Static test of Denso 12 V alternator

[Guineafowl]

I know a real test of this unit would be to mount it and run, checking DC and AC voltage, current etc. The plan is to mount it on my Super Dexta, but this would need some work as there's currently a dynamo which is starting to fail. I was given the alternator free and would like to bench test it before I bother to do the conversion. Denso A127 alternator.

It was removed from a JCB wheel digger as not charging. So far, I have done the following tests:

Regulator: Hooked up to bench power supply, then connected 12V bulb across brushes. Checked bulb glows as voltage rises to 12V, then goes out at 14.3V. I say the reg is OK. Plenty of brush left.

Rectifier: Desoldered and checked the six diodes with multimeter diode test, forward and reverse. There is also a component labelled 'DIDE' which has four connections - one to each phase and one to the +ve supply to the regulator - I assume this is a diode pack for 'bootstrapping' the regulator. There is a 0.5V drop between each of the phase connections and the output to the reg.

Rotor: Cleaned slip rings - in good nick - and checked resistance between - 3 ohms. No short to body. OK?

Stator: Desoldered three (double) phase wires from rectifier, then separated the three pairs of wires. Checked each of the six wires had continuity to only one other. No short to case. No visible damage.

The only thing I found 'wrong' was the slip rings being dirty - the digger had sat for a while. As far as I can see, the alternator checks out OK, pending a dynamic test on the tractor. Am I right?

 

[Tom.G]

Sounds like you've got it mostly covered. Other than misuse, the most common failure is the brushes. Either they are worn to the extent they don't maintain good contact with the slip rings or there is dirt in the brush holders, holding the brushes away from the slip rings. Another very remote possibility is the slip rings may be out-of-round or not concentric with the shaft. This would have the brushes floating and/or bouncing at higher speeds. In high vibration environments, winding shorts occur; but you've mostly covered that. There could be a rotor short that occurs only while spinning

If there is a motor or generator repair shop in your area, they might be willing to do a functional test.

Or you can do your own test; although it may be more work than the actual installation. Find a way to connect a motor to it and see if it has enough output to light up a couple headlights (or charge a mostly run down battery) and maintain 14.3V to 14.4V. Be sure to mount both it and the motor fairly securely if you try that load test, there will be a fair amount of reaction torque.

A somewhat incomplete test would be drive it with a motor and just measure the output voltage without a load. The only advantage is a less secure mounting can be used, the disadvantage is you won't know if it will charge a battery.

Let us know what you find.

 

[Guineafowl]

Sounds like you've got it mostly covered. Other than misuse, the most common failure is the brushes. Either they are worn to the extent they don't maintain good contact with the slip rings or there is dirt in the brush holders, holding the brushes away from the slip rings. Another very remote possibility is the slip rings may be out-of-round or not concentric with the shaft. This would have the brushes floating and/or bouncing at higher speeds. In high vibration environments, winding shorts occur; but you've mostly covered that. There could be a rotor short that occurs only while spinning

If there is a motor or generator repair shop in your area, they might be willing to do a functional test.

Or you can do your own test; although it may be more work than the actual installation. Find a way to connect a motor to it and see if it has enough output to light up a couple headlights (or charge a mostly run down battery) and maintain 14.3V to 14.4V. Be sure to mount both it and the motor fairly securely if you try that load test, there will be a fair amount of reaction torque.

A somewhat incomplete test would be drive it with a motor and just measure the output voltage without a load. The only advantage is a less secure mounting can be used, the disadvantage is you won't know if it will charge a battery.

Let us know what you find.

Thanks. I connected it to a battery in the proper way, with a casing earth, warning light and B+ connection, then spun it with a drill. I put my meter in series with the -ve cable, on amps. Stationary, the current drawn was about + 350 mA for the warning light. When spinning, the warning light dimmed and the current reversed to - 1.125 A. I take it this means it's putting out some charge.

Of course, taking it to a shop or even just mounting it on the tractor would reveal if it's good or not, but this is more of an interesting exercise than a practical solution.

I can't get my head around the stator windings - there are three phase connections, but each one is made of two wires soldered at the ends where they meet the rectifier. Does this mean it's a delta-configured stator?

