1. There are three terms used in this context: Load angle, Power angle and Torque angle.
The load angle primarily used for synchronous generator and torque angle is the same thing for synchronous motor. Power angle is also same in context of synchronous machines but it is more generic – also used in power transmission line.
Let me explain what they are:
Load angle (or Torque angle): For a synchronous generator, the magnetic field rotates at synchronous speed and the rotating magnetic field is created in the stator. These two fields are not fully aligned. The stator field lags the rotating field. This lagging expressed in angle is called load angle. The power developed by the generator is directly proportional to sine of this angle. This angle plays an important role for the stability of the generator. If the angle goes beyond 90º, the generator becomes unstable. This may happen when sudden change of large load occurs or fault sustains longer time. The generator instability is one of reason for massive blackout in case of major fault occurs in transmission line.
For the case of synchronous motor, the angle is called torque angle and the rotating field lags the stator field in this case.
Power angle: For a generator, the power angle is the difference between the generator induced voltage and the generator terminal voltage. The value of the power angle is same as the load angle. So, in context of generator, power angle and load angle mean same thing.
For the case of transmission line the power angle is the angle between the angles of the voltages at two different points (bus). The transfer of power between the two points of power system is proportional to the sine of this angle.
Though there is a distinct difference between these three terms, they are used synonymously in many cases in power system.
See the links below for additional help:
2. Power transfer is
P = [(E1 x E2) sin (a)]/X
where
E1 and E2 are the sending and receiving end voltages
a is the angle between E1 and E2
X is the reactance of the circuit
This expression says that will voltages constant, the power transfer through a circuit is a function of a. If a is zero, there is no power transfer. Maximum power transfer occurs when the a is 90 degrees. Because power transfer is a function of a, a is also known as the 'load angle'.
If you have an array of generators on a system, the fact that the system operates at a single frequency means that the generators must all be rotating at the same speed. (Well - almost. All two-pole generators will be at one speed, while all four-pole generators will be at exactly have that rotational speed, etc.). But there will be a small difference in the physical position of the rotors of those generators - the rotors of generators that are more heavily loaded (in per unit of their individual rating) will lag the rotors of the more lightly loaded machines. These angular differences are the load angles of the individual machines and can be calculated from the power transfer expression above
3. In synchronous system while generator generates rated power and load increase in system, Than frequency start goes to down respectively same as rpm of the all generator start reduce but the speed of the rotating magnetic field is same, So the angle created between rotating magnetic field and generator rotor, This angle are known as load angle of generator.
4. Torque is the rotating force, right??
eg: you can pull 50 kg of load to a distance 100m in 1 min by appling 100 newton energy ( your torque)
60 kG to 100 m in 1 min by 150 N
if 60 kg is your full load and 150 N is your max torque then
100 kg to 100 m in 3 min by 150 N
Similarly this torque angle is the angle between stator pole and rotor pole.According to design in a 4 pole motor the stator and rotor pole exactly matches. Since there is a rotating stator mag field and stationary rotor magnetic field, the rotor poles will get locked to the stator poles and rotates in a speed equl to stator mag field called sync speed. ok
If there's a extra load on the rotor shaft. although the rotor wont loose its mag lock with the stator poles and slightly lags in its pole matching. we term this unmatched displacemnt of rotor from stator pole match in terms of angle called torque angle. even with the created torque angle, rotor runs at sync speed.
This torque angle is because that the motor need more power to drive the increased load. i.e field should be strengthened. so in order to decrease the load torque sync motor creates a torque called sync reactance torque to exactly match the poles ( nothing but decreasing the load angle).
You can further decrease the torque angle by strengthening the field but it is limited by the maximum load that the machine can take.
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