Maker Pro
Maker Pro

Motor / Generator

In "Motor/Generator Analysis Redux" I wrote, but nobody replied to:

On 10 Jun 2005 15:00:48 -0700, [email protected] wrote:




















I've picked up this thread late so I've probably missed important
bits. However the following comments may be useful.

If I've understood the post correctly you are aiming to use an
impedance protected 36 pole motor as a self excited induction
generator.

Self excited induction generators rely on the tiny residual
pattern of magnetisation of the rotor being reinforced by the current
flowing in the near resonant stator winding circuit. It has to be
operating close to resonance for the current build up to be large
enough to reinforce the rotor field pattern. It has to be on the
capacitative side of resonance to permit the phase angle of the stator
current to reinforce the rotor field pattern.

It is a positive feedback regenerative system and on a large
efficient motor the output can build up to far beyond its rated motor
power until limited by magnetic saturation. This effect is sometimes
used for regenerative braking of single and three phase motors and can
result in a spectacularly short stopping time.

With a care and control of speed, self excited induction
generator systems are possible but they're pretty touchy devices.
If you're unlucky with the the rotor iron they may not retain enough
initial magnetism to enable the output to build up (manufacturers
strive to reduce this because it degrades the efficiency when used as
a motor) Also it must use a reasonably efficient motor for the
magnetic feedback to exceed the system losses.

Efficiency is your major problem. An impedance protected motor
means a motor with deliberately large leakage inductance so that the
impedance of this inductance limits the current that flows when the
motor is stalled or overloaded. With limited stalled current the
starting torque (already poor because it is a capacitor run machine)
has to be boosted by the use of a high resistance rotor and this
results in your observed very high slip speed. Even if there were no
other losses of any kind the motor efficiency could not be any better
than the % synchronous speed - 56%. With other losses taken into
account the motor efficiency is probably no better than 40%.

With the uH to mH correction your sums are OK but this level
of efficiency is too low for a succesful induction generator.

Jim



Many thanks to Jim and other contributors to the thread
"Motor/Generator Analysis".

I have put a lot of money and time into this, and I want to give it my
best shot, but I don't want to whip a dead horse, so to say.


Frankly, I don't understand magnetics. At least not as I understand
resonance. I'm an amateur musician; I understand resonance and know a
little about phase shifts near the peak. I do understand that because
the slope of the curve is negative on the high-frequency (capacitative)

side of resonance, loading of the generator, within limits, will result

in additional power to meet the load.


But B x I makes my head spin. I'm fine in 3 dimensions. So I get some
of it. And I get that in the cylindrical coordinate system, B and I can

be locally orthogonal, and can vary in time, with phase shifts, while
being wrapped into a connected topology. I just don't feel that the way

I feel resonances. It's not intuitive.


Would replacing the rotor "windings" with copper wire or bus bar
(easy), and rewinding the stator with bigger wire (hard) have any
chance at all of working together by lowering the leakage inductance
and rotor resistance to allow resonance?


That's my best question; is there any hope at all?


This is a one-off demo, not a production prototype!


Yours,


Doug Goncz
Replikon Research
Falls Church, VA 22044-0394
 
R

Rich Grise

Jan 1, 1970
0
In "Motor/Generator Analysis Redux" I wrote, but nobody replied to: [some stuff]
That's my best question; is there any hope at all?
...
This is a one-off demo, not a production prototype!

Yes, I think there's hope, but first you must do some experiments
and some study of basic electronics, I think.

If you are unable to interpret the results of your experiments
so far, then all that that means is that you need to do some more
study on basic electronics.

Google is your friend here. Try various keywords - find out what
shows up!

Good Luck!
Rich
 
In "Motor/Generator Analysis Redux" I wrote, but nobody replied to:
SNIP
I have put a lot of money and time into this, and I want to give it my
best shot, but I don't want to whip a dead horse, so to say.


