Re: AC Motor as an AC Generator



Induction generators use the system as a source of excitation. The generator is brought up to rated speed in a direction of that opposite to motor rotation and then closed into the system. It may jump a pole but the system pulls it into phase. You of course need a source of mechanical torque to do this. Induction motor pumps have routinely been used to operate as hydo generators but the hydraulic efficiency is lousy.
Regards,
John Phillips
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On Mon, 25 Aug 2003 11:33:01 -0400, "anon"

The machines at PGS are three phase, synchronous, 13.8 kV and around 30 MVA. Mode reversal is achieved at rest by swapping two of the phases.
Regards,
John Phillips
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Huh?
My understanding is that and induction motor runs slightly or somewhat below sync speed. To cause it to supply energy to the network, you just speed it up in the same direction. Once it is ABOVE sync speed it is a generator.
A DC generator with permanent magnet field and a commutator works the same way. It can start as a motor but if your cause it to run faster then at some point it will supply power to the network.

Huh, again.
Induction generators aren't phase sensitive. The DC generator model well describes how it functions from an ENERGY view point.

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On Tue, 2 Sep 2003 00:00:00 -0400, "John Gilmer"

It is certainly the case with a synchronous generator that will spin as a motor driven from the system until sufficient toque is applied. This is not a speed change unless the power system is small however but a current phase angle shift. I was relating to reversible hydro machines that pump in one direction of rotation and generate in the other. It is an hydraulic and not a motor/generator issue.
http://www.qsl.net/ns8o/Induction_Generator.html

You are correct. I recalled starting 30 MVA synchronous generators that used induction starting. The machines were brought up to above 90% speed with the field shorted through a resistor. The main breaker was closed and for an instant it was an induction generator. The field shorting was immediately removed and excitation applied. This is where the machine was pulled into synch.
Regards,
John Phillips
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On Tue, 02 Sep 2003 12:29:56 GMT, John Phillips

John, Do you work for the power companies? No wander we have black outs. :-))
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| Induction generators use the system as a source of excitation. The | generator is brought up to rated speed in a direction of that opposite | to motor rotation and then closed into the system. It may jump a pole | but the system pulls it into phase. You of course need a source of | mechanical torque to do this. Induction motor pumps have routinely | been used to operate as hydo generators but the hydraulic efficiency | is lousy.
This would be ok if you were using a single phase induction motor, which is not sensitive to phase rotation. I Know the type of pump arrangement. When the pump goes one way it pumps and when it goes the other it acts as a turbine.
If this was a three phase machine, You would still have to run it backwards to make the pump work like a turbine, however prior to doing this, you would have to reverse the phasing of the motor / Generator so it would synchronise with the line. Once connected, it would get it's excitation from the line and start generating when the prime mover ( turbine) tried to make it go faster then synchronous speed by roughly the same slip, as it would operate below synchronous speed when operating as a pump.
Tom Grayson
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is
When
Huh? (again)
If you have a three phase inductor MOTOR running happily along and you use some external agency to force the shaft to turn faster than "sync" speed, the motor will become a generator.
Now some water turbines might well be reversed to have them PUMP water in the reverse direction but that's a separate issue.
OT: often, "they" start pumps (except for positive displacement pumps) with the outlet valve CLOSED. Since the motor isn't don't any work, it is easier to start that way.

backwards
would
synchronise
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Gentlemen, The original question did not even mentioned pumps. The question is simply if an AC induction motor would work as a generator or not. The answer is yes a three phase motor would work. The trick is to run it up to speed as a motor, using a three phase power line. Once it is at speed, you need to mechanically rotate it faster than sincronous speed, as Mr. Gilman mentioned, and it will regenerate energy. Do not get lost in the hydraulic world of Mr. John Phillips who does not seem to have the understanding of the AC power motors and generators. Just read what the other people are saying and you will find your answer. :))
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| Gentlemen, | The original question did not even mentioned pumps. | The question is simply if an AC induction motor would work as a | generator or not. | The answer is yes a three phase motor would work.
This is true. and the purest answer to the original question is
"YES"
But that does not really transfer much information to the Person who originally made the enquiry.
Now it appears from the context of the post that he is investigating ways to make his own electrical energy. Now, unless I am mistaken, Wind turbines and Water turbines are probably the second and third most common way of generating your own energy after the most obvious choice of an internal combustion engine. So the subject of water turbines is really not that far off the mark in the spirit of the original post.
| The trick is to run it up to speed as a motor, using a three phase | power line. Once it is at speed, you need to mechanically rotate it | faster than sincronous speed, as Mr. Gilman mentioned, and it will | regenerate energy.
A Question. Why do you need to run it up to speed as a motor, considering you already have a prime mover that should be quite capable of doing it for you ? I am not trying to be smart here, Just trying to find out if there is more to this then I am aware of.
| Do not get lost in the hydraulic world of Mr. John Phillips who does | not seem to have the understanding of the AC power motors and | generators. | Just read what the other people are saying and you will find your | answer. | :))
True again, But what harm is there in trying to point out to Mr Phillips where he is in error.? Maybe he will learn something too !!
Tom Grayson
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Tom, I totally agreed with what you and the other people said in here. I just felt that talking so much about the hydraulic pumps and reversing them, we confused to death the original poster. In my mind, I see the guy taking a relatively small motor connected on some kind of "dyno", turning it and wandering why he is not getting output power. The answer is the same like the answer to your question:

The induction motor, as you know, does not have any excitation built in (permanent magnets or electrical excitation). Even if we rotate the rotor, there is not enough magnetic flux in the motor to create any significant rotating magnetic field. Is just like moving a few wires inside of a coil: there is no magnetic field changing and will be no electric current generated. By connecting the motor to an existing AC power source, the motor will rotate because the existing AC power will supply the rotating magnetic field. The actual speed of the induction motor will be somewhat lower than the sync speed (ex: sync speed at 60Hz, 4 pole motor = 1800RPM, real full load speed is apx 1750RPM or so, depending on the design of the motor) The difference between the magnetic field rotational speed and the rotor speed is known as the "slip" and it is this that generats rotor current, rotor current which creates its own magnetic field, called otherwise excitation. Now if we "over speed " the motor with an external mechanical force, when the motor reached exactly 1800RPM will not take or generate current anymore because the slip will be zero. Once we exceed the sync speed, the rotating magnetic field generated by the power source will create again a current in the rotor but because the direction of the "slip" between the rotor and stator has reversed, so does the current induced. The back EMF will generate a voltage that will exceed the voltage level of the power lines and the current will start flowing the other way, back to the power source. Practical prove that this is how it works: In inverter applications, when one tries to decelerate the motor too fast, the motor generates a lot of energy in a relatively short time, energy goes back to the DC bus pumping it up quite a bit, even blowing up the inverter, hence the need for dynamic breaking, which is in the cheapest form nothing else but a resistor connected across the DC Bus to absorb the extra energy. I am not so good at motors, but I know quite a bit about the power electronics that make them move. Regards, :-))
On Thu, 04 Sep 2003 23:41:40 GMT, "Tom Grqyson"

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| The answer is the same like the answer to your question: | >Why do you need to run it up to speed as a motor, considering you already | >have a prime mover that should be quite capable of doing it for you ? | The induction motor, as you know, does not have any excitation built ..........
true enough. Perhaps I should have modified my question to be .....
" Why do you need to run it up to speed as a motor, PRIOR TO CLOSING THE BREAKER, considering you already have a prime mover that should be quite capable of doing it for you ?"
I didn't think the question, as it was posed, would convey the impression that I didn't know that an induction motor had to be connected to the line in order to get excitation. But it looks as if it came across that way.
Tom Grayson
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Sorry Tom, I didn't mean to be unrespectful. I just miss understood. My regards, :-))
On Fri, 05 Sep 2003 21:25:46 GMT, "Tom Grqyson"

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| | | > | > This would be ok if you were using a single phase induction motor, which | is | > not sensitive to phase rotation. I Know the type of pump arrangement. | When | > the pump goes one way it pumps and when it goes the other it acts as a | > turbine. | | | Huh? (again) | | If you have a three phase inductor MOTOR running happily along and you use | some external agency to force the shaft to turn faster than "sync" speed, | the motor will become a generator. |
I am not disputing this. Did I say I was? In fact, further in my post I said just the same thing.
The comments about direction were added in because it APPEARS that Mr Phillips has the idea that the reversing direction is important from the motor generator point of view, Whereas, it is really related to the function of the pump / turbine operation.
The purpose of my post was to support Mr Phillips because he was actually describing a real operation of something he has seen. ( and that I have also worked on, 40 Mw in fact) I was simply trying to clear up the issue about why the thing had to go backwards to act as a generator. It was not an electrical issue, but an hydraulics issue only.
| Now some water turbines might well be reversed to have them PUMP water in | the reverse direction but that's a separate issue. |
Actually it WAS the issue in this case :o)
| OT: often, "they" start pumps (except for positive displacement pumps) | with the outlet valve CLOSED. Since the motor isn't don't any work, it is | easier to start that way.
And even easier to start if the water is removed completely. In the system I worked on, in which the pump was below water level, They pump in high preasure Air to force the water level down below the turbine so the starting was done in just air. Once the thing was started, the air was vented and the water flooded in, The outlet was oppened, and the serious work began.
Tom Grayson
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wrote:

Well, just a nit to pick, if the machine is only running 90% of sync when the main breaker is closed (before exciting the field), it would be running as an induction motor.
It *is* common on such machinery to run it up to about that speed (or ~95%) as an induction *motor*, then remove the shorting resistor and apply DC to the field. From this speed, the synchronous machine can 'pull-in' and lock into phase.
daestrom
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wrote:

daestrom,
No, it was simply generating negative energy.
Technically, you are correct but by definition it was in the generator direction as opposed to the motor direction because of pump-turbine hydraulics. Hydro machines are routinely "motored" so they can be used in the synchronous condense mode for voltage control. With non-reversable machines, this, of course, is in the generate direction also by definition.
If we go back to the original question of using a purpose built small induction motor driven pump as a hydro generator, I am not sure if it would work at all in the former pump direction hydraulically and it might simply stall. The best efficiency solution by far though would be to reverse the direction of rotation. I have never attempted this with a single phase induction motor by reversing the polarity. With a three phase motor, obviously two phase connections can be swapped.
Regards,
John Phillips
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You need to drive the motor using your three phase power lines up to speed, as a normal motor. To generate energy then you need to have a mechanical device (water, wind, thermo turbines, or another motor) to turn your motor faster than the "normal" (syncronous) speed. Once your motor exceeds the sync speed, the magnetic slip reverses direction and the motor will become a generator, the current reverses direction and flows back into the power lines. Anyone working with inverters has probably seen this when trying to stop the motor too fast, the inverter blew up (uncontrolled fast regeneration). Does it answer your question?

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On Mon, 01 Sep 2003 13:26:32 GMT, John Phillips

John, Let me point out that the generators rotate in the same direction as the motor would in the same system, just faster than the motor (induction gererator - motor that is) or at the same exact speed (syncronous generators). What it matters is the slip between the mag field of stator - rotor. Regards,
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