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Anonymous Poster

Three Phase Generator Winding Got Burned

02/13/2011 1:30 PM

hi gents,

would you please help me with an explaination for this:

its an old three phase 25kva 415v 35amps 50hz brush type ac generator, the three phase winding are placed on the rotor and we get the main power from qty#4 brushes, 3phase+ neutral.

its regulator is not electronic AVR but its something like three phase transformer with the main power cables passing through it so i thing when the amps is more, the output to the rectifier is getting higher.

it has been long time not running, so i fitted box with meters and main breaker.i did insulation test on all of the windings and the excitation transformer and they were more than 1G ohms

when i started the gen- there was an output but its not 415v, its only 340v on all of the phases when the gen rpm is 50hz. it will go to 415 but only if i increase the rpm to 54hz.

so i run on 50hz 340v and connected it to load bank , i run it for about one hour and the load was 25amps on the three phases. in the next day i run it again on load about 20 amps but it only last for 5 seconds then i saw spark on the main winding and found that two wires of the winding toutched to each other and cutted ''burned'' and there was no output after that.

to be honest, i am not good electrician to know what is the reason caused this but i think its the old boor insulation of the windings,

some electrician siad that the cause of the problem is that because i run it on load while its output is low ''340v'' and he said that it should be 380 or 400v, but in my mind i think its not the problem as when the generator volts is 415v it can carry 35amps so when its output is 340v it should carry about 42amps. so the load which was the gen carrying is almost 60% of the rating amps.

would you please correct me if i am wrong as my friend is plaming me and said that i am the one who caused the generator to get burned

sorry for this long story and realy appreciate your advice

thanks

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Guru
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#1

Re: three phase generator winding got burned

02/13/2011 1:35 PM

<...two wires of the winding toutched to each other and cutted ''burned'' ...>

This could be either a cause or an effect. It's very difficult to see which it is from here.

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Anonymous Poster
#2
In reply to #1

Re: three phase generator winding got burned

02/13/2011 2:21 PM

hi PWSlack,

thanks for your reply

are you saying that the reason of the problem is not the low volts output?

thanks

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Guru
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#7
In reply to #2

Re: three phase generator winding got burned

02/13/2011 3:24 PM

#2 is concise.

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#3

Re: Three Phase Generator Winding Got Burned

02/13/2011 2:28 PM

Most likely the reason of the flash between 2 phases took place because of an internal insulation failure that could be happened because you loaded the generator with lower output voltage as the current goes higher,and generated heat proportion to square current value (Jule = I^2*R ).

Your excitation transformer is very old and it depends on the flux that crossing the coils , you could increase the output voltage by increasing the gap between the primary and the secondary of these coils.

Also if your generator xd" was > 15% it would restrict the short circuit currents and make it more stand-able to higher currents.

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Anonymous Poster
#4
In reply to #3

Re: Three Phase Generator Winding Got Burned

02/13/2011 2:42 PM

hi,

so is it happened because the insulation failure which caused by the the low volts output?

the speed is still the same 50hz, when first i run on load for about 1 hour, i did not notice any extra heat on the winding because i did touch them!

thanks

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Anonymous Poster
#5
In reply to #3

Re: Three Phase Generator Winding Got Burned

02/13/2011 2:50 PM

i understand that the amps will go higher with lower volts IF the load was three phase motor or something doing some mechanical action, but the load was salt water tank and the three phase were inside the tank, the phases currents were 25 amps only '' 10 amps less than the generator limit''.

will it be right in this load case to say that with higher volts will be higher amps and with lower volts will be lower amps, as its same as three resistors load bank?

thanks

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Active Contributor

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#8
In reply to #5

Re: Three Phase Generator Winding Got Burned

02/13/2011 3:50 PM

No you can't say that, as load is always current and not voltage, your generator should maintain the output voltage no mater what was the load, because when load increase the voltage will drop but the excitation circuit will regulate the output again and keep it stable.

In your case the losses has increased inside the coils causing the insulation to brake down and that probably ended up with a short circuit between two phases.

Sometimes the dust inside the stator and humidity can reduce the efficiency of the insulation as well.

Also it might happened because the short circuit happened at the load side and that forced the generator to produce more current or maybe at the brushes holders.

These are the reasons I can think about them from here.

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Anonymous Poster
#11
In reply to #8

Re: Three Phase Generator Winding Got Burned

02/14/2011 2:54 AM

hi M-Salem

well, i noticed something after the winding got burned, the winding were not that hot, its not looked that they were overloaded, but it looks like sudden short and boom.

thanks

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#6

Re: Three Phase Generator Winding Got Burned

02/13/2011 3:17 PM

It doesn't look as though overcurrent caused this; I would be inclined to suspect simple age, or possibly an abrasive particle got into the rotor and nicked some winding insulation.

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#9

Re: Three Phase Generator Winding Got Burned

02/13/2011 5:53 PM

Re: three phase genset

You said that the genset is already old. though you got a good insulation of winding with respect to the ground, it does not mean you have a good insulation between conductors of the winding. Insulation of conductors in the winding and its binders DETERIORATE in the course of time without proper maintenance. At your first load test, as the winding(STATOR) spun, there was vibration to cause the "rubbing" of conductors in the winding which shortcircuited resulting to lower voltage output. The insulation of the winding finally worsen at second test that resulted to flashing/burning of the stator winding. Hope this one helps.