My meter also offers conductance in nanosiemens - would this be a good way to test for shorts to casing?

 

[jim hardy]

I can't get my head around the stator windings - there are three phase connections, but each one is made of two wires soldered at the ends where they meet the rectifier. Does this mean it's a delta-configured stator?

Possibly, but the ones I've disassembled were WYE. If there's a fourth junction of at least three and probably six wires it's WYE. I'd guess it is Y wound with double strands of wire for high current capacity.

Your tests sound thorough and proper. Nicely done.

Good luck ! old jim.

 

[Tom.G]

I'm with @jim hardy on the WYE connection being on all I've played with.
If you can supply a make and model number there is a chance to find out for sure. "Denso" same as "Nippon Denso" of Japan?

My meter also offers conductance in nanosiemens - would this be a good way to test for shorts to casing?

Yes. That might detect a high resistance short (electrical leakage) that gets worse during operation, especially on the rotor. However, to cause trouble, any short would be less than, oh, maybe, 100 Ohms (10 millisiemens).

"nanosiemens". Wow! What kind of meter is it? (make and model)

There is also a component labelled 'DIDE' which has four connections - one to each phase and one to the +ve supply to the regulator - I assume this is a diode pack for 'bootstrapping' the regulator. There is a 0.5V drop between each of the phase connections and the output to the reg.

Did you check reverse polarity also? If a diode is shorted it may show as a low value resistor and give a 0.5V drop depending on current.
Although not likely, it could also have 6 diodes in it. Any indication of a fifth connection, such as a metal case?

 

[jim hardy]

Typical automotive alternators I'm used to

The diode trio powers the regulator and field .
Old Bosch and GM and Fords in my day(70's) didnt have that wire from Ignition Switch, or that S(ense) wire.
Bootstrap was through the "Alt Lamp on Dash"which had a low ohm (maybe ten ohms) resistor soldered across its rear terminals. Without that resistor that tiny lamp won't pass enough current to get the alternator voltage up high enough to overcome the diodes until you wind the engine way up to about 4500 RPM.
So lots of alternators I'm sure have got replaced because of that ten-ish ohm resistor behind dash ALT light burning open. I hope you got one of those , it'll be okay. You might look at a new brush set, for when they wear down to about 1/3 of their original length you can't get full output anymore.

Some 60's Fords had a wire from Neutral connected to a relay that disconnects alternator from Battery+ when there's no voltage Neutral to Frame, i guess to prevent wrecking the diodes should somebody hook up jumper cables backward.

Nowadays some have their field controlled by the ECU, a return to the simple alternator with external regulation. Others have an internal microcomputer that communicates with Mr Engine so as to not surprise him with a heavy load at an inopportune time.

old jim

 

[Guineafowl]

Possibly, but the ones I've disassembled were WYE. If there's a fourth junction of at least three and probably six wires it's WYE. I'd guess it is Y wound with double strands of wire for high current capacity.

Your tests sound thorough and proper. Nicely done.

Good luck ! old jim.

There is no fourth junction, and each of the doubled wires, when separated, is continuous with a wire on a different pair. So when I label the wires 1-6, with 1&2 a pair, 3&4, 5&6:

1 connects to 4, 2 to 6 etc.

From what I've found about windings, this says 'delta'. There being no junction and no kind of common connection to the case seems to support this. However, I realize I'm examining a faulty unit, so it's hard to be sure.

 

[Guineafowl]

I'm with @jim hardy on the WYE connection being on all I've played with.
If you can supply a make and model number there is a chance to find out for sure. "Denso" same as "Nippon Denso" of Japan?Yes. That might detect a high resistance short (electrical leakage) that gets worse during operation, especially on the rotor. However, to cause trouble, any short would be less than, oh, maybe, 100 Ohms (10 millisiemens).

"nanosiemens". Wow! What kind of meter is it? (make and model)
Did you check reverse polarity also? If a diode is shorted it may show as a low value resistor and give a 0.5V drop depending on current.
Although not likely, it could also have 6 diodes in it. Any indication of a fifth connection, such as a metal case?

The original label says Denso A127. But there's some incest going on as that label was covered by a Bosch remanufacture label, and the upper case says Magnetti Marelli. To add to the confusion, I watched a Westronicsirl Youtube vid showing a rebuild of a Lucas A127 alternator and that was identical to my unit.

