# Is DC-Generator Commutator a Rectifier?

In Theory of DC-Generator(Simple) a split ring attached to a brush in contact .....the split ring called commutator acts as a Rectifier
But is a Commutator a rectifier really?,,,,,How can a split ring ring be called a Rectifier?
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On Sun, 27 Jul 2008 09:19:44 -0700 (PDT), Gopi

It really can't. There was never any AC to be rectified. A DC generator makes DC by keeping the flux going the same way at each segment of the winding
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snipped-for-privacy@aol.com wrote:

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).
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Paul Hovnanian snipped-for-privacy@hovnanian.com
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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.
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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.
Don Young
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Surely a vibrator power supply is the opposite of a rectifier... its an inverter. see: http://www.radioremembered.org/vpwrsup.htm

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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 rectification.
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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 buzzing sounds.
Frankenstein would have been proud.
K

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Nope!
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.
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It could be considered a synchronous rectifier. ;-)

That's a pretty narrow definition of "rectifier". "Diode", I would agree, but not rectifier. Your definition completely rules synchronous rectifiers out.
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Keith

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------------------ 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.
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Don Kelly snipped-for-privacy@shawcross.ca
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John Gilmer wrote:

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 devices.
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Benjamin D Miller, PE
www.bmillerengineering.com
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