# How does a 110v portable generator with brushes generate electricity?

Discussion in 'General Electronics Discussion' started by ldcarter, Nov 25, 2016.

1. ### ldcarter

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Nov 20, 2016
Perhaps my age, but I can't seem to grasp the concept of a generator. Reads lots of theory but still lost. It has a rotor that rotates, a stator that stays still, wiring, voltage regulator, residual magnetism, excitation wiring, field wiring, power winding, and of course something to rotate the rotor.

You crank it up, the rotor begins to rotate, residual magnetism (in the rotor?) creates a few volts!

What happens next, in operating order, to the various components (excitation, field, power windings, voltage regulator, etc.), where does the reference voltage come from, how does it sense that 110v is the reference? Is the excitation winding on the stator? I really need some help on this. Please don't reference another website, I'm websited out, I want someone that I can ask questions of.

2. ### Bluejets

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Oct 5, 2014
davenn likes this.
3. ### davennModerator

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Sep 5, 2009
nice video, but I'm not sure it's specific enough to answer the OP's Q's
it doesn't go into different windings, brushed systems etc

4. ### Minder

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Apr 24, 2015
Look up Flemings right hand rule.
The residual you are talking about is in the stator, when the rotor is turned, the windings in the rotor cut through the residual magnetism in the stator, this induces a voltage in the armature and creates a small voltage output from the commutator, because the field winding is connected across the armature, this small output voltage is impressed back into the stator (field) coil, which then causes the generator to pull itself up through self excitation.
Called shunt field connected.
We have an electrical museum here that has an original Edison DC generator. Where it is demonstrated.
M.

Last edited: Nov 26, 2016
5. ### ldcarter

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Nov 20, 2016
M. thanks for the reply, let me say up front that I am not challenging anything you have said and I apologize in advance if anything I say sounds that way. I am only trying to get this generator thing to where it all makes sense to me.

This is where I'm getting lost, a generator consists of a motor and a alternator (properly called a genset not a generator). My understanding is that the alternator has a revolving field (rotor) which contains the armature and brushes, and the stator which supplies AC output. I don't understand the concept of "flashing the field" (connecting 12v DC to the brushes) if the magnetism is in the stator. Would not this DC applied to the rotor cause magnetism in the rotor? Also isn't the excitation voltage converted to DC and used to increase the magnetism in the rotor?

6. ### Colin Mitchell

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Aug 31, 2014
Generally an AC generator is brushless.

7. ### hevans1944Hop - AC8NS

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Jun 21, 2012
An AC generator or alternator energizes the rotor winding with DC, applied through a pair of slip-rings on the rotor connected to two spring-loaded stationary brushes mounted on the housing. One brush is insulated from the housing and the other is connected to the alternator frame. The rotor current will produce a magnetic field between salient poles on the rotor and poles on the stator. Rotating the rotor with a motor will cause this magnetic field to sweep past the pole pieces of the stator, inducing a voltage in the stator winding. See Minder's post #4.

There are usually three stator windings spaced electrically 120 degrees apart to produce three-phase electrical current in the stator windings. If AC is all you need, three-phase power can be extracted directly from the stator winding. However, the frequency of the AC will be a function of rotor rotation speed.

In an automobile alternator, the stator windings are connected to a six-diode full-wave bridge rectifier to produce a relatively smooth (low ripple) DC output. The current in the rotor is varied to control the voltage produced in the stator. This rotor current is a function of load and rotor rotation speed for a given design output voltage.

The system is quite efficient, compared to a conventional DC generator, and can produce a higher output current at a lower rotor revolution speed. This is why alternators replaced DC generators in automobiles. An alternator, at the additional expense of six cheap power diodes, provides significant power even when the auto engine is at idle speed. This keeps the car battery charged, even in winter stop-and-go traffic, where a DC generator doesn't provide sufficient power.

