Author Topic: 3 phase versus single  (Read 5439 times)

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Offline Wolvenar

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Re: 3 phase versus single
« Reply #15 on: January 13, 2012, 10:39:03 PM »
Baby Geniuses   ;D
Trying to make power from alternative energy any which way I can.
Just to abuse what I make. (and run this site)

Offline WindyOne

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Re: 3 phase versus single
« Reply #16 on: November 24, 2012, 10:36:30 PM »
Quote
Anyone of the single phase's (by itself) put out ,..21.4Voc, ..connect 4 lights,..7.3Vdc,...9.3amps=68w

Does anyone else think the 14 Volt drop, from the Open Voltage of 21 Volt) down to the Loaded Voltage of 7 Volts, is excessive? Is 9 Amps optimal if the Voltage drops that much?

Offline niall

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Re: 3 phase versus single
« Reply #17 on: November 25, 2012, 08:51:38 PM »
erm ...i think the given load will always clamp the voltage ....so the load will always win  ...ish ..i think

but its interesting to think about single phase ....i like single phase ...lumpy ..yes ..and it will shake things about a bit , ....but all things considered ...why not ?

3 phase seems to lock you into two set v points....star/delta....with single phase there can be a lot more ....a 12v stator could possibly be run at /24/48 ...even 96v...(depending on the number of coils)

as long as the prop is happy ..?

 

Offline WindyOne

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Re: 3 phase versus single
« Reply #18 on: November 25, 2012, 10:49:47 PM »
Quote
erm ...i think the given load will always clamp the voltage ....so the load will always win  ...ish ..i think
Please explain how the load of "4 lights" used by ARTV, "will always clamp the voltage".
I am not following your logic.

Quote
....so the load will always win  ...ish ..i think
HUH?




Offline niall

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Re: 3 phase versus single
« Reply #19 on: November 26, 2012, 10:53:56 AM »
mmm...yes its not the best logic

the open circuit voltage will be high ....the bulbs each add low r into the circuit ...the more bulbs in parallel the lower the resistance ....the alt at a given rpm can only produce a certain amount of amps ...not enough for those bulbs so the voltage falls back ...increase the amps and it will rise back up....

go the other extreme and put 20 bulbs on it ....now the v will be very low ...its all being lost as heat into the load .....( thats a little vague but i think?  its ok to say that )

its all down to ohms law ....but i,m hopeless at that

another shot is ...the amount of resistance added by Artv to the open v will directly effect it  ?       

maybe someone else can chime in on ohms law .....

Offline WindyOne

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Re: 3 phase versus single
« Reply #20 on: November 26, 2012, 09:25:58 PM »
Quote
its all down to ohms law ....but i,m hopeless at that

Well yes kind of, but actually, my original point was the huge difference between the Open Circuit Voltage of 21v and the Loaded Voltage of 7v at 9 amps. Therefore, the generator is wasting about 126 watts in heat to deliver 63 watts to the load. Is that an optimal load for that generator ?

Offline rossw

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Re: 3 phase versus single
« Reply #21 on: November 26, 2012, 09:32:10 PM »
my original point was the huge difference between the Open Circuit Voltage of 21v and the Loaded Voltage of 7v at 9 amps. Therefore, the generator is wasting about 126 watts in heat to deliver 63 watts to the load. Is that an optimal load for that generator ?

The open circuit voltage dropping to only 7V under load in no way "means" that there is 126 watts "wasted in heat" to deliver such a modest power to the load.

Many devices can (and do) generate very high open circuit voltages. A current transformer for example, may produce hundreds or thousands of volts if unloaded - but that doesn't mean they're "wasting" hundreds or thousands of watts just because once loaded their voltage is millivolts to volts.


Offline oztules

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Re: 3 phase versus single
« Reply #22 on: November 27, 2012, 02:43:11 AM »
Some times it pays to stand back and look at what we are measuring.

The 21v open circuit has NO wattage associated with it, as Watts = volts x current.... or 21 x 0 = 0watts.
So we havent wasted anything yet.

When we load it with a load, the current flows in the load and the stator equally... so whatever voltage is forcing the current through the combined resistance of the stator and load , will dictate the output current

ie if 5 amps flows in the load, it must be flowing in the stator as well, so the resistance of the circuit is the load plus the stator.

So we know to get a current  flowing in the load, we need current of equal value flowing in the stator.

This is not just an ohms law problem however, and that explanation will only  work for alternators with no reactance or impedance from other factors.

