Electronic – So the brushes in a commutator are fixed and assume a constant polarity, so how does the commutator reverse the direction of current

There is no such thing as "polarity" in AC power generation. There is only "phase" - and as with all power generators wind turbines are phase-matched before being linked to the grid.

Also, wind turbines only spin one way. They pivot so that they face into the wind, and the wind can only turn the blades in one direction. The blades work on the same principle as aeroplane wings. If they could be blown in either directions then aeroplanes couldn't fly.

Commutator segments = Commutator bars. Segments are made of copper bars separated by mica.

Coil B is connected to commutator segment b and c. C to c & d. 4 segments connected to 4 coils. Each segment connects two coils. All coils are connected in series around commutator. Brushes connect commutator in parallel.

As your rotor spins, Faraday's Law applies. Whenever the flux linked or associated with a circuit changes, a voltage is induced in the circuit.

So in Figure 4.7, coil A and C are moving parallel to the flux. No flux lines are cut, so induced voltage is 0. Coils B and D are moving perpendicular to the flux, so maximum voltage is induced. The text says 20V. \$E_B = E_D = 20V\$.

And how come the voltage in the first picture is eb+ec?

This is not correct.

Bottom of p74.

Consequently, the voltage induced in these coils is at it maximum possible value (20V, say). That is also the voltage across the brushes at this particular instant.

So in Figure 4.11b, we have the same size coils, producing 20V at maximum. Coil A and B produce 0V. Coil C and D produce maximum or 20V. The 18V coil is \$ 20\ sin (60°) = 17.3V\$. The coil is cutting the flux lines at roughly 60°. The 7V coil is \$ 20\ sin (30°) = 10V\$. So: $$10V + 17.3V + 20V + 17.3V + 10V = 74.6V$$

The coils are not fully at 30° and 60°, so voltages are less. Or 70V. But this illustrates where we are.

I disagree with the authors Figure 4.8, which covers Figure 4.10, but this has more to do with understanding the theory of how it works. Two coils at 45° will produce more than 20V. No DC Generator commutator has 4 segments.