Hi everyone,

In gas power plants, they will start the synchronous machine as motor and later turned to generator mode. My question is...while starting as a motor, they will provide a field excitation first and some emf will be induced in stator (VL1-L2,VL2-L3,VL3-L1). based on these voltage levels, they are detecting the pole position of rotor and they will start to give ac supply to stator based on pole position ...but how they are detecting? how they are calculating the pole position with these voltages? please explain with 2 pole machine.

 

See my reply to your latest post on thread http://www.control.com/thread/1351224915#1351654329

 

chandru,

In <b>some</b> gas turbine-generator power plants the synchronous machine is used as a starting means (motor) to accelerate the shaft from zero speed to near synchronous speed, and then use the synchronous machine as a generator. This is done through the use of a device that can produce variable frequency AC, which, when applied to the stator of an AC machine will cause the rotor to spin at a speed that is directly proportional to the frequency.

Your machine (9FA) uses a two-pole rotor, which means the North and South poles are 180 degrees apart from each other. The stator is a three-phase stator, meaning there are three sets of windings through which AC voltage and -current can flow.

During starting the rotor is being turned by the turning gear at a slow rate of speed; that's where the motion comes from. As you have told, excitation (DC) is applied to the rotor field to produce a magnetic field, that is turned by the turning gear mechanism induction motor. When there is relative motion between a magnetic field and a conductor (or conductors) a voltage is developed in the conductor. As the strongest part of the rotating magnetic field (in your case the magnetic field produced by applying DC to windings is the rotor) passes a conductor the voltage in that conductor reaches a peak (positive as one rotor pole passes the conductor(s); negative as the other rotor pole passes the conductor(s)).

The LCI (Load-Commutated Inverter), also called an SFC (Static Frequency Converter) among other names, looks at the magnitudes of the voltages in the three stator windings and uses that to know where the rotor poles are. Again, for a two-pole machine the rotor poles are 180 degrees apart--opposite each other.

For "pictures" and animated representations of this, just use your preferred Internet search engine to look for AC electrical fundamentals and you can find single-phase as well as three-phase descriptions and drawings and animations any number of places on the World Wide Web.

You're asking how the variable frequency control system knows where the poles are: by sensing the magnitudes of the voltages induced in the stator winding conductors as the turning gear mechanism rotates the rotor windings past the stator conductors (watching when they rise and peak and start to decrease, basically). It's like the variable frequency control system has it's own oscilloscope and can "see" when the waveforms peak--which is when the rotor pole locations are known.

It's important to know where the rotor poles are because if the stator is energized when the rotor is about 90 degrees out of phase with the stator field produced when the stator is energized it might rotate in the wrong direction by the force of magnetic repulsion. But if the resulting stator magnetic fields are not attracting the rotor magnetic fields, whether the forces of magnetic repulsion spin the rotor in the right or the wrong direction they are going to violently spin the rotor for one quarter of a rotation and then stop it very quickly, and that's not good. Not good at all.

Or, worse, if the North pole of the rotor is directly above a North pole of a stator field (which would mean the South pole of the rotor is above a South pole of a stator field) the forces of magnetic repulsion could spin the rotor very fast in either the right or wrong direction--which would be very damaging to the machine--and then stopping it almost as violently. Again, we all know how strong the forces of magnetic repulsion are or can be. And in this case the rotor would be spun very quickly almost half a rotation before being brought to a very quick halt by the forces of magnetic attraction--and, well, that's not good.

Unfortunately, we can't use pictures or drawings on control.com, and it would be extremely difficult to try to explain in words how three-phase windings are distributed in a synchronous machine's stator. But the good news--about this point--is that if you want to understand how voltage is produced in a synchronous generator there are many great sites on the World Wide Web that can demonstrate this using all manner of descriptions, drawings, animations, and even videos.

The variable frequency drive is just making use of basic electrical fundamentals to determine where the rotor poles are before applying current to the stator windings, to protect against rotating the machine violently--in either the correct or the incorrect direction--and then stopping it almost as violently, and causing damage to the machine. (This is called "slipping a pole" when it happens when torque is being applied to the generator rotor--and it's even more destructive then!)

 

CSA,

I like your explanation, but would take issue with one detail.

The peak voltage actually occurs 90 degrees after the pole passes (the quadrature axis of the rotor, not the direct axis). Since the emf induced in the stator is proportional to the rate of change of flux, we know that the rate of change of a sinusoid is greatest at its zero crossing (which is 90 degrees after the peak).

Chandru,
CSA is correct in that by looking at the waveform of the stator voltage while the machine has not been connected to load, we understand where the rotor poles are. As the voltage sine wave is negative and crosses zero and becomes positive, at that zero crossing, the North Pole of the rotor is theoretically aligned with the phase conductor.

However, as soon as you connect load to the stator, the poles shift relative to the stator voltage, and the amount they shift is referred to as the load angle. At this point, it becomes very difficult to determine the position of the rotor poles, and has been the subject of much research, and many PhD dissertations.

 

nic,

Thanks very much for the clarification. There are mechanical degrees and electrical degrees, and I try not to mix the two and sometimes get mixed up myself. I should really look at some references sometimes before I write these things.

However, the concept of looking for rate of change (fastest or slowest) is the same when trying to determine where the rotor poles are. I've been in the Generator Collector Compartments of many F-class machines observing starts and have seen many rotors "jump" just a few degress (mechanical degrees!) when the LCI energizes the generator stator windings. It's almost possible to feel the "bump" in one's feet. It's not an exact science, but it keeps the rotor from being violently spun and almost as violently stopped!

 

Hi CSA,

Thanks a ton for your answer. I too guessed the same, but i am not sure. But now, i am clear. Thanks again.