Author Topic: lister fatboy voltage sag  (Read 262 times)

Mtour

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lister fatboy voltage sag
« on: November 29, 2017, 11:48:40 PM »

  I have a mid 50's 2kw fatboy, no load voltage of 133v-62hz and with a 10amp load voltage drops to 110v-60hz. It is 8 dc brush head, 4 large brushes and four smaller. The 4 smaller brushes are connected to two wires these are the are attached to a resistor. I was under the impression that the resistor connected to the smaller brushes was to adjust for voltage drop, it seems to be providing all the current for excitation. If the resistor is disconnected voltage drops to 2v. When the generator is outputting voltage it is producing 20vdc from the ssf winding.

 Any thoughts...Thanks

 


Mtour

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Re: lister fatboy voltage sag
« Reply #1 on: November 30, 2017, 10:25:11 PM »
 
 Noticed today that the 4 small brush holders are pointing counter clockwise, looking at other fatboys they all seem to be pointed clockwise.

starfire

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Re: lister fatboy voltage sag
« Reply #2 on: December 01, 2017, 01:48:14 AM »
That one is doing pretty good, at 20 volt drop at a 1.3kW load. The resistor is in series with the exciter to fields and does adjust the exciter current. With only passive voltage regulation, thats working great.
Otherwise, I dont know what you are asking.

Mtour

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Re: lister fatboy voltage sag
« Reply #3 on: December 01, 2017, 10:55:16 AM »

 I have another fatboy on a sl1 and have restored another as well both of these with the same 10amp load drops about 5v under load. The small brushes are getting about 3v per pole. The fatboy with the large voltage drop the small brushes are getting 17v to one pole and 5v to the other pole measured at the brushes.

 The other fatboy I have here has issues as well, it has a dead short across the starting terminals. It will not produce ac voltage unless 12v is applied across the resistor terminal for a second, then it produces ac fine. Both of these generator where purchase from the same fellow and seem to have been worked on together(same wire crimps)

starfire

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Re: lister fatboy voltage sag
« Reply #4 on: December 01, 2017, 12:51:32 PM »
The exciter relies  on residual magnetism to pull itself up.
Needing to apply  a voltage to get things started implies the residual magnetism isnt holding, you may need to flash it with a higher voltage when stationary.
Any bearing troubles can cause vibration that can destroy residual magnetism as well.
Disconnecting the AC load before running it down may also help to keep it active.
The other machine with the voltage differential sounds like it has either an open field, or shorted turns. An oscilloscope trace will identify the latter, an ohms test the open field.
Otherwise, having several identical units allows you to swap components to hit the faulty part  by elimination.
Generally, voltage sag of around 20 percent from zero to full load is acceptable for these machines, they are generally set to supply 10 percent high at no load, and 10 percent low at full load..... (voltage). 
These were made well before electronic AVRs were invented.
The brushes are angled away from the direction of rotation to minimise wear and reduce noise when a commutator is used.... copper segments,  DC, and usually at right angles with slip rings.....AC, so maybe one of your machines was set up for counter rotation?
On some larger units, the brushes can be rotated slightly a few degrees around the commutator to alter the magnetic pole position in relation to the armature, this sets the AC voltage no load to full load change, and can be tricky to get right.
The actual position of the magnetic flux changes, leads or lags depending on load, and is the basis for the passive voltage regulation. Sometimes salient poles were used also to distort the magnetic field to further improve regulation, but probably not used on your machine.
« Last Edit: December 01, 2017, 12:54:02 PM by starfire »

Mtour

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Re: lister fatboy voltage sag
« Reply #5 on: December 01, 2017, 11:58:55 PM »
 Thanks Starfire for the in-depth reply.

   I had another look at the brush placement between the two fatboys, with one having the small brush holder pointing forward and the other in the opposite direction there is a 1 1/2" offset to the brush locations. The Big brushes holders point the opposite direction of the small.

 There is a tag for direction on the generator that indicates it was made to operate in the opposite direction then it is setup to run now. The generator was made in 1955 and the SL1 that is powering it was made in 1960.  So would running the generator backwards cause this issue, I have a multi meter but no oscilloscope.

The grey fatboy is the one that needs help..



 


starfire

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Re: lister fatboy voltage sag
« Reply #6 on: December 02, 2017, 01:45:32 AM »
I cant give specific answers as I havent had experience with your flavour of generator, but general principals apply to all.
So, sorry, I cant tell you to do this and then it will be fixed.
What AC voltage and at what current is the grey unit producing when running?
Also measure the exciter voltage and current by putting a DC amp gauge in series with the resisitor.  An old car type ampmeter is useful for this.
Do the same with a working unit.
Reverse rotation will effect efficiency and indirectly AC output voltage.
The AC side cares not on direction, but the DC exciter section usually does.
Having another working unit is good, it allows you to compare measurements.
By lifting the brushes and isolating them away from the armature with paper or plastic, you can then ohm the fields, armature windings and AC sections independently without overly dismantling the machine. These measurements can be compared also to your working unit, and repairs made.
A good test is to remove the wires from the exciter (DC) brushes and voltage test across the brushes when the thing is rotating, his will measure the residual starting voltage.... anything over 5/6 volts should be fine.
Be methodical, and take notes for future reference.... they will come in handy in future breakdowns.
That should help track down the problem.
« Last Edit: December 02, 2017, 01:48:36 AM by starfire »

Mtour

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Re: lister fatboy voltage sag
« Reply #7 on: December 03, 2017, 02:48:19 PM »
Thanks again Starfire for your reply...

 It turned out to be a real easy fix, the ring that holds the brushes has a wedge in it that is attached to a nut. Backing the nut off loosens the ring, so I was able to turn it to match the placement of the brushes to the green generator. Now with no load I get 115v at 61hz, and at 10.5amp load I am getting 117v at 60hz.

 The name plate on the dog house on the fatboy is Panther Ranger and has a date code of 1955.

 Thanks for the help....


starfire

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Re: lister fatboy voltage sag
« Reply #8 on: December 04, 2017, 01:40:17 AM »
Thats very good news. The fact that the voltage is increasing slightly under load means its slightly overcompensting, but well within accepted specs. Another slight adjustment will cure that, but hell, I would be very happy with that.
This also confirms just how good these early designs were.
Now, call me a conspiracy person, but Im failly certain the age of the electronic AVR made for cheap and inferior designs of alternators, manufacturers took advantage and  compromised them to aid profits.
Your "small" machine will likely start an electric motor without trouble, whereas many new models twice that rating will not, lacking in copper, iron and sheer mass to supply any sort of short term overload current.
« Last Edit: December 04, 2017, 01:48:02 AM by starfire »

oldgoat

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Re: lister fatboy voltage sag
« Reply #9 on: December 04, 2017, 11:06:16 AM »
These generators are very sensitive to the aux brush position for load compensation. They are a bit slow to respond with inductive loads such as electric motors. When these were built most people didn't have air conditioners etc Probably the highest motor load in those days was a 150 watt fan. Mine is 4kva 240 v built in 1959 and still going strong after an armature rewind.( very time consuming job)