Synchronous Motors 1

[innomation]

I would like to know if I can use an 800 HP synchronous motor as a hydro-electric generator (connected to the grid)
If it's possible what are the advantages and dis-advantages?
Thanks

 

[ScottyUK]

In principle yes you can, if the machine is compatible mechanically. Hydro machines are fairly specialised, usually being big low speed vertical shaft designs.

Advantages and disadvantages relative to what?



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

 

[innomation]

Thanks for your reply ScottyUK. I'm building a small hydro-electric project in northern Ontario and I got this motor for free. It was rewound 13,000 hrs. ago. I want to use it with a Kaplan turbine running at 330 rpm and the motor is running at 885 rpm. My question really is whether there will be any issues controlling output that I wouldn't have using a generator or a different type of motor.

 

[ScottyUK]

So far as I know there are no electrical reasons why you couldn't do this, but on hydro generation I've got very little experience. I spend my time working with large 3000 rpm turbo alternators, which are an very different breed.

You will need to consider the excitation arrangements. Most generators of this size use a PMG on the main shaft to raise excitation power. Motors don't always have this capability because by definition they have an available power source.

You will need to investigate protection for this machine, and again my limited knowledge of hydro applications comes to the fore. On a high speed machine with a turbine as prime mover, or even on a diesel or gas engine set, the protection is much more complex than that for a motor.

What are you planning to control it with? There are a lot of things to consider in this area too which would not be relevant with a motor.


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

 

[innomation]

Please indulge me, my knowledge in this area (motor design) is limited, my specialty is assembly machine design.
There are brushes and 3 slip rings for excitation, but no generator. Primary volts 2300 at 180 A, 3PH. That I understand. Secondary rating is 625V and 575A. What will the secondary requirements be to operate as a generator?

 

[jplinn]

You can use a synchronous motor as a generator. However, for hydro application over speed becomes a critical concern. Upon a load rejection the turbine generator can easily reach speeds of 2 times normal running speed.

In the event of load rejection and failure of the wicket gates to close the runaway speed can reached, typically higher than rejection speed and sustained for a longer duration. We use a hour.

These operating senerios require stronger rotor parts and the bearings have to be oversized to handle heat generation from the sustained overspeeds.

These are just a few of the items that need to be considered.

 

[innomation]

Thanks jplinn.
I don't know yet how quickly the turbine/generator will accelerate, but I plan to stop the water flow within 5 to 10 seconds of a load rejection. Is that fast enough? The rotor has a lot of inertia.

 

[bigamp]

innomation,

If your motor has three sliprings and a secondary rating is stated then it sounds like it is not a synchronous type; it may actually be a wound rotor induction motor.

The key thing with a synchronous motor is that the rotor winding is DC (you apply a DC voltage to it) so there would be only two sliprings. Synchronous motors operate at the exact synchronous speed for the number of poles they have. Induction motors are always a bit slower.

Induction motors can be used as generators and when used as such they must always run a bit faster than synchronous speed. There are many induction generators in service. They need a source of reactive power to operate (often taken from the grid or from a static capacitor bank). If your motor is an induction type, I'm not sure what would need to be done to the sliprings to use it as a generator. Maybe short them out or have some resistance in there?

 

[alehman]

You say the turbine is 330 rpm but the motor is 885?
Do you have a suitable gear box? Surely the motor is rated 900 rpm.

 

[mc5w]

There is a book named Collier Electrician that has a section on using a wound rotor motor as a synchronous motor. When the wound rotor motor is up to a certain speed the DC excitation is connected across 2 of the slip rings. You then remove the rotor resistance or change it to a value that will not waste too much excitation power.

In your application you would need to leave the rotor resistance connected so that the windings will act as an amortisseur or damping winding that will keep the rotor from oscillating when load changes. This will waste some excitation power.

Your motor will not work as well as a machine that is built as generator because a generator uses a low resistance squirrel cage winding as the damper winding which works better.

I also want to know how you are going to afford a gearbox that will step up 800 HP from 330 RPM to 900 RPM. I know that Fairbanks Morse uses generators in the speed range of 400 to 1200 RPM for their diesel generators, so you ight be able to go to their source and get what you need.

 

[innomation]

Thanks guys;
bigamp must be right, the motor has to be a wound rotor type rather than synchronous. It has 3 slip rings, and as alehman pointed out the rpm is 885, not 900.
As for the reduction, I have the original flat belt drive. This motor was used to drive a tree chipper at a pulp mill in Sturgeon Falls, Ontario until last year.
The question that remains is what to do with the rotor windings. I'll try and find the book that mc5w mentioned.

 

[mc5w]

The book is published by England's National Coal Board. You will not find it at Amazon. I already tried that. I loaned my copy out to somebody in 1985 and it had been buried at 1 of his houses since then.

 

[ScottyUK]

England's NCB has been a distant memory for more than a decade now. You'll be lucky to find a copy of that book anywhere - it's prized by the UK's engineering fraternity and when the odd copy comes up in a used book store it gets grabbed pretty quickly. No wonder your friend wants to hang on to it!