In truth, the alternator suffers synchronous impedance, which is frequency and load sensitive, as it means we need the losses from resistance in the stator, armature reactance in the alternator, and impedance caused by the waveform through the inductance of the stator.

To keep it simple we can try to use just ohms law in the stator and load.

Now this may work well for air core alternators with strong flux, very low resistance stators,and large air gaps, but for any unit that relies on steel laminates, and poorer magnetic fields, and tight tolerance air gaps.......the reactances and impedances will current limit the alternator at some point, rendering the ohms law approach useless in the extreme.

At current limit (from reactance etc) it will yield ever higher open circuit voltage as the rpms rise, but the loaded voltage and current will remain the same from that point on.................. we are still wasting the heat in the stator at this point from stator resistance and current flow,  but are NOT using any more motive force  after current limit occurs.

So if were lucky, and this point is arrived before the destructive heat  loses occur, we have a bullet proof alternator heat wise, but we won't be able to stop the prop by shorting it out either...

Does that now explain where the power didn't go?.... it wasn't there in the first place when open circuit...... or short circuit for that matter.

When short circuit,  the motive power used,  purely drives the loses in the armature and stator, as no voltage emerges from the alternator, so the output is also  zero watts.... but maximum current (I max) .... opposite to open circuit (V max), but same power output.

The power absorbed in the alternator is now defined by the current  x  current x  the resistance of the stator,  ( I^2R) as the reactances dont behave like resistance does, there are no real loses in reactance voltage limiting, verses resistive current limiting, same for capacitive reactances and their effects on voltage dropping schemes in electrical circuits that don't want to use transformers because of weight or space requirements.

The capacitive and inductive reactances impede current, and may be expressed in ohms, but they are frequency sensitive, and don't follow ohms law.

Think of your welding transformer. The primary may measure 2 ohms or less. At 240vac 50hz input this should mean 120 amps flow through the primary..... but no, only a few amps flow (shows as magnetising current) through the winding. The AC sees the inductive properties of the core and windings, and so (until you short the secondary with your welding rod) use very small current to idle. A  2 ohm resistor is a totally different animal...I=E/R   = 240/2  = 120A..... W=ExI = 240 X 120 = 28800 watts  ( or use I^2R).

In simple terms, current will appear as torque, and voltage will appear as speed (rotation). A shorted alternator has heavy torque to drive it and no output voltage, and an open alternator has ( high) speed, but uses no torque (bearing and iron losses excepted) and no current output.

Under normal (whatever that is) resistive loads, it will be a combination of torque and speed that gives us power output, and an ideal alternator will have a linear power curve, but nothing is ever ideal.



....................oztules
Flinders Island...... Australia

Offline WindyOne

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Re: 3 phase versus single
« Reply #23 on: November 27, 2012, 08:00:22 AM »
my original point was the huge difference between the Open Circuit Voltage of 21v and the Loaded Voltage of 7v at 9 amps. Therefore, the generator is wasting about 126 watts in heat to deliver 63 watts to the load. Is that an optimal load for that generator ?

The open circuit voltage dropping to only 7V under load in no way "means" that there is 126 watts "wasted in heat" to deliver such a modest power to the load.

Many devices can (and do) generate very high open circuit voltages. A current transformer for example, may produce hundreds or thousands of volts if unloaded - but that doesn't mean they're "wasting" hundreds or thousands of watts just because once loaded their voltage is millivolts to volts.

I do not agree with your analogy of comparing a Wind Powered Generator to a Current Transformer. At a fixed RPM, a Wind Powered Generator is in the "Constant Voltage" class of power sources. A "Current Transformer" is not in that same class. And, as you pointed out and I agree with, a Current Transformer has a very different set of Open Circuit characteristics. Under normal conditions Current Transformers are not typically operated with an open secondary. An Apples vs. Oranges comparison, at best.


Offline WindyOne

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Re: 3 phase versus single
« Reply #24 on: November 27, 2012, 08:28:29 AM »
In truth, the alternator suffers synchronous impedance, which is frequency and load sensitive, as it means we need the losses from resistance in the stator, armature reactance in the alternator, and impedance caused by the waveform through the inductance of the stator.

Now this may work well for air core alternators with strong flux, very low resistance stators,and large air gaps, but for any unit that relies on steel laminates, and poorer magnetic fields, and tight tolerance air gaps.......the reactances and impedances will current limit the alternator at some point, rendering the ohms law approach useless in the extreme.

