Synchronous Motors
Synchronous Motors
(OP)
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
If it's possible what are the advantages and dis-advantages?
Thanks






RE: Synchronous Motors
Advantages and disadvantages relative to what?
----------------------------------
If we learn from our mistakes,
I'm getting a great education!
RE: Synchronous Motors
RE: Synchronous Motors
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.
----------------------------------
If we learn from our mistakes,
I'm getting a great education!
RE: Synchronous Motors
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?
RE: Synchronous Motors
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.
RE: Synchronous Motors
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.
RE: Synchronous Motors
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?
RE: Synchronous Motors
Do you have a suitable gear box? Surely the motor is rated 900 rpm.
RE: Synchronous Motors
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.
RE: Synchronous Motors
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.
RE: Synchronous Motors
RE: Synchronous Motors
----------------------------------
If we learn from our mistakes,
I'm getting a great education!
RE: Synchronous Motors
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
RE: Synchronous Motors
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
RE: Synchronous Motors
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!
RE: Synchronous Motors
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.
RE: Synchronous Motors
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) :(
RE: Synchronous Motors
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...
RE: Synchronous Motors
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.
RE: Synchronous Motors
In the event of a load rejection and correct operation of wicket gates (they close) a balance needs to be reached between maximium overspeed and pressure rise in the penstock.
Using the calculated maximium pressure (+ safety margin)the penstock can handle and the maximium rotor speed a series of unit trips are performed.
Install a high speed pressure transducer into the penstock, and quick responding speed transducer on the unit shaft and if available a turbine flow transduser (ie winter-kennedy). The pressure needs to elevation adjusted.
Perform a series of trips starting at full speed breaker open than full speed breaker closed. Continue the trips at higher loads. Trips are initiated as a breaker opening.
Perform a balancing act, adjust the gate trip closing time to get the lowest over speed and lowest penstock pressure rise. Adjusting the cushion stroke should have little effect on pressure rise. After completing the testing adjust the numbers for maximium head.
This is general outline, you will need to develop specific test plans.
Overspped limits per NEMA MG-1 1993
Syn Speed % Overspeed (<1 minute)
1801 and over 20
1800 and below 25
jpl
RE: Synchronous Motors
RE: Synchronous Motors
Simon