Residential Wind by Way of Illustration - Wind Power for the World: The Rise of Modern Wind Energy - page 582

becomes below the battery voltage, as part of the cycle the turbine
is short-circuited, that is, the voltage is zero then. For the buck it
becomes higher than the battery voltage, as part of the cycle the
turbine

is allowed to accelerate with the battery being

disconnected.
The emf (= open circuit voltage) of alternator of given
construction depends on rotational speed. The alternator and the
controller are parts of the same circuit. If alternator emf is higher
than the voltage on controller input, it creates current in the circuit
(i.e., mechanical torque on alternator shaft) that slows down the
alternator and blades. If alternator emf is lower, it can't create
any current and torque—the alternator and blades accelerate.
Eventually there is equilibrium in the circuit: the rotational speed
of turbine is stable for the given wind velocity and input controller
voltage.
On the stage of initial testing the characteristics of turbine are
taken; for each value of wind velocity the rotational speed of blades
providing for maximal power from the wind is specified. As the
turbine is tested at diferent wind velocities, the peak power curve
images; it is a combination of numerous points—maximal power
at certain rotational speed for the given wind velocity, the later as
a parameter. It is not a secret for an educated reader that it is a
cubic curve.
For practical use, the curve is recalculated the following way.
Rotational speed is converted into voltage on the output of turbine,
same as input voltage of controller. Power divided by rotational
speed is torque. It is converted to current based on alternator
characteristics. The result is a modified peak power-tracking
curve: voltage as a function of current. It is stored in controller
memory.
The controller has a current sensor and it sets its input voltage
as a function of measured turbine DC current. Unlike PV industry,
where universal controllers are common (they look for the power
peak), the wind controller is intended for a particular turbine with
specified characteristics. Searching for peak power usually leads
to unnecessary oscillations of turbine speed.

26.6 Slow Mode

Of-grid customers know the painful truth: there is no correlation
between their need for electricity and power production of PV