Frankly, I don't understand magnetics. At least not as I understand
resonance. I'm an amateur musician; I understand resonance and know a
little about phase shifts near the peak. I do understand that because
the slope of the curve is negative on the high-frequency (capacitative)

side of resonance, loading of the generator, within limits, will result

in additional power to meet the load.


But B x I makes my head spin. I'm fine in 3 dimensions. So I get some
of it. And I get that in the cylindrical coordinate system, B and I can

be locally orthogonal, and can vary in time, with phase shifts, while
being wrapped into a connected topology. I just don't feel that the way

I feel resonances. It's not intuitive.


Would replacing the rotor "windings" with copper wire or bus bar
(easy), and rewinding the stator with bigger wire (hard) have any
chance at all of working together by lowering the leakage inductance
and rotor resistance to allow resonance?


That's my best question; is there any hope at all?


This is a one-off demo, not a production prototype!


Yours,


Doug Goncz
Replikon Research
Falls Church, VA 22044-0394

You've been wading in pretty deep water - although, at first
sight a self excited squirrel cage alternator sounds like a simple
device they're a positive feedback system. They're so touchy that
they're pretty well only used by dedicated amateurs - I don't know of
a single commercial application.

Note - this refers only to SELF EXCITED alternators. The same
machine driven at over synchronous speed and connected to a power grid
works fine as an induction alternator and will feed back to the power
grid the equivalent of it's mechanical input power less its generation
losses. This works well in commercial wind power sytems.

I really don't think you'll have much luck unless you move to a
decent size 4 pole machine and even then I doubt that you'd be very
happy with the results. A small permanent magnet DC motor is soooo
much easier and cheaper!

I'm sorry if this is a bit disappointing but it's all useful
experience and, sadly, experience is usually gained the hard way. You
picked a very tough one for yor first project - better luck next time.

Jim
 
R

Rich Grise

Jan 1, 1970
0
Note - this refers only to SELF EXCITED alternators. The same
machine driven at over synchronous speed and connected to a power grid
works fine as an induction alternator and will feed back to the power
grid the equivalent of it's mechanical input power less its generation
losses. This works well in commercial wind power sytems.

I just had a thought - I'm not that conversant with induction
alternators, but I've heard the term "slip" - which, driving
the shaft mechanically, faster than the synchronous speed, will
generate power - now, my quesion is, if, say, you're on a
windmill, the amount of slip will increase as the RPM increases,
right? Is there some kind of formula or graph - I'd think that
if the "slip" gets up to, like, 90 or 120 degrees, that your
generator efficiency would go back down somewhere, or am I
letting the drugs interfere with my common sense?

Thanks,
Rich
 
J

JohnM

Jan 1, 1970
0
You've been wading in pretty deep water - although, at first
sight a self excited squirrel cage alternator sounds like a simple
device they're a positive feedback system. They're so touchy that
they're pretty well only used by dedicated amateurs - I don't know of
a single commercial application.

Note - this refers only to SELF EXCITED alternators. The same
machine driven at over synchronous speed and connected to a power grid
works fine as an induction alternator and will feed back to the power
grid the equivalent of it's mechanical input power less its generation
losses. This works well in commercial wind power sytems.

That's one I've been wondering about. I thought it should be as you say
but had not come across anything to confirm it.

You really put out some good information concerning electricty- many
thanks for it.

John
 
T

Tony Williams

Jan 1, 1970
0
Rich Grise said:
I just had a thought - I'm not that conversant with induction
alternators, but I've heard the term "slip" - which, driving
the shaft mechanically, faster than the synchronous speed, will
generate power - now, my quesion is, if, say, you're on a
windmill, the amount of slip will increase as the RPM increases,
right? Is there some kind of formula or graph - I'd think that
if the "slip" gets up to, like, 90 or 120 degrees, that your
generator efficiency would go back down somewhere, or am I
letting the drugs interfere with my common sense?

