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Rectifier question. #773978 05/17/19 7:37 am
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TR7RVMan Offline OP
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Hi All, Quick question. Has anybody ever taken a rectifier apart to see what holds it together & see what's between the discs in the center bolt?

Bike in question is '73 T140, but I think they're the similar for many years.

On this bike changing batter I noticed the 2nd disc from the front (end that's not the mounting end) can slightly rotate on the shaft light the end nut may be not pinching stack tight.

The diode wires to next disc are intact. The charging system is working ok at the moment. All original alternator, rectifier & Zener etc.

Photos if possible would be very helpful.

Thanks, Don


1973 Tiger 750
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Re: Rectifier question. [Re: TR7RVMan] #773980 05/17/19 8:31 am
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Stein Roger Offline
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Originally Posted by TR7RVMan
Hi All, Quick question. Has anybody ever taken a rectifier apart to see what holds it together & see what's between the discs in the center bolt?

Bike in question is '73 T140, but I think they're the similar for many years.

On this bike changing batter I noticed the 2nd disc from the front (end that's not the mounting end) can slightly rotate on the shaft light the end nut may be not pinching stack tight.

The diode wires to next disc are intact. The charging system is working ok at the moment. All original alternator, rectifier & Zener etc.

Photos if possible would be very helpful.

Thanks, Don

There's no such thing as a "quick question" on here Don! What were you thinking? confused
But seriously, no I haven't dismantled one but they are as you say, merely a stack of plates. as long as the wires between the diodes themselves aren't broken, they usually work.
The things between the discs would simply be insulating spacers I guess? You know Don, I might just sacrifice an old one just for you, watch this thread!

There are people on here with a lot more experience than me, but I've never come across a rectifier that doesn't work even if it looks like a wreck, twisted and bent.
(I sometimes straighten, blast and paint them them just for looks.)
Likewise, I've only come across two failed Zeners, both killed by silly mistakes. (one by me..)
I'm sure there are many out there that have failed in service of course, I'm just saying that in my own experience they are pretty reliable components.

Re: Rectifier question. [Re: Stein Roger] #774011 05/17/19 3:59 pm
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quinten Online Content
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Yes , just large diodes attached to heat dissipation fins .
Half the fins are insulated from the mounting stud and half are not .( so the stud has polarity)

When new , there is enough tension in the stack to keep it from twisting ... and also often .... some coating ro add rigidity

If it loosens ...more and more "work" is transferred to the solder joints to keep both electrical and mechanical order .
... just like in any other wiring connection , some electrical efficiency could be lost by
the mechanical looseness of the Stack.

This pic is not your rectifier , but it shows a typical stack , diodes and insulation arrangement .
[Linked Image]

If you are careful , while keeping proper fin ailment ( stress off solder spots )
you can used 2 wrenches , one on the top-bolt and one on the bottom-nut .
... tighten the nut that tensions the stack ... and then repaint to insulate.

Or buy a more efficient modern 35amp "cube" Rectifier .
They can be purchased dirt cheap off the net .



Re: Rectifier question. [Re: TR7RVMan] #774040 05/17/19 8:43 pm
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As Quinten said, the diodes themselves are embedded in the fins, as they are the only heat generators that require cooling.

The wires from the diodes are thick, rigid copper wires for efficient conduction of routinely more than 10A.
It is critical that those wires are not subjected to vibrational stress, as they will fracture at one end or the other. A fracture at the diode end is unlikely to be repairable, without damaging the diode.

The discs between the plates are insulators as you suspected, so a little looseness between them would not cause electrical problems with the exception that the ground plate obviously must be connected to the mounting stud.
Any looseness in the stack will allow this crucial connection to be compromised, both by poor connectivity and as time goes by corrosion between the surfaces, irrespective of vibrational damage to the connecting wires mentioned above.

As said, you have little to lose by clamping the plates carefully in soft vise jaws, and nipping them up with 2 spanners. Or just leave it to die naturally.

Modern replacements are cheap enough.

