Compensating Winding

 Compensating Winding

• The cross magnetizing armature reaction effect that causes concentration of flux under one pole tip is caused mainly by the conductors that lie under the pole are.

• When the machines are heavily loaded, the flux density at these tips become very high resulting in higher than normal induced voltage between concerned adjacent commutator segments.

• This may cause a spark over between adjacent commutator segments more so because these coils are physically close to the commutation zone where air temperature is high and favorable for spark over. This may lead to other segments also getting involved and resulting in fire over entire commutator segment.

• Also if load is rapidly fluctuating (e.g. rolling machine), L(di/dt) voltage of coils may became high enough to start a spark over between adjacent commutator segments. This phenomenon will start from the coil under the pole center and it would have maximum.

• This problem is more acute when load is decreasing in generator and increasing in motor.

• The above problems may be overcome by use of compensating winding. Compensating winding consists of conductors embedded in pole faces and carry armature current in a direction opposite to armature conductor current under pole arc.

• The compensating winding may be designed to completely neutralize the armature MMF of the conductors that lie under the pole are resulting into restoration of main field flux.

• The presence of compensating winding reduces the duty of inter-pole winding to the extent of armature MMF of conductors under the pole are and therefore can be set to aid commutation.

• For compensated machine

             Fcomp. = [ { (Z/2) / P } * { pole arc / pole pitch } * { Ia / A } ] AT/pole

       

             Ncomp = [ { (Z/2) / P } * { pole arc / pole pitch } * { 1 / A } ] turns

             Finterpole =  [ { (Z/2) / P } * { 1 - ( pole arc / pole pitch ) } * {Ia / A } + { ( Bint / μo ) * Linterpolar gap } ] AT/pole

• Compensating winding reduces armature circuit inductance and therefore improves time response. Because it neutralizes AR under pole faces only.

• It increase cost of the machine but works satisfactorily under all conditions.