Speed= 0 Speed= Sync Speed= 2xSync
Slip = 1 Slip = 0 Slip = -1
| | |
| |
+Torque | |
| _ \|/ |
| / \ | |
| / \ | |
|_____/ \ | |
|____________\|_____________|
| \ ______
| |\ /
| | \ /
| | \_/
-Torque

/|\ /|\ /|\
|---Motoring--|--Generating-|

Plot a normal Torque-Speed curve from 0 to Sync Speed.

If the shaft is then driven faster than Sync then the
Torque-Speed is a mirror image of the first quadrant.
 
G

Glen Walpert

Jan 1, 1970
0
Speed= 0 Speed= Sync Speed= 2xSync
Slip = 1 Slip = 0 Slip = -1
| | |
| |
+Torque | |
| _ \|/ |
| / \ | |
| / \ | |
|_____/ \ | |
|____________\|_____________|
| \ ______
| |\ /
| | \ /
| | \_/
-Torque

/|\ /|\ /|\
|---Motoring--|--Generating-|

Plot a normal Torque-Speed curve from 0 to Sync Speed.

If the shaft is then driven faster than Sync then the
Torque-Speed is a mirror image of the first quadrant.

That applies to grid-connected induction machines where grid frequency
and number of poles in the machine determine the sync speed.

In self excited induction generators which are not grid connected then
sync speed is not constant, it will vary with shaft speed.

Induction machines (motors or generators) can only operate with a
lagging power factor (required to produce rotor field), typically
about 0.8 at full load. The connected "load" must therefore have a
leading power factor to match. In the case of grid connection
generators and distributed power factor correction capacitors provide
the required leading power factor. In the case of a self excited
induction generator enough capacitance must be connected to insure the
required leading power factor.

The required capacitance varies depending on shaft speed and
electrical load. There are large commercial windmill generators out
there which use proprietary controllers to optimize load power factor
for maximum output power at any wind speed within range, rectify the
output to a DC bus and invert to grid frequency. (There was an
article on wind power in Mechanical Engineering magazine a few years
ago which discussed these but I can't find it now).

While the mfgr's are not disclosing any details, I am fairly sure they
are using a PFC type rectifier with the current control set point not
exactly tracking voltage but leading it by an adjustable amount.
Should be reasonable to adapt this strategy to a small battery
charging windmill induction generator, at least for someone with a
solid understanding of PFC methods. I would be inclined to start with
a DSP implementation of a PFC controller like the one that TI provides
sample code for as part of a UPS reference design, but other
approaches are possible.

For the hobbiest who wants to experiment with induction generators
without learning DSP PFC methods, simply connecting varying values of
power factor correction capacitors should provide a means of adjusting
output with changing speed, at least with a resistive load. Not sure
how well this will work with a three phase diode rectifier battery
charger, but it should work well if a PFC battery charger is used.

Sorry if the above is redundant, I missed most of this thread.

Glen
 
Glen said:
For the hobbiest who wants to experiment with induction generators
without learning DSP PFC methods, simply connecting varying values of
power factor correction capacitors should provide a means of adjusting
output with changing speed, at least with a resistive load. Not sure
how well this will work with a three phase diode rectifier battery
charger, but it should work well if a PFC battery charger is used.

Sorry if the above is redundant, I missed most of this thread.

Glen

I've been trying this without success. It's kind of like shooting a
rifle in the dark at a squirrel whose position you don't know.

I'm planning on building a "universal" self-excited induction generator
(SEIG) workstation. It will have an SPDT switch to put the motor to be
tested across the line, or across a *monster* cap sub box in parallel
with a GC Electronics 20-102 cap sub box I picked up on ebay for twenty
bucks. GC's 20-102 design uses caps rated at 200 WVDC, enough for some
AC experiments. If I were to hit resonance, and hold it by driving the
motor at sync speed, they could blow, but (1) I probably won't hit
resonance exactly, and (2) the motor will blow its load of flywheeling
kinetic energy before it blows a cap (I hope). Caps do take a bit of
surge.