Re: Rectifier question. [Re: TR7RVMan] #774049 05/17/19 10:33 pm
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These days the replacement is this type of thing, better/higher current rating and more reliable/robust.

https://www.eBay.com.au/itm/50A-1000V-Metal-Case-Single-Phases-Diode-Bridge-Rectifier-KBPC50WG/273728030457?hash=item3fbb7646f9:g:K6QAAOSwSIJaqMOZ

https://www.eBay.com.au/itm/Rectifier-with-3-Phase-40A-1000V-Diode-Bridge-SQL40A-P8B8/253668303330?epid=722902786&hash=item3b0fcf21e2:g:U9oAAOSw95lbFQWx&frcectupt=true


Or think about using a proper regulator/rectifier along these lines.

https://www.eBay.com.au/itm/12V-Motorcycle-Voltage-Regulator-Rectifier-For-Honda-PC31-CBR600-CBR900RR/401641918851?hash=item5d83b95183:g:CRMAAOSwB9Fb65ok

https://www.eBay.com.au/itm/Regulator-Rectifier-Male-Plug-Connector-For-Honda-CBR600-F2-F3-F4-CBR900-893-919/232134044324?hash=item360c446aa4:g:9IQAAOxyOlhS1gGB&frcectupt=true

Re: Rectifier question. [Re: TR7RVMan] #774051 05/17/19 10:54 pm
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I have both the 3 terminal (49072A) and 4 terminal (49072B) bridges here. The middle four spaces are insulators because they isolate each plate, which is a terminal, from the next and the central stud. The top and bottom plates do not have insulators above/below because these plates connect together through the stud (+).
Presumably Lucas figured out that the stud connection as a ground point does not work well since it depends upon the part that it is mounted to (the coil bracket on the OIF) having a good connection to the engine and the loom connection and added the fourth terminal.
Modern regulators still need a heat sink since they shunt the excess current to ground or directly back into the generator coils.
Too modern of a regulator might not work since many new electrical systems use LED lights and more efficient ignitions so have smaller capacity generators.

Re: Rectifier question. [Re: TR7RVMan] #774055 05/17/19 11:31 pm
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A modern 'small capacity generator' is most likely about twice as powerful as the unit fitted on the average old britbike
but yes that side of things should be considered.

Re: Rectifier question. [Re: TR7RVMan] #774056 05/17/19 11:40 pm
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"Modern regulators still need a heat sink since they shunt the excess current to ground or directly back into the generator coils."

Of course modern rec/regs generate heat, but very little in comparison with a wasted power system like a zener diode, where the alternator always gives the best it can do at any particulat RPM.

The SCR's of a modern reg/rec are only switching on for appropriate parts of the sine wave, governed by the system voltage. When the system voltage is high (say 15V) then the SCR's are switched off, which is effectively disconnecting the alternator wires.
In this condition, the alternator windings are under no conceivable duress. The only heat generated in the reg/rec is by the usual tiny components in its ciruits. This doesn't compare to the possible 50W that a zener may be expected to cope with. It is an entirely different system.

Re: Rectifier question. [Re: TR7RVMan] #774065 05/18/19 12:59 am
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Koan.
Not really, the scr's are the 'shunt' switches, they in fact 'short' the windings not open the circuit.
Opening a PM alternator winding at high rpm will cause excessive voltage across the stator and
may break down insulation. The largest heat generator in a 'modern' regulator is the forward volt drop
across the bridge. That's why they are going to mosfet replacement for diodes thus reducing the
drop to the small on resistance of a fet. This method can be employed on either the positive only
side of the bridge or with a little more work, both sides. Manipulating the on time of the fets can
achieve the 'shunting and the rectification of the supply.

In the case of most standard regulator units,
The heat generated by the thyristors is relatively small once the shunt is on as the current collapses
in the stator coil, the main losses are during the switching time and the initial load.
You still use, as you say, a phase angle trigger, but shunt, not open circuit the staor.

One of the many 'ideal diode' controllers available.
https://www.analog.com/en/technical...oller-with-reverse-input-protection.html

Last edited by NickL; 05/18/19 1:16 am.
Re: Rectifier question. [Re: TR7RVMan] #774076 05/18/19 3:47 am
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"Opening a PM alternator winding at high rpm will cause excessive voltage across the stator and may break down insulation."

Is this reality or scaremongering?

So the alternator wires being disconnected will damage the insulation, if that happens at high RPM (presumably when the engine is exploding)

I think this is nonsense. What was the point of this NickL?

