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Generators & Electric Motors General Discussion Antique Generators and Old Electric Motors: Questions and answers about restoring and showing old power generation systems.

Generators & Electric Motors General Discussion

Unable to Field Flash


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  #1  
Old 12-15-2018, 03:17:48 PM
Cavalry Cavalry is offline
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Default Unable to Field Flash

I have a 6KW brushless gen head on a light tower that I cannot get to field flash. I successfully retrofitted from a transformer AVR to a SX460 knockoff last year but due to my negligence burned up the AVR after a couple hours use. I tested the windings and found they were not damaged so I picked up another SX460 but no matter what I tried could not get it to flash. I tried the AC drill method, static 12vdc flash to field, etc all with no success. I maintained correct polarity when applicable.
If I apply 12VDC across the field with the AVR disconnected and engine running I get generator output, so I believe the gen head to be mechanically sound. THe AVR appeared to be working during the same disconnected field test the field outputs had current. As soon as I reconnect the field to the AVR I am back to 5VAC output at the generator.
The stamford manual suggests field flashing while running and the AVR connected via 12VDC to field with a blocking diode. I tried this and instantly smoked the AVR.
I have yet another AVR on the way. The only thing I can think of now is to get generating again with 12vdc directly to the field and let it run awhile, maybe with some resistive load on it to help "set" the magnetism of the field?
Any other suggestions?
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Old 12-15-2018, 05:15:15 PM
Vanman Vanman is offline
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Default Re: Unable to Field Flash

What was wrong with the transformer compounding system? That is a vastly superior system from the standpoint of being rugged and reliable while being perfectly adequate. Do you still have those parts? We can help get that working, and ditch the Chinese electronics.
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Old 12-15-2018, 05:28:33 PM
turtmaster turtmaster is offline
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Default Re: Unable to Field Flash

One suggestion, if the is a diode bridge, before the voltage regulator, is it partially shorted, or open?
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Old 12-15-2018, 06:35:39 PM
Cavalry Cavalry is offline
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Default Re: Unable to Field Flash

Quote:
Originally Posted by Vanman View Post
What was wrong with the transformer compounding system? That is a vastly superior system from the standpoint of being rugged and reliable while being perfectly adequate. Do you still have those parts? We can help get that working, and ditch the Chinese electronics.
Story short, mice set up residence on top of the terminal strip and made a direct short blowing the rectifier and transformer. There is no support or parts from the manufacturer and the labels on the components were not legible.
The Chinese parts while not as robust are pretty cheap. At $25 each and only a minute to swap its not the end of the world if they only last a year or two. Having a spare on hand would be likely. I wouldn't be against buying a real stamford if it was known that they are superior.
I priced out transformer parts from a similar sized set and got a little sticker shock. Even if I was sure it would work it was 1/3 the price of another used light tower.
Here is my original post about the mouse incident
https://www.smokstak.com/forum/showthread.php?t=174847

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Originally Posted by turtmaster View Post
One suggestion, if the is a diode bridge, before the voltage regulator, is it partially shorted, or open?
I guess I am not following you on this? The original rectifying circuit was deleted. I am using the AVR in its place.
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Old 12-16-2018, 03:42:16 PM
Vanman Vanman is offline
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Default Re: Unable to Field Flash

I see, bummer about the mice.

If the generator is brushless that means that it has a separate exciter generator, and there is a shaft mounted rectifier between it and the main generator field. If that fails, the generator will be unable to build up.
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Old 12-17-2018, 08:01:49 PM
Cavalry Cavalry is offline
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Default Re: Unable to Field Flash

Quote:
Originally Posted by Vanman View Post

If the generator is brushless that means that it has a separate exciter generator, and there is a shaft mounted rectifier between it and the main generator field. If that fails, the generator will be unable to build up.
I desoldered and checked both rotor rectifiers, both test good
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Old 12-17-2018, 10:14:45 PM
nblack nblack is offline
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Default Re: Unable to Field Flash

the exciter winding checked OK? Only other thought is how were the diodes tested, with a multimeter, or something that could test their reverse bias voltage capability. I have seen diodes pass on a multimeter, but fail in circuit because the reverse voltage in-situ made them conduct in reverse when they should'a been open.
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Old 12-18-2018, 06:26:13 AM
Cavalry Cavalry is offline
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Default Re: Unable to Field Flash

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Originally Posted by nblack View Post
the exciter winding checked OK? Only other thought is how were the diodes tested, with a multimeter, or something that could test their reverse bias voltage capability. I have seen diodes pass on a multimeter, but fail in circuit because the reverse voltage in-situ made them conduct in reverse when they should'a been open.
Yes, I checked each exciter winding individually and the math in total works out.
I too have had issues testing diodes even with a high quality meter in "diode mode" I checked not only with a meter but with a 12v test light.
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Old 12-18-2018, 06:35:42 PM
Rich Mc Rich Mc is offline
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Default Re: Unable to Field Flash

One or several turns shorted together among many can still indicate the correct resistance of the total winding.
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Old 12-18-2018, 08:06:14 PM
Cavalry Cavalry is offline
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Default Re: Unable to Field Flash

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Originally Posted by Rich Mc View Post
One or several turns shorted together among many can still indicate the correct resistance of the total winding.
I checked each field stator coil individually, each was the same at 5Ω

I found a manual that is about the same vintage as my gen head and from Baldor who bought out Powr Gard, for the most part its more alike than anything else I have found (4 lead generator, rotor rectifiers in the right configuration with 7 leads). I'm using it as a reference and not gospel.

https://www.allmand.com/content/dam/...ting_Guide.pdf

I checked the rotor windings
+ to - 1.4Ω
+ to + 0Ω
- to - 0Ω
∿to ∿ .4Ω even across 3 legs
no chassis to ground issues

The only thing that kind of red flagged for me was the +to+ and -to-. The "spec" was .1Ω, I was getting 0Ω on two different meters. Its a pretty small amount, possibly just the resolution/accuracy of the meters? The +to- at 1.4Ω was less than the "spec" of 3-5Ω I'm assuming 2 field windings on the rotor and both being equal its ok?

At this point unless anyone has any other troubleshooting, i'm going to throw it back together, pop in the new AVR and see if it takes a flash. I guess its possible I just had a bad AVR out of the box and have been digging into this just for fun.
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