At current limit (from reactance etc) it will yield ever higher open circuit voltage as the rpms rise, but the loaded voltage and current will remain the same from that point on.................. we are still wasting the heat in the stator at this point from stator resistance and current flow,  but are NOT using any more motive force  after current limit occurs.

....................oztules

oztules,
OK, yes there is a considerable amount of iron within this car alternator being tested.
That means the Inductive Reactance of this alternator would be higher than a Dual Rotor Air Core Axial Flux generator. RIGHT?
But how much higher?

How does the Inductive Reactance compare to the DC Coil resistance for this type of car alternator?
Is it ?
 a) significantly less
 b) about equal
 c) significantly more

Agreed, the DC Coil Resistance would cause energy to be wasted as heat
and the Inductive Reactance should not waste the energy as heat.

If there was significant Inductive Reactance (as compared to the sum of the coil resistance + load resistance) then could this be observed and measured because the AC Voltage would be "out-of-phase" with the AC Current ?

Offline Wolvenar

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Re: 3 phase versus single
« Reply #25 on: November 27, 2012, 10:12:09 AM »
I'm guessing those numbers, if not randomly picked, were characteristic of that machine.
Then it's dependent on how the genny was made.

So unless I missed something in this thread..
We are not privy to wire gauge (or the metal makeup of the wire used ie copper clad aluminum, copper etc), core makeup, amount of wire involved, or even how much the load reduced the blade speed.

So answering this
Quote
Anyone of the single phase's (by itself) put out ,..21.4Voc, ..connect 4 lights,..7.3Vdc,...9.3amps=68w

Does anyone else think the 14 Volt drop, from the Open Voltage of 21 Volt) down to the Loaded Voltage of 7 Volts, is excessive? Is 9 Amps optimal if the Voltage drops that much?


Becomes tricky
Trying to make power from alternative energy any which way I can.
Just to abuse what I make. (and run this site)

Offline Wolvenar

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Re: 3 phase versus single
« Reply #26 on: November 27, 2012, 10:15:30 AM »
I am however interested in how Artv's machine is turning out.
Or was this posted elsewhere?
Trying to make power from alternative energy any which way I can.
Just to abuse what I make. (and run this site)

Offline WindyOne

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Re: 3 phase versus single
« Reply #27 on: November 27, 2012, 01:18:34 PM »
I am however interested in how Artv's machine is turning out.
Or was this posted elsewhere?

I am not sure how the machine turned out.

"... modified car alt ..."

I am not sure how the PMA was modified either - no details were posted ...
 a) The electromagnetic rotor was pulled and replaced with a shaft containing permanent magnets.
 b) The stator was re-wound for more volts at a lower RPM
 c) All of the above
 
 I would like to know the RPMs of the PMA when it generated the 7.3 volts at 9.3 amps = 68 watts.
 I am going to guess between 350 rpm and 400 rpm.
 Is that roughly a 5' diameter blade with 20 mph winds?


Offline rossw

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Re: 3 phase versus single
« Reply #28 on: November 27, 2012, 02:26:00 PM »
I do not agree with your analogy of comparing a Wind Powered Generator to a Current Transformer.

You don't have to. I made an observation of fact based on an incomplete and unspecified question.

Quote
Under normal conditions Current Transformers are not typically operated with an open secondary. An Apples vs. Oranges comparison, at best.

Under normal conditions wind turbines are not typically operated with an open output either.

Offline oztules

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Re: 3 phase versus single
« Reply #29 on: November 27, 2012, 03:40:25 PM »
"oztules,
OK, yes there is a considerable amount of iron within this car alternator being tested.
That means the Inductive Reactance of this alternator would be higher than a Dual Rotor Air Core Axial Flux generator. RIGHT?
But how much higher?"
Magnitudes in some cases. from uH to mH


"How does the Inductive Reactance compare to the DC Coil resistance for this type of car alternator?
Is it ?
 a) significantly less
 b) about equal
 c) significantly more"

Ok, there is a bit here to look at.
The impedance caused by the inductance of the circuit is frequency dependant. So at low frequency, the difference between stator R and stator R+XL is insignificant.
As F increases, so too will the impedance caused buy the inductive reactance of the circuit.... thats why it is an impedance not a resistance, it is dynamic in nature.

So the answer is all of the above... depending on the frequency.

Which brings the next part of the question.
"
Agreed, the DC Coil Resistance would cause energy to be wasted as heat
and the Inductive Reactance should not waste the energy as heat.