My experience of stators is that they are incredidibly resilient, as long as you're not silly with them. I don't even pretend to understand what NickL meant by the top statement.

Re: Rectifier question. [Re: TR7RVMan] #774083 05/18/19 7:31 am
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A rotating Permanent Magnet alternator will generate voltage relative to speed.
If you measure your alternator output with the rectifier and zener disconnected
at about 2-3k rpm you will get about 100-140 volts P-P. most of these old stators
have little better than shellac type insulation on the windings. They are designed
to be run under load so the voltage never gets higher than around 20v.
That's why 'shunt' type regulation is the norm.
Try it if you don't believe me.............. I'm sorry if you feel i am 'scare-mongering'.
An scr or any series semiconductor regulator will in effect be opening the stator
to regulate voltage, that is generally thought, bad news.

Yes the stators are resilient if treated reasonably but they have their limits.
Same with dynamo windings, even Lucas specified that when testing a dynamo
by connecting the field directly to the armature, 'do not exceed 20v' at the output.
It's because the winding insulation is not rated any higher.
It equates to the same thing as operating an ignition coil with no spark plug attached.


As far as i know, no-one manufactures a PM regulator that works as a series
device ie. opening the primary side for regulation without having quite extensive
over-voltage suppression.

Re: Rectifier question. [Re: TR7RVMan] #774087 05/18/19 8:46 am
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I've been foraging around to find a perhaps more 'learned' explanation than
mine as you seem to doubt it. (God knows how i managed to hang on to a job in the electronics game eh?)

The chap 'Crutschow' on this electronics forum is a very clever guy, and about 100 years more up to date than me.
His description of PM regulation is very good and apparently he is a motorcycle enthusiast. Perhaps you will believe him.

https://forum.allaboutcircuits.com/...hunt-regulating-a-3-phase-stator.128316/

Re: Rectifier question. [Re: TR7RVMan] #774090 05/18/19 9:38 am
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Stein Roger Offline
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And I thought I was clever, to come up with "insulating spacers"... laughing

Re: Rectifier question. [Re: TR7RVMan] #774094 05/18/19 10:29 am
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Permanent magnet alternators are not very efficient in my opinion...The alternator is always at full output regardless of load and just shunts the excess . Like running with the throttle wide open and using the brakes to hold the desired speed...Auto types that regulate rotor "magnetic strength"are better but won't function in an oily environment because of the brushes..ATV/ SxS PM alternator stators can have a short life from getting hot from because there's often little load and lots of rpm..Some use oil squirters to keep the rotor cooler...Some bikes also use the same
Years ago I used to rebuild truck/bus alternators as sort of side business..They were about 80-120 amps back then..Had a large military surplus adjustable load device and a 6 HP gas engine to spin the alternator..Turn up the load and hear them alternators moan in protest and then stall the gas engine.


79 T140D, 96 900M Ducati ,2001 Sportster....On a bike you can out run the demons..
Re: Rectifier question. [Re: TR7RVMan] #774095 05/18/19 10:36 am
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The permanent magnet alternator is not at full load when short-circuited. Not the Brit bike type anyway.


Amateur Loctite enthusiast.
Re: Rectifier question. [Re: triton thrasher] #774096 05/18/19 11:44 am
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Originally Posted by triton thrasher
The permanent magnet alternator is not at full load when short-circuited. Not the Brit bike type anyway.


...I see it as when the engine is running at high rpm with no lights on for example, the regulator needs to shunt away enough power to keep the volts around 15...That could be 50 % or more of it's rated watts that is nor being used by the ignition??


79 T140D, 96 900M Ducati ,2001 Sportster....On a bike you can out run the demons..
Re: Rectifier question. [Re: Hillbilly bike] #774118 05/18/19 8:06 pm
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Originally Posted by Hillbilly bike
Originally Posted by triton thrasher
The permanent magnet alternator is not at full load when short-circuited. Not the Brit bike type anyway.


...I see it as when the engine is running at high rpm with no lights on for example, the regulator needs to shunt away enough power to keep the volts around 15...That could be 50 % or more of it's rated watts that is nor being used by the ignition??