My meter is a Fluke 87V - an eBay bargain (shop clearance and the owner didn't know what he had - boxed and nearly new for £100). I'd had enough of cheap Draper meters dying on me so I decided to get a 'meter for life'. Also much safer for working on the 240V mains we have here, and I like to repair 12V inverters for a hobby, so the true RMS, low-pass filter and capacitor check are handy.

Yes, I checked reverse too. It appears the same as the diode trio on the schematic (thanks for that, by the way Jim). I can't see a fifth connection - the component is a flat case with only four flat conductors emerging from it. No other identifying marks other than DIDE.

 

[Guineafowl]

Typical automotive alternators I'm used to

View attachment 203911
The diode trio powers the regulator and field .
Old Bosch and GM and Fords in my day(70's) didnt have that wire from Ignition Switch, or that S(ense) wire.
Bootstrap was through the "Alt Lamp on Dash"which had a low ohm (maybe ten ohms) resistor soldered across its rear terminals. Without that resistor that tiny lamp won't pass enough current to get the alternator voltage up high enough to overcome the diodes until you wind the engine way up to about 4500 RPM.
So lots of alternators I'm sure have got replaced because of that ten-ish ohm resistor behind dash ALT light burning open. I hope you got one of those , it'll be okay. You might look at a new brush set, for when they wear down to about 1/3 of their original length you can't get full output anymore.

Some 60's Fords had a wire from Neutral connected to a relay that disconnects alternator from Battery+ when there's no voltage Neutral to Frame, i guess to prevent wrecking the diodes should somebody hook up jumper cables backward.

Nowadays some have their field controlled by the ECU, a return to the simple alternator with external regulation. Others have an internal microcomputer that communicates with Mr Engine so as to not surprise him with a heavy load at an inopportune time.

old jim

Well, the digger is from the 90's, but I never thought about looking at the warning lamp on the dash. If there is a resistor there, and it's gone, the new unit won't work either. I'll keep you posted on that!

The brushes and slip ring are suspiciously unworn, and that combined with the Bosch reman sticker suggests they've been replaced.

I did once reconditon the alternator on my wife's Land Rover which has a BMW engine. It had three connections, B+, L (light?) and DFM. The latter is, apparently, Digital Field Monitor and connects to the ECU, but I'm not sure if it was for monitoring or control of the field voltage.

Had a discussion the other day about alternators and heavy loads - many say you should never jump start a modern car, but I think the problem happens when you disconnect the jump leads with both cars running - the 'good' car, having been under heavy load charging the 'bad' car's battery, suddenly has much less load and its alternator/ECU cannot respond quickly enough and produces a voltage spike.

My solution is to shut off the good car before disconnecting the leads.

 

[Guineafowl]

Further to this, I've just bought an old Megger BM10 insulation tester. When it arrives I'll disassemble the alt again and check the stator and rotor windings.

I often get given DC brush motors that are burning and can't do more than check the brushes, armature and field in the normal way. Hopefully this fun little gadget will give me a more positive go/no go result than an ohmmeter.

 

[jim hardy]

I wouldn't stress over the wye-or-delta question. If there's no discoloration of windings from heat, just clean it up and give it a try.

You know to not megger with regulator connected...

 

[Guineafowl]

I wouldn't stress over the wye-or-delta question. If there's no discoloration of windings from heat, just clean it up and give it a try.

You know to not megger with regulator connected...

I've never meggered anything in my life! I assume it's best used on isolated windings in motors, generators, transformers, etc. How do you know what voltage to go up to?

 

[jim hardy]

I assume it's best used on isolated windings in motors, generators, transformers, etc. How do you know what voltage to go up to?

Our rule of thumb for industrial machinery was twice rated plus a thousand volts.
On your alternator i wouldn't go above 500 volts, and that much only on a dry day. i think 200 would be plenty adequate if your tester has such a low setting.
Again be careful, meggers are for testing insulation and they'll fry electronics.

 

[Guineafowl]

It does 50-1000V

 

[jim hardy]

Since alternators are capable of around 200 volts on a load dump as you described above,
i'd use that 250 volt setting. Might risk 500 if it's clean and dry.

