The flux (generated by the field coils attached to the stator) doesn't
change direction. But as the rotor rotates, the flux direction w.r.t.
each rotor winding does change. Each rotor winding does in fact generate
an AC voltage. The commutator connects each rotor winding to the
generator's output only during that part of its rotation when it
produces the desired polarity (and magnitude).
Paul Hovnanian firstname.lastname@example.org
Yes, a commutator is a rectifier. Furthermore, the DC output has a
commutation ripple on it just as though it came from a poly phase AC source,
which, in fact it did.
If one were to connect multiple slip rings to the windings instead of the
slotted commutator, the output would be AC and the generator would be an
alternator. The main difference is the DC generator has many phases not just
one or three like a regular AC alternators. Depending on the windings a DC
armature may 20 or more internal phases.
A simple, split ring as you describe is apparently a single phase rectifier
with only one single phase armature winding. The ripple would be very high,
going to zero on each commutation. Actual DC generators are not made this
way but have multiple slots.
There are other mechanical rectifiers also. One type of vibrator power
supply used in old auto radios had one set of contacts to chop the DC into
the transformer and another set to rectify the AC out of the transformer.
There are also synchronous motor driven switches used to rectify very high
voltage AC. One of these I saw was to energize a electrostatic precipitator
at a power plant.
Some vibrators only had one set of contacts for chopping DC, the inverter
function you mentioned. But, many vibrators had dual contacts, the other
set for rectifying the transformed AC back to DC. These were common in the
days before silicon or germanium rectifiers especially in car radios of the
1930's and 40's. See fig 4 in your above URL.
I once saw a 1920's battery charger that had vibrator contacts for
I worked in a steelworks in the 1970's where we used a series of mercury arc
rectifiers to change 3 phase 440v to 440v dc for our overhead cranes. These
devices were spectacular and gave off a blue glow and emitted crackling and
Frankenstein would have been proud.
It's a series of "switches" which are operated as the rotor rotates.
That's why it as the name it does.
A rectifier conducts or not depending upon the voltage across it. The
commutation conducts only when the shaft has a particular orientation.
Were some outside agency to change the direction of the field winding
current, the comutator would still conduct or not regardless of the voltage.
It is not electronic- and predates any electronic rectifier but..
It is a rectifier in that it converts AC to DC. That is where the name
"rectifier" came from and the accepted definition of "rectifier".
It can also act as inverter in that it converts DC to AC in a motor.
Don Kelly email@example.com
remove the X to answer
Definition of "rectifier". Note the last three lines!
Columbia Encyclopedia: rectifier,
component of an electric circuit used to change alternating current to
direct current. Rectifiers are made in various forms, all operating on the
principle that current passes through them freely in one direction but only
slightly or not at all in the opposite direction. One early type of
rectifier was the diode electron tube. Semiconductor rectifiers are
essentially diodes made large enough to safely dissipate the heat caused by
current flow. For heavy currents, they are often equipped with cooling fins
or heat sinks. Rectifiers are commonly used in power supplies for
electronics. There are two kinds of mechanical rectifiers. One, for
polyphase alternating current, is a rotating switch that is synchronized
with the fluctuations of the alternating current. The other uses a
synchronized vibrating reed to change single-phase alternating current into
pulsating direct current. Both have been largely superseded by solid-state
Benjamin D Miller, PE
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