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Rectifier / Regulator and cooking your alternator

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I have had an RM24 alternator on my 750 for years with the 3-ph regulator rectifier which acts in place of the old rectifier and zener diodes. It gets warm normally because it has to 'dump' much of the power the alterntor is producing.

I recently purchased and 850 which had a 3-ph alternator with one phase failed. I replaced it with a new RM24 and 3-ph R/R as per my 750. But.... the alterntor cooked itself. No mechanical interference, just slowly melted the alternator resin.

It turns out that the more modern 3-ph R/Rs don't use zener diodes, they sense the voltage and then 'turn off' the power from the alternator - this means they run cool - they don't dissipate any power. Some are 'series' type - they open circuit the alternator (disconnect the phases) when the voltage is too high and reconnect when it is too low. The are 'shunt' type (such as Wassell, Podtronics) and these turn off the power by short circuiting the alterntor. So, unless your electrical load is somewhere close to what your alternator can produce, you then literally cook your alternator.

I've learned that there are very few 'series' types on the market and they are more expensive - such as the Shindengen SH755. But the other thing I've learned is that there are several 'Chinese' copies of this device sold from China, Italy and Poland that are cheap (and sometimes not cheap) immitations that are in fact 'shunt' types.

The price of the genuine article seems to be about £60 (if you look in the US) but try to find one in the UK and you keep coming back to ebay and all the rubbish.

The genuine ones are fitted to Yamaha and Polaris but if you find the right part on Polaris UK, the price is £200!

I have had to purchase another stator (£100) and now I'm hoping to find a regulator for a bit less but not at easy. At least with the current lock down, I feel I've got a bit of time to explore.

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 That the regulators went open circuit and therefore there was no load and it was the Zener system that 'shorted' the alternator and dumped the excess as heat, but you learn something every day. I suggest that you need to talk to Al Osborn and enquire about his reg/rect units?

For what its worth,  my 650 has a 3-phase alternator and it is happily controlled by a reg/rect from a Kawasaki 1100.

Regards, George 

 

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I thought that they are pulse width modulated at a constant frequency, with an on/off ratio depending on feedback of output voltage. So heat dissipation from the unit are a combination of diode forward voltage drop, typically about one volt, some other internal resistance and switching losses when changing between on and off state. This guess is based on the fact that TriSpark recommend a filter to suppress RFI (radio frequency interference) if you have a Podtronics unit. They also have put a high power MOSFET reg/rec on the market. Probably because MOSFETs have shorter transition time between on and off.

I don't understand why a shunt type reg/rec should cook the alternator more than the original rec/zener, some of them have been working for decades.

Hopefully, Al will explain and lift the clouds.

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The original zener type dumped the spare power - i.e it acts as an adjustable load which dumps any power that the rest of the system doesn't need. Current flows through it a 14.7V and develops power. But the alternator is 'seeing' that as a 14.7V as a voltage to overcome before producing power. If the regulator give is a dead short, it doesn't have any voltage to overcome when it is shunted mode. With an internal resistance of 0.6 ohms, and no voltage to overcome, an alternator is going to be producing a lot more current than it is designed to produce and hence heat in its windings.

 

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I am keen to stop things from getting too confusing here.

 

You should refer to a ‘short-type’ regulator/rectifier instead of using the term ‘shunt’

...this is because all regulators (including your original zener) shunts excess power some where in some way.

So with that in mind, talking about shunting can confuse people.

 

The behaviour of ‘short-type’ regulator/rectifiers shorting out the AC input side is not specific to three phase units.

The single phase units do this too.

The most common examples are Podtronics, Boyer Bransden Power Box and Tympanium, and they all work this way. And don’t be too concerned - they have been doing this for years with no issues at all.

 

But do bear this in mind - we are all under the impression that bigger is better - and have been conditioned to think that way. To that end, many of us have fitted higher output alternators to our bikes thinking that it's going to be a big improvement and all of our problems are going to go away. Actually it just introduces a new set of challenges. The original charging system on an old brit bike was designed to charge at around the same rate as the electrical loads on a bike draws - look at it as kind of a balanced system.

A Commando will be typically drawing around 10 amps when it’s up and running (with headlight on)

So when you are upgrading to a super duper charging system, maybe ask yourself as you are bolting up that high power single phase 200 watt, 16 amp RM27 (LU47239) or three phase 180 watt, 14.5 amp RM24 (LU47244) what is going to happen to all that extra power you’re now making!?!?

And if you don’t ride with your headlight on, or are fitting LED lamps as part of your upgrades, then once again think twice about your upgrade!

 

 

The ‘series-type’ regulator/rectifiers by Shindengen are the models SH775 and SH847 NOT SH755 as stated by the OP.

This type of unit is also referred to by some manufacturers as ‘open-type’

The components used to make these are no more expensive or exquisite than anything else available in the market, but they haven’t been widely implemented yet, which means production numbers are low (hence the higher cost)

Now Yamaha are rolling this technology across their range, as are Polaris on their ATVs, jetskis and snowmobiles hopefully the price will start to come down.