If you have it back together though, why not just spin it up ? My local O'Reilly's auto parts store has a test stand for customers' carry-in alternators.

 

[Guineafowl]

Since alternators are capable of around 200 volts on a load dump as you described above,
i'd use that 250 volt setting. Might risk 500 if it's clean and dry.

If you have it back together though, why not just spin it up ? My local O'Reilly's auto parts store has a test stand for customers' carry-in alternators.

It's apart again - I'm a terrible fiddler.

I've nothing to spin it up on, unless I mount it in the tractor, but that would involve making up new mounts and converting it from positive to negative earth. Pending the arrival of my megger gadget, I'll get it done and let you know. Many thanks for your help.

 

[jim hardy]

It's apart again - I'm a terrible fiddler.

I know the behavior !

converting it from positive to negative earth.

If it's like my 8N Ford that's just reversing the wires on battery and ammeter. And new coil & light bulbs if you're going from 6 volts to 12 .

 

[Guineafowl]

I know the behavior !If it's like my 8N Ford that's just reversing the wires on battery and ammeter. And new coil & light bulbs if you're going from 6 volts to 12 .

It's a Fordson Super Dexta - 12V diesel. Won't the starter motor need reversing internally?

 

[jim hardy]

Won't the starter motor need reversing internally?

Only if it's a permanent magnet field which is unlikely.
Reversing polarity on a wound field machine reverses both field AND armature , product of two negatives is positive.
Starters are almost always series wound field machines.

 

[Averagesupernova]

I know the behavior !If it's like my 8N Ford that's just reversing the wires on battery and ammeter. And new coil & light bulbs if you're going from 6 volts to 12 .

I would never change out the coil in a case like this. Use a ballast resistor to lower the voltage so as to not damage the coil. Then have a bypass to eliminate the resistor when starting for a bit of an extra hot spark. This will definitely improve starting performance.

 

[OCR]

It's a Fordson Super Dexta - 12V diesel.

This guy..?
We had one of these...
A good site to bookmark...

 

[Guineafowl]

This guy..?
We had one of these...
A good site to bookmark...

Yes! A very interesting site - thanks for that. She still gets used for grassland work in the summer, and logsplitting in the winter. The dynamo light only goes out above idle, so I guess it's on its way out, hence the alternator fiddling.

You still see quite a few Majors around here - these older tractors are much-loved and spare parts are readily available.

 

[jim hardy]

The dynamo light only goes out above idle,

That's normal for dynamo equipped automobiles. I don't know about tractors, though. My 8N was swapped to alternator by previous owner.

 

[Guineafowl]

That's normal for dynamo equipped automobiles. I don't know about tractors, though. My 8N was swapped to alternator by previous owner.

It seems to be getting worse - the light was always out during use, even at idle. Now I have to have it revving ever higher to get it to go out.

Is a dynamo a permanent magnet stator with brushed rotor? If so, other than remagnetising, is there anything I could do to refurbish it?

 

[jim hardy]

It seems to be getting worse - the light was always out during use, even at idle. Now I have to have it revving ever higher to get it to go out.

Once light goes out does it stay out ?
How worn down do the brushes look ?

Is a dynamo a permanent magnet stator with brushed rotor?

I'm accustomed to wound field and electromechanical voltage regulator.

i'd take the top off the regulator and watch the relays operate. Measure at what voltage does the cutout relay close.
Have you a photo of your regulator with top removed?

My brother once had a Triumph roadster with Lucas dynamo*. The regulator looked a lot like this

and had inside the standard three coil setup - a voltage regulating coil, a current regulating coil, and a "reverse current cutout" coil.

What you describe sounds like excess resistance in the field circuit. I'd open the regulator and wipe the contacts with either a correct burnishing tool(NOT a file or sandpaper) or some rough Kraft paper as from a brown grocery bag.
Then check condition of the wires from field to regulator and regulator to ground.

Here's a most excellent tutorial on electromechanical voltage regulators.
http://www.navioneer.org/riprelay/The Navion Files/Delco_Remy_Generator_Regulators.pdf
it's 13 meg but i saved and printed a copy, it's only twenty pages. Very informative.

