Newbie question

[pim123]

I have searched the forum but couldn't find my answer so I am asking it here: What is the difference between a dynamo and an alternator (as far as I understand they both generate ac ...) and how can I see wich of the two I have ?

[8009STEVE]

The alternator is short and silver. The dynamo is long and black. Is the car positive or negative earth? if it is negative, then it is probabley neg earth.

[pim123]

Ther is a huge sticker: Warning: Negative Earth ! (so it's probably NE )
I have the short silvery thing

[pim123]

So I have an alternator, but does it different from a dynamo and why is it better ?

[minor_hickup]

The alternator puts out more current, especially at low rpm.

[pim123]

Ok, I why do we want that ? (I am going for the complete brain-drain now )

[bigginger]

Because it'll continue to charge the battery in (for instance) slow moving traffic.

[ColinP]

Pim asked:-

What is the difference between a dynamo and an alternator (as far as I understand they both generate ac ...)

Dynamo has a set of rotating coils in an (electro)magnetic field which generates a current.
By arranging the contacts to the several coils on the rotor, it actuallly produces a DC current. This clever bit is where the brushes (carbon sticks) and teh commutator are involved. Over time the brushes wear, and the slots in the comutator (which separate the different coils) get dirty, and may lose thheir insulation.

Alternator does the same as the dynamo, but has a smaller pulley (so produces more current at lower rpm -and may require a different drive belt) It doesn't have the fancy comutator/brush device, it produces ac whiich is then rectified be a set of power diodes in the alternator.

Colin

[alex_holden]

(as far as I understand they both generate ac ...)

They both put out lumpy DC, hence why you need a battery attached all the time to smooth things out, not just when you're starting the engine. Car alternators put out DC because they have a diode bridge rectifier in them, and dynamos put out DC because they have a commutator that mechanically rectifies the current. Alternators also have an electronic voltage regulator in them which reduces the current through the field winding when the voltage gets too high, whereas with a dynamo you need a separate voltage regulator (the funny brown box with Lucas written on it). Without a regulator the voltage could get too high and overcharge the battery and/or burn out sensitive electrical devices.

An alternator will take more power away from the engine, but I doubt it'll be enough to notice the difference, and they have the advantage that you're much less likely to find you have a flat battery in the middle of winter because the dynamo hasn't been able to keep up with the power you've been using to start the engine and power the lights, heater motor, radio, etc.

[pim123]

Ok, thank you guys for explaining the diffference and the actual working of both systems ! This forum is just what a newbie Morris owner as me needs !! I already have ordered the Hayes book on Morris minors and I am now looking for another book about the basic working of car engines in general.

[ColinP]

Pim wrote:-

looking for another book about the basic working of car engines in general.

Try to get hold or "The reluctant car mechanic" - out of print for some years, but well worth it.
It goes through the whole car set-up, with nice straightforward explanations (both ancient & modern meanings of "nice" for those who read all the posts).

Otherwise, it's basically "suck, squeeze, bang, blow" .....

Colin

ps.

http://www.waterrow.org.uk/motoringbooks/design.htm
go get it for £5!

[minor_hickup]

Was there not a thread recently by Wibble Puppy about good resources for learning about cars? I can't seem to find it.

If you can't wait for delivery this is a good site: http://www.chris-longhurst.com/carbible ... tives.html

[Welung666]

I think this is the post you meant.

[minor_hickup]

Thank you Mr.Welung666, I hope that thread is useful

[pim123]

Tnx guys, I can buy me a nice set of books now !

[pim123]

Follow up on the book thing: just ordered 'THE RELUCTANT MOTOR MECHANIC' , 'AUTOMOBILE WORKSHOP PRACTICE' and 'AUTOMOBILE ENGINES -- QUESTIONS AND ANSWERS'
That will keep me off the street for a while !!