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Anonymous Poster
#10
In reply to #9

Re: Three Phase Generator Winding Got Burned

02/14/2011 2:48 AM

hi,

thanks for your reply

and i think you and mr salem did not understand what i mean.

the generator output was 340v even without load, i mean i can get 415v but only when i increase the rpm of the engine to make the hertz 54hz.

when i run the generator on load, the volts drop but only 10v, so it was 330v when its loaded 25amps.

i dont know why is this but i think there was something wrong with this generator even before we run it on load as the output is only 340v, while it says 415v on the name plate.

hope you got what i mean and thanks

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Guru

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#12
In reply to #10

Re: Three Phase Generator Winding Got Burned

02/14/2011 8:04 AM

Your generator has a compound transformer as the voltage regulator. This transformer type is 3 phase wound and there is an air gap on the magnetic circuit: i.e., the laminated iron sheets form the letter E with a bar (Laminated also) crossing the EI. The 'I' part can be removed and add some thick paper (cardboard) on each branche of the E before reassebling. The thickness of the cardboard determines the air gap. By trying different (increasing) gaps, you will be able to adjust the voltage.

N.B. These transformers usually have tappings (one per phase) that can be used to increase or decrease the Voltage setting. Try to understand/examine this avenue before the need to increase the air gap. In any case, very small air gap increases will increase the voltage fast!! be conservative in the air gap increase at the start!

The output of this transformer is rectified by a 3 phase rectifier bridge and sent to the Field winding on the stator.

If you hear/saw a spark only and there is no sign of overheating on the rotor windings (Darkened insulation varnish or cracked insulations ... you might have had a localise breaking of the connecting wire to the slip rings (?). Have a reliable re-winder guy to check the windings and confirm if the repair is simple or more elaborate.

In any case, as stated before, the fault was not dur to the voltage since the current was lower than the rating of the unit.

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Anonymous Poster
#14
In reply to #12

Re: Three Phase Generator Winding Got Burned

02/14/2011 9:54 AM

hi mr lucke,

thats the answer i am looking for, because every thing i read on your answer, i feel its true and i can see it in the generator.

exactly, there is transformer, and there is paper between the E and the I parts, as you called them.

i will do as per your advice and will show it to rewinder, but can you please help if you know how this transformer regulate the volts, because i see the main power cables are passing through this transformer coils, does it mean when there is more amps will be more output to the bridge rectifier and then to the exciter field?

can you please explain to me how it works or maybe you have some manual for it.

i appreciate your advice

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Guru

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#16
In reply to #14

Re: Three Phase Generator Winding Got Burned

02/14/2011 2:01 PM

Brief explanation:

The phase currents pass through the transformer using a coarse wire winding. The whole current of the load will go through it.

The 2nd windings are fine wire with a relatively very big number of coils and connected in STAR, giving 3 phase outputs, with about 3 tappings each. These are used to vary the excitation current to adjust the voltage required. This is a small variation adjustment, within a narrow range usually.

These 3 output will generate currents in proportional to the load current on each phase. The 3 phase bridge rectifier will convert the lot into DC current that feeds the Field (Stator winding here). The Number of coils etc is worked out to match the Generator size and give a reasonably accurate voltage regulation but there will be a voltage regulation within 5% from no load to full load (Typically).

The air Gap will influence the saturation level of the iron path of the magnetic flux. The bigger the air gap, the easier the saturation, then the less impedence to the main load current flow (less resistance), the more ampere turns in the 2nd winding, thus increasing the voltage output of the generator.

I hope this is enough to help understand the trend. If more details, then you need some litterature about magnetic fields and flow...

Today, these voltage regulators have been replaced by electronic V.R. which are more precise and cheaper to make (in principle), but less robust for systems that do not require very stable and precise voltage regulation.

Certainly not practical for brushless alternators (except for complicated ones ???? using magnetic amplifiers?).

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Anonymous Poster
#17
In reply to #16

Re: Three Phase Generator Winding Got Burned

02/15/2011 3:06 PM

hi, thanks for these informations.

it realy helped me specialy with a picture drawing.

i appreciate your help and advice

thanks

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#13

Re: Three Phase Generator Winding Got Burned

02/14/2011 9:52 AM

Not running for a long time? Sounds like it could have gotten "mouse-bit".

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#15

Re: Three Phase Generator Winding Got Burned

02/14/2011 11:42 AM

If it failed do to over load and heating of the windings. The winding should show this as heat damaged to the insulation through out the length of it. If it's just a spot deterioration at the short it's most likely age. That the insulation deteriorated and crumbled away letting the wires short together. With out a close inspection of the windings it hard to put the cause to it or place blame.

If he wants to put the blame on something tell him mice did it. Must have gotten in while it was setting so long. Chewed the insulation off the wire that's why it shorted.

Seems like no ones really looked as to cause.

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