dynamo field coil control 6 of one and 1/2*12 of the other ?

[Thunderchild]

I'm designing a dynamo controller and was wondering: as the original controller used the dynamo output to feed the field coil could I used the battery to provide the power to the field coil (via the controller), would it get me more power from the dynamo at low speed ?

my worry is not making any more (to charge the battery with) than I'll possibly drain in the first place.

 

[Nigel Goodwin]

As far as I'm aware that what dynamos have always done? - the dynamo field coil is only disconnected from the battery by the controller to prevent it discharging the battery

 

[MikeMl]

As far as I'm aware that what dynamos have always done? - the dynamo field coil is only disconnected from the battery by the controller to prevent it discharging the battery

That is the way that automotive generators and alternators work. If you excite the field from a battery, then you will get more output from the genny at low rpm.

 

[Thunderchild]

well on my friends classic car the field coil is fed off the dynamo output unless I'm mistaken, I know that running the field coil from the battery will mean that we will need an extra wire as there is no way within the regulator itself for it to know if the engine is on of off, it will need running of the ignition switch

 

[Nigel Goodwin]

There's a contact in the regulator box that switches the power from the battery to the field coil.

Have a look at:

The pages of 'Bee' and 'Vee'

 

[Thunderchild]

Nigel, if you look at that diagram you will notice that the ignition switch is actually connected to the battery and that the regulator box is merely acting as a conector block, the field coil control power is not delivered from the battery or at least that diagram will not confirm that, somewhere I have a vintage car electrics book i should find and re read it

 

[Nigel Goodwin]

Nigel, if you look at that diagram you will notice that the ignition switch is actually connected to the battery and that the regulator box is merely acting as a conector block, the field coil control power is not delivered from the battery or at least that diagram will not confirm that, somewhere I have a vintage car electrics book i should find and re read it

The regulator isn't merely a connection block, it's a system of relay type contacts, one of which connects the battery to the field coil - it's this contact opening and closing that regulates the output from the dynamo. When the dynamo starts up the contact is open, and the dynamo starts producing power based on the residual magnetism left in the field coils.

If you wanted to convert such a car to -ve earth (as they were +ve earth back then), you simply flashed a wire from the battery to the field coil, reversing it's residual magnetism. Instead of a wire, you could just take the top off the regulator and manually press the field coil contact, doing the same job.

 

[Thunderchild]

I'm not saying that the regulator box is just a connection box (I've already taken one apart and refitted it with one prototype) but in the diagram you linked to the ignition is connected to the battery terminal on the reg as is the battery. as the inside diagram of the box is not shown, I'm not sure about different boxes though there seem to have been a few types. I did get my previous electronic version to run from self exitation of the dynamo and thats what my friend prefered although I want to put the case to him of using battery power as it should get more power ast low revs as we said above

 

[Hero999]

well on my friends classic car the field coil is fed off the dynamo output unless I'm mistaken, I know that running the field coil from the battery will mean that we will need an extra wire as there is no way within the regulator itself for it to know if the engine is on of off, it will need running of the ignition switch

How does it get going?

Does it just use the residual flux?

That sounds like it'll give poor voltage regulation. As the voltage increases the current in the field will also increase which will increase the voltage further. If no load is connected I can see the voltage rising until one of the following happens

• The torque slows the shaft
• Core saturation occurs in the field coil
• The voltage causes the insulation to breakdown
• The excessive current causes the field coil to fail.

The first two possibilities won't destroy the dynamo, the voltage and current will reach an equilibrium. The last two will obviously destroy it which is where my bets lie. In other words, don't operate it without a load.

In a normal alternator, PWM is used to regulate the current through the field coil which is adjusted to keep the output voltage constant. A diode in reverse parallel keeps the current flowing through the coil in-between pulses.

Many regulator ICs exist I'd recommend using one, more information can be found using your favourite search engine.
alternator regulator IC - Google Search
http://www.datasheetcatalog.org/datasheet/CherrySemiconductor/mXtuwrw.pdf

 

[Thunderchild]

it does use residual magnetism, and the original regulator is suprisingly stable and my electronic version was too, if the voltage increases over the limit it just stops supplying the field coil, in actual fact I doubt very much that there is little difference between the regulation of a dynamo and an alternator, the other thing to remeber is that the ignition lamp is connected between the battery and dynamo (bypassing the regulator) so I would assume provide some exiting current

 

[Hero999]

How did the original regulator work? My guess is, it was just a shunt regulator i.e. a giant zener diode.

Please post the schematic for your design?

 

[Thunderchild]

The original regulator was a couple of specially designed relays, one made sure that if the dynamo output was less than the battery it would keep them disconected (known as the cut-out) the other controlled the field coil, it simply kept the field coil connected until the target voltage was reached and then disconect the field coil and when the voltage fell it would reconect, when it disconect a 70 ohm resistor keeps the field coil connect so some current still flows, basically it buzzes at between 70 and 150 Hz and in effect was an electromechanical PWM circuit, this design existed long before semiconductors were used (pre WWII and right up to the 70's when semiconducters powerful enough became cheap and reliable), the whole principle this was based on is as much the mecanical properties as the electrical properties of the relay's and there is a lot of info around if you google.

my design is simply an electronic replica: a simple comparator driving a mosfet, thing is I have to find a way of getting it to not activate the mosfet with the engine stopped or it will discharge the battery without adding more connections than there already are. the circuit will find it's own operating frequency with will probably be governed by how fast the rise and fall of the battery voltage reacts to the applied voltage

 

[Hero999]

That sounds a sensible solution, in fact the regulators I linked to work at just a bit higher frequency.

I don't see anything wrong with powering the circuit from the battery, assuming you can do that. If you keep the current consumption of your circuit low enough then it should never discharge the battery, even if the MOSFET is permanently on; remember a MOSFET gate is an open circuit.

What's the size of the car battery?

Assuming it's 100Ah and your circuit uses 5mA, on a full charge it should last for 20,000h which is over two years sitting in a garage. The TL431 will work down to 1mA and an LM393 only draws 2.5mA so <5mA is easily possible.

You should easily be able to reduce the current to below 20µA but there's no need as the internal discharge will dominate.

The LMC7211 uses just 7µA
https://www.electro-tech-online.com/custompdfs/2010/02/LMC7211.pdf

The LM2936-5.0 has a quiscent current of 15µA and a dropout of 100mV with a load of under 100µA.

 

[Nigel Goodwin]

It's been 40 years or so since I last played with dynamos, and they were a LONG way from been current technology back then

However, they worked fine then, and still do now - so why mess something that isn't broken?.

They worked by switching the field coil in and out, and started up with the field coil switched out, so as not to drain the battery - as far as I'm aware they didn't power the field coil from the output, but only from the battery. Presumably once the dynamo is providing enough current to make it worth while, the field coil is then energised from the battery to increase it's output, and switched off again if the battery is fully charged.

Making a solid state replacement you need to duplicate it's behaviour - feeding the field coil from the battery is how the mechanical ones worked, and is how the solid state one needs to work - it's just a question of exactly when the field coil is powered or not.

 

[Thunderchild]

Nigel, I know that dynamos were used right up to the 70's but i have a friend that dables in cars that go back to pre WWII, people that are nuts about these cars want to have an original looking reg and these today are bought from india and are poorly made only lasting months in some cases, he wants me to replace his with a solid state version that will be built into the original case so he is happy that he still has an "original" looking one, he has shown me the defects of the modern remakes and yes they are not as good quality build as the originals infact like most stuff that comes out of asia made to not last.