawing current.) I thought to myself Why Did The Lights Go Out and where is the current going?!? I told the electrician to check the gen in the penthouse. That’s when we found out that the gen was being driven by the utility. After the gen was shut down the ATS was re-wired. After a second test, amazingly every thing worked fine and no damage was done to the gen! Up to this time I had always heard stories of gens being “motored” and here I had witnessed it first hand!