There will be a place for an LCR meter, one I have or a better one in
the future, and a test load, which can damp generator operation, as
well as a ground, and leads to the 'scope and a voltmeter. I am not
familiar enough with the literature to say this workstation is novel,
but it may be. I certainly have never seen one in any lab. I intend to
try the workstation first with my drill press / circular saw motor
(remember the Crapmaster?), then Burden's motor.

My design load for the motor I am working with is *only* 8 watts. It's
a white LED traffic signal used in rail and dockyard work, I believe,
donated by John Viselli at Dialight (yes, John, I am still working on
this) in exchange for a tip off on a likely stolen traffic light I
bought on ebay, then had doubts about, and out of general high tech
good will on John's part as well.

Now, this is a high tech load and should not damp generator operation,
as it is rated 80-135 VAC unknown Hz, probably 50-60, maybe a wider
range, and I expect, although I have not tried it with a Variac, that
it will go off line while the generator output is building from about 1
VAC, which is what remanant magnetism will provide, through a few or
dozens of cycles of resonance to some value above 80 VAC, at which
point it should come on line and make use of the generated power.

And that is how the project looks today. A couple bits of perfboard
15x12 inches, a few supports about 9 inches long, and the whole thing
will fit in an R-Kive box when I am not using it. The extended cap box
will be 5-50 uf and 50-300 uf for 350 total, and the CG box is 11.111
uf for 361.1 uf or so.

Doug
 
Hey, gang.

On 22 Jun 2005 13:46:10 -0700, [email protected] wrote:
You've been wading in pretty deep water - although, at first
sight a self excited squirrel cage alternator sounds like a simple
device they're a positive feedback system. They're so touchy that
they're pretty well only used by dedicated amateurs - I don't know of
a single commercial application.

Note - this refers only to SELF EXCITED alternators. The same
machine driven at over synchronous speed and connected to a power grid
works fine as an induction alternator and will feed back to the power
grid the equivalent of it's mechanical input power less its generation
losses. This works well in commercial wind power sytems.

I've got a medium sized DC permag motor and not one but two inverters.

Is there a load-sharing or four quadrant inverter I could add to my
collection to experiment with what I do already recognize as much
simpler synchronous, non self-excited operation?

I'd drive the generator with the rear wheel as detailed at
ftp://users.aol.com/DGoncz/Bicycle and in other posts, and invert the
output as I did on January 1, 2005, to the hilarity of my neighbors,
using the sine wave inverter this time, drive the motor on one phase,
and collect power from the other, or, with a load-sharing or four
quadrant inverter, put the ultracapacitors back on line between the
generator and inverter, and use both phases of the motor as a
synchronous generator.
I really don't think you'll have much luck unless you move to a
decent size 4 pole machine and even then I doubt that you'd be very
happy with the results. A small permanent magnet DC motor is soooo
much easier and cheaper!

Been there, done that. See ftp://users.aol.com/DGoncz for the video.

I want, in all this work, to *feel what it's like* in my muscles. I
just have this incredible appetite to gage levels of power from 1 -
1000 watts as muscle power, in convoluted ways. I want to feel the
positive feedback "kick in" when I pedal my generator. That's what I
live for, and for the occasional stunt. It's like having an appetite
for kinky sex, not that I would know what that is like.
I'm sorry if this is a bit disappointing but it's all useful
experience and, sadly, experience is usually gained the hard way. You
picked a very tough one for yor first project - better luck next time.

Jim

No, it's OK.

If I replace the grease in my generator's bearings with light oil, it
will rotate even more easily. This brings to mind the opportunity for
winter operation. That brings to mind room temperature superconductors,
which we don't have yet, but also the thermal coefficient of electrical
resistivity, which is a known property. It may be at some low
temperature, with the right bearing lube, R may indeed by less than
sqrt(L*C), allowing resonant operation.