They are also catching on with the Harley boys too.

A fews misconceptions here Mr Martin-Yes the original Zener dumped excessive 'power' This cannot be adjustable as the Zener starts to conduct at its voltage ie 14.7 (if you are lucky) nothing to do with alternators having to 'overcome' this to product power. The way it works is this, the alternator produces AC (variable frequency depending how fast your engine is going) way out of specification of your '50 cycs mains' calibrated multymeter-so don't try to measure it with any accuracy. This AC is rectifier with a full wave bridge. We now have DC to recharge our battery. As of course the ignition and lights run off the battery. The Zener starts to cut in at its specified voltage, draws current so 'holding the available DC at a safe voltage below that that would other wise over charge the battery and/or blow bulbs and might even cause an ignition miss fire (due to excessive 'battery' voltage. IF on the other hand you have a low output alternator,(tired rotor) and you runaround with your lights on and maybe even have a sidecar with a car battery in it that can take the extra charge current, then you won't need a zener.

With regard to alternators producing 'a lot more current than it is supposed to' not so, if the traditional Lucas manufactured alternator is run into a short circuit as per the modern regulator/rectifier the voltage across it drops to a couple volts hence power generated by the alternator is often 10 Amp at 2 V ie 20W-not an issue. As LUCAS alternators used to stand being shorted out any modern failures with the potting needs to be investigated properly, if it was made in the Far East and sold under a WW part number then I suggest you contact that company-let me know the results.

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You say that your 750 works well with an RM24 and a reg/rec. Most people seems to have no problem with that setup.

Then the 850 came with a faulty alternator (RM24?). Why had it failed?

You fitted a new RM24, which melted.

Two obvious questions. Could a faulty reg/rec have caused death to both alternators? Was the new RM24 faulty from start?

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Be careful here as confusingly Lucas list two types of RM24 alternator stator:

  • RM24 10.5 amp stator (LU47252)
  • RM24 14.5 amp stator (LU47244)

I would suggest that the 10.5 amp unit is a great fit for the Pre-MK3 Commando.

 

The high power unit should only be considered if you have an electric start and your charging system is continually recharging a depleted battery.

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I run a RM23 single phase, a Tri Spark mosfet rectifier and a BSM from Al. On the MK3 even with full beam on the BSM still indicates green. This suggests that the RM23 output is more than adequate once you have a system that can effeciently use what it generates. 

The podtronic fitted prior to the Tri Spark rectifier was not as efficient as the BSM used to glow orange red at night when the revs dropped and that was with just dipped beam on. 

Unless you are going to tap and use the extra a 3 phase stator will give you, then it seems pointless heating the atmosphere. 

I don't think you're correct Alan Oz. It is the current I'm talking about and not the voltage.

The power dissipated in the alternator stator is produced by the current going through its internal resistance. The resistance stays constant but, as the current goes up and the power dissipated is proportional to the square of the current. You are therefore going to dissipate a lot more power in the stator with a shorting type and (in my case) enough to melt the resin.

If the voltage produced by the alternator is enough to drive 14.5 Amps into a load at 14.7V, think what the current will be when you replace the 14.7V with a short (0V).

I've looked at the Norton workshop manual and it's advice on testing an RM21 in situ. Looking at the numbers, they infer an internal resistance of about 1 ohm and an open circuit voltage of around 18V ac at 3000rpm. If you gave it a dead short, you would expect around 18 amps at that speed. There would be zero power on the load but the current is somewhere around double the rated current and therefore 4 times the power dissipated in the stator.

I fitted the 180W 3-ph alternator and rode through the winter with electric gloves and lights. Then the warm weather came along and I fitted led lighting all round. Suddenly, the alternator is spending most of its time shorted and the stator melted.

If you have an electric starter, ride in winter with heated clothing, don't want to heat up the atmosphere so fit led lighting, you find a lot of variation in alternator load. A series (open) type regulator would give you a run cool RR and a run cool stator and not heat up the atmosphere. A short type regulator give a run cool RR but otherwise fails. It is just barbaric.

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Alan osborn is correct in what he said

the norton alternator is a permanent magnet type what you seem to be thinking about is the princaples of a standard car alternator which is very different

they will charge if the roter is spinning if you short it out it cant develop volts 

i x v = watts so no volts no power to dissipate 

the problems of melted stators is usaly faulty rectifier packs 

google it 

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Hi Robert,

I agree with what you are saying about shorting out will not cause a problem, in which case  what IS the mechanism that causes overheating?

Tried searching for anything relevant without success.