If there was significant Inductive Reactance (as compared to the sum of the coil resistance + load resistance) then could this be observed and measured because the AC Voltage would be "out-of-phase" with the AC Current ?"

The answer is ...yes... need to measure synchronous impedance, and subtract the resistance losses, and then whats left will be mechanical losses,inductive reactance and the real elephant in the room with iron cores... armature reactance. (even though the inductive reactance acts like a choke... current limiter).
There are  many how too's on how to measure the reactive components, open circuit to short circuit measurements will give you most of the answers.


Think of it like this.
If we use a magnet to induce a voltage in a wire, and load that wire, a current will flow in that wire.     Simple enough too....

But, whenever we have a current flowing we create a magnetic field....so now the wire with the induced EMF is now an electromagnet whose field is in opposition to  the magnetomotive force that induced it. (MMF).
We can see that now we have an inducing field and a repelling field produced by that field.... it's war. As long as the inducing field  (rotor MMF) is stronger than the repelling field (Back MMF), we can continue to drive up the rpm, and get power out of the stator.

When the stator is carrying enough current to create back MMF sufficient to prevent further MMF from seeing the stator, we get a state where no matter how fast you spin the shaft, no more EMF can be induced in the stator.... we CURRENT LIMIT at this point.

If the air gaps are large (like in an axial flux with neos) and the stator coils are without iron (axial flux with neos again) and we have a strong magnetising field (neos etc) then we have a machine that will follow the ohms law pretty well, and we can ignore the armature reactance, and the inductive reactances....... but...

In a car alternator we have tight gaps for the field to operate over, and we have iron core in the stator with slots. These slotted iron cores allow much lower magnetising current to be used, and much tighter air gaps ( 1mm instead of 20mm). The stator wire is wound around these slots/poles, and that allows the flux induced by the rotor to cut the coils at one instant, as it flicks and drags from pole to pole........ in an axial, the coil pole is physically spread.....and without an iron core of any kind to focus the back MMF against the magnetising field....  and so the field penetrates the legs of the coils gradually, not all at once.

So a combination of gap (bigger is harder to focus the back MMF), and an iron core focussing the back MMF, we have a completely different animal to deal with.

As a car alternator, when these problems arise, we simply increase the flux in the magnetising field (rotor), and that papers over the problems for the most part.....
A car alternator will still current limit from armature reactance, depending on the max flux the rotor can create with full battery voltage across it ... for a car usually 5A is about it for the 14v driving emf. The rotor resistance limits it to that, and so the AMP TURNS that can create the field is limited to this value.

The inductive reaction from just the frequency in a car alt does not cause quite the results that back MMF does in most cases, as F is not usually so high as to bring it to the fore front, it will usually be just be a creeping output loss on the back of the armature reactance. In a car alt, the turns are very low, the core very small, so inductance is low.

In short, in an iron core machine, it will almost always be the back MMF of the armature reactance (and XL) that defines the upper limit, and the resistance that defines low power performance. in commercial generators, they tend to balance the copper loss with the armature reactance (and XL)losses.
It is really an excellent safety valve, as if it did not occur, then a short in a power alternator would probably destroy it, and bust shafts pronto.

Power factor losses are a vector thinggy, and so should not bother the alternator at first glance.... but it you need say 10 amps available to vectorially get 7 real amps in phase with the voltage, we still have to make the 10 amps.... Now there is no voltage associated with the last three amps so no power... so we should not require any more motor to develop it... but no... sadly.

Current exhibits as torque, so even though no power should be associated with the phantom current, the motor will feel it. Folks charging batteries with no PFC stages will notice this behaviour. The 10 amps needs to be carried to the work site, and the resistance in the wires, fuses, transmission whatever will exhibit a voltage drop, producing a voltage proportionally(albeit small), and our phantom current has a small in phase voltage now... not much in the way of watts, as V is small, but  the 3 amps still exists, and torque will need to be found to support it...... motor labours accordingly.  I would think that the 7 amps to 10 amps will give the ratio of change of torque in this instance...... because torque is proportional to current, as  rpm is to  stator voltage.

It is the stators back MMF fighting  against the rotors MMF that makes the shaft hard to turn when load is applied..... it is only caused by current flow, as only current makes flux.... in ampere turns.... ideally, EMF has no effect ( does in the real world... we need to overcome internal resistance / impedance for the current to flow... but you get my point?)



.................oztules
Flinders Island...... Australia