*Brother's Triumph had suffered internal mechanical damage , commutator was ruined . We mounted a Ford car generator (1/10th the cost) and the Lucas regulator was perfectly happy to control it.

There's a certain charm to seeing an old school generator on an old school tractor... as if we've defied the teeth of time.

old jim

 

[Guineafowl]

Once light goes out does it stay out ?
How worn down do the brushes look ?

I'm accustomed to wound field and electromechanical voltage regulator.

i'd take the top off the regulator and watch the relays operate. Measure at what voltage does the cutout relay close.
Have you a photo of your regulator with top removed?

My brother once had a Triumph roadster with Lucas dynamo*. The regulator looked a lot like this
View attachment 204149
and had inside the standard three coil setup - a voltage regulating coil, a current regulating coil, and a "reverse current cutout" coil.

What you describe sounds like excess resistance in the field circuit. I'd open the regulator and wipe the contacts with either a correct burnishing tool(NOT a file or sandpaper) or some rough Kraft paper as from a brown grocery bag.
Then check condition of the wires from field to regulator and regulator to ground.

Here's a most excellent tutorial on electromechanical voltage regulators.
http://www.navioneer.org/riprelay/The Navion Files/Delco_Remy_Generator_Regulators.pdf
it's 13 meg but i saved and printed a copy, it's only twenty pages. Very informative.

*Brother's Triumph had suffered internal mechanical damage , commutator was ruined . We mounted a Ford car generator (1/10th the cost) and the Lucas regulator was perfectly happy to control it.

There's a certain charm to seeing an old school generator on an old school tractor... as if we've defied the teeth of time.

old jim

Wow, some bedtime reading! You've obviously spent a lot of time getting all that together - a great many thanks for that.

I'll get on to it tonight when I have more time.

And yes, I'm reluctant to replace the dynamo with an alternator - seems a bit rude to shoehorn in a modern device on such a nice old machine.

 

[Guineafowl]

Once light goes out does it stay out ?
How worn down do the brushes look ?

I'm accustomed to wound field and electromechanical voltage regulator.

i'd take the top off the regulator and watch the relays operate. Measure at what voltage does the cutout relay close.
Have you a photo of your regulator with top removed?

My brother once had a Triumph roadster with Lucas dynamo*. The regulator looked a lot like this
View attachment 204149
and had inside the standard three coil setup - a voltage regulating coil, a current regulating coil, and a "reverse current cutout" coil.

What you describe sounds like excess resistance in the field circuit. I'd open the regulator and wipe the contacts with either a correct burnishing tool(NOT a file or sandpaper) or some rough Kraft paper as from a brown grocery bag.
Then check condition of the wires from field to regulator and regulator to ground.

Here's a most excellent tutorial on electromechanical voltage regulators.
http://www.navioneer.org/riprelay/The Navion Files/Delco_Remy_Generator_Regulators.pdf
it's 13 meg but i saved and printed a copy, it's only twenty pages. Very informative.

*Brother's Triumph had suffered internal mechanical damage , commutator was ruined . We mounted a Ford car generator (1/10th the cost) and the Lucas regulator was perfectly happy to control it.

There's a certain charm to seeing an old school generator on an old school tractor... as if we've defied the teeth of time.

old jim

Further to this, I stripped the reg out of the tractor, cleaned the contacts (NOT with sandpaper, as I have done all my life but never knew better!) and all is working. I didn't try adjusting anything. I owe you a pint of IPA. Also, I loved the Delco-Remy guide. You don't see such attention to detail in modern manuals - too often the words "refer to authorised service centre" appears.

This is fortunate because, if you're interested, I've got the Megger and have tested the windings of the alternator. The manual shows proper operation but doesn't really go into interpreting the results, so I'd appreciate your input:

RECTIFIER: 1.1 Gohm @ 50V for all diodes in reverse. Didn't dare go further.

ROTOR: Coil resistance, 3 ohm. Coil-case: 200 Mohm @ 50V, 100 Mohm @ 500V. My verdict: PASS.

STATOR: Confirmed delta config. Separated coil wires before test. Labelled coils A, B, C. All coils <0.5 ohm.

