More generator Q's

Awl --
So I'm amassing a collection of perm. mag. DC motors for my various (de)generative follies, but a friend said he took apart a gas powered generator, and observed no magnets, with both stator and rotor being wound -- suggesting that AC induction motors should provide juice, but mine don't.
Was my friend wrong, or can wound rotors/stators yield juice, and if so, under what conditions?
How are back-up generators generally wound, as well as prime generators, such as coal, hydro, etc?
Will a typical 3 ph motor throw out juice, if driven by a pony motor?
Any primers on this stuff?

Your friend was wrong. Motors need a magnetic field, whether permanent magnet or electromagnet to produce voltage.
Three phase motors are typically induction types and produce the second field of magnetism by induction from the wound fields. Dynamic braking can use the collapsing field while they are turning for a only short time to generate power but then the field is gone.
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"Existential Angst" wrote in message
Awl --
So I'm amassing a collection of perm. mag. DC motors for my various (de)generative follies, but a friend said he took apart a gas powered generator, and observed no magnets, with both stator and rotor being wound -- suggesting that AC induction motors should provide juice, but mine don't.
Was my friend wrong, or can wound rotors/stators yield juice, and if so, under what conditions?
How are back-up generators generally wound, as well as prime generators, such as coal, hydro, etc?
Will a typical 3 ph motor throw out juice, if driven by a pony motor?
Any primers on this stuff?

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Many of the smallish generators (5KW to 50KW) in the Army had wound rotors. They worked by bootstrapping: remnant magnetism in the iron generated a small field current, which increased the generated voltage, increasing the field current.... until the output voltage or field current reached a threshold where some means of regulation engaged. Occasionally a generator would fail to generate because there wasn't enough remnant magnetism to get it going. The solution was to "pole" the field by applying DC to it to re-magnetize it.
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In a way. It must be excited by the proper 3phase AC voltage, but if the pony motor then spins the 3phase motor above synchronous speed, the direction of current flow will be such as to deliver power to the power line. An electric meter on that line would run backwards.

Not completely true. Only reactive power (RVA) will flow back into the line and no "real" power will flow into the line,except for losses due to heat in the "consumption" direction.
The kW and kWh meter will not indicate backwards in this situation. The kVAR and kVARh meters will indicate backward. This is common with elevators that use their own motors for braking on descent.
This is nicknamed "rotating capacitor" by some electric utilities and used to correct power factor on lines and systems.
----------------------- "Don Foreman" wrote in message
In a way. It must be excited by the proper 3phase AC voltage, but if the pony motor then spins the 3phase motor above synchronous speed, the direction of current flow will be such as to deliver power to the power line. An electric meter on that line would run backwards.
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Don't give up your day job, Josepi.

On 03/09/2011 07:42 AM, Josepi wrote:
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Nope, it is called an induction alternator, and definitely can generate real power, as opposed to reactive. They are often used in small hydropwer systems, and use the grid as backup and for regulation. However, there ARE some people who have rigged these up as prime alternators, and use various schemes to achieve regulation of both voltage and frequency.
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There used to be large motors used for phase angle correction in large factories, etc. They had a wound field, and the excitation was controlled by a phase angle meter, to correct the power factor drawn from the utility grid. They got a big break on their electric bill for using such a device. Often called a "rotary condenser".
Jon

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Those are over-excited synchronous motors.

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Not true, or only a little so. A 3 Ph induction motor WILL output power to the grid, WHEN RUN OVER SYNCHRONOUS SPEED. THis is NOT a rotating capacitor! It will still be "consuming" reactive power (vars) just like it was when running as a motor. This is an induction generator. THey are commonly found in wind turbines, and pumped storage systems.
A rotating condenser (capacitor) is a SYNCHRONOUS motor that is excited past unity power factor on it's field winding. [i.e. more excitation than needed when running as a motor] it is rotating at exactly synchronous speed. Attempt to rotate it faster (by sticking a primer mover on it) and you have a generator. Put a load on the shaft and you have a motor.
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jk

Do you have any links for information along those line? . I never worked on "rotating capacitors" for power factor correction usage and would find that interesting . Mostly I worked with smaller motors under 5HP, transformers and transmission lines and communication stuff. The few PH correction capacitors we had were static units under 60 MVARs and I would think the protection circuits would be simpler.
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"jk" wrote in message wrote: This is nicknamed "rotating capacitor" by some electric utilities and used to correct power factor on lines and systems.

On Tue, 08 Mar 2011 23:42:44 -0600, Don Foreman
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For UK residents but may apply elsewhere. The old analogue electric power meters happily spun forwards or backwards to read power consumed or power fed back into the supply. In the UK the supply authorities are now replacing them with digital meters that are programmed NOT to register negative power. Power fed back into the supply will be happily accepted by the supply authority but will not reduce the meter reading.
Not sure why they are programmed this way but the effect with many Solar PV tariffs is to reduce the claimed saving In the nett electricity bill. Power that is registered outside daylight hours cannot be reduced or cancelled by daylight solar energy.
Jim

I haven't ran into that situation here. I doubt it would be approved...yet...until somebody thinks of it...shhhhhhh.
It would make sense as they do not want your extra energy in off peak hours. They are try to give the stuff away, already.
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wrote in message For UK residents but may apply elsewhere. The old analogue electric power meters happily spun forwards or backwards to read power consumed or power fed back into the supply. In the UK the supply authorities are now replacing them with digital meters that are programmed NOT to register negative power. Power fed back into the supply will be happily accepted by the supply authority but will not reduce the meter reading.
Not sure why they are programmed this way but the effect with many Solar PV tariffs is to reduce the claimed saving In the nett electricity bill. Power that is registered outside daylight hours cannot be reduced or cancelled by daylight solar energy.
Jim

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A motor that has both a wound rotor and a wound stator is not, as far as I know, an AC induction motor. Induction motors have a rotor that's almost always made up of very short conductors.
Old DC motors often had wound rotors and stators. Until powerful permanent magnet came along, that's the way all but the smallest DC motors were made. More than magnet strength, the issue was magnet permanence. All-wound DC motors generally were of one of three types: series wound, parallel wound, or series parallel combinations.
That's also the way many DC generators were made. I had an old WWII dynamo set from a radio Jeep that was made that way. It supplied wire for many of my ham radio coils back in the '60s.
Used as generators, they need a way to provide initial excitement to at least one of the coils. There were various setups for this, including relatively weak magnetized armatures, start batteries, and so on. Once they were generating current they typically were designed to be self-exciting.

Ed Huntress wrote:
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Via parallel capacitors? http://www.qsl.net/ns8o/Induction_Generator.html
--Winston

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That's a different situation, Winnie. Those are being used to provide initial excitement to *induction* motors used as generators. That's a tricky thing, and I didn't answer EA's question about three-phase induction motors because it's something with which I have no experience.
But it may well be used to excite some all-wound DC generators, too. 'Don't know. They were mostly before my time.

Ed Huntress wrote:
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Three phase induction? This looks promising: http://ronja.twibright.com/exciter /
Though the most I could ever accomplish was 70 W for half an hour a day, so I don't expect to run my house from one of these.
(That parallel FET Q1 cannot be the best possible way to regulate output power!) Yeesh.
--Winnie

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http://www.frenchriverland.com/Motors_As_Generators_N_Smith_UK_1994.pdf

PrecisionmachinisT wrote:
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Perused and copied.
Thanks PM!
--Winston

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"Requires a massive heat sink". Yes, I'll bet it does! Lake Superior comes to mind...

Don Foreman wrote:
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Heh! :)
--Winston

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That's actually not bad. How did you determine that it was 70 W?