I'm surprised that some pedant hasn't pointed out that all alternators /are/ dynamos - which can be AC or DC - it's just a motor trade convention that the word "dynamo" was kept for the original DC generators when alternators came in.
Possibly because the pedants know that a dynamo usually has a commutator and directly produces something resembling DC, an alternator usually has slip rings and produces AC that has to be rectified.
The classic bike brigade often fit LED indicators etc in a bid to save as m uch lecky as possible for the main light.
I wonder if it?s worth doing something similar with a moggy (me, I ?d fit an alternator)?
They are both generators - but the definition of a dynamo is a generator which produces DC by use of a commutator.
As per definition in the wiki article:
Yet the 'dynamo' on my old bike produced AC.
Apparently I should have been calling it a 'magneto'.
Never hear then referring to the Alternators in a power station as Dynamo's!..
"Dynamo" comes from a Greek word "dynamis" meaning force or power. Not that the Ancient Greeks would have known the difference between AC and DC. Or force and power. Heck, they knew hardly anything about electricity: they generated electricity by rubbing a piece of amber on a cat.
That's the very definition of a dynamo (if you more precisely define the commutator as being an integral part of the armature assembly) so there's no possibility of any confusion.
The motor trade called the modern DC generators "Alternators", even though they output DC voltage via their integral 3 phase fullwave bridge rectifier, simply to distinguish them from the earlier dynamo technology.
It was the advent of silicon diode rectifiers with average current ratings of 50A or more which released the industry from the tyranny of the carbon brushed commutator assembly. The output carrying windings could now be placed in a stator where they were no longer subject to centripetal forces, delegating the task of supplying a rotating magnetic field in a field coil rotor assembly that could withstand much higher rotational speeds without danger of burst windings or fragile laminated armature cores flying apart.
Also, the field could be energised via low friction/wear slip rings which only had to deal with 4 or 5 amps of excitation current rather than the 22 amps or more of the classic dynamo's high friction/wear of a segmented slipring to mechanically 'rectify' the armature's internal AC voltage.
In fact, it's quite possible to eliminate even the slip ring connection to the rotating field winding by locating the field coils externally so as to magnetise the rotating poles via conduction of magnetic flux across the small clearance gap at each end of the shaft and into the rotor assembly.
However, this creates a more bulky and intricate design for which the automotive industry would gain no benefit by supplying alternators whose life is limited only by that of its bearings which could well exceed that of the engine's bearings by an order of magnitude.
The 75,000 miles or longer rating of the slipring/brush assembly on a conventional alternator is more than ample for the task, aided by the fact that only the long life brushes themselves need be replaced, normally as a cheap sub-assembly that can be replaced in a matter of minutes for very little cost.
Modern switching voltage regulator technology now allows the use of permanent magnet rotors which not only simplifies construction (no field coil, no slip rings) but also improves efficiency. However, when you've got several dozen or more horsepower to tap into, efficiency improvements are quite low on the car manufacturer's agenda so the traditional slipring energised rotating field coil alternator design is unlikely to be usurped for the sake of a mere quarter to one third of a horsepower's worth of savings.
When it comes to building your own wind turbine, permanent magnet alternators are king. Anyone seriously into such projects would never consider re-purposing a car alternator (or even the more efficient truck alternator with its 28v output) as the core of a wind turbine generator since the vampire drain, feeding the rotor field winding with 4 or 5 amps, seriously cuts into the low wind speed range of such a setup.
When it comes to modern suitcase sized portable inverter generators (typically in the 700 to 2KVA peak output range) they all use a 3 phase permanent magnet (PM) alternator to generate either a nominal 200 or 400 volts DC[1] via a 6 diode fullwave bridge rectifier feeding what is in essence, a bridged pair of class D amplifiers, typically using a sampling rate of 5KHz, to amplify a pure 60 or 50Hz sinewave reference signal to
120 or 230 volts ac (the so called inverter).The whole lot is microprocessor controlled so that the engine revs can be ramped up with load via a stepper motor controlling the carburettor?s butterfly valve (throttle). this takes care of increasing the torque output requirement to match the amperage demand and also to compensate the voltage drop from the PM alternator by increasing the revs.
