Connect with us

Dynamo and Alternator

Discussion in 'Electrical Engineering' started by n o s p a m p l e a s e, Mar 12, 2005.

Scroll to continue with content
  1. What is the difference between the two?

    Thanx
    NSP
     
  2. 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.
     
  3. Dick Alvarez

    Dick Alvarez Guest

    <<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
     
  4. 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...
     
  5. Dick Alvarez

    Dick Alvarez Guest

    Sue wrote <<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 agree completely. I was thinking of the generators in
    power-houses, and I completely neglected automotive generators. Yes,
    inexpensive semiconductor power rectifiers are a great boon to
    automotive technology. I am old enough to remember automotive DC
    generators, armature growlers, undercutting the mica in commutators,
    having to conserve electricity when we drove only at night, and such
    things.

    <<...it wasn't really practical to fit mercury arc rectifiers to
    cars to keep their batteries charged...>>

    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
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-