A-B: 5 kohm @ 50V, 3 kohm @ 500V.
A-C: 10 kohm and 5 kohm.
B-C: 20 kohm and 9 kohm.

All coils to case: 3.5 Mohm @ 50V, 4 Mohm @ 500V. Not sure why, assume difficulties in reading scale at 50V.

My verdict: FAIL due to inter-coil shorts.

Thoughts? I did test at 1000V afterwards, and the needle jumps to near zero.

 

[jim hardy]

A-B: 5 kohm @ 50V, 3 kohm @ 500V.
A-C: 10 kohm and 5 kohm.
B-C: 20 kohm and 9 kohm.

All coils to case: 3.5 Mohm @ 50V, 4 Mohm @ 500V. Not sure why, assume difficulties in reading scale at 50V.

My verdict: FAIL due to inter-coil shorts.

Thoughts? I did test at 1000V afterwards, and the needle jumps to near zero.

That interwinding resistance sounds too low to me, though it probably was functional. I'd expect tens of megohms. Perhaps it was wet or very dirty. One can wash them with soap and distilled water then dry in an oven about 120 degF and sometimes improve the readings a LOT.

After the 1kv test, does it read shorted with plain old ohm-meter?

Anyhow - congratulations on your great adventure inside the Lucas regulator.
Isn't it great when a mysterious device becomes a familiar one ? The ingenuity of those 1930's era design guys is awesome.

Furthermore, you've kept your tractor impervious to EMP attack.

old jim

 

[Guineafowl]

That interwinding resistance sounds too low to me, though it probably was functional. I'd expect tens of megohms. Perhaps it was wet or very dirty. One can wash them with soap and distilled water then dry an an oven about 120 degF and sometimes improve the readings a LOT.

After the 1kv test, does it read shorted with plain old ohm-meter?

Anyhow - congratulations on your great adventure inside the Lucas regulator.
Isn't it great when a mysterious device becomes a familiar one ? The ingenuity of those 1930's era design guys is awesome.

Furthermore, you've kept your tractor impervious to EMP attack.

old jim

I should have mentioned that the readings were taken after washing in soap and (tap) water then drying on top of the stove.

It doesn't read shorted with the multimeter now, but if I squeeze the coils gently the reading will jump around a lot. Remember this is a faulty unit that I haven't found a real fault on until now. I'm guessing when the alternator warms up and is in use the insulation breakdown becomes a problem. Now I need to find some other uses for the Megger unit...

What struck me about the regulator is the ruggedness and adjustability of an electromechanical unit. I sometimes wish I'd been born a little earlier and able to work daily with these sorts of things. Oh, well, back to peering through my jewellers' loupe at yet another little sod of a surface-mount capacitor with no markings...

 

[Tom.G]

It doesn't read shorted with the multimeter now, but if I squeeze the coils gently the reading will jump around a lot.

Yup, intermittent coil-to-coil short. Hope you can find a motor rewind or alternator repair shop nearby. It's possible to do it yourself but kinda touchy, and often hard to get the right size wire and insulating paper if used.

Please keep us updated.

 

[Guineafowl]

Yup, intermittent coil-to-coil short. Hope you can find a motor rewind or alternator repair shop nearby. It's possible to do it yourself but kinda touchy, and often hard to get the right size wire and insulating paper if used.

Please keep us updated.

Sadly, the little old rewind shops are getting rare these days. Since I assume a bad stator is a less common fault than, say, a bad rectifier or regulator, I may be able to find another similar unit and make two into one.

Anyway, I can bide my time now as the old dynamo is back on line!

I'll certainly post an update - very frustrating to view a thread that is not resolved.

 

[jim hardy]

It doesn't read shorted with the multimeter now, but if I squeeze the coils gently the reading will jump around a lot.

Sometimes, if you're lucky,...
you can poke the winding with a soft wood stick (ladies' orange stick for cuticle maintenance work well) and get the wires separated .
Then saturate the winding with varnish.
Proper material would be electrical varnish or industrial "Glyptol" electric motor paint made for the purpose.
I have had success with both Polyurethane wood furniture varnish from the hardware store and high temperature engine enamel from the auto parts store. For under the hood i think i'd prefer the engine enamel.