Cogging reduced by magnetic design

Tom Shelley reports on some simple design enhancements that greatly the smooth running of electric motors A major maker of small electric motors has found it possible to almost eliminate cogging by shifting magnets slightly within a pole pair and by changing their shape. In miniature motors, a similar effect is achieved by introducing small rubber magnets between the two main ceramic magnets. Cogging is caused by changes in the magnetic energy within the air gap between magnets and laminations as the armature rotates. This leads to preferred rest positions for the armature and noise and vibration as it rotates. Servo motor makers have found solutions that include slotless and skewed armatures but for the makers of small, high volume motors, these solutions have not always been cost effective. Skewing is not effective to three, five and seven slot armatures. Johnson Electric has patented a solution in which the curvature of the pole piece of the magnet is an arc whose centre of curvature is outside the centre of the motor. This increases the air gap at the acuate ends of the magnets. This reduces even harmonics. In addition, cogging can be further reduced by angularly off setting the location of the magnets in a pole pair by one quarter of the slot pitch, in advance for one magnet and in retard for the other. This reduces odd harmonics. This has been applied to a 300W to 350W motor with 2 poles and 12 slots, and an output torque of 4 to 6 Kgcm (0.4 to 0.6 Nm), with an initial maximum peak to peak cogging torque of 0.05Kgcm (4.9mNm). Tests show that the modifications applied reduce this figure to about one fifth of its former value. In another development programme for miniature permanent magnet DC motors, reductions in cogging have been obtained by introducing small rubber magnets between the two main ceramic magnets. This approach changes the shape of the magnetic field inside the space in which the armature rotates. By changing the shape of the field, it is possible to reduce the changes in total magnetic flux passing through the armature at various angular positions. The rubber magnets can be compressed to aid assembly, but once charged, they stiffen, creating a firm interference fit between the ceramic magnets and the rubber magnets, fixing the magnets to the housing. The combination of the two types of magnets allows for many variations in the total magnetic field. Through experimentation, an arrangement can be found that substantially reduces cogging. Roger Baines of Johnson Electric notes that there are also many other causes of noise and vibration in motors that should also be addressed. While Johnson Electric was originally found in Hong Kong in 1959 to make miniature electric motors for toys it is now a global enterprise with a production capacity of a billion units per year, many of which are used in cars. Johnson Electric Nick Pearson at Durham Pipeline Technology Pointers * Even harmonics can be reduced by shaping magnet poles so that air gaps are greater on the sides * Odd harmonics may be reduced by deliberately misaligning the magnet pair * Placing rubber magnets between the main ceramic magnets in a PMDC motor reduces cogging in miniature motors