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ags
United Kingdom
275 Posts |
 Posted - 26/05/2008 : 22:35:25
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Hi All,
Following from the topic, "Early M-type Small Dynamo Improvement" I would like to ask some basic questions of anyone, particularly my American or German colleagues, who have experience of the Bosch or other dynamo conversions.
I do not recall anyone quantifying the improvement in dynamo output obtained. "Amps in Abundance" is very encouraging - but how many, please?
My particular concern is the night driving situation, and improving headlamp light output. Whereas twenty years ago my dark adapted eyes were able to see with two thirty-six watt beams well enough to maintain cross country averages in the 45 - 50 mph bracket, I was not then liable to be faced with 100 - 200 watts output from oncoming traffic. Even quick flashes will take away the eye adjustment leaving a driver relatively unseeing.
To start the discussion the standard dynamo will give a maximum of 9A reliably, and having made the mistake, I know that even as small an increase as to 9.5A will lead to solder throwing and dynamo failure at the revs. which I tend to use.
The problem with the MMM dynamo is waste heat generation leading to the above situation, which I believe will be the same for either third-brush or external regulator control under full output conditions. A quick glance at Wikipedia has just told me that, the "maximum efficiency is only about 60% for the best of these generators, 40% is more typical" so for ease of calculation when quantifying I shall assume 50% efficiency for our dynamos. This indicates that the standard dynamo can dissipate about 9 x 12 = 108W in safety, but that 114W is marginal or worse.
Now for a bit of speculation, by assuming that any increase in dynamo output be supplied to the headlamp filaments. A 15A output would allow the headlamp power to be doubled to 2 off 72W bulbs. That is useful but the dynamo will now need to disperse 15 x 12 = 180W waste heat. This is way beyond the limit estimated above.
So how can another dynamo give an output like this? I can only think of four different ways to get such outputs;
(a) more heat resistant construction, for example crimped or other mechanically restrained wiring
(b) better efficiency of generation, but the quoted figures indicate little gain is to be had here
(c) better cooling, the centrifugal fans behind the pulleys of the most recent dynamos are examples. Certainly there should be some gains to be had here, but is anything obtainable whilst retaining the original appearance viz monolithic casing and solid top plate?
(d) better efficiency of utilisation. Regulator control will improve matters in the daytime by switching the dynamo to lower outputs for long periods but it will not help in my defined problem area of high constant output for night time running. In this condition I doubt if there is any difference between the heat produced by third brush or regulator controlled machines. (Comments please on this guess.)
Are there any other ways of getting rid of this heat?
So if none of these, or any combination, are sufficient, how is output significantly increased?
I should point out that I am a physicist, and worse, have an ingrained tendency to check the teeth of gift horses. I do believe that the Laws of Thermodynamics and Electromagnetism apply here and that these laws can be summarised as - Free lunches are always paid for in one way or another.
So, how many amps?
PS - The 9A output needs (9 x 12)/0.5 = 216W so more than 1/4 HP taken from the engine but not propelling the car. Thinks - a switch in the field circuit to save this loss on the track or for a hill-climb might be well worthwhile.
More technical ramblings from
Andrew Smith MMM571 |
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Blue M
United Kingdom
1472 Posts |
Posted - 27/05/2008 : 09:59:03
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Regarding cooling the dynamo, mine is ventilated at the top. The tin inspection cover has been altered by a previous owner who has cut out the metal where the holes in the casing are, and replaced it with gauze.
Although nowhere near as effective as a fan system I suppose it must be better than nothing.
Would cooling fins on the outside make any difference?
Ian |
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ags
United Kingdom
275 Posts |
Posted - 27/05/2008 : 13:57:58
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Hi Ian,
My guess is that fins on the outside would not make much difference, but good thinking nevertheless.
My reason for this statement is that the most heat is generated within the rotor, and this is also where the damage occurs. The heat generation will be equal to V?/R for both rotor and field coils. The voltage is the same for both sets of windings under full charge conditions and though I do not have the exact resistances to hand I have measured them in the past and a ratio of about 1:10 comes to mind. In other words about ten times more heat is developed in the rotor than in the static field coils. The heat conduction path from rotor to external fins would also be rather indirect, via ball bearings and other fitted joints.
