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I am restoring a fairly old alternator, single phase, 4 poles, 230V, 8kVA, cos phi=0.8, 50Hz @ 1500 RPM.

I measured a couple of resistance values and I am wondering whether the alternator is healthy (Please see the pictures):

Slip ring No. 1 to 4 resistance (Ohms):

1-2: 3.2
1-3: 3.2
1-4: 1.8
2-3: 3.2
2-4: 1.8
3-4: 1.8

None of the slip rings is shortened to ground. Electricity is delivered from the armature via slip ring coals to terminals.

enter image description here

There are four stator windings A - D. Every stator winding pack is connected to an adjacent stator winding pack. Stator winding pack A and stator winding pack D both have their terminals leading outside to a small connector block at the side. Based on the corrosion of these terminals is seems as there has never been anything connected to these terminals.

Resistance between the cables leading to these terminals from stator A and D is 8 Ohm. No shorting to ground. Couldn't measure individual stator packs resistance as all wires are isolated and I didn't want to remove the isolation).

enter image description here

Based on these characteristics:

  • Is the alternator healthy?
  • How do I have to connect the leads from the slip rings to get L1 and N?
  • What is the auxiliary stator connector block used for? (Assumption: excitation via 12V battery when alternator has no remanence?)
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It looks like your 3 field windings are good, I am guessing that 4 is an exciter but on the units I have worked on this is normally a separate coil with a permanent magnet. To make sure you really should use a megger (high voltage ohm meter) from each leg to ground, the windings look like they have been quite hot or are just very dirty. You would want a minimum of 10 Meg ohms at 500v, a 1000v would be better. If the motor is in good shape +300M ohms at 1000v is common. If the unit has been in a damp area and reads between 1 and 10 mega ohms. At 1000v it may needed to be dried out a small electric heater blowing warm air through the motor can dry things out but keep the temp below 150F to be safe some books say up to 225F but I have overheated windings at this heat. Note it may take a couple of days but if you recheck and see the resistance to ground increasing let it keep going. I have recovered up to 200 hp motors doing this that started out at approx 5 meg breaking down at 700v after 3 days I had 2.1 Gig at 1000v.

  • Why three field windings? It's single phase. Further I think this generator is excited from the stator surrounding the armature. As it is a very old one, in former times the stator was used for excitement. Therefore four slip rings, otherwise two slip rings would suffice to feed the excitement current? – JohnDoe Sep 22 '18 at 13:01
  • It may not be hooked up as 3 phase currently but 4 slip rings is normal for a 3phase generator 3 hots and a neutral. Most of the generators I have worked on have a small but separate exciter that works with a perminant magnet to generate the exciter voltage, some use the battery voltage but on large generators the perminant magnet exciter is the most common. A single phase alternator usually only has 2 slip rings. I have worked on both single phase and 3 phase and have found several 3 phase that were setup to provide single phase power.. – Ed Beal Sep 22 '18 at 15:52

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