Alternators

[fixitflav]

Something I've wondered about for a while. I've asked various electrical engineers in the past, and never had a convincing answer.

Why do alternators not need a current regulator, just a voltage regulator, whereas dynamos have both? I'm talking about automotive kit here. Maybe bigger kit like gensets and power station alternators are different.

As I'm sure most on this forum know, with a dynamo the field current is fed via the control box, which has two sets of trembling contacts in series, one limiting output current, the other voltage (earlier control boxes had a single regulating coil, but it had both a voltage and a current winding, so the effect was similar). An alternator only has a voltage regulator (built-in these days), it is self-limiting for current. As in both cases the main coils consist of heavy copper wire it would seem an alternator would also overload on current if connected to a big enough load, but it doesn't.

Can anybody explain why not?

 

[Burnerman]

As I see it, the voltage regulator keeps the battery supplied with a 'controlled electrical charge'. This unit would give the battery an unhealthy burst of current should it fail, therefore it fails safe and burns out to prevent that happening.
The voltage regulator controls the field current going to the spinning rotor within the alternator. When no current is applied to the field, there is no output.
When the battery voltage falls below 13.5, the voltage regulator applies current to the field and charging commences.
When the battery voltage reaches 14.5, the voltage regulator stops applying current to the field and charging output ceases.
This is how the voltage output is regulated and the current output is largely regulated by the battery condition. If the battery is in a low charge condition, the EMF is not strong enough to hold back charge to the field, and charging continues. When the battery is fit again, the EMF is strong enough to oppose the charge to the field, and charging ceases down to zero.
(My explanation is based on no scientific fact whatsoever - its what I have assumed is happening in a car charging system. Maybe someone with a brain cell will come along and confirm.)
Calling Peter N. !
John

 

[fixitflav]

This is how the voltage output is regulated

Yes, the voltage regulation works the same way in a dynamo as an alternator.

the current output is largely regulated by the battery condition.

If that's the explanation it would apply to both, but it doesn't. The dynamo needs a separate current regulator, otherwise when the battery is low, eg just after starting, the current would go too high and damage the dynamo. If you bypass the voltage regulator on an alternator and give it some revs, the current rises to the alternator rating, but no higher. On a dynamo, if you hold both trembling contacts together the current goes as high as you like and damages the dynamo if you don't release them.

 

[fixitflav]

John

Thanks for your comments.
Had another think about this, and maybe it's to do with the inductance of the stator coils. With a typical alternator rated output 55amp, to limit it to that would need impedance about 14/55 = 0.25 ohm. At an alternator speed say 5000rpm ~ 500rad/s the inductance = 0.25/500 = 0.5millihenry. But I don't have a feel for whether the stator coil would have that sort of inductance, I might try to estimate it sometime.

 

[ericmark]

Some alternators do have current regulations, the CAV AC203 for example the 440 regulator has M1 and M2 terminals which connect to a steel shunt to measure current which often also works the ammeter, mainly found on buses, designed to give either 60 or 80 amp but at rather low revs so it will still maintain the bus battery at tick over. And they are rather large beasts.

However car alternators have the current producing part on the outside, and the control bit on the inside so there is a lot of cooling on the outside bit, where a dynamo has the field on the outside and the current producing bit on the inside, so it is a lot harder to cool a dynamo to an alternator.

There are other factors as well, but in the main it's simply the hot bits are larger so easier to cool, there is also the fact it is working at quite a high frequency, I remember when they arrived in the 70's every one wanted to fit them as far better, however unless you also used a smaller pulley so it turned faster it was little better than the dynamo it replaced.

Early dynamos did not have current or voltage regulation, there was a free wheel in the pulley and when you turned on the ignition the dynamo would start to motor, once the engine started it would be turning faster than the dynamo would motor so it would start to charge, the charge rate was controlled by moving the third brush on the dynamo to increase or decrease the field current.

However we started to remove the skill required to drive, and the two bobbin regulator increase the output 19 amp could be squeezed out of a standard length and 22 amp out of the long version of the C40 dynamo, we also fitted automatic advance and ****** instead of hiving it on the steering wheel. The tractor version of the C40 had just 11A output as no air cooling.

