Generator ATS transitions

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Joethemechanic

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No, at least not in my case. This is happening when the utility is restored and the ATS shifts back to a very stiff utility supply.




No, not in my case.

When I said more than one generator I meant we may have an emergency and also a optional standby generators supplying different loads.




I don't think we are talking about the same thing.

I am talking about a typical open transition ATS switching between a utility source and a back up source.

Once any time delay perimeters have been met the switch itself is as fast as it can move. In some cases this is very fast. Unless you program in an additional delay between break and make
Going back to poco power can be lots faster because it's already back on when the switch decides to transfer. And yeah this thread is kind of mixed up with the other one that he is referencing. In that one there are posters saying that the breakers are tripping because the "induction motors are operating as generators" during the time they are winding down. But they can not operate as generators unless there is added capacitence to supply them with reactive power.
 

don_resqcapt19

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The following is from a transfer switch technical document. I have found similar statements for most brands of larger transfer switches.
When large inductive loads ? loads consisting of large motors and/or transformers ? are transferred between
two live power sources (e.g., a normal source and an emergency source) which may not be in synchronism,
efforts should be made to eliminate line current transients that occur during this type of switching. This
situation exists either when re-transferring from the emergency source to the restored normal source after
a power outage, or transferring in either direction while testing the emergency power system. Through the
proper selection of transfer switches, emergency/backup power system designers can dramatically reduce
and effectively mitigate these transients and the problems associated with them.
The two common methods to eliminate this problem is a delay when transferring between two live sources or a closed transition transfer switch that syncs the two sources before briefly connecting them together.
Now that does not explain the tripping of single pole 20 amp breakers as they would not be supplying "large inductive loads".
 

Joethemechanic

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Philly Pa burbs
The OP (Wayne), I believe is questioning that an induction motor rotating at partial speed is going to draw a higher amperage than one starting from a standstill. I know what Wayne believes to be true is true
 

steve66

Senior Member
Location
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Engineer
Joe; If you are intersted in this, I'd suggest you go to asco.com, and go to their transfer switch site (they also make soleniods), and look up their white papers on in-phase monitoring.

I think they had several white papers that will walk you through the problem, and solutions.
 

Joethemechanic

Senior Member
Location
Philly Pa burbs
Joe; If you are intersted in this, I'd suggest you go to asco.com, and go to their transfer switch site (they also make soleniods), and look up their white papers on in-phase monitoring.

I think they had several white papers that will walk you through the problem, and solutions.

I am seeing some "in phase monitoring" for ups units. But what the OP is talking about is induction motors connected "across the line"

Am I looking at the wrong documents?

As far as I am concerned the closer to it's synchronous speed an induction motor is when you close the switch, the lower the current will be.

Now if we are talking about breakers tripping from transformer induction inrush, that is a whole different thing.
 

don_resqcapt19

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Joe,
In the case of motors and a transfer switch transfering between two live sources, the "open" time is very short....and the motor is generating(yes it can generate without a connection to a reactive source, for a short time) and you have an out of sync connection of two sources. This can result in very high currents and trip breakers...there are even cases of broken motor shafts when the motor physically tries to match the new phase angle.
 

iwire

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Location
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The OP (Wayne), I believe is questioning that an induction motor rotating at partial speed is going to draw a higher amperage than one starting from a standstill. I know what Wayne believes to be true is true

If the switch closes at the wrong point in the sine wave the inrush will be very high for a moment. It would be like bringing two unsynchronized sources together.
 
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steve66

Senior Member
Location
Illinois
Occupation
Engineer
I am seeing some "in phase monitoring" for ups units. But what the OP is talking about is induction motors connected "across the line"

Am I looking at the wrong documents?

As far as I am concerned the closer to it's synchronous speed an induction motor is when you close the switch, the lower the current will be.

Now if we are talking about breakers tripping from transformer induction inrush, that is a whole different thing.

My mistake. I assumed it would be easy to find on their website, but I see any kind of search goes back to Emerson Power - and includes all their UPS stuff.

I've got them saved somewhere, but I can't seem to find them.

But to put 20 pages of info. into one sentance, Bob and Don have it right. If the motor is spinning (even at almost full speed), and power is suddenly applied, and the phase of the applied power doesn't match the rotor's position, you get a larger surge that can trip a breaker. In some cases, this could even be enough to sheer a motor shaft.
 

