Generator - flashing the field

[PITCAIRN]

I have an AC Generator , diesel engine driven (its a Mariner 6000, made by MASE with a Rugerini diesel engine)
It all of a sudden stopped putting out AC power. Whilst the generator was on, there was a sudden grinding of metal and quite a bit of heat and some smoke.

I immediately closed down the diesel.

Just some basic background on this Generator, very simplified..... It consists of a rotor, 2 pole, magnets spinning within the fixed Stator which is made of various copper windings. The Generator is 'Self Excited', in other words it produces its own DC current which is fed into the iron core of the Rotor to get sufficient magnetic flux produced in order to get enough ( 220V ) AC generated in the Stator windings. It is brushless. There is no separate Voltage regulator (like you would find in an Alternator)

After letting everything cool down, I noticed some 'deposit' sitting on the floor underneath the end cap of the generator housing. It looked like a wax like substance.

Anyway, I took off the end cap of the stator housing. There is a bushing, or should be a bushing, located in the centre of the end cap of the housing, into which the spindle of the rotor fits and sits whilst the rotor is spinning (In the manual it is described as a Vullkollan bushing. Apparrentely 'Vulkkollan' is a synthetic substance from which a bushing is made, never heard of it myself)

This bushing was totally trashed, all chewed up.

So I am thinking... as this bushing gave way so the rotor vibrated enough for it to 'jar' and jump and rub against the fixed and stationary Stator. Hence clash of metal and heat. It may be that the Stator and or Rotor are totally trashed, but I want to try to do some tests first and see if I can salvage something.

So what to do. I have tried replacing the capacitor - still no AC.
I have tested the output wires of the Stator for continuity and that is ok , but of course this test would not tell me if there was a short within the windings - maybe an Ohms test would, but what Ohms figure am I looking for?

Next I am going to check and change the diodes which attach to the rotor and are part of the mechanism whereby DC current is produced for Self Exciting.

Lastly, I think I am right in saying that sudden heat and or vibration can cause the iron core of the Rotor to 'loose' its residual magnetism, which it needs in order for it to begin the cycle of producing a Magnetic flux of sufficient size in order to kick in AC generation on the Stator windings...without residual magnetism in the Rotor no Generator would be able to produce AC (well not enough to be of any use, i.e. 110V in each winding so developing 220V).

So how to get back the residual magnetism. This can be done by 'flashing the field' i.e. pumping some DC into the Rotor whilst it is turning, which I believe gets some magnetism back into the iron core of the Rotor. I could not begin to explain the Physics here and its beyond my experience.

Does anyone have any knowledge and experience of doing this - what exactly is the procedure ? And if anyone has any other ideas on what I can do please let me know.

Cheers

 

[david_bagshaw]

Flashing a rotor can usually be done while the set is stationary. Ziese suggest using a 9v pp battery. only needs to be done for a second.

I would rather expect the windings have been trashed though if heat and smoke were involved.

 

[superheat6k]

If the rotor touches the stator the magnetic field gets corrupted and the output ceases, as happened.

The field resistance will be very low, and would require a Wheatstone bridge or perhaps a Megger with ohm meter reading to 2 places of decimal.

Have you replaced the bearing ?

If not there is a firm in Bolton called Beldam Crossley who supply a ptfe product called Rulon. They might be able to offer a suitable bearing from this material, or else try to fit a normal sealed roller bearing.

To restore the magnetism in a two pole rotor twin using a compass to identify the N & S poles then make a simple electromagnet with a coarse coil of copper wire and connect it to a DC source, using again the compass to identify the poles, wrap this over the rotor and try inducing the field back into the rotor. Might work.

