Suggestions on Improving Efficiency (Motor-Generator ) 1

[jimmynora]

(Drawing of the setup attached)

Background:

We conduct electrical tests in our lab on electrical components (circuit breakers, switchgears,etc). In the lab we have a short-circuit Generator that generates the electricity for the tests. The generator is powered by an asynchronous motor with a slipring rotor. We are trying to improving the efficiency of this process by measuring our electrical consumption and comparing it with how much we actually need. A suggestion is to feed-in the excess electricity back into the grid.

Specifications of Motor/Generator:

6KV
2500KW
280A
Pf= 0.9
f= 50Hz (R= 2975/min)

I need suggestion on how we could improve the electrical efficiency of the testing procedure.

Q) Any other feasible methods on how to save the excess electricity apart from feeding-in the excess electricity into the grid ?

Q) How would we need to modify the circuit (best method) and what additional components would we need to install to allow us to feed-in the excess electricity ?

Q) During shut-down of the generator or braking of the generator to slow it down (eg. 1000/min) can we use this mechanical energy to provide electricity to feed-back into grid or circuit. How can we do that and what components will we need to install (In this case the frequency will drop below 50Hz required frequency of the grid.)

 

[wolf39]

Gunnar:

The rotational energy is proportional to the mass (50%) multiplied with the square of the velocity, not the revolutions per minute.

With my Itaipu calculation I was trying to open a discussion whether Jimmynora's MWh figures make sense or not. As you said already, all data given are very confusing and I for myself am reluctant to spend my valuable time to solve a problem which still is beyond my comprehension. But it seems that others are still trying hard.

Wolf

http://www.hydropower-consult.com

 

[Skogsgurra]

Wolf, they are the same thing. And you know it. There's only a constant to factor in.

Now, our friend jimmynora says: "The drive system controls the speed of rotations, hence the frequency of AC".

What drive system? There's none!? Not in the 'drawing' and none mentioned in the text before 18:33. What is going on? A (subtle) leg-pulling? Or what?

Our friend jimmynora also still thinks there is a mystical excess energy in the system and that he can feed that energy back to grid.

I give up.





Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[jimmynora]

The drive system is connected in the Generator-Motor circuit. It can power on/off the motor & generator, or reduce the speed of the RPM.

In the control room I can see the displayed RPM at any moment and any load. As for how it does its speed control that I something I am also investigating.

 

[wolf39]

Gunnar:

There is no constant factor, you have to take into account the physical dimensions.

The Itaipu generator in question has a stator bore diameter of 16 metres and a speed of 90.9 rpm (66 poles). The rotational velocity (based on the stator bore diameter) is about 76 m/s. A large 3000 rpm turbogenerator may have a bore diameter of 1.2 metres and this gives us a rotational velocity of about 188 m/s (again based on the stator bore diameter). The velocity ratio therefore is 188 : 76 = 2.5, whereas the speed ratio is 3000 : 90.9 = 33.

Wolf

http://www.hydropower-consult.com

 

[Skogsgurra]

Yes, done those calculations several times. It is part of the dimensioning of winders for steel, Al foil and paper and it is not just a question of weight and dimensions. You have to calculate I [kgm^2] and take the resulting I times radians/second squared.

So you need to use RPM squared and the factor I mentioned is the factor needed to get radians/second from RPM.

Even if I gave up on jimmynora, I have been thinking. He probably doesn't have the set up he describes in his 'drawing'. I am thinking that he has a W-L set that feeds a large DC motor which, in its turn, feeds the short-circuit generator. That is the standard for test generators in 'mature' (old)installations.

He probably hasn't seen more than part of the machinery and I wouldn't be surprised if what he describes as slip rings actually is the commutator on one of the DC machines.

You are right. No use discussing this when the OP doesn't seem to have any idea what he has got and how it works.



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jimmynora]

I haven't seen the internal machinery. I am working on the information provided to me.

It is an asynchronous slip-ring rotor Motor running as a pony-motor. Its purpose is to power our 2-pole short-circuit Generator to provided the electricity for our tests.

On recommendation I was able to get from this forum is a slip-recovery system in which we would replace the traditional resistances of our motor and generator which we use to control the speed. Part of the solution may also be replacing the motor with a thyristor based solution, which will be part of our slip-recovery system.

This way we can regulate the RPM of our generator and control the energy demand of our load while putting back any additional energy that currently we loose as heat in the resistors.

 

[Skogsgurra]

ARE YOU USING THE RESISTORS FOR SPEED CONTROL!!!???

That's about the dumbest I ever have heard. No wonder you have losses.

If you had told us that from the beginning! The resistors are usuallly used to start the machine and then short-circuited.

