The procedure how to transform an asynchronous motor into a PM-generator is described in my report KD 341 which you find at the menu KD-reports on my website: www.kdwindturbines.nl
. The maximum torque level of a generator or a motor for which a certain magnetic flux is realised in the air gap, is about proportional to the armature volume lying within the stator stamping. The mechanical power is proportional to the product of the torque times the rotational speed. So a 6-pole asynchronous motor which can supply a certain power at a rotational speed of about 964 rpm (at f = 50 Hz) needs an armature volume which is about a factor 1.5 larger than for a 4-pole motor which can supply the same power at a rotational speed of about 1450 rpm.
However, a direct drive PM-generator made from an asynchronous motor, is normally used at a much lower rotational speed than the original nominal motor speed. The windmill rotor supplies a certain maximum torque depending on the wind speed and the safety system which limits the power above a certain wind speed. So the armature volume of the PM-generator must be at least that large that it can take the maximum torque level of the rotor. It doesn't matter if the motor has originally 4, 6 or 8 poles, it is the armature volume which counts.
However, if the original winding winding is used, the open voltage which is generated at a certain rpm, is proportional to the pole number. So the open voltage of a 6-pole PM-generator is a factor 1.5 higher than for a 4-pole PM-generator. If the PM-generator is used for 24 V battery charging, it might be that this is possible to use the original winding of a 4-pole motor but that the voltage of the original winding of a 6-pole motor is too high or that a 6-pole PM-generator with the original winding can only be used for 48 V battery charging. It is possible to connect the first and the second layer of the winding in parallel in stead of in series and this halves the voltage and doubles the current. The voltagre can also be reduced by a factor square root of three, if the winding is rectified in delta in stead of in star. However, rectification in delta has the disadvantage that the sticking torque is rising faster than for rectification in star because higher harmonics can circulate in the winding and this may give starting problems (see report KD 78 for measurements of different onloaded torque curves).
Theoretically it is possible to rewind the generator completely with a number of turns per coil and a wire thickness such that the matching in between rotor and generator is optimal for a certain nominal battery voltage. However, a special winding may have extra cost more than that of a complete new motor with a standard winding.