Commutation in Universal-Type Motors

The problem of commutation has always been one of the most important problems in the satisfactory operation of both d-c and a-c commutator motors. The universal motor has the problems of both a-c and d-c motors. The many factors influencing satisfactory commutation such as electrical and magnetic conditions, mechanical condition of the armature, carbon brush condition, and in some applications such as portable electrical tools, atmospheric conditions may be such as to influence commutations. In both the d-c and a-c motors, the armature conductors cutting across the magnetic field set up by the combined armature and field winding generate electromotive forces in the short-circuited coils which are proportional to the product of the speed of the armature, the flux which the armature conductors cut, and the number of turns in series. However, in the a-c motor, there is a voltage generated by the reversals of the main field flux just as the voltage is generated in the secondary of a transformer. In the noncompensated motor, commutation can be somewhat improved by using a strong main field and weak armature field and moving the brushes backward. However, this has no effect upon the transformer voltage. In the compensated universal motor there is a compensating winding distributed at 90 electrical degrees to the main field and opposing the armature field, or in the single fied distributed winding the brushes are moved back a predetermined amount which compensates for the voltage due to the armature field.