your question doesn't make 100% sense to me but I think I know what you are talking about. A while back I was reading about universal motors, which are basically a series wound DC motor with low reluctance laminations so that it can efficiently run on AC. The material I was reading said that the commutator acts as a mechanical rectifier. I do not agree with this statement. if you take a series wound motor and apply DC to it, it will spin ,say clockwise. If you then reverse the polarity, it will still spin clockwise. it's not until you swap the ends of the field winding that the motor will reverse direction. the series wound motor does not care about polarity. so if you connect AC to it, it doesn't care if the polarity swaps 50 or 60 times per second, it spins always in the same direction. So there is no rectification going on there, mechanical or otherwise.Can someone help me understand how does the commutator act as a mechanical rectifier of the AC voltage ?
I guess I was thinking of a motor instead of a generator, which doesn't really make sense when talking about a rectifier. Thats why I assumed he must be talking about the universal motor.There is a mix-up of ideas here. The commutator of a DC generator is indeed a sort of rectifier (if the armature output came out via slip rings, it would be AC).
This is separate from the explanation of why a universal motor can run on AC as well as DC, which is because armature and field reverse at the same time.
Can you link to one of these articles? I couldn't find it. I looked in these:Wikipedia has tons of references to Mechanical Rectifier being built into commutators and brushes.
I just did a search for it, and it seems somebody added that in every article they could.
While this is true, not many people wanting to rectify AC to DC want to run it through a motor to get the result they are looking for.In addition, to avoid confusion when reading reference sources, where so many fail to address the state of the machine, I hope this will help.
When the machine functions as a motor the commutator is a mechanical switch in order to preserve opposing polaritys. Conversely, if the reciprocal function holds true and it's a "Jenny" then yes the commutator functions as a mechanical rectifier.
Faradays law will always hold when a conductor "cuts" a magnetic field. Amid the pole shoes (pole pitch) no voltage is induced in the coils. In effect, the commutator input will have a tendency not to be a pure sinusoid but a rectangular waveform changing polarity over a period of time. Once the induced EMf is being furnished as an output it will alternate between it's maximum value and zero due to the insulating medium between segments.
In a two segment commutator this is AC superimposed on the DC output and is not desirable. To compensate for this you increase the number of segments until the output tends to pure DC. If the opposite is true, and the state of the machine is a motor, the extra segments will provide a uniform "current sheet" and will furnish smoother torque that does not fluctuate.
Unfortunately, still being a "kid", I am not aware who would want to rectify AC to DC through a motor.While this is true, not many people wanting to rectify AC to DC want to run it through a motor to get the result they are looking for.
That is why I am confused at the number of references to mechanical rectifier on the wiki page. I would think the mechanical rectifier would be a buzzing 60Hz relay, similar to the "buzz relay" used in early DC-AC inverters.
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by Jake Hertz
by Jake Hertz