Back Feeding AC Generator? What would happen?

Status
Not open for further replies.
J

jaywbyrd11

New User
Location
Baltimore Md
Occupation
Electrical Foreman
What would happen if voltage got sent into a non running 3phase Ac generator? Would this cause phase to phase or phase to neutral/ground shorts? Or would nothing happen at all.
 
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.

The motor is first started as a slip ring induction motor. The resistance is gradually cut-off as the motor gains speed. When it achieves near synchronous speed, DC excitation is given to the rotor, and it is pulled into synchronism. Then it starts rotating as a synchronous motor.Oct 25, 2020

Synchronous Motor Starting - Electrical4U​
Click to expand...

Don't do it, you could end up frying something
 
Well, with the rotor not moving through the stator fields, the stator windings are just big coils of wire with a little resistance. Fir the most part they will look like a short circuit.
 
Jraef said:
Well, with the rotor not moving through the stator fields, the stator windings are just big coils of wire with a little resistance. Fir the most part they will look like a short circuit.
Click to expand...
I'm not so sure. The rotor would be an open circuit and so might as well not be in the magnetic circuit except for the soft iron. I think the generator's stator coils would behave like a transformer's primary windings with no load on the secondary.
 
I'm not sure the rotor would be an open circuit. The rotor would be a transformer secondary connected to the excitation control circuit. So I'd expect voltage induced in the rotor to be applied to the output of the excitation circuit, and the results of this experiment dependent on the details of that excitation circuit.

-Jon
 
winnie said:
I'm not sure the rotor would be an open circuit. The rotor would be a transformer secondary connected to the excitation control circuit. So I'd expect voltage induced in the rotor to be applied to the output of the excitation circuit, and the results of this experiment dependent on the details of that excitation circuit.

-Jon
Click to expand...
Hmmm, with a brushless generator it might create a current that would blow the diodes. Like putting AC to the DC terminals of a bridge rectifier.
 
I've seen utility put into a non running single phase generator twice that I can think of.

The first one resulted in me hollering to TURN IT OFF! And they did, no damage that I know of.

The second happened when nobody was looking. It was left on for a good amount of time. It took out the ROTOR of all things. The stator seems to be fine. It was never repaired/placed back in service so it is not completely verified. I guess the field from the stator transformed into the rotor and heated it to the burning point.

Generally speaking, what I would expect to see happen is : 1. A loud buzzing noise 2. A loud bang 3. Smoke.
 
Yeah, the rotor would be the item to fail, I think. The diodes would be forward biased on half the cycle unless/until they shorted. The windings in the rotor would see more current than designed for and then burn up.
 
Last edited:
But there is a bigger picture to consider. What kind of distribution system would allow this to happen? Whatever it is would allow genny power to go out on the grid! Or allow genny power to be shorted to utility. I've seen this crap done in industrial situations where a xfer switch was not installed. Breakers were racked out and locked out as a precaution, but then you are relying on human procedures. Kirk keys in such a situation is a good idea.

One situation I know of resulted in a contractor getting a 480V shock. (Ok, really a 277V shock). No injury as far as I know.
 
When FPL guys are working on power lines after a hurricane goes through, the sound of a home generator starting up gives them pause. I know to shut off my main breaker before backfeeding my box, but not every homeowner in my area has a similar understanding of what they are doing.
 
Those interlocks are cheap, easy to install, and effective.

I installed one at my house, and realized that it would be pretty easy to defeat, but that it was an effective memory aid, and an aid is all that is needed. No one wants to try to energize the grid with their home generator.

With many of these you need to drill holes in the deadfront in just the right place. I had some 'blind hole spotters' that made marking the holes a breeze. See for example
Center-Locating Punch for Unthreaded Closed-End Hole 3/16" Diameter https://www.mcmaster.com/product/3414A215

Jon
 
JimInPB said:
That's a cute little interlock gizmo. I'll have to see if I can find one that fits my box. If not, I'll make one. Thanks for posting.
Click to expand...
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.
 
TwoBlocked said:
But there is a bigger picture to consider. What kind of distribution system would allow this to happen? ...
Click to expand...
Any co-generation system. The wiring for backfeeding must necessarily be there.

TwoBlocked said:
... Kirk keys in such a situation is a good idea. ...
Click to expand...
I was not aware of these before today. Interesting product. Thanks for the reference.
www.kirkkey.com

Kirk Products & Support

North Canton Ohio US. Kirk Products & Support. Established in 1932 Kirk designs and manufactures over 70 000 safety components…
www.kirkkey.com www.kirkkey.com

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.
 
drcampbell said:
Any co-generation system. The wiring for backfeeding must necessarily be there.
Click to expand...

A co-generation system would be designed to backfeed the grid, presumably with an interconnection agreement.

This is a very different situation than a backup generator designed only to be used when the grid is disconnected. The OP never specified, but the discussion went down the path of discussing backup generators.

I presume a co-generation system would be designed to tolerate inadvertent connection of a non-running generator to the grid, or there would be rigorous controls to prevent this, or perhaps some systems actually start up by connecting the non-running generator to the grid, temporarily running the generator as a motor.

-Jon
 
Status
Not open for further replies.
Top