As with my patience.
Power factor correction experiment
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zbang
Senior Member
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- Roughly 5346 miles from Earls Court
But it has been somewhat amusing to read, like a novel serialized in the newspaper, I just have to check back every day. It's also interesting to see how people approach a problem and present data.
Running way back to the original idea- common sense, reading my electric bill, and a smattering of EE knowledge tells me that these devices are simply a way to part me, the consumer, from my cash.
(I was really energetic, I'd bundle one of the Chroma AC supplies and it's monitor into the car, maybe add a scope, take them home, and plug in my 'fridge.... then sequentially connect some caps and 200' of 12g SO while taking readings along the way. But I'm far too lazy for that, those Chroma's are heavy! :lol
mivey
Senior Member
I'm not sure what else you could add, but you keep suprising me with more examples. Very nice ones, I might add.
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T.M.Haja Sahib
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Hi mivey,
You appreciated Besoeker.But you did not tell even a single word of appreciation to me for my efforts, as self-assumed manager of this thread,in moving the thread in a specific direction.I feel let down.
Now coming back to your remark
''I'm not sure what else you could add,''
I want to say Besoeker as an expert has concealed some important information so as not to concede his defeat.I hereby expose that secret information to you.
The torque-speed characteristic of the induction motor displayed in the diagram of post#170 is based on what is called open loop control of induction motor.In that method,motor speed control is feasible within a very narrow range,say,1.2%.But there is another method called phase controlled induction motor drive with feedback control by which the induction motor can be run at any speed below its synchronous speed by reducing the supply voltage alone.
A block diagram of phase control drive is shown below.
http://www.st.com/internet/com/end_application/375.jsp
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Thank you kindly, sir.
I think mivey might have been referring to my real world examples taken from my own personal experience of those actual cases.
Do you have any similar of your own to share with us?
Well, thank you. That gave me a good laugh - nearly fell off my chair!
Wrong again. What I gave are the motor characteristics. They are what they are regardless of method of control.
It isn't a method of speed control. It's just the motor characteristics. They are what they are.
So are you now conceding that your comment from post #151:
is incorrect?
Yes there is.
A variable frequency inverter.
Single phase, no speed feedback. No credibility.
It reduces the voltage just the same was a conventional light dimmer. To paraphrase the cooking shows......here's one I made earlier..
But hang on a minute....
Isn't this the very technique about which you commented thus about speed control?
Do you read Shakespeare?
It seems to me that these lines from Hamlet are appropriate to your position in this discussion:
"There's letters seal'd: and my two schoolfellows,
Whom I will trust as I will adders fang'd,
They bear the mandate; they must sweep my way
And marshal me to knavery. Let it work;
For 'tis the sport to have the engineer
Hoist with his own petard; and 't shall go hard
But I will delve one yard below their mines
And blow them at the moon: O, 'tis most sweet,
When in one line two crafts directly meet. "
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- Massachusetts
T
T.M.Haja Sahib
Guest
Modification motor characteristics are one of the fundamental rules of control theory in achieving speed variation over a wide range of induction motors.
My comment is based on phase control drive with feedback loop method of induction motors in which variation of motor speed down to zero is achievable.I based my request for you to learn more on that.
See the black arrow pointing from the motor to the control unit.It represents the feedback loop.
A conventional light dimmer is no match for it,because the same type of controller with additional circuit for slip recovery scheme (to feed the rotor power back to source to improve efficiency) is used in motors of MW range in water pumping stations
Please make a study of phase control drive and you will realize that speed of the induction can be controlled over a wide range with that control method by reduction of applied voltage, unlike your post #170.In fact, which water pump installation will need your pump motor control with almost fixed speed?
Good....it was intended as an attempt at humour. British and I wasn't sure that it would travel well.I do not know why but that really did make me laugh out loud.
As it happens, I did that (and quite a few others like it) for a practical requirement. A different division of the company to mine makes dimmers mostly for the commercial market - hotels, concert halls, cinemas and the like. None of the circuits takes a whole lot of current on its own but some of the larger installations can be in the hundreds of kW and the aggregate harmonic load can become a problem. I sometimes get called upon to look at that.
T
T.M.Haja Sahib
Guest
I hope I made you aware of your need to learn more.
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T
T.M.Haja Sahib
Guest
I already gave you a real life example at post#151.
Oh!You knew nothing of it?!.Sorry for assuming you were an expert!
Wrong again. The motor characteristic are determined by the design and construction. They are what they are. The method of control, or none, does not alter them one whit.
Now remind me of how many induction motor control systems you have designed and built, tested, and done so to meet guaranteed performance requirements?
With a standard induction motor you can't. If you don't believe me, try it.
That completely misses the point. The power side shows a triac.
The method of control is by phase control to give voltage reduction which is exactly what a dimmer does. And the very method you said "is feasible within a very narrow range".
No it isn't.
Your system shows a power control device on the supply to the motor. For the sub-synchronous static converter cascade, a more modern approach to the Scherbius speed control system, all the control is done on the rotor side. It isn't an additional scheme. It is the scheme. And ours is an up-market version of it. Well, I would say that, wouldn't I.
Here's the power schematic from a technical paper I presented, by invitation, on the subject a few years ago:
Obviously, you will have discerned that it's a 12-pulse system. It has reduced harmonics compared to a 6-pulse system and we could have gone that way in this particular case. But it was a belt and braces approach. If the star side failed, the system could run on delta alone.
The paper was based on the energy savings for yet another real life application where we had designed, manufactured, tested, and supplied two such drives for centrifugal fans. At 2.25MW they were sort of medium sized for us for this type of drive.
