# Generator Keeps losing excitation



## magmash (Jul 20, 2012)

Hi

Got a generator with this weird problem. The generator is a 60 KW Brushless Diesel. The generator didn't put out any voltage when i first got to check it, Tested the AVR on another generator and it worked fine, tested the exciter resistance (19 Ohms) Which is up to specs. The weird thing is that when ever i flash the exciter it works fine, but when i shut the unit down and let it sit for 25 minutes and start it up again the generator doesn't put out any voltage, so i have to reflash it again to get the voltage back. the exciter is some how losing the magnetism . What do you guys think the problem is / Any theory ???.


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## backstay (Feb 3, 2011)

magmash said:


> Hi
> 
> Got a generator with this weird problem. The generator is a 60 KW Brushless Diesel. The generator didn't put out any voltage when i first got to check it, Tested the AVR on another generator and it worked fine, tested the exciter resistance (19 Ohms) Which is up to specs. The weird thing is that when ever i flash the exciter it works fine, but when i shut the unit down and let it sit for 25 minutes and start it up again the generator doesn't put out any voltage, so i have to reflash it again to get the voltage back. the exciter is some how losing the magnetism . What do you guys think the problem is / Any theory ???.


I've seen this also. I don't know of a fix.


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## macmikeman (Jan 23, 2007)

Borrow one of those blue pills from your oldest co worker and toss it into the fuel tank.


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## magmash (Jul 20, 2012)

backstay said:


> I've seen this also. I don't know of a fix.


What did you end up doing ?, Replacing the whole alternator ????


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## EM1 (Oct 25, 2014)

I've seen this on a couple of little portables that rode in the service trucks all day. Not sure if it was vibration or what, but they had to be flashed if not run on a regular basis. 

Have you run it under load after flashing the field? Long enough and heavy enough load to give it a normal duty cycle?


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## bill39 (Sep 4, 2009)

magmash said:


> What did you end up doing ?, Replacing the whole alternator ????


We had that on an old generator and would use a 2 D cell flashlight continuity tester to flash the fields. It would be OK for several days. If it sat unused for awhile then we'd have to do it again.

I don't know how to permanently fix it.


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## telsa (May 22, 2015)

Speculation:

Considering your location (Dubai) and its climate (toasty) I'd guess that your diodes in the circuit are TOAST.

The way the device works is that residual magnetism -- feeble though it is -- is used to unduce current into the excitation logic. 

I'd guess that this starts out as a feeble AC current that needs to be rectified, then re-injected back into the windings to massively amplify their strength.

From that point forward, the machine boots up -- always bleeding a trivial amount of generated power from its AC wave form to provide DC excitation -- under positive control back into the machine.

If this DC power source is frustrated from the first, then it never can boot up.

To have this weird trait, the breakdown in the diodes/ rectification has to be partial.

It passes enough current at start to not rectify the AC -- yet is still able to block the current when the flow picks up. It's leakage is, thus, only so great. It's the equivalent of a wire that has just the beginnings of corona discharge breakdown. 

Its ability to have this dual state personality of leaking a low current and blocking a full current has you spinning. Like a transistor, it's changing conduction states on you.


(BTW, the diode might actually be something more complicated -- but still in the solid state family.)

In sum: it has a marginal DC power source to boot up from.

When you show up with an independent source/ battery -- you are jumping it past its leakage.

You MAY find that for a long term fix you're going to have to install a small cooling fan -- the type you might re-purpose from an old PC power supply -- to blow air across the solid state excitation circuitry -- probably totally independently of the motor's state. The fan would have to be running even as the machine was cooling down after a run... fully stopped. ( Even water cooling -- as seen in exotic PCs may be necessary.):blink:

*******

As a general tip: ANYTHING solid state in Dubai needs to fear heat.

As a tech, you should look at excess heating as being the NUMBER ONE source of trouble for virtually everything you touch. The gear you service is designed by fellows who could never imagine the heat that you have to work with and live in.

PS

You may want to check out the temperatures of the machine, too.

For efficiency, steels with lower hysteresis have become pretty common. In this situation, such a trait would work against the boot-up sequence: residual magnetism might be amazingly feeble.