DC generators are obsolete, for all practical purposes, as a means to convert mechanical power into electrical power. Any periodically varing magnetic field always produces an AC voltage. You can think of this as "what goes up must come down" if you wish. In a DC generator, the stator provides a fixed magnetic field through which the rotor windings rotate inducing an AC voltage in each winding. The AC induced in the rotor windings is converted to DC (inefficiently) by the commutator segments to which the rotor windings are connected. Basically, the commutator electrically reverses each of the rotor windings twice per revolution as the induced AC goes through zero, thereby producing a DC output.

Edison liked DC generators, although Tesla's three-phase AC method was much better. But Tesla required cheap semiconductor power diodes to make the "DC alternator" practical, and I don't think he lived to see that happen.

Last edited: Nov 26, 2016
8. ### Minder

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Apr 24, 2015
The method is called self-exitation.
The difference between a dc generator and an alternator is on the DC gen the field is wound on the stator, in an alternator the field is on the rotor and connects via slip rings.
With a DC generator initially it is flashed as you say with a dc source such as a 12v battery in order to create a residual field , but you flash the Field, NOT the Armature.
Once the field exists, when the rotor is externally rotated the coils on the armature pass through this residual field and creates a DC current in the armature as per Flemings L.H. rule.
This current is coupled to a wound field in the stator which further increases the generated power therefore the output increases in ratio to the rpm.
There are two methods of controlling the output, the older method is for a governor to control the rpm of rotation of the driving motor in order to keep output at a steady level regardless of load.
The second is for a high rpm and the field is electronically regulated for the correct output.
The alternator is essentially identical in operation, the rotor field is fed with DC via two slip rings.
But the stator output is 3ph AC which is then rectified and fed back to the rotor in a regulated fashion.
Also a Alternator usually refers to AC generation and a Dynamo and Generator to DC.
It is misused in modern jargon.
BTW, I have experimented with an automobile alternator in order to try and make it self excite, with no luck.
M.

Last edited: Nov 26, 2016
9. ### ldcarter

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Nov 20, 2016
That certainly helps with the clarification. One of the ways I read about flashing was to use a 9v battery (the small one) and connect it to the receptacle being careful to make sure the + side is on the hot leg and the - to the ground side. Since they are connected directly to the stator it would induce a small amount of magnetism in the stator. One of the others was to connect household power to a receptacle on your genset along with some lamps in series, too scary I thought, but there are claims it works. A third one was to use a reversible drill set on reverse and spin it by hand. So the 12v DC to the rotor through the brushes may be the easiest because of the fact that they are convenient to get to.

I read somewhere that most automotive alternators and some high dollar gensets use the 12V battery supply used for starting to excite the windings.

Looking at the attached image I couldn't understand the tie in with the voltage regulator. Now that I have a better grasp of what is happening I think that lines 11 and 22 are a feedback circuit to the voltage regulator used to determine if the voltage regulator needs to increase or decrease the DC to the rotor to control the AC output. If this is correct then I give all involved in helping me an A+. If not unfortunately we have some work to do.

In the words of Satchell Paige, "How old would you be if you didn't know how old you was". Now, so how does the voltage regulator know that it has reached 110v AC and then maintain it? I know that most gensets have a pot that you can use to adjust the output voltage. Exactly how does this circuit work?

Hevans1944 mentioned Tesla, I consider him on electrical workings on par with minder's avatar of Einstein.

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10. ### Minder

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Apr 24, 2015
Auto alternator regulators have evolved over the years and are fed off of the battery, the first kind was a simple Relay operation, the next was electronic version of the relay type, and worked in the electronic version by a sensing circuit to detect 14.5v, when the output of the bridge reached this value, the rotor (field) voltage was removed, and of course the voltage went down, as soon as 14.5 was reached, the field was reconnected, this has the effect of rapid switching and the length of time on or off was decided by the load. This creates a mean level of field CURRENT.
The problem with this method was it was found that the random switching caused all kinds of interference in the car radio.
So the modern version was designed around PWM control, which has a fixed switching rate, which lessens the radio problems.
As far as the way the residual works, again, the effect is very simply explained by Fleming.
Incidentally some years ago I designed a electronic regulator for the field in 240vdc magnet mobile crane DC generators used in R.R's and scrap yards, The method I used was along the lines of the simpler electronic type, (there is normally no radio in the crane cab!).
M.