Most of these inverter gensets have an eco-mode setting to not only reduce fuel consumption under very light loading conditions, but also to mitigate the noise pollution. However, even when eco-mode is disabled and the engine is running at a higher rpm, the revs will still increase with electrical loading.
In order to keep these small (1 to 2 KVA peak) inverter gensets down to a manageable size and weight, they typically use a small single cylinder
4 stroke 50cc engine running from just under 4000rpm up to around 4600rpm or so (classic 50Hz gensets in this power range run a 45 to 80cc single cylinder 2 or 4 stroke engine at a steady 3000rpm).You might think the higher rpms would create more of a noise nuisance but it's worth bearing in mind that the higher the noise frequency, the easier it is to soundproof as Honda with its eu series have demonstrated for many years now with their enclosed suitcase designs (sadly, their much cheaper imitators rather fall down on this aspect of enclosed suitcase inverter genset design - the recent Generac design excepted).
[1] When I was researching ways and means of quieting my Parkside inverter genset, I came across many forum postings which not only dealt with the noise pollution issue (very badly imo) but also included seeking advice on home built inverter generators using car alternators with a heavy duty 12v sine wave inverter, either using a repurposed lawn mower engine or else as a repair or an upgrade to an existing conventional genset.I rather pitied the (misguided) fools for even considering such an inefficient way to produce sine wave quality mains voltage power. The rectifier volt drops in the alternator alone represent at least a 12.5% loss of efficiency before you then have to deal with at least another 10% loss in even a very efficient 12 to 120/230v sine wave inverter. Ignoring the vampire load from the field current, you'd be looking at an electrical efficiency from the alternator ac output through to the final
230v 50Hz ac output of a mere 78% at best.There's an excellent reason as to why a PM 3 phase alternator output voltage of 200 or 400 volts was chosen by the inverter genset manufacturers. Firstly, it generates the required 170 or 350v peak voltage with a little something to spare for the 'inverter' to generate the mains voltage peaks of a 120/240vac sine wave, leaving the 'inverter' with merely the task of turning this source of DC voltage directly into a
50/60Hz sine wave of 240 or 120 rms volts using pulse width modulation switching (effectively a class D amplifier with an unusually low sampling rate, circa 5KHz for improved efficiency).The rectifier volt drops for even the lower 200 volt alternator case will now only represent a mere 1% loss (the 230/240 inverter genset's
400vdc alternator output reduces this to a 0.5% loss). The inverter losses are unlikely to exceed 2 or 3 % making such a genset a whole lot more efficient than even a simple basic single phase 230/120vac 50/60 Hz alternator driven directly at 3000/3600 rpm let alone a franken-genset comprised of car alternator and 12v to 120/230v inverter box.This, BTW, was a heads up for any of the more ambitious DIY enthusiasts monitoring this thread who might happen to be contemplating homebrewing their own inverter genset 'on the cheap'. :-)
Mmm. The Lord of the Dynamo?
of course that isn't true if you let the alternator produce more than 12v
NT
snip
That was very long but very very interesting. Full of facts.
Bill
Thanks for that, Bill.
Pretty well all of that info is available[1] over the internet (wikipedia is a good starting point for those interested in such technicalities[1]). Indeed, until I checked out 'my facts', I hadn't quite realised just how serious an issue there was in drawing power via the commutator/brush gear - at least half of each of those old Edison
100v dynamos you see archive pictures of are just to accommodate the several banks of commutators/brush sets required to draw their designed amperage. [1] Blimey! I managed to spell "technicalities" correctly first go, yet still managed to misspell "available". :-( :-)
If you're prepared to run the alternator fast enough and replace/modify the regulator, you could probably get away with letting it output 28 or maybe even 42 volts (assuming you limit the excitation voltage source to
14v to prevent burning out the field coil). The PIV rating of the rectifier pack is probably 200v or more which should leave some margin for voltage spikes.However, you might do better to start with a 28v truck alternator and double up to 56 volts to feed a 48v inverter module (perhaps one recovered from a defunct SmartUPS2000 or another 48v battery backed UPS). In this case, the excitation voltage source can be limited to the higher
28v max limit but there's no guarantee that the PIV rating will be any higher than that of a diode pack used in a 12v alternator - you might need to upgrade the rectifier pack to one with a three or four hundred volt rating. It is possible to test the PIV ratings of silicon diodes when you can't identify them in order to look it up on a datasheet.All this assumes a very keen polymath DIY enthusiast prepared to assemble such a franken-beast of a genset made up of cast off bits of
24/36/48 volt battery backed UPS inverters and cobble up a controller using an Arduino or RPi with a stepper motor bolted onto the carburettor body. Somehow or other, it just seems so much easier to simply[1] spend 99 quid on a Lidl Parkside PGI 1200 B2 inverter genset and have done with it. :-) [1] It's a relative term as I eventually discovered during my own genset purchasing saga just last month. The trouble with "Lidl Specials" like this is they may well have clocked a few thousand miles in the back of 38 tonners going up and down UK motorways before you even get your grubby mitts on one at your local store.Such treatment often leaves them with a couple easy to fix (once you know how) stock faults for the more nervous purchaser to return as DoA faulty goods in the small hope of getting working genset in exchange or else a full refund (small consolation for those who truly appreciate just what a bargain a *working* 1200W peak (30 seconds, not 5!) inverter genset is at this price).
