No Output Voltage

how do you measure winding resistance? it should be free from any connection if you do so.

if you did it right, then there could be an internal short to your winding.

Check thouroughly the rectifier. Might be defective, when broken they give short circuit, and end up no output voltage on your terminals.

The exciter field wdg. is measured by disconnecting the field wdg. from bridge rectifier and connecting the leads of digital meter to the two terminals of exciter field. For your kind information,there is no fault in bridge rectifier and rotating diodes. We even replaced all the electronics component by new one.The alternator gives voltage only if continuous dc supply is given.The main suspect with the weakening of residual magnetism. We even placed iron rod to the core of exciter field. We can hardly feel the magnetic effect to the rod. Is there any possibility behind weakening of residual magnetism?

Hint...If you can get the machine to output rated voltage by simply injecting a current in to the pilot exciter field winding without exceeding its rating then you should be looking at all the circuitry that normally feeds dc to that point. It would also help if you RTFM (Read The Fine Manual) or take a course on how these excitation systems work.

Residual magnetism is very weak by nature. You cannot feel it like you would with a normal magnet.

There is what is called "Automatic Voltage Regulator", or AVR. You did not mention anything about it here!

1- Residual magnetism will generate some current that will feed the exciter field and enhance the field (snow ball effect) until the main voltage has reached the set value for the generator. Then the AVR will control the current being fed to the field.

2- if this is not happening automatically, but you are getting a voltage when you connect a 6V dc to the field, directly, without the field being connected to the AVR, then the AVR electronics (if it is an electronic type), is not working. It needs replacing probably.

In KEL made brushless alternator, the electronic AVR is available only above 100 KVA along with compounding transformer which give dual regulation. But,below 100 KVA there is only compoubding transformer for regulation. All the compounding transformer gives continuity. The problem of no output voltage used to appear from time to time. At that time, we used to charge field by 6V dc and it used to give the required output voltage. This made us suspect that there is loss of residual magnetism.Can it be possible even the alternator is in running condition? But this time, the continuous dc only gives the output voltage. Doesn't that mean the loss of residual magnetism or weak magnetic field?

Residual magnetism depends upon the material and construction of the rotor, and state of the machine at the moment of de-energization. Salient pole rotors can have very distinct residual magnetism, modern smooth round rotors will have the magnetic field more dispersed. In either case a magnet on a string or a compass will point the way (pun intended) to the poles, and it is unlikely that the residual magnetism will go to zero except after a very long time of inactivity.

The fact that it can generate a voltage with dc power on the field (aka as flashing the field), and it collapses when it is removed, indicates that part of the excitation system is not doing its job of applying sufficient dc to the main field (the snowball effect that LAA mentioned).

You already answered your own question when you said that the field winding reads only 1.2 vs. 9 Ohm from the data sheets. That indicates that you may have a pilot exciter or main rotor whose field has many shorted turns or a winding to ground fault, either of which will reduce the resulting main rotor field current by up to 87%!!

As Ramconsult explained. Residual magnetism strength depends on the material used for the iron core, and the winding ...

You can lose the remanent magnetism if the current going into the field winding, at the time of shutdown, reverses (i.e. not DC as at start), or there was an electrical overload on the generator such that the magnetism was cancelled (this can happen ...) just as the field collapsed.

Also: if the exciter field is damaged (hence the low resistance reading you mentioned), then the small current that is normally generated by any remanent magnetism, will be sinking into the field and producing more heat than enhancing the the magnetic field, like shorting the compounding transformer. the rectifier bridge at the output of this transformer, will be able to handle this current (in a short circuit mode) without damage. Since a lot of the windings seem to have shorted, the Amp-turns of the exciter field will be very unsufficient to promote the snow ball effect beeing seeked.

Naren,

No previous experience with Kirloskar alternators, but when you are getting rated output by separately exciting the exciter field with 6V battery - that means that there is no problem with exciter, the main fields and diodes etc. Problem is on AVR side. Therefore following checks can give you the clue to problem:

(1) Measure 3 phase residual voltage induced at sensing terminals of AVR. If available, next check the (2) output voltage of AVR which is connected to exciter field - if available, check (3) continuity between wires to exciter field - if you are still getting 1.2 Ohm then AVR is defective, otherwise physically trace leads between output of AVR and exciter and you may find either the leads are open or might be that there is 7.8 Ohm series resistance (normally used as matching resistance many a times in 3 phase alternators when output impedance of AVR is more than resistance of field winding)..

If results of check 1 are negative, then either no remanance in the machine or leads from out put of alternator to AVR are opened. If no remanance, you need to excite at slightly higher voltage or for longer period.

If results at 2 are negative, If there is a remote voltage adjustment potentiometer connected to AVR - AND THIS IS OPENED CIRCUIT - WILL RESULT IN NO BUILD UP OF VOLTAGE ABOVE REMANANCE VOLTAGE. Otherwise either fuse of AVR is blown or AVR needs replacement.

Also would suggest that to measure DC resistance with surety in such cases use low resistance meter (Ductor). If not available, easiest is to connect Current scale of Multi-meter in series, measure current and from simple Ohms Law find out the DC Resistance. If 1.2 Ohm is the correct resistance of exciter field winding, when 6V DC was applied, you shall read approx. 5A in multimeter.

To check there is no short in exciter winding - with machine not running, apply 6V DC to exciter - take a blade of Hacksaw or similar iron blade and touch against shoe of each pole - the blade will stick to pole. Even if there is partial short in coils - difference in pull of field poles on you hand will give you the idea if any pole is partially shorted (Interturn short). But Interturn short will not change the resistance from 9 ohm to 1.2 ohm.