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Suggestions on Improving Efficiency (Motor-Generator )

Suggestions on Improving Efficiency (Motor-Generator )

Suggestions on Improving Efficiency (Motor-Generator )

(OP)
(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.)

 

RE: Suggestions on Improving Efficiency (Motor-Generator )

You are chasing your tail. Turn this unit off when it's not being used if you want to save energy. Replace the motor with a more efficient one if you want to save energy. Investigate removing or reducing the motor fan if you want to save energy. If you make this motor-generator set easier to keep spinning you will save energy.

You still seem "stuck" on this idea that the generator is always producing its rated output power while running, ie 2500kW, meaning you need to find some means to "save" this energy. Forget about it because it's not.

Q1 - Suggesting powering a motor coupled to a genset connected back to the source power is a way to save power is supporting an over unity scheme.

Q3 - it's not worth the expense considering the small amount of energy you will recover.

What really surprizes me is that a company which is making/testing circuit breakers and contactors is asking or considering such things.

RE: Suggestions on Improving Efficiency (Motor-Generator )

This setup does not consume much power above what the device under test consumes.

Is it the difference between rated power and actual power that is your concern? Not much to be done about that.

When decelerating, the rotation energy in the motor and test specimen will automatically be fed back to the grid. That is how a genset works. Genuine four-quadrant drive.

Has some bean counter told to improve the efficiency of this test rig? If so, you can safely tell him that efficiency is good and not much that can be done to improve it further.

If the tests are running continuously and if the existing test rig is very big for the tested device, it may be a good idea to buy a test rig more suited for low-power applications.
 

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Suggestions on Improving Efficiency (Motor-Generator )

You bet me to it, Lionel. I think this guy is a victim of some romantic ideas.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Suggestions on Improving Efficiency (Motor-Generator )

(OP)
The motor/generator runs even at time when no electrical tests are taking place or in between tests. The actual tests take only 5-10mins individually. That's why saving energy is a big challenge for us.

Starting up the generator takes time and energy that's why turning it off is not always the automatic reaction. Part of this project is to find out when is it more feasible to turn off the generator and in which cases is it better to slow it down... or feed-back the electricity.

The issue of replacing the motor is possible but very unlikely. At first we want to see what we can do with what we have. I am monitoring how much electricity the motor draws from the 6kV supply and it is huge (MWh range).

What about components like Regenerative variable-speed drives for feedback ?

If we remove the fan the motor will over-heat and burn-out during the braking process.

How can the generator be automatically feeding back electricity into the grid while decelerating. None of my current measurements show anything coming into the grid. Plus we have no frequency converting equipment to accommodate the electricity at a different frequency than the standard 50Hz.  

RE: Suggestions on Improving Efficiency (Motor-Generator )

Every motor is a generator when driven. That is how asynchronous generators in wind turbines work. If you cannot see any energy fed back when decelerating, it is probably because your losses are bigger than the probably small amount of energy available during deceleration.

If stopping the motor isn't an option, then you either have to accept the losses or find another piece of equipment for your tests.

What tests are you performing? Could it be done using a simple transformer? I understand that you do not need a variable frequency - just a variable voltage or current. In that case, a Variac may suffice? Or a thyristor controlled transformer? Or soomething else?

Please give us some details. Voltage, current, duration of test etcetera.

Your location can also be of interest - there are different possibilities in different countries.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Suggestions on Improving Efficiency (Motor-Generator )

(OP)
Im working in the testing department of a huge company in Germany.

There are many test being conducting on a whole range of electrical devices to test them to see if they adhere to IEC/ANSI/IEEE specifications.

Tests like breaking and making current on circuit breakers (capacitive and inductive currents), Lightning tests, short-circuit current tests,etc...

I know every motor can act as a generator but so far I do not see any energy being fed back in the grid.

The generator is a two pole short-circuit generator.
 

