Dynamo and Alternator

This isn't my field (small pun) but, AFAIK:

A dynamo produces dc and has a commutator with brushes to connect to the coils in the rotor. An alternator produces ac and has slip rings to connect to the coils on the rotor.

Dynamos were used prior to the availablility of inexpensive semiconductor power rectifiers - which allowed the use of alternators.

Dynamos were used in the earliest days of electrical power distribution, before alternating current was fully appreciated.

Alternating current soon came into use because it is much easier to generate alternating current than to generate direct current; because alternating current it can be transformed in Voltage for efficiency of generation, transmission, and use; and because alternating current motors are much simpler than direct current motors for most applications.

Ease of generation: In a dynamo, the load current is generated in the rotor, and must be conducted out of the rotor through a commutator and brushes. In an alternator, the load current is generated in the stator, and is taken out directly by fixed wires, rather than through a commutator and brushes. Similarly, alternating current motors are much simpler than direct current motors for most applications.

Voltage transformation: In large alternators, alternating current is generated at medium Voltage for efficiency, then transformed up to high Voltage for efficiency in transmission over long distances, then transformed down to low Voltage for ease of consumption in homes.

Alternating current came into use long before inexpensive semiconductor power rectifiers became available. In early alternators, slip rings conducted the excitation current to the coils on the rotor. Slip rings, in alternators, had two big advantages over commutators, in dynamos: First, the slip rings had to conduct only the relatively small excitation current, which made the rotor into a rotating electromagnet, rather than conduct the full load current, as in a dynamo. Second, slip rings were smooth, not segmented as in a dynamo, and thus there was relatively little wear and arcing between the slip rings and the brushes. Still, slip ring wear and arcing were significant problems in alternators. Furthermore, all medium and large alternators had separate DC exciter generators (little dynamos), with commutators and brushes and their attendant complications, but at least the exciter generators were much smaller than the dynamos that would be needed to generate the same amount of load power.

Later, inexpensive semiconductor power rectifiers made it possible to eliminate the separate DC exciter generators, and couple the excitation power into the rotor by magnetic induction, rather than by conduction through slip rings. But in the rotor, that induced excitation power was in the form of alternating current, not direct current. Additional semiconductor power rectifiers were installed right on the rotor, to convert the alternating excitation current to the direct current that is needed to make the rotor into a rotating electromagnet. Very occasionally, those rotating rectifiers fail, either electrically or mechanically, but that is rare. So inexpensive semiconductor power rectifiers have simplified electric power generation significantly. But alternators came first, and inexpensive semiconductor power rectifiers came later.

Dick Alvarez alvarez at alumni dot caltech dot edu

Perhaps I should have prefixed "for the majority of generators," to "dynamos were used.." - The majority of generators are fitted to motor vehicles, boats, etc and I was addressing that most common situation in stating that semiconductor rectifiers have allowed alternators to be used in preference to dynamos. As you explained so well, there is a lot of reasons to prefer to use alternators - but it wasn't really practical to fit mercury arc rectifiers to cars to keep their batteries charged...

I do remember seeing a mercury arc rectifier (connected to a battery charger) in my distant youth - in its own little room. Quite beautiful and far, far more exciting that a few bolts with wires coming out of them...

Sue wrote >

During the late 1950s, I noticed an alternator on a fire engine, with apparently a selenium three-phase bridge rectifier, I suppose for powering the big flood-lights.

Dick Alvarez alvarez at alumni dot caltech dot edu