Your previous owner was on the right lines, but did not arrange for a directed flow path. Similar holes at the base of the casing, also gauze covered, would take advantage of vertical convection within the housing as well as any induced centrifugal outward flow at the top. Going back to modern dynamos, the fans and open end housings seem to be the only thing that makes the difference between our 9A and typical 30A recent dynamo (not alternator) outputs. In other words they disperse (30 x 12) - (9 x 12) more heat, about 250W. A third of this above original would allow the 15A target to be reached. Worth looking to see if there is any room for those low level openings. And it would keep the appearance relatively standard.
Anything more from anybody else?
And what about those higher safe output claims?
More technical ramblings from,
Andrew Smith MMM571 |
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talbot
United Kingdom
718 Posts |
Posted - 27/05/2008 : 14:28:54
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Andrew,
Thanks for initiating a good topic. Isn't the problem the way the regulator controls the dynamo? With a 12V system powering 72W of headlamps the current draw will be about 6A. If a 40 amp hour battery is fitted this should power the lamps for about 6 hours. I suspect, but would welcome your comment, that the regulator tries to charge the battery as quickly as posible. Wouldn't it be better to spread the charging over a longer time so less amps are used? When there is a heavy electrical load the battery would discharge but this would be partially replaced by the dynamo. When the load lessens the battery would recover. I know this is similar to the old summer / winter charge system but couldn't the concept be automated?
I considered fitting an alternator to my M Type. I turned a sliver off the casing of a Denso alternator from a 1000cc Kawasaki motorbike so it was a push fit into the original MG dynamo case. The Denso unit has an integral fan and I had considered drilling holes in the original casing and leaving the brush band cover off. I was unable to find what the output of the alternator was though so bit the bullet and bought a new one from Wood Auto.
Cheers
Jan T |
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Bob L
Sweden
50 Posts |
Posted - 27/05/2008 : 16:02:23
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The VW generator I used for my conversion had a rating of 14V, 30A when running in the VW motor with good ventilation. I have run it for about 2 years without trouble however I do not know exactly how many Amps I draw from the generator when all the electrics (lights) are on. The ammeter in my car is not that accurate. I have not increased the headlight wattage.
As an educated guess I would say that the generator could now give 15A continuously without trouble when running without forced ventilated (fully enclosed) in my MG PA.
The reasoning behind this is:
-The heat generated at 15A is a quarter of the heat generated at the original 30A rating.
-The rotor coil ends are brazed to the commutator, not soft soldered as in the original generators and hence withstand a higher temperature.
-The modern coil insulation in the Bosch generator is much thinner allowing better heat transfer and it will withstand higher working temperatures than the coil insulation in the original generators. This is true for both the rotor windings and the field windings.
Bob |
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Bruce Sutherland
United Kingdom
1564 Posts |
Posted - 27/05/2008 : 17:11:57
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Hi All,
".......... Worth looking to see if there is any room for those low level openings."
Like you Andrew, I've been pondering how to reliably get a higher output generator. Bob's description (see doc.dwn.loads) of his VW Beetle Bosch dynamo conversion looks very promising and has obviously worked well for him.
To address the additional cooling requirements with higher outputs I've been looking at Barry Foster's article in the 2004 Yearbook which shows a forced convection cooling arrangement with a small 12V centrifugal fan attached to the brushes service-access band with air exhausting through holes at the base of the dynamo housing.
There are modern 12V centrifugal fans which could be used to do this - see http://www.diamondelec.co.uk/info.cfm?SubCatID=Dccompactblowers&ID=6 . The Bosch dynamo plus a fan could give 15+ Amps??? Worthy of further investigation?
Bruce. |
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Bob L
Sweden
50 Posts |
Posted - 27/05/2008 : 19:06:15
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A further point to my comments above is the fact that the thinner modern insulation allows an increase in copper diameter in the available slot space. This lowers the coil resistance by the square of the diameter increase which in turn results in the decreasing the losses (heat generated) by the square of the diameter increase.
About 20 years ago I rewound the rotor of the generator for my model T Ford. The rotor of the generator had burnt out. The original insulation was double cotton enamel. The modern insulation was single modern enamel so I used up the extra available space in the winding slots by increasing the copper diameter and then I used modern varnish. It is still running without problems.