The old mechanical one.
(DC generator)

Last edited: Nov 26, 2016
11. ### ldcarter

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Nov 20, 2016
So possibly the MC1741CP1 in my circuit is being used as a comparator to determine the output voltage in relation to the desired voltage and turn on/off the bridge rectifier as needed to achieve 110v? Attached is a view of the voltage regulator with the labeled components.

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12. ### Minder

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Apr 24, 2015
Actually I used a LM311 comparitor to achieve the one I designed!
Compare the output with the set voltage and the comparitor switches the field on and off.
It was a very simple circuit.
M.

13. ### Minder

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Apr 24, 2015
BTW, usually the field is controlled NOT the direct control of the bridge.
M.

14. ### ldcarter

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Nov 20, 2016
By George, I think I may have it. Kudos's to all those who have helped me with this. By the way, went to school for 6 months on Uncle Sam's money in 1974. They taught me about about all the components, what they did, how they were used and how circuits were made. Went to work, they assigned me to work on weather radars. There were 8 or 10 boards in the radars. All we did was change boards until it worked. Thought to myself, they didn't need to send me to school for 6 months for this.

Just read your new post as I was typing this. Thought controlling the DC to the rotor did control the field, but regardless I can see how the alternator works and how it is controlled. My next step is to check all the components and attempt to draw the circuit using the image I attached above. If anyone following this post wishes, I would appreciate them looking at my post "T3J81 (Zener Diode ?) in the Datasheets, Schematics, Manuals and Parts Forum to see if they recognize that part and what it is and perhaps know a substitute.

Thanks to all.

15. ### Minder

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Apr 24, 2015
That is correct as it applies to a alternator as the field is on the rotor.
In a DC generator the field is the stator.
Hope that clears it up now.
M.

16. ### Slick

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Jul 27, 2013
I've been involved in repairing all sorts of portable gas generators, welding equipment and besides owning my own shop was foreman at a local motor shop. I just want to add to the conversation that generators ( actually altenators ) will run at 3600 RPM which is a two pole system and 1800 RPM for a four pole unit. Speed is important due to the way it effects the 60 cycle power. These numbers do vary due to magnetic strength, ect. I always used a frequency meter to set the rpm, or out in the field used a volt meter to set the engine for rated voltage output. As you can see a 1800 rpm engine only runs about half the speed, but the engine will last a long time. Most of the portable generators for home use run at 3600 rpm. It takes more power to run the same load at 1800 rpm, but from my experience I'd rather pay a little more for a unit that runs at 1800 rpm that doesn't just sit there screaming. Problem is trying to find a company that makes four pole generators. Generators for the most part are old school. Almost everything has gone to altenators. Even your portable welders are altenators that put out ac curent and rectified DC. Some also have seperate windings that produce 110 volt. 4 poles X 1800 rpm = 7,200 devided by 60 cycle=120 volt, or 2 poles X 3600 = 7,200 devided by 60 cycle = 120 volt

Last edited: Dec 3, 2016
17. ### ldcarter

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Nov 20, 2016
Slick, when I got this genset I noticed it had a 18hp twin cylinder Briggs. Thought to myself, that engine must just lope along. Checked the tag on the alternator and it specified 3600 rpm. It will put out 9600 watts (43 amps) of 220v). Seems like a lot of engine, but it is a Industrial model Dayton 7500 4W113 (sold by Grainger, the alternator made by Dayco, the voltage regulator by Generac). If you are familiar with this type of generator I would appreciate it you would look at my post in this section about converting from a voltage regulator to a generic AVR. It has the reason I would consider this.