RE: Suggestions on Improving Efficiency (Motor-Generator )

Our of curiosity, can you give us nameplate info for the motor and generator?  Is there a flywheel?   How often do you run it and how long? You mentioned MWh range.... over what period of time (1MWhr over one hour or some longer period?).

=====================================
(2B)+(2B)'  ?

RE: Suggestions on Improving Efficiency (Motor-Generator )

for jimmynora:

For what possible reason do you want to recover energy? At your "Huge Company in Germany" I would imagine that this test room and MG set has been in operation for many years if not decades.

These types of MG sets, usually with a flywheel and/or massive construction of the generator were a standard item for SC labs.
It is possible in fact to brake the induction motor to a stop by injecting Low Voltage DC into the stator winding and dissipating the energy in the already existing resistor setup in the rotor circuit, but the energy is wasted as heat. This method was/is used to brake large synchronous condensers to a stop by using the starting motor (aka pony motor, a wound rotor machine)in this manner. Stopping time reduced from more than 2 to 3 hours to around 10 minutes. Reduces possibility of bearing damage in case the HP oil pumps fail during a stop.

Extra switchgear and rectifier required. Complicated.

Why now is this needed, if in fact it has been running for years?

I am sure that the "above company" could in fact engineer some kind of VFD system that would do a smooth start up and energy-recovery stop of such a machine fully electronically. In fact they have done this all before for hydro units in pumped storage plants. As well as thir competitors...

rasevskii

RE: Suggestions on Improving Efficiency (Motor-Generator )

That huge company ought to have expertise on the kind of equipment you are working with available somewhere in-house.

Wouldn't it be smarter to ask them for help?

I thought you were working in an area with limited electric supply. But I don't think that is the case in Germany.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Suggestions on Improving Efficiency (Motor-Generator )

Quote:


Starting up the generator takes time and energy that's why turning it off is not always the automatic reaction
A common misconception is that starting takes significantly "more" energy than leaving it running for long periods of time. Although the starting current looks high, it is at very low power factor at first, so the actual kW consumed is not as high as people think. Once running and unloaded, the PF is again very low, same issue. So as was mentioned, you are not consuming that much energy when the motor is running idle, all it is consuming is the kW necessary to overcome it's own inertia and not slow down. You'd probably be surprised how little that is.

In addition, your prime mover is a wound rotor induction motor I take it. if so it will not inherently regenerate unless being over-driven by the generator, which is unlikely to happen. That would explain why you are not seeing any regen. There are slip recovery systems for the wound rotor that will recoup some of the no-load losses but as others have said, in the use profile you have the savings will be difficult to justify the expense.

About the only suggestion I would have is to see if, when you don't need as much test energy, there is a benefit in running the prime mover at a lower speed. If so, you may consider a line regenerative VFD for the prime mover. That will also allow you to almost fully regenerate and recover the acceleration energy, then you can leave the unit powered off in between tests. Very expensive though if it's a large motor. If you work for Siemens though, your internal cost for a fully regenerative VFD will be a lot lower than it is for the rest of us!
 

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
  
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RE: Suggestions on Improving Efficiency (Motor-Generator )

Would this large facility have the initials I, P, and H by any chance?
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Suggestions on Improving Efficiency (Motor-Generator )

Is there any requirement to stop the unit between tests?

Obviously, you're not wrong stating the motor is consuming power in the MWh range because it will do so if measured for enough time. But, that's certainly not an hourly number and you're leaving off the important detail. A 100W light bulb will consume 1MWh of power if powered on long enough. Tell us how many hours it takes to consume 1MWh of power. Tell us what percentage of the time you are testing vs time the machine is idling. Then, we can have an idea of the losses of the machine.

When it is just spinning idle I'd expect the losses to be something like 6% or 150kW for a 2500kW motor. This is 150kWh for each hour it runs idle. At $0.2/kWh this would cost $30 an hour to leave it idling during the work day.