How low do I have to go?

That is, do I need a Dewar of Freon, Nitrogen, Helium, or Hydrogen to
lower the coefficient of electrical resistivty to the point resonance
is possible? Dr. Majewski would certainly support such an effort.

I'll go check now....

Doug
 
Copy of letter to Dr. Majewski:

Dear Dr. Majewski:

I am going to send this email to Dr. Brandt at FSU.

Does it look OK?

Are you alright with the way I've stated our association?

There is a problem with the math. I get some very low sub-ohm figure
for what R *should* be. Like 0.008 ohms. That doesn't hold out much
hope against the possiblity of cryogenic generator operation, does it?
I think not, but maybe I am doing something wrong.

Shall I sign up for PHY 298?


Doug

Dear Dr. Brandt:

I have been developing a small, slow self-excited induction generator
with Dr. Walerian Majewski at NVCC, nearby.

The problem I am up against now is that the stator DC resistance, R is
still greater than the square root of the product of the winding
inductance, L, and the resonant capacitor C. Units are ohms, henries,
and farads. Values are 23 ohms, 136 millihenries, and 51 microfarads.

What I'd like to do is carefully dip a sample stator in whatever
cryogens you have available in approximately five liter quantities, and
measure the stator DC resistance at these boiling cryogen temperatures,
to determine whether R might in fact be less than sqrt(L*C) at some
temperature. If a "hit" is found, research can be conducted for a low
temperature bearing lubricant, and a live test made, as the machine has
only one moving part, plus two ball bearings.

Yours,

Doug Goncz
Replikon Research
Seven Corners, VA 22044-0394
Student member SAE
[email protected]
 
G

Glen Walpert

Jan 1, 1970
0
I'm planning on building a "universal" self-excited induction generator
(SEIG) workstation. It will have an SPDT switch to put the motor to be
tested across the line, or across a *monster* cap sub box in parallel
with a GC Electronics 20-102 cap sub box I picked up on ebay for twenty
bucks. GC's 20-102 design uses caps rated at 200 WVDC, enough for some
AC experiments. If I were to hit resonance, and hold it by driving the
motor at sync speed, they could blow, but (1) I probably won't hit
resonance exactly, and (2) the motor will blow its load of flywheeling
kinetic energy before it blows a cap (I hope). Caps do take a bit of
surge.

I trust you are using a three phase motor for your experiments - I
have never heard of a sucessful single phase induction generator, and
strongly suspect that it is either completely impossible or requires
separate control of the start and/or phase-shifted run windings.

Suggest you study induction machine theory a bit before diving into
cryogenic experiments and the like. "Principles of Alternating
Current Machinery" by Ralph R. Lawrence, 4th Edition, 1953, is a
classic on the subject and often available inexpensively from the
usual used booksellers. This book discusses the theory of induction
generators in detail, something rarely found in more recent texts.

Glen
 
Copy of letter to Dr. Brandt:

Dear Dr. Brandt:

I have been developing a small, slow self-excited induction generator
(SEIG) with Dr. Walerian Majewski at NVCC, nearby.

The problem I am up against now is that the stator DC resistance, R is
still greater than the square root of the product of the winding
inductance, L, and the resonant capacitor C. Units are ohms, henries,
and farads. Values are 23 ohms, 136 millihenries, and 51 microfarads.
Capacitance to 361 microfards will be available soon.

What I'd like to do is carefully dip a sample stator in whatever
cryogens you have available in approximately five liter quantities, and
measure the stator DC resistance at these boiling cryogen temperatures,
to determine whether R might in fact be less than sqrt(L*C) at some
temperature. If a "hit" is found, research can be conducted for a low
temperature bearing lubricant, and a live test made, as the machine has
only one moving part, plus two ball bearings.

