On Saturday, November 16, 2013 10:12:00 PM UTC-5, Ignoramus26083 wrote:
As an engineering student I was on a tour of one of the Roseton NY power plants.
Got to look through a small window at the astonishingly small alternator at the end of the turbine shaft.
Stuck onto the end of the alternator shaft was an even smaller excitation generator.
It is possible to a asynchronous generator/motor auto-excite utilizing the
same VA rating of generation with capacitors. but the voltage and
frequency is hard to control this way.
with no eletrical grid it is better to utilize a permanent magnetic
generator. with a electronic inverter or an generator/motor with rotor
field coil, with the last it is possible to lock the generator speed same
to mains frequency, and control the voltage regulating the field current,
very like the car alternator.
but to close circuity with the mains will be necessary to sync the phase.
the permanent magnet has better efficiency.
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On Sat, 16 Nov 2013 20:35:37 -0600, Ignoramus26083
Similar to what you're talking about, see these 12v systems.
Look at some of the hand powered generators on YouTube, Ig. Take an
old automotive alternator and create backup power from it.
http://www.prestowind.com/ American made systems. Good pricing.
Perfect to back up a solar/battery power system.
We are always the same age inside.
-- Gertrude Stein
It's been 40 years, so I can't give you any details...but I
remember something about the optimum angle between the field and the
armature being different whether a device is used as a motor
or a generator.
There's a guy on the Colorado River that is many miles from the
grid. But, he has guaranteed water rights to some insane number
of cubic feet / second of the river included in his deed.
So, he built a micro hydro system. He used a modest 3 phase
induction motor, banks of caps to ensure excitation, and a
huge tapped resistor grid to regulate the thing. I think there
was a flyball governor that controlled RPM by throttling the
water flow. He apparently had this running for at least a decade
when I read about it in the Mother Earth News back in the '70s.
Apparently his regulation was so good electric clocks ran accurately,
and only needed resetting every few weeks. he sold excess power
to his neighbor. Since the electricity was free, he used it for
EVERYTHING, electric resistive heating, etc.
There are neighborhood micro hydro systems in various off-grid
areas of the US that have been running for years, although they
may not be quite as sophisticated as the Colorado river one.
I think the deal was that the power output had to be constant.
So, if he wasn't using much power in the middle of the night, the
resistors had to absorb the maximum power he would use during the
day. There are probably more intelligent (efficient) ways to
regulate it. But, his energy input was free and constant, so
he didn't care.
"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote:
Better answer, depends on the motor design.
Technically any motor can act as a generator, but efficiency
will likely not be optimal.
Locomotives use their traction motors as generators to energize
dynamic braking systems for example. If the motors and
resistors were 100% efficient, they could stop a train without
using brakes, but that isn't the case. They do generate quite a
bit of energy though, which is dissipated as heat.
As you say above, something must excite the fields.
On Sun, 17 Nov 2013 11:18:53 +0100, depucca wrote:
Pretty much any standard 3 phase induction motor can be used as a
generator, provided that it has a load with a leading power factor
(normally the controller will connect as much capacitance as required for
this) or is paralleled with the utility line and driven above synchronous
speed. Control is a bit tricky when not line connected, but a search on
induction motor generator should lead you to information on how to do
it. A wound rotor or permanent magnets are not required, unless you want
to be able to generate power without using a complex controller.
You can find a fairly complete mathematical analysis in the chapter on
induction generators in "Principles of Alternating-Current Machinery" by
Lawrence, 4th edition, 1953.
Synchronous AC motors work well as generators- and are often used that
way in industry-having the advantage of var control. Their excitation
comes from the DC field.
Induction motors also can be used as motors-however an external
excitation source is needed- preferably the power grid. In Some places
(e.g Scotland), in remote areas, small streams drive induction
generators- once up to speed, they are connected to the grid and feed
power into the grid (drawing reactive from the grid). Capacitors can be
used once running and the grid connection could be removed- but voltage
stability is a problem.
An extreme case that I know of was where a farmer with a single phase
supply had a large induction motor which he connected mechanically to a
smaller single phase motor and excited one phase with the result that
he could generate 3 phase which he used to drive another 3 phase motor.
It worked well and was cheap because the polyphase motors were surplus
units. Sure the energy all came from the single phase supply but it was
a lot less expensive than getting 3 phase in a remote area or buying a
large single phase machine (and not a great deal less efficient).
Nowadays there are electronic drives that do the job better for less
money but then, many weird and wonderful arrangements and machines were
Nightcrawler's reference is quite dated but includes material not
generally taught these days (less emphasis, except in machine design
texts, on some factors, and circle diagrams went out when or before
electronic calculators came in, and transient response was generally
ignored). It brings back some memories as much of the same was done in
the early 50's. Approaches in texts after the 50's have changed
considerably- emphasizing that essentially a motor and a generator are
the same except for the direction of the energy flow and taking into
account the various advances in control and power electronics.
Induction motors - that are designed to be motors - make lousy generators,
regardless of how well 'tuned' their capacitive excitations circuits are,
if they're used for any varying loads, or - for the most part - on any
They're about useless for anything but purely resistive loads like
incandescent lights or resistive-limited battery charging.
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