To regenerative brake the motor you will have to install a 2500kW, 6kV VFD. This could run something like $300k by the time you get it installed. That would take well over 5 years to pay off if you worked 250 days a year and 8 hour a day and shut the motor down every time you were not testing. It would also use more power while the motor is running, which would further hurt your payback time.

You will have to do some calculations like the above to figure out how much energy you could save so you can confirm if the solution is in fact valid. You measure the kW (not kWh) of the motor when running idle and calculate your losses due to leaving it running idle. Then, decide how much of the running time can be changed to off time. Your savings will be due to changing idle running to off time. This really isn't that hard a project which, once again, is why I'm stunned that this can't be figured out internally.

Also to repeat myself again, forget trying to "feed back" electricity while the machine is left running between tests. It's nothing but a fool's errand, that is unless you are a firm believer in over unity or perpertual motion.
 

RE: Suggestions on Improving Efficiency (Motor-Generator )

(OP)
@electricpete
The info on the nameplate I wrote in my opening post. The Generator runs as long as the tests are being done. Usually a test procedure goes for 1 hour. When there is a long break between procedure the generator is either turned off... or slowed down to like 1000 rotations/min.

I am taking measurement from the 6kv feed-in for the motor that runs the generator.From my measurement today I can give you an example of what I saw:
2 tests procedures each of about 1 hour were conducted.
Total electricity used overall 2.3MWh
Total electricity consumed during test procedures: 1.6MWh

Clearly shows that even ignoring the start-up electricity consumed to get the generator up too full speed twice today there is a potential to somehow save electricity in some way.

@rasevskii
Yes the generator is very old and uses mechanical braking to slow it down. A fan keep the system cool. One suggestion is to use the mechanical braking energy to feed-in energy if it is feasible.
We have a drive system used to maintain a constant speed and control the frequency of the generator but clearly not very high-tech. New VFD or regenerative VFDs seem like an interesting solution.

@jraef
The generator has to rotate at 3000rpm because we need a 50Hz AC frequency from our 2pole generator. Can you please explain this slip-recovery system that we can use to regenerate electricity ?

So a fully regenerative VFD to power the generator instead of the asynchronous motor (slipring rotor) is the best bet ?



 

RE: Suggestions on Improving Efficiency (Motor-Generator )


I'm surprised that six of the most competent members are responding to a post of this calibre.

By the way: I've calculated the rotational energy of one of the world's largest hydro generators. The rotor of a 825 MVA Itaipu unit is weighing about 2000 tons but the energy stored at rated speed is equivalent to only 1000 kilowatthours.

Wolf
www.hydropower-consult.com

RE: Suggestions on Improving Efficiency (Motor-Generator )

(OP)
Ok so far there have been 2 concrete recommendation. Correct me if I am wrong

i) Slip Ring Power Recovery Sytem (SPRS):
 
A variable speed drive for slip ring induction motors. It recovers and delivers the slip* dependent rotor power from the motor to the grid.

This sounds more promising since it can be integrated in the normal circuit during the operation of the motor/generator.

ii) Regenerative Variable Frequency Drive:

I guess this may be a solution for generation of electricity during the braking process. Apart from that I do not see how we can use this. Any suggestion ?  

RE: Suggestions on Improving Efficiency (Motor-Generator )

I think I am getting even more confused than jimmynora now.

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).

In your 16:20 post, you say that "the generator is either turned off... or slowed down to like 1000 rotations/min" and that confuses me a lot. How do you reduce speed to 1000 RPM when running an insduction motor with rated speed 12975 RPM? I think that we all need to understand what you mean by that statement.

There is an obvious need for improvement - your numbers show that.

"2 tests procedures each of about 1 hour were conducted.Total electricity used overall 2.3MWh. Total electricity consumed during test procedures: 1.6MWh"

Does that mean that each test consumes 800 kWh and that (assuming a fully utilized generator) each test has a 20 minutes duration. Would that imply that the 700 kWh is consumed by the idling machine during two 40 minute periods? Then, the losses are huge. More than 500 kW loss power. That is twenty percent losses. No wonder your company wants to do something about it.

