Radio Frequency Interference from Small a.c. Commutator Motors

Abstract

Small a.c. commutator motors as used in domestic appliances inherently generate unacceptably high levels of Radio Frequency Interference (RFI) and thus require suppression. The factors affecting the generation of RFI in the motor design parameters and the influence of mechanical variations in the motor assembly have been studied by experimental means. The relative importance of various factors, in terms of the interference level which they generate is established by isolation of the individual RFI sources so far as is practicable. In addition a case-study of the manufacture of domestic appliance motors at Hoover plc (the industrial sponsors of this thesis) was carried out in order to identify the causes of motor to motor variability in RFI levels from mass produced motors. It has been found that commutation even without the influence of the short circuited coil parameters generates high RFI levels. The additional influence of the short circuited coil parameters is to further increase RFI. It is shown that the construction of the armature and field can be engineered to reduce overall RFI levels from small motors. The contact stability of the brushes has been isolated as a major factor in influencing RFI levels. The allowable mechanical tolerances in the motor construction to minimise brush vibration and thus reduce RFI have been determined and these are compared with those used in manufacture at Hoover plc. It has been found that motor to motor variability in RFI levels results from manufacturing tolerances achieved on the shop floor. Recommendations for there duction of this variability are stated. It is concluded that improvement of brush stability by appropriate mechanical design improves commutation which in turn; reduces overall RFI levels, reduces suppression cost, minim uses motor to motor variations in RFI levels and reduces brush wear.