OK, I understand now.

I said in 17:10 "There is a 'drawing'attached to the 31 May 11 8:37 post. It shows a three-phase motor fed from a 6 kV grid. No indication as to what kind of motor it is but the text says slipring motor and the specification says 2975 RPM and PF .9, which is plausible. There is no easy way of controlling the speed of a standard asynchronous motor (if it doesn't have a cascade arrangement, which has not been indicated)"

The crucial point is: "if it doesn't have a cascade arrangement, which has not been indicated". A cascade does what you are asking for. It connects the slip-rings back to the grid via a controlled rectifier. Both Siemens and ABB are using that technique for large pumps.

It would have been so much better if you had given us all information from the beginning.

Yes, there are several solutions. Burning energy on a MW scale in resistors is just dumb.

I finally understand your problem. And it is easily solved.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jimmynora]

I will check with the companies if they have an internal solution. As far as I know we do not have any cascade arrangement hence no way to feedback electricity.

The solution I am looking for with the slip-ring recovery system should replace resistors in the motor and ideally also the generator resistor so we can then rectify and convert the energy before feeding it back into the grid.

 

[Skogsgurra]

Yes. Correct. That's how a cascade works.

I am glad that I didn't give up on you completely. But it was very close - actually did so. Wolf brought me back.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

Hier kannst Du ein Wenig vorbereiten:

http://www.transresch.de/fileadmin/Dateien/Fachartikel/Fachbericht_usk_zur_energieeinsparung.pdf

Google 'Stromrichterkaskade' for more.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[jimmynora]

Danke ! Nie wieder aufgeben

 

[Skogsgurra]

Genau!
Tschüss!

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[rasevskii]

for jimmymora:

You mean that you are actually in the plant and do not know what you have got? Or are you at some remote office and getting the info second or third hand from others? Now why not post the actual schematic electrical dwgs (remove the name of the suppliers and plant) It can be a resistor started wound rotor motor which at full speed changes over to a Scherbius machine for fine speed control (is this the case?) Otherwise a speed of 3000RPM to get 50Hz from the syn machine is impossible due the motor slip even when all the resistance is shorted out.

The losses in this high speed machine will be mainly windage and bearing losses plus copper and iron losses in the motor. The generator will have excitation losses (several KW)and iron losses when excited. Plus copper losses at load.

None of these losses can be recovered or even reduced. It is the Nature of the Beast, as they say. It must be a rather noisy machine, likely in its own room. There have to be pressure-lubricated bearings and oil pumps, more losses. You said that it has a mechanical brake. I doubt that, there must be electrical braking of some sort already foreseen. The mechanical brake if existing is only for the final 10% speed, impossible at 3000 RPM.

I think you are looking at an impossible goal. No amount of new controls will help. There is no recoverable energy at all.

Unless, as said above, you are actually using the rotor resistor bank (or is it a liquid resistor) for speed control, then a new control method might be worthwhile.

rasevskii

 

[jimmynora]

We use resistor banks for speed control. That's why energy is wasted as heat.

 

[rasevskii]

jimmymora:

In that case, there is a big potential to save energy by not using the resistor banks, except for initial start up. Look for an in-house solution from the department that is involved with drives or outside the company from other suppliers within the electrical industry. Salesmen will soon be banging on your door...

rasevskii

 

[LionelHutz]

We use resistor banks for speed control. That's why energy is wasted as heat.

I supposed you expect others to be mind readers or psychic. Good job keeping everyone guessing and wasting there time. The other were right to be skeptical about the waste of time.

You posted that you always needed to run the motor at 3000rpm (or as close as possible) to get 50Hz for the tests. Now, you need to run the motor at part speed?

Actually, good luck since I probably won't bother opening this thread again.

 

[rasevskii]

jimmymora:

As an afterthought why on earth do you ever run the unit at partial speed (1000RPM)at all? That is a total waste of energy done by cutting in resistance and letting the unit run down in speed. Just shut down altogether until needed for the next tests, then restart.

Likely the unit has been run that way for years. Someone has been doing that all wrong and giving you (I assume you are new there)all the wrong information. Old habits die hard.

Therefore any kind of new slip ring energy recovery system is a waste of money. Period. You do not need to run at partial speed, ever.

rasevskii

 

[jimmynora]

We let the generator run down from 3000 to 1000 RPM by turning off the motor.

This is done to avoid large start-up power between test intervals if they are happening in rapid succession.

The slip-ring recovery system will allow us leave such primitive energy saving methods and we can simply run the generator as long as it is needed while the load energy demand and the speed of the rotor will be balanced.