On Friday, 18 May 2018 22:45:49 UTC+1, Johnny B Good wrote:
On Fri, 18 May 2018 14:03:24 +0100, Dave Plowman (News) wrote:

In article ,
Max Demian wrote:
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.

They are both generators - but the definition of a dynamo is a generator
which produces DC by use of a commutator.

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 carburettors
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'. :-)

of course that isn't true if you let the alternator produce more than 12v


NT