Synchronous machine fall into the category that stator carries AC current and rotor carries DC current (inverted operation found in small machines) The shaft is driven by a turbine or a suitable prime mover in a case of the generator. In principle, the stator magnetic field and the rotor magnetic field should not have a relative speed on all machines. In synchronous machines, the stator field ( due to three-phase current) is rotating at a speed of 60fprpm. Therefore to obtain this same speed in the rotor field we must rotate the rotor at the same speed 60f we know is synchronous speed.
F- Frequency of the stator of three-phase current
P- pole pair number of the stator winding
The terms synchronous machine is used to recognized fact of the rotor speed is equal to the synchronous field speed.
To excite a generator, for example, to provide a DC current to its rotor winding. There are several methods of excitation is used such as following.
- Using external DC and slip rings
- Using generated DC and slip ring
- Using generated AC and shaft sounded rectifier.
Real power control of synchronous machines
The real power output is controlled by adjusting the mechanical power output from the turbine. This is done by the spectral unit which was known as turbine governor. The turbine governor is inlet valve control mechanical 5 m with a user interface. There are several types including electronic governor the principal of operation is explained using the good old mechanical governor. The distance ‘x” determines the inlet valve opening of the governor and this opening of governor has two operation modes such as.
- Pre- setting operation
- Free running operation
Pre-Setting means to set the initial value of x and this is done by raising or lowering the whole assembly on the center shaft. Free running means self-operation on a given preset point that meant the self-decreasing of x when the speed N is going higher of self-increasing x when speed N is going lower. In order to choose a preset point in this governor.
The real power output from a generator connected to a larger system will be controlled by alternating. The governor preset as the speed N is fixed by a system.
Reactive Power Control of Synchronous Machine
Reactive power output from a generator is controlled by adjusting the DC current to the rotor. To raise the reactive power we should increase the excitation or to lower reactive power we should decrease the excitation. Excitation control is initiated via unit called Automatic Voltage Regulator (AVR).
If we want to produce some additional AVR purposely we can apply a direct input. In if free running operation, only reactive power device cause terminal voltage (Vt) to fall and causes the AVR to pass more field current (If) to the generator field. Then the internal emf (E), go up and produces more reactive power in order to restore (Vt)