Lucas was the odd one out, you fed the field with current to get output, all the other makes to grounded the field to get output, so Lucas was easy to swap pos to neg earth, with other makes far harder.

Early alternators would destroy themselves if battery became disconnected as the regulator transistor could not vary the field current it would burn out, so it used a mark/space method switching fully on or fully off and used the battery to smooth the output, latter zenor diodes were added to try to stop them burning out, there were some with mechanical regulators specially the French and Japanese ones, I have actually measured 110 volt from a 24 volt alternator when the battery became disconnected and it gave me quite a kick.

In the early days there were some really odd alternators, French split phase, most were three phase, and USA Delco with auto transformers giving both 12 and 24 volt from same alternator, Delco and Paris Rhone both made brush less alternators but not like the mains voltage brush less with rotating diodes, but the field coil was static and the soft iron fingers rotated between field coil and stator.

Dynamos also had some odd variations, the bus dynamos had bucking coils, in the field wound in reverse direction as the field would retain too much magnetism and even with no field current could over charge the battery without the bucking coils which stopped the build up of permanent magnetism.

Many of the alternators fitted to CAT tractors would run quite happy with no battery connected. And Bosch started to rectify the neutral point to increase output and reduce heat. And as said Paris Rhone split phase (single phase double winding) designed to be cheap had just two diodes and a fuse, latter ones three diodes and output was half wave rectified. They would often burn them selves out if over sized batteries were fitted, the Savium trucks had a lot of problems, they got such a bad name it was changed to Renault.

 

[fixitflav]

Thanks for the history. On another forum, CR4, there was a query about using an alternator regulator to control a dynamo. Obviously a standard modern alternator regulator wouldn't do it, as there is no current regulator, and there's the cut-out issue. Somebody posted a link which explained alternator self-regulation, as being due to the inductance of the stator coils. So it looks like my speculation was correct. I tried to find it again but searched a while with no joy. If I manage to find it I'll post.

 

[fixitflav]

Thanks for the history. On another forum, CR4, there was a query about using an alternator regulator to control a dynamo. Obviously a standard modern alternator regulator wouldn't do it, as there is no current regulator, and there's the cut-out issue. Somebody posted a link which explained alternator self-regulation, as being due to the inductance of the stator coils. So it looks like my speculation was correct. I tried to find it again but searched a while with no joy. If I manage to find it I'll post.

If anybody's interested, I found the article about alternator current being self-limiting - below.

From the practical point of view, dynamos are different from alternators in two very important aspects. Firstly, when an alternator isn't rotating, it won't take any current from the battery. But if a dynamo isn't rotating, it would drain the battery flat very fast, and very possibly burn itself out in the process! So, electromechanical dynamo regulators disconnect the dynamo from the battery as soon as the current starts flowing in reverse, a delicate and all-important task that often fails to work, and kills dynamos. In the modern world, of course, we have diodes, and putting a single big diode in series with the dynamo output solves this problem once and for all time.

Secondly, dynamos are not self-limiting, while alternators are. When an alternator runs faster, higher voltage is induced, but also the frequency of the AC is higher. Since there is a large inductance in the windings, the reactance increases with frequency, the two effects roughly compensate, such that a correctly designed alternator can never deliver a dangerous current! It limits its output current to a safe value, regardless of RPM. Dynamos instead just don't have this nice feature. As long as there is enough mechanical drive power, they will deliver whatever current a discharged battery can take, and will easily self-destroy if the current isn't limited externally.

Voltage regulation, on the other hand, can basically be implemented in the same way for dynamos and alternators. So, a dynamo regulator is basically an alternator regulator with additional current regulation and reverse current blocking.

 

[ericmark]

No alternator current is not self regulating, because the stator has a larger area than the armature on a dynamo it can better get rid of the heat than a dynamo. But as to if it can dissipate enough heat depends on many factors, rectifying the star point does it seem remove harmonics and so decrease the running temperature. The reverse is also true, single phase alternators would burn out if battery size was increased.

