This is a question about a wind turbine generator

[Dan78]

Homework Statement

1. This is a question about a wind turbine generator
a) The blades of a 20 kW, 400 V machine rotate at 2 revs per second at rated
wind speed

i) Calculate the speed of the generator if the gear box ratio is 1:10.
ii) Calculate the current generated at rated power.
iii) This four pole machine has a lap wound armature consisting of 16 coils
each with a resistance of 0.3 ohms. Calculate the resistance of the
armature.
iv) Draw the circuit diagram for the generator with this armature winding
resistance and the shunt resistance is 50 ohms.
v) Calculate the winding losses and hence the efficiency of the wind turbine at
rated wind speed.

b) The actual wind speed is lower than rated, so that the blades rotate
at half the speed. The output voltage is now 190 V.

i) What is the induced emf at rated speed?
ii) What is the emf at actual speed ?
iii) What is the actual output current?
iv) What is the actual power generated?

Relevant Equations

I have only completed part (a), once I know I'm on the right track I'll complete b. But I am little bit confused with ii) iii) iv) from part b.

 

[Babadag]

If rated emf=415 V[400+50*.3] at 20 rps and 8 A excitation [the new iexcitation =190/50=3.8 A] then:
the new emf it has to be rated emf/rated rps*new rps/rated iexcitation*new iexcitation=415/20*10/8*3.8=98.56 V
In this case the new current is negative [impossible!]. So, in some way the shunt excitation is still supplied by 8 A [?] and the new emf=415/2.