The decay in magnetic alignment -- after running -- is sped up the hotter the metal. And your machine must run the hottest of all.

You may have to provide a separate DC excitation source as a permanent field modification. (Li-ion batteries?)


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## Joefixit2 (Nov 16, 2007)

Go to this site and sign up, go to the electrical forum and post up your question. There is a guy over there named Duane B who knows his generators inside out and will gladly help you figure it out. http://weldingweb.com/


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## triden (Jun 13, 2012)

Does there happen to be a lot of capacitance in the system? Too must of a leading power factor will kill excitation. Things like harmonic filters are known for doing that.


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## magmash (Jul 20, 2012)

telsa said:


> Speculation:
> 
> Considering your location (Dubai) and its climate (toasty) I'd guess that your diodes in the circuit are TOAST.
> 
> ...



There is 4 other identical generators at the same site running fine under the sun, so they are doing a god job under the sun. What are the most know symptoms for bad diodes on a brushless generator ?


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## telsa (May 22, 2015)

magmash said:


> There is 4 other identical generators at the same site running fine under the sun, so they are doing a god job under the sun. What are the most know symptoms for bad diodes on a brushless generator ?


You're in real luck.

When given a chance, swap the relevant sub-systems between the machines.

If the problem follows the sub-assembly -- you've hit paydirt.

Solid state devices are mass produced, mass tested, yet still have significant variability.

One example out of many: Intel microprocessors. The first Pentiums -- decades ago -- were sold as 60 mHz chips. They were designed to be 66 mHz chips -- but were slowed down because Intel's production yield was far too low at the higher specification -- at that time. 

Within months, the very same chips (design wise) were being mass marketed as 66 mHz Pentiums. (The later, higher speed chips were re-shrunk and not the same die size.)

The same exact production logic hits everything semi-conductor. Performance variability is much higher because of doping, etc. than anything otherwise manufactured. (nuts, bolts, washers, ... ) The variability is much more towards that of Lumber.

So you might well find that even the very same gen-set (from the outside) has different (batch) semi-conductors or that this specific circuit has been "value engineered" a constant feature in all solid state items.

In sum: your diode bridge/ silicon controlled rectifier (SCR) may have been cooked or marginal straight from the factory. 

https://en.wikipedia.org/wiki/Rectifier

You'll note the HEAT SINK that they all come with. Look to see if any of these puppies are used in your excitation power supply. They are common. They are also prone to being cooked. (Hence, the heat sink)



It's time to put your eyeballs on it -- and see if there's any design difference, or date of production difference.

One thing you know, something's 'off' with your excitation circuit -- that does not affect it once it's up to power.

There are only so many things that would frustrate excitation boot-up.

( I'm making the assumption that your excitation energy is being bled off of the main power flow. 

Perhaps -- for the excitation circuit, alone -- you've got a whirl of permanent magnets. (Mono-pole generator) These can be de-magnetized if they get too hot. )

https://www.google.com/patents/US20060038456

{ The common vehicle alternator is a three-phase induction device that boots up because of the starter battery -- usually -- but can also boot-up if the vehicle is rolled forward after which the clutch is popped closed, thence to rev the engine. Even at that low of a speed, there's more than enough magnetism to boot up the alternator's excitation and fire the spark plugs. 

I'm making the big assumption that your machine uses such a simplistic feed back scheme to boot up.}

[ The three phases are rectified by three diodes into ripply DC at 14 volts. So outside of the alternator, the entire system is strictly DC. ]

If worse comes to worst, you could always cobble together a DC battery and simply imitate the boot up approach of an automobile.

Based upon other responses, battery injection is the universal field fix for such a problem. Just slap on a common battery (correct voltage) with a charger, and have it at the ready to inject the excitation by remote. (buddy relay/ auxiliary contacts)

Such a fix is so much cheaper than sending the unit back for service, or anything else one might imagine....

I assume your basic machine appears to be okay once it's up and running.

&&

If you're curious about mono-pole generators... so is NASA. They were used to generate astounding amperages to test heat shields (plasma arcs) and power electro-rockets. (plasma arcs, again)


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