The latter dynamo regulators used diodes instead of cut outs, As said the CAV 203 was current regulated. There were specials using oil cooling, and by setting the max magnetism of the rotor the max output could be set to below the self destructive limit, but it is not always the case, to get low speed output the AC203 was built so it could produce over the 60 or 80 amp output which is a fair output for a 24 volt alternator.

 

[fixitflav]

I was referring to modern 12 volt alternators on cars and similar. No doubt things could be different on HGVs, and various designs have been used historically.

No alternator current is not self regulating

If it isn't, it behaves as if it is for practical purposes! There is no current regulator, only voltage.

The latter dynamo regulators used diodes instead of cut outs

I've seen adverts for dynamo regulators which in addition to diode instead of cut-out contacts have transistorised current and voltage regulation. Aimed at retrofit market for classic vehicles. Whether it gives a big improvement I wouldn't know, but the traditional vibrating contact type lasted years enough. I don't know about any being installed as original equipment, better to fit an alternator.

 

[ericmark]

For most alternator yes the field is designed so no current regulation is required, early alternators uses contacts for voltage control just like the dynamo, specially the Japanese ones, there were some very basic models with French cars, split phase, some with three diodes and some with just two.

I have not seen a proper brush less with motor vehicles or plant, although they were brush less, but no rotating diodes.

 

[fixitflav]

there were some very basic models with French cars, split phase, some with three diodes and some with just two.

Yes, the French often do funny things

early alternators uses contacts for voltage control just like the dynamo

A friend has a 70s motorbike, can't remember make, something Japanese, which has alternator with an external trembling contact voltage reg.

I have not seen a proper brush less with motor vehicles or plant, although they were brush less, but no rotating diodes.

I believe some motorbikes have them, but they're not "proper" brushless, with shaft-mounted exciter like big alternators (as you said). Might have permanent magnet rotor. Can't find anything on a quick web search, but I assume cheaper and lighter than car type. I believe control is worse, perhaps involving dumping of excess output.

 

[ericmark]

I think Paris Rhone did an alternator where the fingers of the rotor went less than half way across the coil the latter held by a thin web so fingers rotated but coil remained static and direct fed without need for brushes, the Delco Remy had rear fingers supported on front fingers again fingers rotated but the coil was static.

Motor cycles had both internal (centre) rotating permanent magnet and outer rotating magnets often with no regulation, the latter often 5 coils (6th position had points) one for each function horn, brake lights, side lights, head lights latter often used output of two coils, the centre rotating magnet output rectified to DC so it could charge a battery with a Zenor diode to sink excess into heat.

Although above was brush less, what is considered as brush less is where a static outer coil generates current in a rotating coil which is rectified to DC with rotating diodes which feeds a second rotating coil which generates an output in the second stator, worked on 4 x 400 MW in Connah's Quay and 2 x 650 MW in Sizewell, with the Connah's Quay one there was also a permanent magnet generator at rear of all that, but it was just to run the lubricating oil pump, and also a load of brushes but these were for sensors over heat etc, not actually controlling output of alternator. Found same method used down to small 68 kW generators, smaller 3.5 kW and 7 kW tended to use brushes and output was from rotator not the stator reverse to all motor vehicle alternators I have worked on, however like the AC203 I am sure there is an exception some where?

Even at 230 volt some generators did not have a voltage regulator, two field coils one in parallel with a resistor and full wave rectifier set the off load volts, and one in series again full wave rectifier although much larger and a by-pass resistor set the on load volts, they actually worked quite well, used a lot for lighting sets.

Car dynamos were the same at one time, free wheel in the drive pulley allowed dynamo to motor when ignition turned on, once engine started the engine went faster than dynamo so it started to charge, the amount of charge was controlled by moving a third brush closer or further away to the common brush and an ammeter told driver charging rate. There was no regulator as such like the ignition advance and ****** it was down to the driver.

Lucas made a blocking diode that looked like an ignition coil and these were often fitted to old cars to stop the dynamo motoring. One thing I learnt working as a auto electrician is never say it's not done, as some one some where has likely done it, 8 volt batteries to improve lights on 6 volt vehicles, on the floor start buttons which swapped from 6 volt to 12 volt for starting, had to make sure all lights off first or it would blow all the bulbs (Mac 6x6 used for snow ploughing) 12 to 24 volt start relays on mini buses so 24 volt start and 12 volt run.

