GenSet Rotor Rectifier Question - Page 1
Forum section: electronics / repair

author: Trailboss
author_rank: Contributor
timestamp: March 09, 2018, 04:31:46 am
content: Hi Group,I have an 18hp 8 or 10 KW portable home generator, circa 1990, brand unknown with open diodes in the rotor windings.This is a brush-less self exciting set using an exciter stator/ capacitor for regulation.The field rotor has four winding's on two bobbins, the winding's are in series and at each junction is a diode.The diodes connect to a plate on the rotor shaft.I am not sure of the polarity pof the diode arrangement.?I am also not sure of the diode ratings.?I have researched this in depth and cannot find any info on a four coil arrangement, everything I read is three phase windings.Any help with the diode selection (common cathode/common anode) and EI ratings appreciated. I was guessing a Vishay VS series 10 or 20 amps at 1200 volt in a DO-203AA (DO-4) package.If anyone can reference literature on this rotor arrangement I would be much appreciated as I would like to understand how this ckt operates in terms of current flow etc.....Many Thanks

author: Jeroen_Bezemer
author_rank: Contributor
timestamp: March 10, 2018, 08:06:37 am
content: Quote from: Trailboss on March 09, 2018, 04:31:46 amI am not familiar with low voltage Generators, as I'm working with high voltage 10.5/13.8 kVOn those the rotor is formed by an exciter and a main magnetcoil. The exciter will generate AC which will be rectified and feed to the main coil.So you will need to seek two (or three) small coils and 2 large. Then just make a bridge so the large coils have DC and forms 2 or 4 magnet poles N-S or N-S-N-S on the circumference.The small coils will have a seperate winding on the stator, as that is for the regulator.Verstuurd vanaf mijn SHIELD Tablet K1 met Tapatalk

author: tpowell1830
author_rank: Frequent Contributor
timestamp: March 10, 2018, 11:55:18 am
content: In my experience with those type of diodes, since they are so big, there is usually a part number clearly printed on them. Also, they often have a diode symbol on them showing the polarity. I can see from the photos that your diodes are very dirty, but perhaps you could clean them enough to see a marking.Hope this helps...

author: tautech
author_rank: Super Contributor
timestamp: March 16, 2018, 07:56:37 am
content: Quote from: tpowell1830 on March 10, 2018, 11:55:18 am+1Stud diodes commonly come in either polarity configurations so Pt#'s are needed to indicate if stud = anode or cathode.To reduce the risk of scuffing off any remaining ID marks I'd look at a chemical cleaning solution.

author: Jeroen3
author_rank: Super Contributor
timestamp: March 16, 2018, 09:16:49 am
content: The number of phases of the exciter rotor does not really matter. All that matters is that the output is rectified into DC for the main rotor.800 to 1200V diodes are typical. Sometimes they are mounted on a disk that can be replaced in one go.There is also a varistor on the main rator protecting the diodes.Isn't there anything on the machine that can be used to identify it? Do you have a full picture or type plate?

author: Stray Electron
author_rank: Super Contributor
timestamp: March 16, 2018, 04:56:27 pm
content: Does this generator use a battery? If so you can trace the + lead back to the diodes and determine their polarity. The same goes for the capacitor, it's polarity should be marked. I bought a largish generator like this that didn't work and the problem was the capacitor and not the diodes. The cap was an electrolytic and had simply dried out and lost it's capacity.

author: james_s
author_rank: Super Contributor
timestamp: March 18, 2018, 06:19:51 pm
content: I haven't had any caffeine yet today so I could be missing something obvious, but does the polarity even matter? The generator rotors I've seen are symmetrical, and they're spinning so it seems that as long as multiple diodes are all the same type the absolute polarity shouldn't be important.

author: floobydust
author_rank: Super Contributor
timestamp: March 19, 2018, 04:08:47 am
content: Polarity is important if there is a pilot excitor magnet in the rotor. If so, OP can use a compass with small current in each coil to ensure it matches the magnet polarity.The stud-mount diodes don't get a lot of heatsinking here on a small steel flange, but big voltage spikes. You can remove the flange rust with automotive bodywork "rust converter".I don't see the need for four diodes. Anyone else figured that out"...When this type of alternator is started, residual magnetism in the rotor core produces a magnetic flux which induces a small alternating current in the excitation coil. The capacitor connected to this coil phase shifts the induced current to produce a magnetic flux that is opposed to the flux from the rotor. The flux from the excitation circuit in turn induces a current in the rotor coil which is rectified by the diode to strengthen the magnetic field on the rotor. At the same time, current induced in the output coil coupled to the generator load increases the rotating electromotive force in the air gap between the rotor and stator. The alternator thereby bootstraps its operation until the magnetic field reaches a quiescent level."edit: added drawing