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What's generator residual magnetism??

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Mark

06-27-2001 10:07:30




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Will be going overseas and would like to store my DeVilbiss 8KW generator. Manual says to run it every 60 days to prevent the loss of residual magnetism that produces the electricity. Never heard of this. What is it in simple terms. Should I just sell the thing since I can't run it for a year or two??




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bill b va

06-28-2001 14:16:10




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 Re: What's generator residual magnetism?? in reply to Mark, 06-27-2001 10:07:30  

mark... i looked up your question so as not to give my verison of it from 50 years ago when i did a home study course in automotive electricity {remanent magnetism}(physics),magnetism which remains in a body that has little coercive force after the magnetizing force is withdrawn, as soft iron;-- called also {residual magnetism}. the guys below mean well but they remind me of the story of the ten blind men describing a elaphant each was touching a different part of the elephants body . i cant explain why the generator would loose its residual magnetism so quickly . automotive generators can set idle for years and still work . as for the difference between a generator and a alternator there is little difference in how they produce electricity. it is all in how the power is taken from the unit.in an alternator the juice is taken continously as the field rotates past the stator thus producing power that alternates negtive to positive the frequency of which is based on the rpms . in a generator the juice is taken off at near the peak of a half cycle by the brushes and communutator and thus produces mechanically rectified power . bill b va

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Terry Johnson

07-23-2001 08:32:05




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 Re: Re: What's generator residual magnetism?? in reply to bill b va, 06-28-2001 14:16:10  
Residual magnetisim (permanence) allows the generator to when coupled with a complete field and motion to generate power. This is typically done in the exciter section of the generator and inducted into the rotor. This field is in turn inducted into the secondary stator and is used as output and a monitoring point for the voltage regulator. If the voltage output is below or above the reference voltage established in the Voltage regulator, then the VR output F+ is increased or decreased accordingly. Loosing the residual magnetisim in the exciter of the generator would disallow voltage buildup and output. To re-establish the output one would disconnect the F+ and F- leads from the voltage regulator. (engine should be stopped) The F+ and F- leads can then be connected to a 12 VDC batteries positive and negative terminals respectively for a short period of time. 10-15 seconds. This should in most cases re-establish sufficent magnetisim within the exciter to allow proper operation after the unit is re-connected.

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Bob

06-28-2001 09:17:00




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 Re: What's generator residual magnetism?? in reply to Mark, 06-27-2001 10:07:30  
Electrical current cannot be generated without a magnetic field. Alternators do not have this therefore must be excited from an external source before creating current of their own.
Generators have magnets within them. That’s why they are heavier and require more current to generate power. Normally a generator will retain magnetism for years if it isn't banged around. Worry about the engine. The manufacture is being overly protective of his product unless this is a very inexpensive unit with soft metal magnets. Not likely! Other wise you can polarize. Good luck!

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Franz

06-28-2001 11:05:02




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 Re: Re: What's generator residual magnetism?? in reply to Bob, 06-28-2001 09:17:00  
Bob are you kidding, or you just totally confused?
The primary difference between a generator and an alternator is as follows, generators have a stationary field, and alternators have a rotating field. Alternators are considered superior because the output of the machine is taken from a stationary winding, thus eliminating the need to conduct heavy current thru slip rings from the rotor.
Both generators and alternators produce electricity by converting magnetic flux to electricity by rotating a coil of wire thru the magnetic field. In the case of gas powered generators and alternators, it is simply the conversion of chemical energy to electricity via a gasoline engine spinning a coil of wire thru a flux field.
Residual magnetism is the remaining magnetic gaus residing in the iron of the field coil, be it stationary or rotating. Unless the machine is built with embedded ceramic magnets, like the ones on the flywheel of a small gas engine, it is a virtual guarantee there will not be sufficient residual magnetism to initialize generation if the machine sits idle for an extended period.
Finally, there seems to exist a confusion in this thread between flashing and polarizing. Flashing is the process of inducing a flux field into the field of the machine to cause the machine to produce electricity. Polarizing is the process of creating the initial flux field in a DC machine.

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Bruce

06-27-2001 17:50:41




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 Re: What's generator residual magnetism?? in reply to Mark, 06-27-2001 10:07:30  
Mark, I have never heard of it either. I did some looking and I think if you go to www.davidsonsales.com/bulletin2.htm you will find what you need. Hope this helps.

Bruce



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Franz

06-27-2001 13:17:57




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 Re: What's generator residual magnetism?? in reply to Mark, 06-27-2001 10:07:30  
Generator residual magnetism is what makes a long line of cars in my driveway every time we have a good storm that knocks out the power around here. It's the remaining magnetic flux stored in the iron laminations in the rotating field of the alternator, the stuff that makes the initial burst of power that causes the rotating field to make power, witch in turn comes back to the field thru a rectifier, and sufficiently energises the field to make the electricity you want and need.
Since I'm not acquainted with your machine, but you have the instruction book, look to see if there is also an instruction for flashing the field. If there is, you'll be able to get the machine making electric again, even if you loose all the residual.
The problem occurrs more often these days, since the manufacturers all figured a way to reduce production cost by minimizing the amount of iron in the rotating field, witch in turn allowed them to eliminate the rear alternator bearing. That's what they mean by "new and improved". The only improvement was to the service man's income stream.
Best thing you can do is find a buyer who will pay sufficiently to make you happy, and sell him your problem. Chances are, unless this machine is stored in a dry climate, without running every 60 days, when you get back it will need to be hammered apart just to clean enough rust off the rotor to make the machine turn.

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Not an electrician

06-27-2001 12:00:08




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 Re: What's generator residual magnetism?? in reply to Mark, 06-27-2001 10:07:30  
I believe that all you would have to do is to polarize the generator if you should start it up after storage and find out that it is not charging. I would be more concerned about getting the engine properly prepared for long term storage,and closing up all the openings to prevent the animals and weather from damaging the generator windings.



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