But, these numbers are not very realistic. They mean that your efficiency is a low low 80 percent when idling! And probably a lot less when running fully loaded.


See why we think that you should check both your numbers and your thinking? And - how the heck can you run the machine at 1000 RPM? That's the biggest mystery.


And what about cooling? It must be getting pretty hot in that room. I guess that cooling costs are even higher than cost for electricity for the motor.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Suggestions on Improving Efficiency (Motor-Generator )

Yes wolf - it surprises me too.

But you know how it is; you think that you have all the facts and that the OP understands basic electricity.

By the time you discover that he doesn't, you sometimes leave him alone or you try to understand what the problem is.

I have now reached confusion state. I hope to get some facts soon.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Suggestions on Improving Efficiency (Motor-Generator )

OK, but the Itaipu machines are not spinning at 3000 RPM. Speed squared, you know.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Suggestions on Improving Efficiency (Motor-Generator )

(OP)
@LionelHutz
The breaks between testing procedure are usually because of rest breaks for employees. There have been cases that the Generator runs 4 hours and the actual time for testing is only 2 hours but spread over this interval. This is why we are looking at if and when we should turn off or keep it running and but back the energy in the grid (either at normal speed or slowed down).

I am a bit sceptical about the braking regenerative theory because I doubt it will be feasible.  I am more favouring a slipring recovery system kind of solution. Allowing us to feedback the excess/unused electric energy when the generator is running and no test is occurring.

@Skogsgurra

It is a normal asynchronous (induction) motor having a slipring rotor. It is being used as a pony-motor to power up the generator.
The drive system controls the speed of rotations, hence the frequency of AC.

When there is a pause of about 30minutes or more between tests sometimes the generator is turned off. Or in some circumstances the speed is just lowered down to 1000 rotations/min so decreasing energy taken from the grid. Our drive control displays the rotational speed and when can run it at any speed we want. Keep in mind we have a mechanical braking system apart from ways to increase the resistance of the circuit to slow down the rotor.

The statistic i wanted to share is that about 400KWh of energy we wasted today (after subtracting startup energy and other losses)during which the generator was running idly. Theoretically this is the energy we want to save some-way... perhaps by feeding back in the grid.








 

RE: Suggestions on Improving Efficiency (Motor-Generator )


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
www.hydropower-consult.com  

RE: Suggestions on Improving Efficiency (Motor-Generator )

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

RE: Suggestions on Improving Efficiency (Motor-Generator )

(OP)
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.

 

RE: Suggestions on Improving Efficiency (Motor-Generator )


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
www.hydropower-consult.com  

RE: Suggestions on Improving Efficiency (Motor-Generator )

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

RE: Suggestions on Improving Efficiency (Motor-Generator )

(OP)
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.

 

RE: Suggestions on Improving Efficiency (Motor-Generator )

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

RE: Suggestions on Improving Efficiency (Motor-Generator )

(OP)
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.

RE: Suggestions on Improving Efficiency (Motor-Generator )

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

RE: Suggestions on Improving Efficiency (Motor-Generator )

(OP)
Danke ! Nie wieder aufgeben :)

 

RE: Suggestions on Improving Efficiency (Motor-Generator )

Genau!
Tschüss!

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Suggestions on Improving Efficiency (Motor-Generator )

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

RE: Suggestions on Improving Efficiency (Motor-Generator )

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

RE: Suggestions on Improving Efficiency (Motor-Generator )

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

RE: Suggestions on Improving Efficiency (Motor-Generator )

Quote:

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.
 

RE: Suggestions on Improving Efficiency (Motor-Generator )

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

RE: Suggestions on Improving Efficiency (Motor-Generator )

(OP)
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.

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