7.5kw ST Head Stopped Producing Electricity

[mike90045]

Your diode bridge may have fried, if the battery causes you to get AC at the power terminals (even low volt AC)

[38ac]

I am probably the most electric challenged guy on this forum but when my 15KW St head quit it was doing the exact same as yours.  Ended up the rectifier (bridge diode) was fried as Mike just suggested. Somebody else will have to tell you how to test it as I cannot remember   Replacements are dirt cheap buy some spares.

[ronmar]

Well if the brushes were not working, you would have gotten no output when you applied the external power, as the current you were feeding into the generator went thru the brushes to get to the rotor windings to build a field.  If the set didn't sustain an output after you removed the external power you were flashing with, you either have a problem with the diode pack, or the "Z" winding feeding the energy to the diode pack.  You don't have a regulator, at least I didn't see one in your pictures. 

First thing I would do is to check the resistance of the harmonic "Z" winding.  Disconnect the two wires connected to the ~ terminals on the diode pack and measure the resistance.  You should have I am guessing around 5-7 Ohms. Someone please chime in with the correct number if you have it.  If it is really low(1-2), the wiring down to the windings, or the windings themselvs may be shorted.  It it is open/no circuit(infinite or OL), the wiring to or winding may be open.  Also measure from each lead to ground, which should be open/no circuit.

The bridge recitfyer pack has 4 diodes, one between each lead(along each edge of the square).  These can be easilly measured with an ohm meter.  Each pair of terminals should measure a low resistance in one direction and infinite/open in the other.  If it is bad, there are a few places to get replacements. 
Check all the wiring and crimps looking for a bad connection...
   

[Listerational]

Great help you all. I will check the bridge rectifier.

ronmar your video of the interior of your 6/1 helps drive home to me at least the importance of having the correct amount of engine oil in the crank case.

[Listerational]

You all were right, it was the bridge rectifier. It was a radio shack item and radio shack had one in stock. Funny, the new bridge rectifier was about half the thickness as the old bridge rectifier.
Thanks once again for the timely advice.

[38ac]

Glad to hear you got it up and going again.  Those rectifiers build heat when under load that needs to be dissipated.  Some of them are surrounded with finned aluminum housings, this is what I had that failed. My replacment 50 amp job is just an small aluminum square with four posts for the wires. If yours is like as shown here you need to be sure it is against something to absorb and dissipate the heat. There is also a paste sold to help with the heat transfer I believe it is sold at RS also. Mine is working fine so far with it being bolted to the junction box and using the heat sink gook

[ronmar]

+1 for what 38ac said.  If it dosn't have a metal case to bond and dissipate heat, it probably will fail.

Radio Shack had one in stock in a store?  Last I looked thru one of their component drawers, one that I would say is adequate for this application was not a stocked item...

One like this would work from Surplus Center.  Buy 2, they are cheap, and the closest thing to an active component(other than the brushes) you are going to find in a harmonically excited ST head...

http://www.surpluscenter.com/Electrical/Transformers-Bridge-Rectifiers/Bridge-Rectifiers/35-AMP-1000-VOLT-KBPC3510-BRIDGE-RECTIFIER-22-1206.axd

Ron

[Listerational]

+1 for what 38ac said.  If it dosn't have a metal case to bond and dissipate heat, it probably will fail.

Radio Shack had one in stock in a store?  Last I looked thru one of their component drawers, one that I would say is adequate for this application was not a stocked item...

One like this would work from Surplus Center.  Buy 2, they are cheap, and the closest thing to an active component(other than the brushes) you are going to find in a harmonically excited ST head...

http://www.surpluscenter.com/Electrical/Transformers-Bridge-Rectifiers/Bridge-Rectifiers/35-AMP-1000-VOLT-KBPC3510-BRIDGE-RECTIFIER-22-1206.axd

Ron

Great stuff 38ac and Ron thanks.
I will just run and pick up a couple more of the kind that I have at Radio Shack and some sink gook.
Ron when you wrote harmonically excited are you referring to the same type of harmonics referred to in the electrical industry?
Thanks for all the help to all. It was nice getting the generator producing again.

[bandmiller2]

I noticed last time I exersized my 10kw ST that it took a few seconds to build up voltage. Is that a sign that the residual magnetism in the core was week.?? Thanks Frank C.

[38ac]

I noticed last time I exersized my 10kw ST that it took a few seconds to build up voltage. Is that a sign that the residual magnetism in the core was week.?? Thanks Frank C.

I dont know but my 15 KW head that is used for home stand by does that when it sits for more than a few weeks between test runs. My 5KW head has set for a year and powers up as soon as it is spun.

[dieselgman]

Different heads and different environmental conditions (especially humidity) will cause varying results... This is, in part, why some plants in harsh environments have heaters installed for keeping the windings dry. The standard for a standby plant is to exercise once a month with applied load. This is a recommended part of routine preventative maintenance.

dieselgman