What type of generator is involved and what is it connected to? If the generator is disconnected from ALL loads and still gives zero output there are several possible reasons depending on the type of machine:
For any type machine, an open circuit or shorted winding could be causing the problem.
In a permanent magnet machine, the magnets could be demagnetized. This could happen due to e.g, high temperature.
In a wound field machine the field supply could be inoperative, or the field circuit could have a shorted or open winding or bad slip ring(s).
If the machine is an induction machine, it needs external excitation on the stator windings to create the rotating field. This type of machine will not produce any output if not connected to an energized A.C. system, or at least to a capacitor bank to bootstrap some reactive current.
As you can see, a lot depends on the type of machine involved. Can you give us more information?
Also - it would be helpful to give your questions a meaningful title. "ELECTRICAL ENGINEER" doesn't convey much information to a casual browser or to someone searching the site. ... Just a suggestion.
If you have a small portable generator, i.e. Coleman powermate or such, check the field excitment circuit. They use a DC power supply to produce the electormagnet. Usually a full wave bridge retifier and large capactor as the DC supply. I have found the bridge retifier to be the weak link. Discharge the capactor and check the retifier with a multimeter.
Automobile generators although they give a DC output they are are internaly 3 phase AC machines which are fitted with 6 diodes to rectify the output to DC.
They operate by having a magnetic rotar rotating adjacent to the three stator coils.
To function the coil on the rotar must be supplied with a DC supply to to make it magnetic this current once the generator is up and running is supplied by the rectified output from the diodes via a small electronic control unit.
The charateristics of diodes are such that until the voltage across them reaches about 0.7v (remember there are two in series they pass virtualy no current) and the residual magnatism in the rotar is not sufficient to get things going.
To overcome this problem a small current is fed from the battery to the rotar and electronic control unit via an 'ignition warning lamp' hence if this lamp is dud or if there is no battery things will not get going as you found out!
If the generator has a closed circuit and load on the output the short condition will not allow the residual magnetism to build a voltage. Verify the out leads are not connected to a load. There should be some type of voltage at speed I.E. 480 volt 3 phase nominal there should be 30 to 70 vac.
Completely disconnect the the voltage regulator from the field, sensing and power circuits and run up the generator. Verify the field leads has a ohm reading. Make sure you have a AC voltmeter on the output leads and apply 12 volts to the field. The output voltage should go up, depending on the field requirements it may not reach nominal. If it does not remove the 12 vdc to field.
If this fails to regenerate the residual magnetism in the main rotor magnets then there may Diode problems on the exciter. Float the diodes to very. Also verify that the is a circuit on the main rotor magnets. This will be two leads that attach to the (-) and (+) on the exciter.
There will never be enough residual magetism in the the rotor to generate enough voltage to get past the diodes an external source of current is always needed to get things going.
not only has the current for excitation to get past the power diodes it has to be sufficient to get the control electronics working.
I am sure that many servicable generators have been trashed after they have been tested by the un-initiated without a source of start up current
Are you interchanging Alternator with Generator?Big difference.With the 6 diodes, it sounds as if you are talking Alternator.
The alternator is capable of self-generation of field excitement.A standard alternator can be modified to a "ONE WIRE" alternator, however,it does require a higher initial RPM to start output, usually not a problem in automotive applications, but with retrofitted farm equipment, the engine must be revved up high initially to start the auto-excitation process.
One wire alternators are commonly available at all automotive parts suppliers.
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"A man never stands so tall as when he stoops to help a child." "Never argue with a stupid person.They will drag you down to their level and beat you with experience" "To create an apple pie from scratch, first you must create a universe"
It does not seem appropriate to me to refer to these machines as alternators when in fact their output is DC whatever go's on inside.
As the original poster pointed out the residual magnetism in the rotor was not sufficient to get the machine running (call it generator or alternator as you like).
in a brush type DC generator there are no diodes and the small residual magnetism in the field structure is normally sufficient to get things going
I suggest you purchase a one-wire automotive alternator and study it dilligently in order to be able to argue this matter Intelligently.
In Very large alternators, the field current is supplied by a separate external source, either motor driven or rectifier supplied from line voltage.
__________________
"A man never stands so tall as when he stoops to help a child." "Never argue with a stupid person.They will drag you down to their level and beat you with experience" "To create an apple pie from scratch, first you must create a universe"
I agree that there would be no problem in building a one wire Generator/Alternator it is only a matter of constructing the rotor of materiel that retains a fair amount of residual magnatism, it is simply that none of the vehicles that I have owned have used one.
I have in this discussion have mostly been concerned to explain why the origonal posters machine failed to work
It depends on type & size of gen. Big ones have Exciter-units while smaller are self-excited. More over in Alternator-Rectifier type embedded Diodes normally go faulty.
Generator Problem
Re: ELECTRICAL ENGINEER