Bob |
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Gerhard Maier
Germany
873 Posts |
Posted - 27/05/2008 : 21:05:47
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Although I use to drive in European countries where headlights are obligatory during the day, (which is even more important for safety, when you have a small car in british-racing-green), I must confess that I am sometimes at the limit, but I canÆt say that I really have a problem with scarcity of electric supply.
In the headlights there are 2 x 35 watts, but I pay attention, that the silvering/chroming of the reflectors is in good condition, and the bulb well focused.
For the side, tail and brake lights modern diode bulbs can help saving electricity.
I adjusted my 3 brush dynamo well to the limitation of 9,5 amps, and then on the road, the amp-meter on the dashboard stands on zero, which is ok all the day, when I started the journey with two full 6 volt batteries. At home the batteries are always hanging on one of those modern battery-conditioners, which charge and slightly discharge the battery by turns. (I have also the feeling that so battery-life is extended)
Problems can arise, when windscreen-wipers are needed, or the electric fan in climbing the Alps. But usually the running period of those is short enough.
I must say, that perhaps the greater heat conduction to the air of the some bigger and more voluminous N-type dynamo (with aluminium or brass top), helps a bit to keep the temperatures on the saver side ?
Gerhard
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ags
United Kingdom
275 Posts |
Posted - 27/05/2008 : 22:17:21
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Hi Bob and Bruce,
Thanks for your replies.
Bob L, I had consulted your notes and drawings before starting this thread and I was hoping that you had done some checks with a reasonable ammeter. Still, better cheap and nasty than nothing at all. Do you have any readings from the car ammeter? With 36W bulbs in both headlights my own used to run at a steady 2A (car ammeter) discharge at night with everything going except windscreen wipers. This was the figure that I had calculated from all the bulb wattages etc and the set output of 9A and so I am reasonably happy that my car ammeter is not a dramatic liar. Given that the standard long MMM battery has a large capacity of 50Ah this did not worry me at all. (I recall a Phil Bayne-Powell story somewhere of setting off for a week's work away from home in the ND and then returning after twice daily starts on the battery, all on just the initial full charge, but with the dynamo u/s and not giving a single amp in assistance.) However turning this into a six to eight amp constant discharge to reach my target of 15A consumption would be pushing things a little too far, I think.
Also Bob, your note of the brazed wiring is covered in my suggested method (a) for improvement and will be important but I need a bit more time to think about your comments on thinner insulation, thicker conductors. Everything you have said is correct but I am not yet sure that it is the whole story. To avoid the electrical analysis was one of the reasons that I based my calculations on efficiencies.
Bruce, I was not aware of Barry's suggestion, time to update my collection of Yearbooks beyond 2001, I think. His scheme might work even better with a reversed flow, if that is possible. Why oppose natural upward convection when it could be used to increase airflow and assist the cooling?
Also there is of course the search for more efficient (ie more light per amp) lighting units as suggested by Sam Christie in the original thread. Tungsten/Halogen units will not help here since they are still incandescent in operation and merely allow more light output from the same sized, or smaller unit, but no more per amp. Time to look up LED sources unless they prove for our purposes to be in "the realms of fantasy".
Keep the suggestions coming, and how about you Nick? Expertise in "Electrics and P-types" is exactly what is needed here.
Thanks all, more technical ramblings from
Andrew Smith MMM571 |
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Bob L
Sweden
50 Posts |
Posted - 28/05/2008 : 08:42:13
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Andrew, The ammeter on my car is connected in series with the battery and measuter the net current flow to/from the battery. I think all early MGs were connected in this manner. When running with all electrics on (including wiper motor) my ammeter shows +/-0 Amps or perhaps slightly +. There will be a flick in the needle when the traffic blinkers are running, but that's understandable considering the dynamics of the generator control. If my car has not been running for a few weeks then there will be a charging current for the first few kilometers. Since I have had no trouble since the rebuild, I have not paid particular attention to the charging rate. I tend to keep an eye on oil pressure.
Sam, The outer casing of the generator forms part of the magnetic circuit for the fields and hence must be made of steel or a material capable of transmitting a magnetic flux.
Bob |
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rbm
United Kingdom
141 Posts |
Posted - 28/05/2008 : 13:55:31
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Ordinary incandecent lamps give out about 15-17 lumens/watt, halogen incandecent are arround 20-22 lumens/watt, HID are a wopping 60 lumens/watt but none of this helps û at the end of the day a 55w bulb is a 55w bulb and draws 9 and a bit amps what ever type of bulb it is.
