well i can say i am not from iit or a reputed institute but as far as my knowledge goes it goes like this that its not a multiple of 11 its a multiple of 1.1 i.e. Kf(form factor)look that whenever we are generating the armature that goes through the field coils will always generate a.c. so to get maximum utilization of armature we can generate only voltage that are multiple of the form factor as if u wanna generate say 114kv or 500kv then u will finally end up with slots having conductors that are fractions i.e to say if coil 2 & 6 are made up of same conductors then 2 will be along the entire length of the armature but 6 will be only to a fractional length so total utilization of armature is not possible.But this explanation is only true iff the average value of the voltage is an integer as Kf=(avg val)/(r.m.s. val) so in this case i have a doubt as what to do for fractional avg val?