The ideal position of the brushes on the commutator varies depending on the load. If they are in the "wrong" position, there is a sparking and brush wear.
In days of yore large DC machines had moveable brushes which were manually changed to suit load. Later machines had interpoles or compensating windings in series with the armature which automatically corrected this problem by modifying the field piece magnetic field. Of course this costs money and they are not fitted to small machines. So brushes etc don't last as long and there is arcing.
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The armature current BTW reverses as it passes under the brush ideally which is why there is no sparking. Or it would if it was DC. Not always the case with AC. Which is why power tools have a life of only hours.
Harry's trying and failing to be a smart alec. And he keeps forgetting that there *is* a type of electric motor that works on pure DC, with no internal conversion to AC.
A consequence of the distortion of the magnetic field due to the variation of armature current with loading. On a simple motor with fixed brushes, it's set to a midway compromise position unless it's been designed for use with a reversing switch.
That was fine for large industrial DC motors.
A rather neat refinement you're unlikely to see on a typical sub horsepower universal motor typical of most domestic white goods and electrical tools.
Ok, you 'get it' then. So why nitpick my statements?
The intersegment voltage is pretty close to zero at the ideal brush location and reaches a max at 90 degrees.
I think you meant to say "tens of hours", still pretty short lived but in the context of its use, probably sufficient for a few years of DIY use in most cases.
No you're the crass idiot in saying there is no such thing as a DC motor. Or a motor that won't accept a DC power input.
Next you'll be saying a universal motor running on AC doesn't need a commutator.
Have you tried applying an AC current to opposing segments of a motor. Would you expect anything other than a "hum"?
For a DC motor with a DC field coil, a commutator ensures that a DC current flows in the appropriate section of armature to maintain a torque.
It is true that any part of an armature will see an alternating current, but the motor is still driven with DC and is called in all parlances a DC motor.
It wouldn't run at all, so you are indeed correct for a change. The only way a motor without a commutator will run is if it's designed to only run on AC, which makes it the opposite of a universal machine. The only way to design a motor to run on AC or DC without a commutator would be to incorporate an inverter in the design, and a bridge rectifier to power the inverter, but the motor in this case would be AC only. Brushless DC motors use electronic switching instead of mechanical.
If you examine each segment in turn as it passes the same position, the current always flows in the same direction.
Harry's a troll. Oh he's not the foul mouthed class of troll usually associated with the name Troll but a troll, nonetheless. He's a more intellectual troll (in most news groups, the worst kind).
His exception, which he could so easily have named, is the Faraday or homopolar motor or generator which relies on Lorentz force. This type works on a steady DC current with no internally generated polarity reversals as used by conventional DC motors.
What he states is, strictly speaking, absolutely correct. The common or garden DC motor is essentially an AC motor with a mechanical reversing swith synchronised to the rotational position of the armature. The current flow in each pole winding isn't constant as in the case of the homopolar motor but alternating as in the case of an ac motor. The commutator brush gear just neatly disguises this fact.
See these links for the obvious sources of information on this subject:-
Clearly thinking is not your forte. They are used for example in all commercial electric cars. They have excellent start characteristics. Max torque @ zero revs. Ideal for traction. They are used for many of the auxilliary motors in electric cars too.
They are used in many traction applications , eg electric trains, ships.
Domestically they are appearing in vacuum cleaners and washing machines (elctronics being cheaper than commutators.) "Digital motors". Heh Heh.
There is no difference between a synchronous motor and an alternator. All electric"motors" can be run as generators.
The motor has DC supplied to it. But it doesn't run on DC.
I can't make it any more simple than that for you Go and read some books.
As that last link was supposed to have shown one of the uses for Mega Ampere generators. I'm not sure how I managed to paste my home page link instead of the one I meant to post, here:
Mention was made in a wiki article on the homopolar generator of hundreds of volts output (possibly not at a million or more amps though - 100MW or higher if a 1MA 100 volt generator was involved).
Ah! This is the relavent link:
With regard to Harry's trolling, he just wants to start an argument that he 'knows' he can 'win' on a technicality he hopes is obscure enough to get a pointless and protracted argument going.
He carefully chooses his subject matter for maximum effect when he's the thread starter, conveniently forgetting about practicalities that seem best summed up as "You can't make an omelette without cracking a few eggs".
He seems to have a Bee in his bonnet when it comes to nuclear power, choosing to exclude the most pragmatic of solutions to the conflicting need for more energy and the need to reduce pollution which might cause the global climate to flip over into yet another Ice Age or a Global Warm period (it's far from clear as to exactly what the consequences will be for the climate due to our significant contributions to the greenhouse gasses in the atmosphere - it's not just the carbon dioxide we have to worry about in this regard, there's also the possibility of massive releases of the methane trapped just below the permafrost of the Tundra, a gas that is 60 times more effective than CO2 as a greenhouse gas).
As a result of his wilful ignorance of the real world, most of Harry's spoutings are best ignored and forgotten about. Don't let the technical truths in his postings suck you into a pointless discussion over the impracticalities involved in his suggested solutions (most of which appear to be a case of 'not thinking things through') unless you like going round and round in circles on a journey to nowhere.
There are alot of poorly educated people here. I can't abide ignorance and those that perpetrate stupid theories Especially the people that come up with homespun theories to account for things they have never bothered to research. Easy these days with the internet.
And no-one has yet explained to me how nuclear waste is disposed of. Only the homespun theories and crap about the likes of future thorium Let alone if it can be done economically. I have found plenty of evidence on the internet to the contrary.
If everything was so easy, it would be happening. And it isn't. Plus we have been told so many liesin the past by the nuclear industry.
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