It can't. If such a technique was viable it would have long ago eclipsed the need for variable frequency inverters. Actually, if it were the case, they need never have been developed in the first place. But they were. Think about that.
My post #170 gave motor characteristics. Just that. They are what they are. Nothing there precludes operation in variable speed mode with a suitable variable voltage and variable frequency source. Varying the voltage alone just doesn't cut the mustard.
You still don't get it. My motor and the characteristics I presented have nothing whatsoever to do with the means of control. Nothing at all.
TBH, we don't often get involved with fixed speed projects.
That said, we are currently running a pumping station project that has two 1300kW 11kV and two 300kW 400V fixed speed pump drives along with some smaller fixed speed sump and bearing lubrication pumps. These are the panels and control desks for those that we designed, manufactured, tested, and installed.
It's the real deal.
An upgrade replacing all the control gear and starters whilst retaining the main motors.
The original kit was around the early 1950s and there was little technical data on the original design and what little there was didn't match nameplate data,
I spent an inordinate amount of time trying to reconcile various disparate data to come up with designs to match the requirements.
From the end customers perspective, it was a several million ?UK project between the pump refurbishment, new pipework, and our electrical content.
Huge pressure on me to get the electrical design right despite the distinct lack of data.
Something like that seriously tests your understanding on electrical machine theory.
Whilst it gave me a few worries and some sleepless nights we got it right.
Now,what did you want me to learn?
Please take time to reflect on the copious real life information I have provided you.
T
T.M.Haja Sahib
Guest
I do not know how to convince you with 40 years of experience in the field.However,I'll try a bit.Surely,the motor characteristic are determined by the design and construction but not invariable.The change in applied voltage changes the torque-speed characteristic of the motor as is clear from your own post #170.A very simple auto transformer motor speed controller changing the voltage and hence the motor characteristics is given below.
This is not relevant.I asserted that the speed of an induction motor can greatly change with change in applied voltage.You challenged that it can not be done,because an induction motor is a constant speed machine.I showed above that it can be done.Whether it can be done with an ordinary induction motor or a special induction motor is irrelevant.Both are induction motors.
I only said a light dimmer is no match for a phase control drive with its additional circuitry such as feed back control circuit etc.,I did not say they did not have any thing in common.
You misunderstood me.Please read again my first entry in this post.
This is unfair on your part.You did not mention problems which baffle you and which are not yet solved by you.Then only your question of ' Now,what did you want me to learn?' is relevant.
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A change in the operational point is not a change in the motor characteristics.
If you have any experience of calculating induction motor performance you will be familiar with the Steinmetz equivalent circuit. It is a handy tool.
On the remote chance that you have, not here it is.
I find it jolly useful and pretty reliable. Had it not proved so, I'd have been up to my balls in litigation claims.
And here was I was beginning to think I might have to send you a bigger shovel...I don't.
Take a good look at those curves. Can you see the obvious problem? I'll let you ponder that for a bit. With your superior knowledge of machine theory and clarity of mind, I have confidence that you will get there.
You also said within a very narrow speed range. Ho hum....
Again you seem to miss the fundamental point. Both are voltage control. And you had already correctly accepted that speed control by voltage variation was a very narrow range.
within a very narrow range
Que?
Which problems have I not yet solved?
The meaning of life obviously, the existence of the FSM, steam navigation on the Clyde.... maybe......
But, slightly more seriously, in the context of this thread in this forum, maybe you could explain what I have yet to solve.
For the avoidance of doubt, please do not construe this point to mean that I think have solved all problems. Quite the reverse.
I'm an old fellow. Heck, I might even make it to my next birthday.
The more you know, the more you realise what you don't know.
The R2' takes care of that. It's the transformed R2/s where s is the slip.
http://books.google.co.uk/books?id=osa7ldPgUpEC&pg=PA275&lpg=PA275&dq=steinmetz+motor+equivalent+circuit&source=bl&ots=S5NwRKdPmE&sig=uv74AQ-YNxXlZrmgrHu0Tc9HDNo&hl=en&ei=bhLOTvv1L9SR8gP2w-TbDw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CB8Q6AEwAA#v=onepage&q=steinmetz%20motor%20equivalent%20circuit&f=false
See Fig 6.6 on page 278.
T
T.M.Haja Sahib
Guest
May God bless,'Long live, Besoeker!'.
Well, thank you kindly, young fellow.
The next milestone (or maybe millstone) is but a couple of days away so, at this point, the prognosis looks favourable.
T
T.M.Haja Sahib
Guest
You are talking about the operation of motor in statically unstable region of the motor characteristic.This problem does not exist,when the voltage is sufficiently reduced for speed control.
Yes.That is for open loop control.With feed back control and circuitry to limit stator current,the closed loop control can vary the motor over a broad range.In case of variable torque loads such as pump motor,open loop control is found sufficient (up to a range of motor size), as I already showed in post #193.
The instability problem exists exactly because the voltage is reduced and the motor is operating on the wrong side of the speed torque curve. Reducing the voltage further will not fix that.
What you showed in post #193 actually demonstrates the problem quite well.
If you get on the wrong side of the speed torque curve, you have unstable operation. Look at the motor characteristics I posted in #171. Anything below about 0.97 pu is in that unstable region. No amount of control can change that. It is what it is.
Speed control by voltage reduction, as you have clearly demonstrated, is not viable.
If it was, you would be getting people employing the very much simpler (W3C) soft-start power circuit rather than the double conversion arrangement of a variable frequency inverter for speed control. But they are not. Don't you wonder why?
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