Page 235 - Fluid Power Engineering

202 Chapter Te n

on the axis of the stator with N-S pointing radially and the axis of the
spin coincides with the axis of the stator, as shown in Fig. 10-2.
The rotor is the moving part of the generator. This is connected to
the turbine rotor to which the blades are attached. The different types
of rotors will be explained later in the chapter.

Conversion of Mechanical to Electrical Power
A wind turbine captures linear kinetic energy of wind and converts it
into rotational kinetic energy. This rotational kinetic energy from a tur-
bine is transferred to an electrical generator either directly or through a
gearbox. The electrical generator then converts the mechanical energy
into electrical energy.
Consider a fixed-speed generator with a steady angular velocity
of ω; let E be the amount of kinetic energy per second and ε be the
overall efficiency of power transfer from wind to the generator. The
torque transferred to the rotor of generator is:

E
τ g = ε (10-19)
ω
Next, consider the stator of the generator that is connected to a grid
with terminal AC voltage of V T with a frequency of ω. Power in an
electrical circuit is V T i cos ϕ, where i is the current flowing through the
circuit and ϕ is the phase difference between the voltage and current.

τ g ω = V T i cos ϕ (10-20)

Simplistically, in the above equation, ω and V T are constant. Therefore,
torque governs the amount of current and phase angle. The remain-
ing question is: How is the mechanical torque that is applied to the
rotor of the generator balanced by an opposing torque? If the torque
is not balanced, the rotor will accelerate out of control. The answer is:
The balancing torque is provided by two mechanisms described be-
low. Remember that the stator creates a rotating magnetic field with
angular speed ω and the rotor is rotating with an angular speed of ω.

In a synchronous generator, the magnetic pull between the

rotor and stator provides the balancing torque. This magnetic
pull is the tangential force applied when the opposite poles of
the stator and rotor are not radially aligned.
In an asynchronous generator, balancing torque is provided

by the tangential force experienced by current carrying con-
ductors in the rotor in the presence of magnetic field created
by the stator. For the torque to be generated, the speed of
rotor has to be slightly higher than the speed of the stator’s
magnetic field.