Simple Electrical Question for some of our Electrical Experts

[Larry E.]

This question came up the other day when we where "Bench Racing" with a couple of hot rodders.
We all know that the modern day car has an alternator that produces AC current and the the Diodes tranform the
current to DC to charge the battery. How was this done with for example with the 57 Chevrolet?? It had a
generator that produced AC current. We know at that time Diodes where not around. I know they
had "rectifiers" at that time. I got a pitcture of one below from my American Flyer AC Gilbert train set. I
think we all saw these in some form or another. Did the generator have rectifiers built in?? That would create
a lot of heat though or where the rectifiers hidden someplace in the car. Or What?? Thanks in Advance-Larry

[Gary B.]

Re: Simple Electrical Question for some of our Electrical Experts

Larry,

As a mechanical engineer, it's always been my belief that there is no such thing as a "simple" electrical question. My grade is undergraduate circuit theory class proves that.

Gary

[William C.]

Re: Simple Electrical Question for some of our Electrical Experts

Generators are a DC device (essentially a DC motor being driven) hence no rectification required, Alternators are AC devices and integrate diodes into the housing to do the rectification. Output of either to the system is DC.

[Larry E.]

Re: Simple Electrical Question for some of our Electrical Experts

Generators are a DC device hence no rectification required, Alternators are AC devices and integrate diodes into the housing to do the rectification. Output of either to the system is DC.

OK; that's the answer. For some reason I thought the generators where AC. Thanks--Larry

[Philip P.]

Re: Simple Electrical Question for some of our Electrical Experts

Diodes have been around for quite awhile, longer than some of us would like to remember.
Phil

[Joe R.]

Re: Simple Electrical Question for some of our Electrical Experts

We know at that time Diodes where not around. I know they
had "rectifiers" at that time.

Larry,

Here's the WikiPedia history of the diode, for what it's worth:

http://en.wikipedia.org/wiki/Diode

Rectifier is shorthand for rectifier diode.

Joe

[Bill M.]

Re: Simple Electrical Question for some of our Electrical Experts

more Wikipedia:

A commutator is a rotary electrical switch in certain types of electric motors or electrical generators that periodically reverses the current direction between the rotor and the external circuit. In a motor, it applies power to the best location on the rotor, and in a generator, picks off power similarly. As a switch, it has exceptionally long life, considering the number of circuit makes and breaks that occur in normal operation.
A commutator is a common feature of direct current rotating machines. By reversing the current direction in the moving coil of a motor's armature, a steady rotating force (torque) is produced. Similarly, in a generator, reversing of the coil's connection to the external circuit provides unidirectional (i.e. direct) current to the external circuit.

[William C.]

Re: Simple Electrical Question for some of our Electrical Experts

Diodes -- Block DC current in one direction

Rectifiers Convert AC to DC voltage.

The generator is a spinning coil with a magnetinc filed that creates the DC or AC voltage

[Dan D.]

Re: Simple Electrical Question for some of our Electrical Experts

A diode is a rectifier. They are a 2 element solid state device - hence called a diode.

DC generators employ a split ring commutator on the rotating armature. The current is developed in the spinning armature and is brought to the outside world through the commutator and brushes. Because the current is high the brushes are large and are subject to wear - due to the high current, arcing, the resulting heat, and because the split rings are not a smooth continuous surface for the brushes to ride on.

The alternator, on the other hand, generates the current in the fixed stator (field coil in generators). Therefore the rotor (or armature in DC generator) only supplies excitation current, so the commutator is a 'slip ring' and the brushes are much smaller as the current is much less. But the current developed in the stator is AC and must be rectified, or converted to DC. All automotive alternators are 3 phase - the AC current is phased 120* apart in each of the 3 phases. There are 6 diodes, 3 in series and 3 in shunt.

Alternators first appeared in 1957 (or 1958- not sure) on Chrysler products. It wasn't until that time that high current silicon diodes became available. And they came just in time, as the current demands in automobiles started to rise (air conditioning in particular). To get more current out of generators, they would get much bigger, heavier, and more ugly. Alternators really saved the day. -Dan-

[Scott K.]

Re: Simple Electrical Question for some of our Electrical Experts

Diodes -- Block DC current in one direction

Rectifiers Convert AC to DC voltage.

The generator is a spinning coil with a magnetinc filed that creates the DC or AC voltage

EDIT: This post not 100% Correct because Automotive Alternator is Three Phase. See Posts Below.
The Rectifier that converts AC to DC is 4 Diodes in a diamond shaped setup.
AC is applied at the ends where cathode and anode meet.
DC positive voltage comes out where the 2 cathodes tie together.
DC negative is where the 2 anodes meet together.

[Dan D.]

Re: Simple Electrical Question for some of our Electrical Experts

Scott,

What you posted is correct for a single phase system. But automobile alternators are a 3 phase system, and they do not use a 'bridge diode' rectifier. They are full wave, but only 2 diodes per phase for a total of 6 diodes. They are not at all like your diagram. -Dan-

[Scott K.]

Re: Simple Electrical Question for some of our Electrical Experts

Scott,

What you posted is correct for a single phase system. But automobile alternators are a 3 phase system, and they do not use a 'bridge diode' rectifier. They are full wave, but only 2 diodes per phase for a total of 6 diodes. They are not at all like your diagram. -Dan-

Three phase is Technically the Same principal, 6 Diodes instead of 4.
They still use Diodes.
It would be this Diagram instead.

[Dan D.]

Re: Simple Electrical Question for some of our Electrical Experts

That is correct. R, S, and T are the three phases in the stator.