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?

Reply to
Existential Angst
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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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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?

Reply to
Josepi

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.

Reply to
Ed Huntress

(...)

Via parallel capacitors?

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--Winston

Reply to
Winston

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.

Reply to
Ed Huntress

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.

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.

Reply to
Don Foreman

Generally speaking, older, bigger rotating machines have field windings that need DC excitation. You can regulate the voltage in a generator by regulating the current in the field winding. It's exactly the same idea as a car alternator.

Either with field windings, a motor designed to turn at exactly 1800 (or

3600) RPM to match line frequency, and a regulator connected to the field windings. More recent ones have inverters, and probably permanent generators AC generators whose output gets rectified then inverted.

As above, the bigger and older it is, the more likely it is to be a wound- field synchronous AC machine.

Well, yes, if the stars are aligned right. If you excite it with a voltage, and turn it faster than its synchronous speed*, then it'll dump current onto the line instead of sucking current from it. Older small- time co-generation schemes did this, because maintaining synchronization is _not_ trivial. Nowadays, it's mostly done with special inverters that sense the voltage on the line and synchronize the current to the voltage.

I dunno.

  • Synchronous speed = 3600, 1800, 1200, etc., on down. So an induction machine that has a design speed of 3540RPM has a synchronous speed of
3600RPM, and a slip of 1Hz. Turn it up to 3660RPM, and it'll absorb something close to its rated power while putting almost that much power onto the line along with some inductive loading.
Reply to
Tim Wescott

Three phase induction? This looks promising:

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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

Reply to
Winston

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Reply to
PrecisionmachinisT

Perused and copied.

Thanks PM!

--Winston

Reply to
Winston

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

Reply to
Don Foreman

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.

Reply to
Josepi

Current? maybe but it will be 90 degrees out of phase with the voltage.

It is often stated as putting reactive power back into the supply.

Reply to
Josepi

That is not strictly true. I drove a 72 AMC (with a Motorola Alternator) for months without a battery in it. The alternator retains enough magnetism to self excite enough to fire the coil. This was a carburated straight 6. I lived and worked in a hilly area. If I killed it in traffic, I was hosed.

A newer fuel injected vehicle requires enough current to run the fuel pump and the computer in addition to the coil, so you be walking now.

BobH

Reply to
BobH

A old-style non-PM DC generator usually relies on residual magnetism in the pole pieces to start the cycle. One of the to-do items after working on a VW generator was to connect the battery to the field windings the correct way round in order to get startup polarity correct. Just a quick zap to provide some residual magnetism.

A wound alternator needs some DC to start up, one reason you can't push start a car with an alternator and a totally dead battery. When I was a kid, my dad took me to visit one of the sites he was currently working on, was a rural diesel power plant that had had a crankcase explosion and was being rebuilt. Got the tour from the guys in charge, had a huge V-16 engine attached to this dinky gray cylinder about the size of a garbage can. I asked what that was, they said it was the alternator. Had a tray of batteries sitting on a cart, were the old squarish cells they used to use for doorbells, all connected in series. That was the starting DC for the alternator when they were going from blackout conditions, no juice anywhere.

An AC motor may work as a generator, but you'll have to provide some method for providing and controlling the field current.

Stan

Reply to
stans4

In basic motor and generator theory examples (many online) current passing thru a conductor located within in a magnetic field/influence, or a conductor moved thru a magnetic field are the basic principles. Finding descriptions of how automotive alternators shouldn't be difficult, and will likely explain a lot wrt generation.

Many generators utilize brushes, but induction motors can generate under the proper conditions, as in Winston's referred example.

IIRC, the automotive generators of old cars utilized poles within the stator/field windings, (and brushes).. the poles weren't permanent magnet material, but they would hold in a magnet-like state after being properly magnetized.. same/similar to Don's example. If you can refer to an older automotive service manual (pre-1970s), you'll likely find the procedure to stun? the pole pieces in those old generators.

A very weak/slow-motion motor example would be analog panel meters. In the moving coil type meter movements, the coil (armature) is supported on low friction pivot points (were jeweled bearings in days of old) within a magnetic field. When an appropriate small current flows, the meter needle deflects, and if the moving coil is rotated manually (not a normal procedure), a small current flows. Many new analog meters are packaged with a shorting wire across the terminals to dampen the needle movement during shipping.. an example of braking of the generator effect. Not all analog panel meters employ moving coils, though, and those which aren't, don't effectively generate any current flow.

Over the years, I've seen a lot of DIY websites showing various generating techniques from portable/emergency power to wind generation generators that were made in home shops with powerful magnets (surplus suppliers) and hand-wound coils on forms and later potted in epoxy or resin.

Reply to
Wild_Bill

On 03/09/2011 05:47 AM, Josepi wrote: (top posting fixed)

Nope, sorry, you're just plain wrong.

Using induction machines as generators is an established -- if slightly obscure -- practice. Real mechanical power is transformed to real electric power. There are some inductive VARs, but there's real VARs, too.

But don't argue with me. Argue with the world wide web:

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(and all the myriad other pages that come up when you search on "induction generator).

Reply to
Tim Wescott

(moronic posting format fixed)

I would love to see an example of this where you turn asynchronous motor at

3660 poles per second on a 60Hz line and it puts ***real*** power back into the line and the O/C protection stays intact. That is a completely moronic statement in itself.

You may get some real power out of this machine but not very much. The power is mostly VAR power and this is done on large grid systems, in many areas.

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Using induction machines as generators is an established -- if slightly obscure -- practice. Real mechanical power is transformed to real electric power. There are some inductive VARs, but there's real VARs, too.

But don't argue with me. Argue with the world wide web:

formatting link
(and all the myriad other pages that come up when you search on "induction generator).

On 03/09/2011 05:47 AM, Josepi wrote: Current? maybe but it will be 90 degrees out of phase with the voltage.

It is often stated as putting reactive power back into the supply.

Reply to
Josepi

Don't give up your day job, Josepi.

Reply to
Don Foreman

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

Reply to
Existential Angst

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