Tony

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http://www.zefox.net/~bob/mc/vfr/alternator.html

read it towards the bottom of the article

the other problem is rubbing stators on the magnet (overheating)

The 750 has the high powered alternator too (RM24 14.5amp) but it had two matched zeners one on each z plate - so the alternator ran on nearly constant load. It may be crude, but the original Lucas system was therefore capable of delivering no load through to full load on your electics by running the alternator at full load - all components rated for that purpose. The rectifier failed at one point and I replaced it the zeners with a scooter regulator rectifier (which I now assume is a 'short' type). But I always run the 750 with lights on and they are the old incandescent type - so presenting the alternator with more load. The RM24 is the older brown type resin which might be more temperature tolerant.

The 850 was a completely new set up with new alternator RM24 and rotor - manufactured by Lucas (or shall we say 'sold' by Lucas). The resin is the green type one. The RR is supplied by Wassell and is a 'short' type. But I fitted led lights all round indluding headligh (excellent H4 type, warm coloured and good beam control from classiccarleds). So the alternator is running at next to no load. Result was a slow melt down. You can see from the damage that there was no coming together of metal, just melting of resin. The alternator didn't fail electrically but I started getting a lot of mechanical vibration. When I investigated, I checked out the wiring and the components and all was in order. So I put the old bulb in, switched on the lights and ran the bike with the primary outer removed. I tested the voltage at the battery at several different revs. It was absolutely steady at all revs (14.6V) until I dropped it down to tickover and it dropped to about 12.3V. The RR stayed cool (hardly warmed up). The stator was running in ambinet air and got very warm but no sign of melting.

I replaced the RR with a series ('open') type and tried the exercise again - this time without the lights on. The RR was cool - why all those fins I don't know - but this time the alternator was also cool. Effectively, it open circuits the alternator when it doesn't want power rather than short circuiting it.

It's like a lot of things, you believe it must be OK because loads of reputable manufacturers make the 'short' type. But the moment you start looking, you realise that loads of people have sufferred from this 'modern' issue. The answer is not to 'balance' your load with your alternator - that's a work around for a system that is less capable than the one supplied in the 70s. The answer is to go back to the original (zeners) or replace with something better.

Hope that explains it well enough.

So why aren't all regulators the 'open' type. I'm guessing the reason is (on a 3-ph unit) you can't avoid opening at a time when current is passing through your alternator coil (which is an inductor). You can get into high induced voltage issues where you can zap the gate on your mosfets (I know this from making mosfet based circuits for mag conversions to open the LT circuit on a coil. I wound up using additional components to protect the power fets). Interestingly, with a single phase alternator, you could break the circuit at the point of zero current and avoid the high induced voltages but this means a lot more design that the simple 'close' type. 

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Just as an aside to all this, the zener on my '72 Commando failed open circuit, the voltage rose to heaven knows what and cooked the battery. This resulted in my show-winning bike being sprayed with sulphuric acid over the silencer, rear (aluminium) wheel, aluminium hub etc.

Made my day.

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The Zener going O/C as you say gave the battery excess voltage (won't happen if your lights are on, but we don't all do that) I have an answer, I sell a BSM LED that changes colour with battery voltage hence you would have seen this problem very early on the warning on the BSM. The advantage over an Ammeter is that very often one does not look at the Ammeter but a changing colour on a lamp will catch your eye.

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Hi Alan,

Does your LED fit directly in the headlight shell?

Or is surgery required?

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Just to add my two pennys worth. I agree that the rectifier/regulators are not the best design in some cases.

I do quite a lot of work on Nortons and can cite 2 cases on Mk3s which have had failed stators. Both of them had the higher output stators and boyer power boxes. So it could have been a failure of the boyers or the stators. Both stators got cooked and in one case it de-magnetised the rotor. On both machines the whole charging system was replaced, but with podtronics 200w regulator.

I am very interested in these Shindegen open type regulators. They look like the way forward. Especially with the new lithium type batteries that are finding favour now and electric starter demand.

I do see the stators on some bikes do look more overheated than others. I shall have to see if there is any correlation between reg/rect equipped bikes and those on zeners.

Cheers 

Paul

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Paul, 

The latest lithium type batteries don't need special chargers, just a DC voltage of the correct specified range. There are only two batteries on the market at the moment that are like this and would fit motorcycles. Not cheap though. 

Things move on rapidly these days. 

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As a simple old ‘erbert who barely knows his **se from his volts, I think Grant has a very valid point in saying we have been conditioned into believing bigger is better. This really does call for some back to basics thinking: what is my riding style (ie am I an every day, all-weather rider or a weekend/summertime leisure rider), plus what are the demands of my bike- more ie electric start, powerful/daytime lights etc or less: low demand, LEDs etc.

Having determined that, the range of suitable alternator setups (outputs) can be ascertained, and then determine which of the RR units available are compatible- and in our price range.

Perhaps Lucas got it right first time round, with a 3-wire setup controlling the alternator output directly through the lighting switch and its position…although I would hate to go back to that huge doughnut of a switch in the headlamp shell like my old Tiger Cubs had!

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.. the 3 wire alternator was ideal. But I was very happy with the 2 wire alternator, full wave rectifier (using a modern silicon type) and Zener diode on my Commando. Simple and relatively bulletproof.

 


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