LED > 100 lumens/watt and are not limited by conventinal auto bulb construction / availability.
LEDÆs well at the moment I think you will find that it is not legal to use LED headlights in the EU, it is ok in the USA and IÆm not sure about TroW. (LEDÆs should be EU approved late 2008/2009)
If you replaced the side light and stop tails you can shed a heap of amps but not enough so the question I have is why was this not a problem in period?
Richard |
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Terry Andrews
United Kingdom
546 Posts |
Posted - 28/05/2008 : 14:39:37
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Hi Andrew
With regards to you some of the things you have raised.
1 On a 3 brush system the 3dr brush is the regulating system and this bend the flux within the dynamo. It is a perverse system in that it charges better when the battery is fully charged and charges less when the battery is down. When this is the case the unit is working much harder and is more likely to heat up and through the solder. So you have not used your car for a while the battery has not been on charge. The car starts and off you goà the dynamo is likely to get much hotter.
Thus three brush dynamos used in our cars it is normal for an increase in speed to produce a decrease in voltage due to flux distortion. (Please remember this!).
In practice, our three brush dynamos are designed so that at low speed the effect of the armature reaction is less than at increased speeds. When the car is first started the charging rate will increase with the increase in speed until at a certain r.p.m. has been reached. At a given r.p.m the current output remains approximately constant up to a further given r.p.m. limit. At this second r.p.m point the effect of the armature reaction is greater than the effect of increasing r.p.m with the effect that a decrease in charging rate occurs.
Typical outputs are as follows: 500 rpm = 2 amps
750 rpm = 7 amps
1000 rpm = 10 amps
1500 rpm = 12 amps
1800 rpm = 11 amps
2000 rpm = 10 amps
2300 rpm = 8 amps
This must be taken as a guide as the condition of the dynamo is a factor. Third brush regulation is not so sensitive as the later types of voltage and current regulation which can be kept within strict tolerance levels.
Please note that if the third brush is adjusted at 8 amps at 2.300 rpm YOU WILL DESTROY YOUR 3 BRUSH DYNIMO!!!!! The 8 amps must be adjusted on the rise of the rpm i.e. between the 750 and 1000 rpm marks as per the table above. This system was marginal in 1935. DO NOT USE YOUR CARÆs AMPMETER TO DO THIS.
2 In the scheme of things I have never found the need to put vents in or remove the cover band. On my P type with 3 brush this was never a problemà.. I think because it was correctly adjusted.
3 The 2 Brush dynamos I have made (approx 25 DDS14 style and 5 Rotax style). The DDS style gave 28 amps output and the Rotax copy 22 amps. The cover band is always in place. But they are voltage regulated which is much more precise and kinder to the unit. I have had one of my 2 brush dynamos in my P type since I put in a deferent engine. I have this weekend remove the dynamo for other reasons. It is lovely inside. It has covered 6 k miles of sustained long motorway journeys to Luxemburg and the like.
4 I think the debate or 3, 2 brush or alternators really depends on the cars use etc. What is OK for a day car for Sunday outings is not the same requirement as a trials car through the night. If adjusted correctly a 3 brush can give good service. I only changed as I wanted the finer voltage control and the extra out put for continental touring.
5 Can I suggest that people try fitting a volt meter to their cars. If you look at the voltage across your cars system as well as the ammeter you will get a better indication as to the charging rate and the health of your dynamo.
6 The ammeter fitted to our cars is only an ôINDACATORö they are very inaccurate !!!!!  At best use it to check that you are charging or discharging your battery. Use a good quality moving coil voltmeter as to the state of what is happening. I have a good quality 2 inch volt meter which I have had the modern face changed to MG style in P type brown.
7 The VW armatures are similar to later Lucas ones when the copper is friction welded to the commutator so the can take more heat. Another thing is that on many armatures the ends when the copper meet the commutator they are bound with thin thread and the shellacked. Thus if the solder does melt the copper wire does not destroy the field coils. In many cases the armature can be reclaimed if this thread has been applied to the armature. I always try and do this to protect the armature and field coils.
I hope this puts some light on the subject? 
Regards Terry
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ags
United Kingdom
275 Posts |
Posted - 28/05/2008 : 15:06:05
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Hi Richard,
Thanks for your figures on the comparative efficiencies of various light sources. Late last night I had found most of what you have quoted, but I certainly did not find the vehicle regulations that you point out.