 

[William_H]

As said it is most likely the windings of the rotor have been damaged or blackened wires. I would expect that this will be obvious under close inspection. Brushless alternators usually work well. The collapse of the bearings will however kill it.
I would think the permanent magnet is in the main large part of the rotor. This will have a coil around it to augment the residual magnetism. So the main rotor part with residual magnetism will produce some small output on the AC out side. Some of this will be rectified to provide an outside field to the small part. There will normally be a regulator of some sort here to control final output voltage. Call it the excitor generator. The current induced in the rotor of the excitor gnerator will be rectified by diodes in the rotor between the excitor rotor and the main rotor. This current augments the residual magnetism. Perhaps that is what you explained above.
So look for 2 lots of diodes one rotating one fixed. Look for a regulator of some sort from 240vAC output wires to excitor outer field.

Coils of alternators can get shorted turns. It is unlikely that a resistance measurement will show up the shorted turns. You might be ablke to measure the inductance of the coil but this is only good if you know what it should be.
Professionals use what is know as a "growler". This is an electromagnet feed on 240vAC 50 hertz. The pole pieces are shaped so that you can magnetically couple it to your coil for testing. Normal coil gives no reaction as there is no connection to the ends of the coil. eg commutator on rotor removed from brushes or field coil disconnected.
However with shorted turns you have a transformer with shorted tuns. The excessive load on the elecromagnet causes it to get very upset. It vibrates and buzzes. (growls) a sure sign. So if you suspect shorted turns take the components to an auto electrician who hopefully has a growler or you could try something similar yourself. A small electromagnet fed by AC will produce a significant AC voltage when magnetically coupled to a coil. (be careful it might bite). If you have more than one coil you can compare AC produced. A shorted turn will resul;t inn low AC measured. good luck olewill

 

[Chalker]

The workshop manual on my Fisher Panda generator has a simple method of flashing the residual magnetism to get the genny going. Disconnect ALL AC loads from the generator. Connect 12v across 240v genny output for 4 seconds.
Check to see if AC neutral is connected to DC 0v first, if so make sure that is the DC + that connects to the AC live terminal.
DO NOT DO THIS WHILST THE GENERATOR IS TURNING.If it worked you would have 240v AC connected to the battery. Ooer Mum!!!!!

But given a crashed end bearing, sparks and grinding, I'd want the rotor and field windings inspected first.

Hope that is easier than on my 4000 series Fisher Panda.

 

[PITCAIRN]

Thanks for all those replies. I am waiting for a new bearing which is probably unlikely to be available till September (coming from Italy). I am also going to check the diodes, just need to get the rotor off to get access. Fortunately I do have spare diodes (and varistors, which have to be replaced if the diode is replaced according to my manual). I have also I hope sourced a new Stator, not confirmed yet, may have to come from the USA - so will post again when there are developments.

 

[JOHNPEET]

Hi Pitcairn,

For info, the exciter, rotor and stator windings are all capable of being re-wound by your local motor re-wind company. This would be almost certainly a more cost effective alternative to obtaining new!

I had a similar event to yourself a while back with a 6.5Kva Kohler unit. The stator winding assembly had moved inside its outer casing to the extent that it had come into contact with the rotor. Killed the stator windings. Everything else checked out ok. Got the stator, which is 4 pole in my case re-wound, aligned properly and welded into the outer casing for £350 I then re-assembled the unit, test ran and set up the AVR.

 

[charles_reed]

As suggested by others I'd cut losses and take the alternator to a professional electrical rewinder.

Vulkollan is an ultra-high molecular weight polyurethane, a trade name of Bayer http://www.polyurethanes.bayer.com/en/Products/VULKOLLAN.aspx

Many other generic versions, so you're probably being taken for a ride. Try a local plastics company, get a length of UHDMWPU rod of the correct diameter and take it along to an old-fashioned engineer with an old-fashioned (probably > 50 years old) lathe and have it turned to exactly the correct size.

PS The likelihood of anything arriving from Italy, between 30 July and 30th September is very remote.

 

[PITCAIRN]

Yes Johnpeet, a re-wind may be the way to go, good point. I am in Spain, so not quite as straightforward as if I was back in the UK, nevertheless I will make enquiries.

Also thanks Charles for the info on Vulkollan, yes I am sure there are substitutes available and will keep that in mind.