All this is a direct result of Lidl's policy of never allowing such 'promotions' to last more than a week or two at each of their stores before the arrival of the "Next Promotion" forces the store manager to make room for the 'new line' and ship them off once more to stores in other UK regions. It seems the store itself is its own warehousing space. What's hidden, back of store, is just a small temporary holding pen for stock that's about to be sent on yet another motorway journey.
For all I know, Aldi might be using exactly the same strategy but I don't seem get to visit Aldi stores when such "generator specials" are being promoted. Indeed, it's been several years now since I last saw any type of genset on sale in an Aldi store (mind you, until recently, I could say the same of Lidl).
Indeed, the amount of 'interesting bargains' in electronic tech and genset rarities seem to have become vanishingly rarified over the past few years in both Aldi and Lidl stores. There was a time when I used to look forward to browsing the non-food sections whilst SWIMBI did the weekly shop but not for the past two or three years now. :-(
sure but making something on the cheap makes oh look, I have an alternator sat in the garage, let's use it.
today yes. Gennies used to cost far more. Your £99 job won't last long though.
ditto, I seldom bother now.
NT
I don't know about power station generators, but when I was involved in testing a couple of 3-phase, 60Hz, 11kV, 24MW generator sets for North-sea oil rigs, they consisted of an RB211, exhausing into a DR-61 turbine and direct driving (according to the manufacturer's nameplate) a "Brush AC Generator".
SteveW
Which is a lot easier than rubbing a cat on a piece of amber :)
SteveW
Oooh an expert :)
I always assumed that in those kind of tasks they just used the turbine that normally drives the fan (which I assume isn't there!) Why not? Doesn't it extract enough power?
Andy
Not much of one.
That's basically it.
From memory (this was nearly 25 years ago), an RB211 (for aircraft) actually has three compressor stages (low, medium and high pressure), on three co-axial shafts, driven by three turbines. The turbines are sized to power the appropriate compressor stage and you get a lot of hot, fast moving exhaust. The reaction to the exhaust escaping backwards is to push the engine forwards.
An RB211 (for industrial use) only has two compressor stages (medium and high pressure), on two co-axial shafts, driven by two turbines - probably because the air at ground level is dense and the first stage is redundant. Again, the turbines are sized to power the appropriate compressor stage and you get a lot of hot, fast moving exhaust. In this use, it is actually termed a gas generator (or was by the company I worked for) and you don't want the GG to move. Instead it is bolted down and the exhaust is directed into a power-turbine, which is driven by the gas, before exhausting to a heat recovery plant. The power-turbine drives whatever load you have attached.
There is no direct connection between the load and the GG. It is impressive how well the control system (Woodward governor on those sets) ramped up the fuel control to bring the power turbine up to running speed with only a tiny overshoot and then settled to constant speed. Although slamming a 10MW motor online REALLY made the revs/frequency drop for a short while!
It amused me at the time that the Rolls-Royce manuals referred to allowing the engine to cool before turning it off, by "taxiing for 3 minutes."
SteveW
A bog standard open cycle gas turbine then..
Indeed. Nothing special.
SteveW
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