As you infer, what I as a driver in the dark, am primarily in need of is lumens showing the way ahead and so the amps discussion is intended to find the maximum current that we can reasonably get from our 75 year old mechanisms. After that we should try an turn it into as much light as possible.
Your question, "why was this not a problem in period", deserves a bit more thought than my earlier quick summary. My guess is that stray light of all sorts has increased a great deal over, say, the last fifteen years. Therefore to increase the contrast needed to distinguish details we need to throw more light ourselves. An aspect of this is the change in street lighting design that has taken place. Prior to about 1980 - 1990 (if we have any road traffic engineer subscribers please correct this section if necessary) the general approach was to leave the road surface dark on the grounds that car headlights would then provide the greatest contrast for any obstruction in the beam. Today the opposite is the case, the road and its surroundings are made as bright as possible, leading to the consequence given in my third sentence above. I think that in the Thirties that street lighting was probably very restricted in power and area of distribution so that drivers eyes were generally fully dark adapted, using the different set of more sensitive optical receptors that this implies.
In the bright road scenario (opticians or physiologists please?) I guess that we are probably seeing in daytime mode all the time. All this is in addition to the modern car's higher rated headlamps, leading to dazzle problems for us which would not have occurred when the best that the oncoming traffic could mount would be a pair of P100s with perhaps 40 - 50W bulbs. Furthermore, for the oncoming modern driver, our limited lumens will tend to be lost (lack of contrast again) in the general brightness of everything in their field of view.
What do you think?, more technical ramblings from
Andrew Smith MMM571 |
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ags
United Kingdom
275 Posts |
Posted - 28/05/2008 : 15:27:59
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Hi Terry and Bob,
Terry, thanks for your comments. I did know how a third-brush dynamo regulates itself, by flux distortion, but did not have the details given in the table of figures you provided. I also am quite happy with the third brush system, why carry extra mechanisms which can and do fail? I omitted all of these factors for simplicity, not having time to write a text book as a forum submission.
Once the PB is running again I shall certainly try out your improved dynamo setting up procedure, though my dashboard will remain clear of non-original instruments - desirable for electrical analysis of the system though they may be.
Bob L., thanks for your figures which Gerhard has confirmed. I agree that your guess of a 15A output for the Bosch setup is likely to be a close estimate of an "accurate" figure.
More technical ramblings from
Andrew Smith MMM571 |
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Terry Andrews
United Kingdom
546 Posts |
Posted - 28/05/2008 : 17:55:53
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Andrew,
If you want to know more about how the 3 brush self regulates read my artical in the 1997 Triple-M Yearbook from which part of my first reply came from. I quite agree the 3 bursh will be OK. You could fit a tempory voltmeter to the red and black plugs of the dashboard supply to see how things are doing. This is what I do when I check our other peoples cars. Also take a look at my voltmeter at Sliverstone on my P type.
regards
Terry
WO9320
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Nick Feakes
USA
3372 Posts |
Posted - 29/05/2008 : 02:07:24
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Hi All
some excellent responses all pointing to the inherent limitation of the standard dynamo. It is quite possible that the Bosch dynamo will deliver a higher current due to improved construction techniques used in the armature. One simple solution is to use a larger capacity battery and recharge it frequently (provided you are not planning to drive in the 24Hr Le Mans!). However, a word of warning about Lead Acid batteries, if you want a reasonable life then they should NEVER be discharged below 50% of their rated capacity, so the standard battery should only have 25AH taken from it. It might manage 5A for 5 hours but 2.5A for 10 hours is a better bet (the more current you draw the lower the capacity you realise!).
It is possible to extract more power from the standard dynamo, it does however require some modern electronics. It is possible to raise the output voltage of the dynamo whilst keeping the output current restricted to 8A. Say the output is regulated to 20V at 8A, we now have 160W. Now use a switching regulator to reduce the voltage to 12V, (assuming a switching efficiency of 80%) we can get about 12A.
This represents about a 50% increase but at the expense of non originality and complexity.
I think it is an excellent idea to try and utilize LED sidelights (are they legal in the UK? I seem to remember that lights used to be specified in Watts - is this still the case?) as these are on with the headlights this has to be a good way to go.
Nick |
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