Frankly, I doubt this will happen, but room temperature superconductors
are only an era away, and I could certainly present the resistance
measurements at the SPS spring conference as an "experiment that
failed, but produced results." :(

As far as I know, the literature contains temperature coefficents of
resistance for small pieces of wire, not whole stators, although your
lab has certainly got some big magnets, and I am sure you've
characterized them thoroughly. There are solder joints in this stator,
for example....

Yours,

Doug Goncz
Replikon Research
Seven Corners, VA 22044-0394
Student member SAE
[email protected]
 
B

bz

Jan 1, 1970
0
[email protected] wrote in @f14g2000cwb.googlegroups.com:
There are solder joints in this stator,
for example....

Here is a free idea for you:

Google on 'silver clay'. It fires to produce .999 fine silver.

Develop a way to extrude the wet silver clay, clad in a layer of insulating
ceramic slip.

Extrude your conductors and form your windings while the materials are
still flexable.

Make the end connections with silver clay slip.
Fire the unit. [silver clay fires at a moderate red heat.]

You should have a low resistance set of windings.



--
bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

[email protected] remove ch100-5 to avoid spam trap
 
G

Glen Walpert

Jan 1, 1970
0
Thanks, Glen, that's an excellent reference, and I have ordered one
Lawrence for $18, book rate, to my PO Box.

A bargain, I think you will like it. BTW, induction generators were
discussed in the first edition of Lawrence, copyright 1916.
I suggest

http://www.qsl.net/ns8o/Induction_Generator.html

for single phase induction generator practice, and I have no source for
the theory.

Doug

I took a look at the single phase induction generator info. Learn
something new every day. The performance your source reports is
fairly dismal however, starting a motor only 1/10 the nameplate
rating, and providing significant output only very close to rated
frequency and only into a well matched load, all no doubt due to the
need to match the requirements of whatever phase shifting mechanism
the motor uses to obtain a rotating field from single phase power.

The performance of 3-phase induction generators is sooooo much better,
and used 3-phase motors are so cheap, that it only makes sense to use
a single phase induction generator if you are going to run it from an
engine with speed regulation, and connect only well matched loads. If
you need to match widely varying speeds and powers such as a windmill
generator, then 3-phase is clearly the only viable option, as it can
operate well over a very wide range of speeds and loads.

Glen
 
Dear Jim,

You've been wading in pretty deep water - although, at first
sight a self excited squirrel cage alternator sounds like a simple
device they're a positive feedback system. They're so touchy that
they're pretty well only used by dedicated amateurs - I don't know of
a single commercial application.

A positive feedback system would be unstable.

I do understand that the initial remnant magnetization is fed back to
produce more magentization, but since operation is on the capacitive
side of the Bode plot, I see negative feedback as the primary
characteristic.

That is, loading the generator incrementally produces reduced speed,
producing more output. This is negative feedback, stable
characteristic.

I would be very interested in hearing from you an explanation of which
aspects of the system are positive-feedback in nature, and which are
negative. Anyone else, also, is welcome to comment on this.

Doug
 
Dear Jim,



A positive feedback system would be unstable.

I do understand that the initial remnant magnetization is fed back to
produce more magentization, but since operation is on the capacitive
side of the Bode plot, I see negative feedback as the primary
characteristic.

That is, loading the generator incrementally produces reduced speed,
producing more output. This is negative feedback, stable
characteristic.

I would be very interested in hearing from you an explanation of which
aspects of the system are positive-feedback in nature, and which are
negative. Anyone else, also, is welcome to comment on this.

Doug


It is a positive feedback sytem and it is unstable. With a
sufficiently high loaded Q, feedback builds up the small residual
magnetic pattern in the rotor to produce ever increasing output.

This suicidal tendency is only controlled by both shaft available
input power (i.e. the speed drops) and the rising iron losses which
drop the loaded Q and modify the feedback phase angle.

Jim
 
Top