After many videos and reading numerous articles, they suggest that you adjust the output by frequency. Usually around 62 Hz, while making sure that the AC is just over 120v with no load. Obviously with your background you would be familiar with this procedure, do you think this is how it should done?

I was a lead plant electrician and we always referred to standard house voltage as 110/220 but a wall receptacle was considered to have 120 volts, always thought that was strange but that's how I learned and still call it that way. Most of our work was done on 550 3 phase, then later with newer machinery we went to 480 3 phase.

18. ### Minder

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Apr 24, 2015
That was the original rating, N.A. has been 120/240 for some time now.
M.

19. ### globecollector

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Jun 27, 2011
About a decade ago I used to help a local electrical technician work on all manner of things. He was very proficient with series wound motors in power tools and was decidedly "old school" and knew his rotary machine stuff very well and was good with the mechanical aspects. Sometimes we worked on big, multi Kw D.C. generators where the problems were usually machining the commutator and changing the brushes.
I am decidedly "more modern" and my background is electronics, ANY electronics from audiophile amplifiers to T.V. transmitters to X-Ray machines....the underlying principles are all the same. I have discovered over the decades that my approach is somewhat different than the majority of others in the electronics game. I seek to "get inside the head of the bloke who designed it" and get as full an UNDERSTANDING of its function as I can before attempting any repairs or modifications. This does not go well with this fast-paced money-mad world where the pervading philosophy is get 'em in, "fix" 'em quick and get 'em out.....BUT I get results were most of the others fail to because they make assumptions due to time constraints. whereas I want to KNOW first before I "fiddle". I'm falling behind these digital days because my firmware/software skills are poor...but, hey...life ain't long enough to know it all. My mechanical skills aren't to confident either.
SO...getting to the alternator...a few times I had to work on portable gensets and there was one design that stuck in my mind...it was really clever but had a weakness. How it was clever is that it had TWO alternators on the one shaft, the main one and an excitation one...no brushes were used, so no brushes (or commutators) to wear. This design tended to be used on the cheaper economy models, so the burshless design was a way of getting the manufacturing costs down. The exciter had its stator energized with D.C., its rotor then generated A.C. as it span inside the stator....now here's the ingenious bit, this A.C. was bridge rectified with four diodes...that span on the shaft! The D.C. obtained then energized the rotor of the main alternator in much the same manner as an automotive alternator...but without the slip rings. The voltage and frequency control were pretty rough, but functional...the former achieved by an LM311 turning a MOSFET on an off which fed the stator of the excitor, the latter, a crappy charge-pump arrangement fed a solenoid-spring on the throttle of the internal combustion engine,
The weakness in this design was if the petrol ran out whilst it was under load, which could eaisly happen, and it stopped under load the residual magnetism on the rotor was lost and it would not self excite....something to do with the laminations of the rotor going around the diminishing B-H magnetic hysteresis curve all the way to zero in the middle ...I recall that quite a bit of fiddling about was required to get external D.C. onto the exciter stator before it would "pick itself up again by the seat of its own pants".
Often the diodes on the rotor would get damaged by the forces there and I recall having to replace them...my job as the electronics half of the team, then my electrical technician mate had to mechanically bind, fasten and epoxy them down so they would not fly off when it was started!

Last edited: Dec 3, 2016
hevans1944 likes this.
20. ### hevans1944Hop - AC8NS

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What a wonderful story, @globecollector! It appears you and I think very much alike vis a vis trying to get inside the original designer's head. I especially enjoyed how you analyzed and "fixed" the 12V/5V CMOS interface problem created by having two different designers responsible for two different boards. This process of figuring out the original intent, and perhaps finding a way to make it better, did not serve me well when I was working as a technician. As you said, the "just fix it" management mentality is prevalent. It's only when a marginal design began to fail repeatedly was I then allowed to "improve" it. It is the main reason I continued down the path to an engineering degree, working for ten years as a technician to do so, but finally reaching my goal in 1978. Working as an engineer after that revealed an entirely new set of problems, but that's another story for another thread.