An AC shunt regulator is not shunting power , the potential power is never realized

A 10 amp stator
As an inductor
Can not be its own load .
Shunting an inductor ... by lowering the resistance
pulls the voltage towards zero ... 10 amps x zero-ish volts is ... close to no watts ... there less load ... less work is being done
The load is only the switch load of regulation.
The stator momentarily is a magnetic circuit .
The copper mass is a closed loop ,
an induced magnetic field closed into and on itself .
With less drag than when used to sustain a 14 volt potential to an external load .
The stator is unloaded 40 to 60 percent.
For the phase angle portion used during each shunting .

As this is happening at up to 400 hz ?
A modern ac side shunt regulator can be the busy bee .
The load imposed on the engine by the stator
Is "marginally" easier with AC shunt regulation ... on paper anyway .

It more efficient... but moves the effective burden
on to the efficiency of the regulator.



















Re: Rectifier question. [Re: TR7RVMan] #774121 05/18/19 9:23 pm
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NickL - thanks for the link to the FET fellow's circuit page, I found that very interesting.

I apologise for my delusions about the workings of regulators, you are absolutely correct that they function by shunting rather than disconnecting.
I have been spending too much time dealing with triac/diac circuits for mains control.

Going back to the voltage/insulation problem with a cut-rather-than-shunt mechanism, the potential 80+V that the alternator may generate open circuit is presumably what may be measured across the 2 or 3 alternator output leads at high RPM.

Looking inside the stator, that say 80V is shared between one end and the other, through the many hundreds of windings. Any adjacent windings will surely only have a small potential difference between them (certainly nothing like the full 80V), so where does the threat to insulation come from?

It seems to me that the only wires exposed to high PD's are the output wires. Is this the insulation breakdown between these that is being spoken of?

I expect, as usual, that I have missed something elementary - I am grateful for your illumination and patience.

Re: Rectifier question. [Re: TR7RVMan] #774130 05/18/19 10:30 pm
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There are not that many windings on a typical old Lucas stator that's one point.
The voltage you observe when the stator is run open circuit is only one part of
the problem. When you open a running stator with a regulating semi you are doing it very often
and very fast, it's a large inductor so spikes will be huge, several times that observed
by just running it open circuit. You would need a scope to see but that's the main
threat to the insulation.

I recently had cause to design a large output PM regulator and spent a fair while doing it.
I used fet's as the current requirement was very high 40-50 amps.
I wish i had found that chaps circuit beforehand, it would have saved me exercising my
seized up old brain. Strangely with few exceptions it is very similar to the thing i came
up with, maybe i'm not such a senile old git anyway.

Re: Rectifier question. [Re: TR7RVMan] #774215 05/19/19 7:23 pm
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Hi All, Interesting subject. I had no idea how complex the charging system really is. Overall the factory system in my estimation is extremely long lasting & trouble free.

I understand the balance of consumers to output is critical. That is the weak point I suppose, no real reserve for adding on additional consumers.

Back to the rectifier, I was hoping someone could take it apart to show exactly what the insulator bushings etc. is like. What are they made of? How hard are they?

There is a sleeve nut at front disc that seems?? to hold stack together. Then I have what appears to be a spacer sleeve?? with thick black paint. Then thick flat washer, tool tray, finally lock washer & non locking nut.

So it looks like I'd need to back off mounting nut, counter hold the through bolt head & first plate next to sleeve nut, then tighten sleeve nut.

I have no idea of how tight the discs should be. Interesting only 1 disc can be moved with fingers. It certainly still has some tension on it, but too easy in my mind.

Another question.... Are the discs & insulators bonded to the through bolt in any way with some sort of sealant? Or is there a plastic insulating sleeve around bolt. Obviously some discs must be insulated from ground. Or are the bushings stepped? That's the kind of stuff I'd like to know.

My rectifier has a tab connector under through bolt head with a red wire hooked to it. This wire is not shown on wire diagram. So it looks like the tool tray becomes a redundant ground?? Shop manual calls this connector capacitor ignition terminal. If my bike has a capacitor, I've never been able to find it. Ohm meter shows red wire is hooked to ground.

Stein, please don't feel obligated to take apart a good one. I was hoping someone had a bad one they could take apart.
Don


1973 Tiger 750
Re: Rectifier question. [Re: quinten] #774216 05/19/19 7:31 pm
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Originally Posted by quinten
Originally Posted by Hillbilly bike
Originally Posted by triton thrasher
The permanent magnet alternator is not at full load when short-circuited. Not the Brit bike type anyway.


