# How can I test the rotor winding of a single phase brushless alternator on a generator?

Context Generator runs but no power output from the alternator.

The manual gives Ohm readings for the stator, excitation and rotor windings. The stator and excitation coil are to spec and have no short circuits with respect to the ground. However, there are no wires coming out for the rotor (this I believe is normal for a brushless alternator). So I think I might have to remove the stator to test the winding. It has diodes on it, do I have to un-solder the diodes to make it an open circuit to test the windings? Should I test the diodes too?

If my no power problem is due to a loss of excitation field, I have read that I can flash the excitation winding to create a magnetic field within the metal. However, the guide shows that I have to honor the polarity of the excitation winding and yet the 2 wires are both the same color with no sign of polarity markings. Is polarity important when flashing? I have read that $12 ; V$ for 5 seconds should work. Also, I will order a new multi-meter that is capable of testing the capacitor. If you have any advice on testing I would be grateful to receive it.

It is brushless as stated in the manual and because there are no brushes (unless they are in the prime mover end which would require removal of the stator to access)

Its an AP-85 manufactured by Meccalte

2 black wires that connect to the capacitor match the resistance of the excitation coil specified on page 11 in the manual.

the ground wire goes between the alternator chassis and the receptacle panel

the 4 coloured wires are the 2 stator coils (its a 115/130v alternator so I believe they are connected in series for 230v)

I have tested the stator output directly with the receptacle panel disconnected to rule out faulty breakers and changeover switch.

Manual, it's not great!

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• Brushless rotor winding is something new to me. Single phase is also not very plausible, A guide for "flashing" excitation winding ??!! You'd better post a photo of the alternator with some data, I think you are totally wrong in every aspect. – Marko Buršič Mar 18 at 18:44
• Do you have part numbers? Specifications? Datasheets? – Math Keeps Me Busy Mar 18 at 20:06
• Neither single-phase alternators or brushless alternators are a new concept. This one was manufactured in 2000. Yes I have added manual and data plate to post, thanks. – Tee Jay Mar 19 at 9:47
• thank you for all of your answers, I will see if my additional details and photo's bring any extra light – Tee Jay Mar 19 at 10:01
• Are the diodes physically attached to the rotor? Some non permanent magnet brushless motors have two coils on the rotor, one picks up energy from induction from the stator and feeds the other coil DC through a rectifier. Is there any chance that's what you're looking at? – K H Mar 20 at 0:18

Update: Assuming the windings fuse and diodes are OK, it appears the rotor needs a current kick start to magnetize the coupling and thus self-excitation of this series wound brushless AC generator with DC rectifier option. There does not appear to be any regulation as this design excites the rotor with more load to maintain the output voltage,a sort of unity gain negative feedback magnetic amplifier. It’s like a catch-22, if after not being used for a long time there is no Remanence left in the rotor to increase coupling to the stator, it can’t seem to start the current loop to generate voltage, as there are no permanent magnets. (Not familiar with series wound Motor generator series)

Normal alternators regulate voltage by linear current regulated grounding the excitation single winding.

Yours seems to be different with 2 excitation wires so DC current polarity matters. ( and the rotor diodes take care of that) Voltage will be proportional to RPM and excitation current which requires more excitation with a load is handled by precise design of each coil and gap.

You can compare “load regulated Voltage with a fixed minimum RPM and fixed rated max excitation current. The ratio of load regulation in % will determine your alternator minimum impedance into a suitable battery (both stator and rotor impedances are coupled by the excitation so in series with Diode resistance x2)

If it is indeed brushless, then there must be 2 Hall sensors that go to 0V when flux is detected and pullup high otherwise. then you must externally commutator the stator windings in the correct phase using the Hall sense feedback.

Your “No Power” state may be wrong excitation current for a minimum RPM or a fault somewhere. No need to look at rotor unless no fault elsewhere. (See manual Note added below)

this video makes it look so simple (:

New info from your link: Which not parts have you tested on this diagram? Fuse? Diodes?, Disconnected Cap? Which wire colours match? Mismatch?

Annotate above diagram , SVP.

Note it says:

If the generator does not self-excite, it should be excited by applying an alternating voltage of around 50÷230V to the capacitor heads for an instant..

The Cap is chosen to shift the line frequency toward 90 deg to make two phases from one.
Since the rotor has clamp diodes it will rectify the induced current to polarize the rotor excitation current waves.

• How current go through the rotor without brushes? – user263983 Mar 18 at 21:24
• The excitation current couples magnetic field into the rotor like the 10% excitation current in a power transformer to activate the magnetic coupling and overcome the hysteresis. – Tony Stewart EE75 Mar 18 at 23:48
• So in alternator current go through stator winding, creates magnetic field. That magnetic field creates excitation current in rotor. The rotor excitation current creates magnetic field in rotor , which creates excitation current in stator and you have power output without any initialization energy. – user263983 Mar 19 at 0:36
• May be it is why OP did not have energy ooutput? Could be only with rotor made with permanent magnet. – user263983 Mar 19 at 0:48
• From the manual "AP Series alternators are single phase brushless units. These units do not require maintenance since they do not have slip rings nor slipping contacts." If you have made a comment questioning if this is indeed a brushless alternator, please delete your comment to make the thread less cluttered – Tee Jay Apr 23 at 18:56

On page 8 of the manual it says

If the generator does not self-excite, it should be excited by applying an alternating voltage of around 50÷230V to the capacitor heads for an instant.

I hope this helps

• peter good find! could anyone comment if they think that should be done during engine rotation? – Tee Jay Mar 19 at 15:34