Kindly tell me how generator increase its load by just increasing the fuel of prime mover. Increasing the fuel prime mover should increase its speed, but it does not happen only load increases

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Increasing the fuel flow to the prime mover increases the speed and therefore causes the alternator rotor to move ahead relative to the magnetic field produced by the terminal voltage. Since the electrical power produced by the alternator depends on the sine of the angle between the internal emf and terminal voltage, the electrical power increases.

When the electrical power matches the mechanical power supplied on the drive shaft, there is no surplus power to cause further acceleration and the system comes o new stable state with the rotor rotating at the same speed as before but with increased lead angle.

 

Bruce, Much too wordy, and no exclamation points.

The basic formula for electric power produced by an alternator (synchronous generator) is: P = Vt * Ia, where Vt is the alternator terminal voltage, and Ia is the armature current (the armature is the stationary part of the alternator that is connected to the load). Since the terminal voltage of most alternators is relatively constant, the way to increase power is to increase the amperage. To increase the amperage, one has to increase the torque (since an alternator converts torque to amps).

An alternator is a device for converting torque into amperes. Amperes can be transmitted long distances over wires, and then re-converted into torque in an electric motor. That's why we produce electricity, because it can be easily transmitted and reconverted into torque at remote locations. Early hydro turbines (sources of torque) were located long distances from factories where torque was required.

An alternator is locked into a speed that is directly proportional to the frequency of the grid and other generators with which it is in parallel. Increasing the fuel should increase the speed of the alternator rotor, but the speed can't be increased because the alternator rotor is locked into speed by the strength of the magnetic field interacting with the magnetic field of the stator which is a function of the frequency of the grid frequency with which it is connected. Increasing the fuel increases the torque being applied to the alternator rotor, but because the rotor can't increase in speed the generator converts the torque into amperes.

Now, to your point, Bruce, the increased torque being applied does indeed cause twist on the load coupling, which is called the load angle. More torque; more twist. And the electrical power produced is a function of the amount of torque (amount of twist).

The increased torque is, indeed, trying to increase the speed (and it does so imperceptibly for a very brief time as the torque is increased) but the rotor is locked into a speed that is a function of the grid frequency with which it is connected. It can't go any faster (or slower) than the grid frequency because the rotor is being held in synchronism

 

Hi CSA,

I can add a few !!! if you like.

It's all to do with relative motion - if you are sitting on the rotor going round at 3000 RPM, and have some way of seeing the rotating magnetic field, increasing mechanical drive power with the machine synchronised will result in movement of the rotating poles relative to the magnetic field, and an increase in the angle of the rotor field ahead of the stator field. If the two were perfectly in phase, all the magnetic lines of force would be perpendicular to the rotor and not exert any torque. As the rotor is driven ahead, there is a small component of field which is tangential to the rotor and this is what produces the torque. And if the angle increases sufficiently, and exceeds 90 degrees, then the load torque presented to the shaft falls, the rotor accelerates further, and evil will be the consequences thereof... Torque x speed = power - and since speed is essentially constant, we can equate the 2 approaches. Speed and voltage are also related, and by butchering the equations we can show that current and torque are interdependent as well. (I'm very aware of the interrelationships as I am currently working on getting a BLDC motor with permanent field to work as a synchronous motor using a 3ph PWM generator to drive. Unfortunately the sine wave amplitude has to be modulated as well as the frequency...)

Cheers,

Bruce

 

Can anyone help me on this...

We have synchronized 6 units of generators but not connected to the local utility. When we do load the system, one of the generators has a leading power factor and the rest have lagging. But the generators share equal load though. Please tell me, is there any problem with our system??

 

Hi blackstoneboy,
Power factor depends on the alternator excitation and does not depend on the load. Is there a system to coordinate power factor among the sets? One possibility could be a current sensing system connected the wrong way round.

Bruce