Phase leg identification- damaged generator

if your output terminal is in Y connection and you can return back all the neutral line and tie up together, you can establish to run the generator in open load and use phase rotation tester to get back the proper phasing, or you can use a 3 phase motor to identify the proper rotation. Hope this will help you

Thanks for the response. Since I don't have the special tester, this would not be my easiest option. The 3 phase motor would help but since motor rotation can be reversed by switching any two lines, I am unsure of how to use the electric motor to establish 100% which line is which....Could you please elaborate on this option? Thanks, Dan

Dan if you can use a three phase motor which has a label in each phase, check out the nameplate of the motor what rotation stated in standard connection. Let say T1-T2-T3 are clockwise rotation in nameplate, once you connect in generator and get a result of clockwise rotation you can finally label the Generator terminal. If it runs in reversed direction you can interchange two connection and finally label your Genset Terminal. Thats the only thing i can advice without any mathematical contribution. As far as i know in my small experience, phasing problem are mostly in motor. Once you get the proper (rotation of motor) all connected load either motor, transformer or any three phase load will get the same result without any problem.

Once you get the proper (rotation of motor) all connected load either motor, transformer or any three phase load will get the same result without any problem.

Actually, in industrial settings, the leads coming to the motor are often indistinguishable from one another. Wire it up, try it, reverse any two leads to fix rotation. But as you say, just using a motor to get them in the right sequence should work.

123, 231, 312 are all the same. 321, 213, 132 are the same, but in the reversed direction.

Thanks for the replies. My aim is to be able to parallel this generator with another, and my concern is that with generator paralleling if there is an error in the phasing (normally the genset's leads are labeled) it could cause generator damage to one or both sets. I have been wondering if I could do it this way: narrow the possibilities by observing motor rotation, then using paralleling lights similar to those seen on tactical field sets, trying different combinations until I get it right. A phase rotation indicator sounds like the safest bet. Regards, Dan

Even if the phase rotation is correct, some method of synchronising the three phases between generators is essential if damage and injury are to be avoided.

Suggestion:

• Before connecting the two generators to the same system, connect three voltmeters, one each between the L1 of one and the L1 of the other, the L2 of one and the L2 of the other and the L3 of one and the L3 of the other, and tie together the generator neutrals. Start both generators up and try and match their speeds. At minimum volts on all three voltmeters the risk of damage is minimal. If zero volts cannot be achieved on all three simultaneously then their phases are rotating in opposite directions, so any two phase conductors of one of the generators should be reversed. Once minimum volts is achieved on all three voltmeters, close the parallelling switch at this point.

http://en.wikipedia.org/wiki/Synchroscope

PWSlack has the best approach.

There is a difference to phase rotation and phase sequence.

In single generator island mode operation it does not matter.

In systems where you have to parallel generators, you just identify the phases.

Using PWSlack's approack , you should be OK.

Remember to get the three voltmeters at zero , the terminal voltages must be the same , on most voltage regulator modules you should be able to adjust the terminal voltage from a pot on the Voltage Regulator module.

The trends of the meter movement should be the same even if you cannot get them to zero.

I assume that there are no PTs on this small generator, if there are , you must be cognisant of the polarity of the PTs.

I can see how using 3 analog meters would be more sensitive than paralleling lights. I will try this and post back, I anticipate good results. No power transformer installed. Thanks to all for the expert advice on this question. Regards, Dan

And don't forget to put a HUGE padlock on the thing afterwards, or you'll end up doing this all over again...

Hei Dan... welcome to engineers forum... and I totally agree with the last post... the same happen with you happen in my state... so... do that could be a good idea.

Find out what type of breakers are being used or alternator and get the datasheets for them. Then it should say which lines are connected where.

PWSlack's approach is dead on. I've had to do this a number of times when connecting new generators & lines. Even if you can visually identify the phases, you should still run the phasing test he describes.

One tip: while observing the voltmeters, the needle movement should be identical on all 3 meters. In other words, the reading should increase, reach peak, then decrease and reach minimum simultaneously on all 3 meters.

So is it possible for me to parallel my repaired 17.5 Kw generator to another, with the lines not exactly in the same order as originally, so long as when paralleling: Same terminal voltage on both gensets, test voltmeters between both the generator's lines all three rise and fall together and at 60hz exactly the meters all read zero, assuring proper phase sequence AND proper phase rotation, & same frequency, 60hz ..? This is an odd sounding question but there is so much more to it than I thought but I think I get it. As a side note, this is a Y wound generator and I will be paralleling with other Y wound generators. Thanks again, Dan

You've got it! As long as you are sure you have the neutral side of all 3 phase windings identified, so you are only dealing with the output termination, what you've described is correct. If the phasing is correct, all 3 meters act in a synchronized manner.

Note: The meters may not go all the way to zero, depending on the line loss and reactance between the generators, but they should be within 2-3% of 0v.