...I see it as when the engine is running at high rpm with no lights on for example, the regulator needs to shunt away enough power to keep the volts around 15...That could be 50 % or more of it's rated watts that is nor being used by the ignition??

An AC shunt regulator is not shunting power , the potential power is never realized

A 10 amp stator
As an inductor
Can not be its own load .
Shunting an inductor ... by lowering the resistance
pulls the voltage towards zero ... 10 amps x zero-ish volts is ... close to no watts ... there less load ... less work is being done
The load is only the switch load of regulation.
The stator momentarily is a magnetic circuit .
The copper mass is a closed loop ,
an induced magnetic field closed into and on itself .
With less drag than when used to sustain a 14 volt potential to an external load .
The stator is unloaded 40 to 60 percent.
For the phase angle portion used during each shunting .

As this is happening at up to 400 hz ?
A modern ac side shunt regulator can be the busy bee .
The load imposed on the engine by the stator
Is "marginally" easier with AC shunt regulation ... on paper anyway .

It more efficient... but moves the effective burden
on to the efficiency of the regulator.



Thanks Q. I sort of knew some of that but I’m too ignorant to type it up.
















[/quote]


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Re: Rectifier question. [Re: triton thrasher] #774239 05/19/19 10:00 pm
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Then why does a zener get so hot? it must be conducting high wattage to ground? Whey do "modern" rectifiers/regulators have fins and get hot? Why do the stators on some machines have oil squirters to cool the stator? I'm asking because I thought I knew and now I don't...


79 T140D, 96 900M Ducati ,2001 Sportster....On a bike you can out run the demons..
Re: Rectifier question. [Re: TR7RVMan] #774240 05/19/19 10:05 pm
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These bits get hot because they’re not perfect. The rectifier has resistance, for instance.


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Re: Rectifier question. [Re: TR7RVMan] #774244 05/19/19 10:40 pm
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Originally Posted by koan58
"Opening a PM alternator winding at high rpm will cause excessive voltage across the stator and may break down insulation."

Is this reality or scaremongering?

So the alternator wires being disconnected will damage the insulation, if that happens at high RPM (presumably when the engine is exploding)

I think this is nonsense. What was the point of this NickL?

My experience of stators is that they are incredidibly resilient, as long as you're not silly with them. I don't even pretend to understand what NickL meant by the top statement.

You are correct. Open cct. the alternator will not kill itself.
https://drive.google.com/file/d/0B1JR4IC3eMhZNDBjNjBmNmUtYzVlOS00ZmU5LTliNTMtMTYxZDBhYjM1ZmQ4/view

A generator at a power plant might but that is irrelevant to us.

Re: Rectifier question. [Re: TR7RVMan] #774246 05/19/19 10:49 pm
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The zener on most set-ups is placed on the DC side of the system, so you know what the DC waveform looks like
at that point, raw dc with a hard ripple above the line relative to speed/frequency.
The standard zener is around about a 15v 100 watt device, it's placed directly across the dc rail pos and neg.
With a PM alternator the generated voltage is relative to speed and load, the voltage will go up if rotational speed
is high and load is light, the voltage will drop if load is high or speed is low. Current output is set by design.
At the set zener voltage the device will start to breakdown and conduct lets assume 15v and at that time the generator
is revving at say 2000 rpm. The design of the alternator may be at 2000 rpm it can provide 5 amps.
The bike or lights etc are using say 2 amps.
So the zener must dissipate as heat the difference between it's value and the alternator's output power as watts.
Volts across zener at time of conducting = 15v Current at time of conducting 3a.
V x I = P so 15v at 3amps = 45 watts
This would be the case if the generated power was pure dc and the zener was conducting all the time but in the real
world the zener is only on for the time the actual peak voltage is greater than it's value. so the duty cycle plays a large
role in the sizing of the zener. Say duty cycle of 50% allowing for alternator construction poles, air gap, etc.
So 45 x 0.5 = 22.5 Watts
That's why the good old zener gets so hot. (I've probably been a little over the top with duty cycle.)


Last edited by NickL; 05/19/19 11:46 pm.
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