Twin 80 amp alternators with a alternator to battery inverter control panel to float charge engine starting battery while pulse charging domestic battery (narrow boats) and DC to DC inverters to charge bow thruster battery at front of boat, these are getting more common, Jaguar XE has DC to DC inverter to ensure voltage is stable to use while battery voltage varies depending on if computer has decided it is to charge or not.

We are in a time of change, caravans are presenting a problem, how to charge caravan battery without allowing start current to flow from caravan to car, and also ensure power available for anti snake device, and work fridge when voltage dependent relays no longer work as car switches alternator off when under load and on again on over run, so move to fitting DC to DC inverters in the caravan. However it means car and caravan need to match each other, which can cause problems, with some German cars not supplying power to anti snake devices.

 

[fixitflav]

I think Paris Rhone did an alternator where the fingers of the rotor went less than half way across the coil the latter held by a thin web so fingers rotated but coil remained static and direct fed without need for brushes, the Delco Remy had rear fingers supported on front fingers again fingers rotated but the coil was static.

Motor cycles had both internal (centre) rotating permanent magnet and outer rotating magnets often with no regulation, the latter often 5 coils (6th position had points) one for each function horn, brake lights, side lights, head lights latter often used output of two coils, the centre rotating magnet output rectified to DC so it could charge a battery with a Zenor diode to sink excess into heat.

Although above was brush less, what is considered as brush less is where a static outer coil generates current in a rotating coil which is rectified to DC with rotating diodes which feeds a second rotating coil which generates an output in the second stator, worked on 4 x 400 MW in Connah's Quay and 2 x 650 MW in Sizewell, with the Connah's Quay one there was also a permanent magnet generator at rear of all that, but it was just to run the lubricating oil pump, and also a load of brushes but these were for sensors over heat etc, not actually controlling output of alternator. Found same method used down to small 68 kW generators, smaller 3.5 kW and 7 kW tended to use brushes and output was from rotator not the stator reverse to all motor vehicle alternators I have worked on, however like the AC203 I am sure there is an exception some where?

Even at 230 volt some generators did not have a voltage regulator, two field coils one in parallel with a resistor and full wave rectifier set the off load volts, and one in series again full wave rectifier although much larger and a by-pass resistor set the on load volts, they actually worked quite well, used a lot for lighting sets.

Car dynamos were the same at one time, free wheel in the drive pulley allowed dynamo to motor when ignition turned on, once engine started the engine went faster than dynamo so it started to charge, the amount of charge was controlled by moving a third brush closer or further away to the common brush and an ammeter told driver charging rate. There was no regulator as such like the ignition advance and ****** it was down to the driver.

Lucas made a blocking diode that looked like an ignition coil and these were often fitted to old cars to stop the dynamo motoring. One thing I learnt working as a auto electrician is never say it's not done, as some one some where has likely done it, 8 volt batteries to improve lights on 6 volt vehicles, on the floor start buttons which swapped from 6 volt to 12 volt for starting, had to make sure all lights off first or it would blow all the bulbs (Mac 6x6 used for snow ploughing) 12 to 24 volt start relays on mini buses so 24 volt start and 12 volt run.

Twin 80 amp alternators with a alternator to battery inverter control panel to float charge engine starting battery while pulse charging domestic battery (narrow boats) and DC to DC inverters to charge bow thruster battery at front of boat, these are getting more common, Jaguar XE has DC to DC inverter to ensure voltage is stable to use while battery voltage varies depending on if computer has decided it is to charge or not.

We are in a time of change, caravans are presenting a problem, how to charge caravan battery without allowing start current to flow from caravan to car, and also ensure power available for anti snake device, and work fridge when voltage dependent relays no longer work as car switches alternator off when under load and on again on over run, so move to fitting DC to DC inverters in the caravan. However it means car and caravan need to match each other, which can cause problems, with some German cars not supplying power to anti snake devices.

OK thanks for the treatise Interesting