i have not wrote on this site for a while but nw have the following problem that i have pondered over for a while!
I am currently a service engineer on varable speed 3.6 MW wind turbines and know that they use a asynchronous generator to convert the winds Kinetic energy into electrical energy, then convert the output varying AC supply from the asynchronous generator to DC then from DC back to the grid frequency of 50Hz AC using high power thyristors. My question is with the start up of the asynchronous generator.
The below document i have been reading states;
"Grid Connection Required On the page about the permanent magnet synchronous generator we showed that it could run as a generator without connection to the public grid. An asynchronous generator is different, because it requires the stator to be magnetised from the grid before it works. You can run an asynchronous generator in a stand alone system, however, if it is provided with capacitors which supply the necessary magnetisation current. It also requires that there be some remanence in the rotor iron, i.e. some leftover magnetism when you start the turbine. Otherwise you will need a battery and power electronics, or a small diesel generator to start the system).
now im not sure what it means when it says you need to provide capacitors to supply the necassary magnatism?are these capacitors to create a phase shift to allow the stator to induce current in to the rotor?
also when the above states there will need to be a remanence left over in the stator to start the turbine......is this to charge the capacitors?
any help would be much appreciated.
PS the document i copied the above info is a really good one and can be found here.
The generator works by a moving a conductor across a magnetic field. With a permanent magnet generator,the magnetic field is always there. Therefore all you have to do is move the conductors. In the asynchronous generator, a sufficiently strong enough magnetic field is NOT present until an electric current is run through the stator windings.
The capacitors referenced are just used as storage. Once the rotor is moving, the capacitors are dicharged into the stator windings. At this point you will have a magnetic field plus moving conductors. The generator will start producing electricity and the control circuits will control the stator winding current to regulate the generator output.
If the capacitors do not have enough energy to start the electricity production, that is when you need the power grid or a small generator to provide the "excitation" to get the big generator to start producing usefull power.
An asynchronous generator is identical to an induction motor, in that the current inducing an electric current in the rotor is created by the external power supply. The key difference is that motor loads lag- if the syncronous frequency is 1800rpm, the motor would run maybe 1780rpm. But when it's spun at a higher rate (like 1850rpm, etc), it will create current in the stator as opposed to consume it.
If you're off-grid, an external source like batteries is still required. The talk about capacitors is that if there's some residual ferromagnatism in the rotor from previous magnetization, some current is generated in the stator, and that can be captured via the capacitors to induce greater magnetization in the coil.
I understand the principles of inducing a current into a conductor that passes through a magnetic field.And i also understand that the stator acts as the electromagnet, But the bit i don't understand is yes you now have a magnetic field set up in the stator for the rotor to cut right,but as the output from the generator is fed out the stator and not the rotor well then does the induced current in the rotor then get induced back into the stator then out?
Sorry about this i kind of thought i new this already!!
Does the rotor use brushes or a coomuntator? If not, does the rotor have integrated diodes?
When I used to repair generators, some types had two independent stator windings. One set of stator windings were used to control the "field" windings on the rotor to regulate the output, and the other stator windings were the output. One of the two sets of windings on the rotor produced an AC output which was then rectified to DC to produce the rotating magnetic field which then generated the actual AC output. The advantage to this type of design was NO BRUSHES !! Of course there was a slight hit to power efficiency.
The magnetizing current comes from an external source but once you are creating the power you can use a small part of that current to maintain the magnet. As stated you can control the field strength to vary the output from either the rotor or the stator. You can pull the produced energy as DC of the rotor if a commutator is installed or as AC if slip rings are used. I think most big generators take the output off the stator as the amount of current you can pull off a rotor is affected by user maintainable parts like brushes. I is often easier to use the rotor as the exciter and control and input the magnetic field there. I assume you change to dc and then to AC to control the frequency. I assume the reason the magnetizing current must come off the grid is to ensure you don't put power on to a grid that has tripped or been shut off for any reason like maintenance.
Hi mikesh, yes we change to DC back to AC for matching the output Frequency to the grid Frequency. The magnetizing current is fed off the grid because that is the only sauce of supply available.
I am still lost as to the sequence of events from start up of generator to producing AC output in the stator winding. How can you input a magnetic field onto the rotor if it has no brushes or slip rings?
So sorry but i need to get this because my head is going to expload!!!!!
If you input the magnetizing current into the rotor there has to be a commutator but the current is only required to be enough to create the correct field strength as opposed to current off the rotor to supply loads. Putting a couple of hundred amps into a rotor via a commutator vs taking a couple of thousand amps off a rotor is quite a different challenge. the physical size of the commutator and the brushes is massively different. Add rotating mass of all that added copper and you see that they would be changing brushes pretty regularly. I don't know what current is required to create a strong enough field to generate a large amount of power in the stator but I am sure it is a lot less than what you could physically take from a rotor. A less massive rotor also takes less mechanical energy to turn it too. The other more obvious part is the rotor is turned by mechanical energy from some energy source like a coal fired turbine or water wheel in a hydro dam or wind. Way back in the day we used to find that occasionally motor driven electric welders would cease working. the brushes were good but no output. These welders relied on the residual magnetism in the iron core to start the generation and used a regulator once the field built up to control the output current. I often had to disconnect the field terminals and use a car battery to flash the fields. IE i would create a simple dc electromagnet of the fields. The iron would remember or hold some residual magnetism and after reconnecting the filed wires the welder would work again. It was possible to reverse the polarity if you flashed it wrong polarity so you always had to check that + was positive. If you got it wrong you re-flashed the field reverse to what you just did. You are trying to create north and south poles in the stator of the welders who usually took the dc output off the commutator. Welders use dc mostly so in this case the output was taken off the rotor.
Make sure to put a helmet on when you think of these things so your brains don't get all over the computer when your head explodes ;-)
To really wrap your head around how induction rotors work, consider the Activepower Flywheel:
This flywheel is just a solid chunk of steel floating on magnetic bearings. The flywheel itself contains no coils, no brushes, no permanent magnets, no wires, nothing fancy, just a chunk of steel with some teeth in it that gets spun by induction and generates electricity by induction. The only coils are in the part that doesn't move. Yet, this rotor acts as both a motor and a generator. Ponder that for a while, and when it clicks, so will asynchronous generators
The rotor of induction motors will always have SOME residual magnetic field.
This means that they can be used to generate power, also.
The technique is to feed back the induced current -- field shifting with the capacitors is most helpful -- until the rotor is actively re-polarized. The slip of the rotor versus the field poles will keep it magnetized.
WRT your automobile alternator: it operates exactly this way. It is, in fact, a 3-phase asynchronous alternator with a built-in package of 3 diodes. The field strength is modulated electronically -- other wise you might rev it up to 90 VDC!
Every issue that is involved in your project is replicated -- in a simpler way -- in every automotive alternator produced since the mid-1960s.