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

 

[cloche]

Innomation,
I have no knowledge on generators so I won't be able to help you in this field. I do mechanical calculations for hydro turbomachines and so I have to warn you:
please be extremely careful with respect to the hydro-related parameters, or you might destroy your plant the first time you run it!
1- opening times have to be regulated in order to allow the machine to gradually accelerate. How much depends on all the inertias and on the stresses allowable on the blades, on the blades' bearings in the hub, in the hub itself, etc...
2- calculation of the flexural eigenmodes and critical speeds of the system is crucial
3- calculation of the torsional eigenmodes is crucial to determine the correct torque transmission to the turbine main flange, in case of 2-phases short-circuit, 3-phases short, and anti-parallel
4- closing times are dictated by the plant layout ("rythm" of the conduit, max allowed waterhammer pressure, etc...). You have to do careful calculations in a scenario where you loose any "active" control on the system (it may seem unlikely, but it happens more frequently than one can think...)
5- integrate a system to dis-eccitate the generator over a predetermined speed, so that in case of runaway you won't deal with electric pull. If you can, provide your shaftline with a centrifugal disruptor
6- provide your plant with a so-called "synchronous discharge", that is a bypass valve on the suction side of the runner that will open when distributor closes abruptly (waterhammer case...), to prevent destruction that can incur when downstream side water column, that goes on by inertia, returns back (simply said...). Also, when distributor gets closed "in emergency", the Kaplan blades have to become fully-open in order not to be damaged by this "return watercolumn".
There are also a lot of other issues to take into account, here are only a few... Well, I think that you already know, if you're planning to build your own small plant.
Good luck!
Claudio

 

[itsmoked]

cloche; That was an informational and interesting peek into the big hydro plant realm thanks!

Sounds like I probably shouldn't have been standing at the bottom of the 1,600ft penstock, 2ft from the 500MW wicker gates during the load dump commissioning test for Helms Pumped Storage. hmmmm? Was exciting tho! :0

 

[innomation]

Thanks cloche for your input. You certainly know your stuff.
Our plant has 12 ft. of head and the penstock is only 55 ft. long. We will examine all the hydro-dynamic parameters
carefully and use closed loop control on anything that involves water flow. On a plant this size we can afford to go overkill on control to minimize the potential pitfalls unforeseen due to our lack of hydro-electric experience!

 

[davidbeach]

itsmoked,

If you ever saw the burst penstock from the Helms project, you wouldn't have wanted to be around it even when it was running with normal flow. It's been many years (prior to 1990) since I saw the damaged penstock, but that thick steel torn like a thin sheet of paper gives one an appreciation of the power of flowing water.

 

[itsmoked]

davidbeach are you refering to the burst that gouged that entire canyon out and cut the highway next to the mess hall???

BTW I had to crawl out into the center of the cable mesh to lower a pressure transducer down into the 1,400 tall 5OFt diameter surge tower to monitor the water rise during the load dump. (dropped my wallet 1,000ft to the water)

 

[cloche]

Hi everybody,
yeah, "perhaps" I was a bit "exaggerated", but... 800 HP machine is already not a children toy, many manufacturers have machines like that called "compact-range" machines and they dimension them not far as they do for large plants. Nobody wants to throw away money and work, so...
Well, the calculation principles are always the same; where you can apply simplifications:
- instead of installing more flow control devices, valves, etc, you can make some simple analytical calculations to estimate the force of a back-flow. Then, over-dimension the main plate and the supports... This can not be done with very large machines, but in your case it's more cost-effective!
- as you say, you can incorporate some "tricks" in the control logic of the machine: e.g., you can use the blades themselves as a "brake" in case of incipient runaway by opening them to fully-open, regardless of the position of the distributor (which will close in emergency "as soon as possible" but with parameters of its own). Make estimations to determine if the force upon the blades in this case can be afforded by the components or if it's better to whithstand the centrifugal force of a runaway.
- a simplified eigenfrequencies calculation (two-masses schema, for example) can allow you to verify how much abnormal torque can be transmitted to the coupling flange or joint, or to verify that your assembly is always "under-critical" even in runaway speed. As runaway is a bit complicated to exactly compute, you can prudentially estimate it to 2.0 - 2.2 times the nominal speed. Probably a simple mechanical centrifugal disruptor is the best way to save the generator (= ex-motor in your case, where I suppose this overspeed has not been taken into account in the design) especially as regards bearings and poles fixations.
Never seen the Helms, but I saw a 8 MW low-head Kaplan completely destroyed by back-flow because the synchro-discharge didn't work... Water is powerful, I agree...

 

[davidbeach]

itsmoked,

I don't remember exactly, but I doubt there was more than one such event. What I do remember was all of the damaged penstock lined up along the road. At that point there was still ongoing litigation, so the evidence was left out in the open, a bit too big to take into a courtroom.