Joethemechanic

Senior Member
Location
Philly Pa burbs
But rotor current drops to nothing as soon as an induction motor's armature current falls to zero. So how can it generate without a rotating field. And really an induction motor is never "in sync". even at zero mechanical load there is always some slip.

I just don't see this as being a problem involving a squirrel cage rotor spinning at any speed in the range from standstill up to it's design synchronous speed.


I've jogged too many motor starters while measuring current to ever let myself believe that I could be close enough to being "in sync" with the sine that many times.

Even if I considered some residual magnetism in the rotor causing some generation, first off, it would be very weak, and I just don't see a mechanical switch as being fast enough for that
 
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Joethemechanic

Senior Member
Location
Philly Pa burbs
Sorry you don't see it, but it seems to be a real issue.

But I haven't heard a theory that agrees with it. All I hear is you add a delay and the problem disappears.

If the problem was purely inductance in the circuit adding a delay would cure that.

If the problem was induction motors with caps added for PF correction, I would figure the caps could be supplying the reactive power and then the induction motors could be generating out of sync, and yes a delay long enough for the motors to stop would cure that too.

I don't care who said anything, or wrote something on paper, or what degree or license they have.

The statement was made that Induction motors being spun mechanically can generate power that may be "in sync" or may be "out of sync"

I want to know how a induction motor connected to nothing electrically and having it's rotor spun by way of mechanical input can generate any potential across it's armature leads.
 

iwire

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But I haven't heard a theory that agrees with it. All I hear is you add a delay and the problem disappears.

If the problem was purely inductance in the circuit adding a delay would cure that.

If the problem was induction motors with caps added for PF correction, I would figure the caps could be supplying the reactive power and then the induction motors could be generating out of sync, and yes a delay long enough for the motors to stop would cure that too.

I don't care who said anything, or wrote something on paper, or what degree or license they have.

The statement was made that Induction motors being spun mechanically can generate power that may be "in sync" or may be "out of sync"

I want to know how a induction motor connected to nothing electrically and having it's rotor spun by way of mechanical input can generate any potential across it's armature leads.

Joe, considering the problem happens with transformers and motors. And the fact that motors and transformers are not very different I really don't think the motor spinning (or being spun) has anything to do with it.

I believe it has to do with the collapsing field that would happen even if the motor stopped the instant the switch opened.

But forget about my WAG, lets here your explanation of why putting a delay in works.

You have been given a few outside references to look at. On the other hand you just keep saying 'no' but you have not given us anything more than your disbelief.
 
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Joethemechanic

Senior Member
Location
Philly Pa burbs
I really don't think the motor spinning (or being spun) has anything to do with it.

Then we are in agreement,

I don't even know who brought that up in the original "Poltergiest" thread, but I believe that is what Wayne was questioning in this thread.


My references were already posted, but I'll post them again. Right hand column, first paragraph
 

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Joethemechanic

Senior Member
Location
Philly Pa burbs
I've built induction generators, I was a kid growing up around a repair shop, with a whole salvage yard full of parts to use. I used an old 5 HP 3600 rpm 3 phase motor and a Briggs & Stratton model 19 I took off of an old paving roller. I belt drove it about 125% overdrive so I could get the motor to spin at it's syncroness speed + the slip.

It put out nothing until I added some caps that I salvaged from some old high bay fixtures.

You see I wasn't satisfied with just reading about it in books, I had to build it and see it with my own eyes

(I think if I had gone to school, doing so would be called "A Lab Experiment")
 

don_resqcapt19

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retired electrician
While you can't make a motor into a real generator by spinning the motor without a reactive source, it does generate some power on "coast down". It does not do it for very long, but it does it long enough to cause a problem with fast acting transfer switches.
A long time ago I did a project for a customer that did not want to use a starter axillary contact to prove the motor was running. He had us add a relay with a 480 volt coil across the T leads. This relay would not drop out instantaneously when you opened the starter contacts and it would chatter for a couple of seconds as the motor coasted down. The chatter was the coil pulling in and out on the power generated by the coasting motor.
 

iwire

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Then we are in agreement,

Doubtful.:)

I don't even know who brought that up in the original "Poltergiest" thread, but I believe that is what Wayne was questioning in this thread.

It seems to be you that tried to make this into a 'motors are not generators' thread.




My references were already posted, but I'll post them again. Right hand column, first paragraph

And that disproves the need for transition delays for motors and transformers how?
 
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