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Reducing cogging torque in brushless motors March 29, 2012

Posted by Servo2Go.com in Technical Support Information.
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Selecting the number of rotor poles and slot combinations to reduce cogging is not an intuitive exercise.  Take advantage of these guidelines based on pretested configurations to help you optimize your next motor design.

Cogging torque in dc brushless motors comes from variations in magnetic field density around a rotor’s permanent magnets as they pass the nonuniform geometry of the slot openings in the stator.  In applications such as servosystems and spindle drives, the pulsating speed that cogging generates can blemish machined surfaces or reduce position accuracy. 

Unfortunately, classical electromagnetic calculations do not provide the data needed to determine how much cogging torque might develop in a new paper design.  Although a complete finite-element analysis may be an alternative to manual methods, it usually requires more project time than is available.  In most cases, several prototypes must be made to measure and eventually reduce the cogging torque.  Thus, it is critical to have a simple check list of major factors that determine cogging torque during the initial design procedure so several iterations can be made before finalizing the drawings.

Major factors affecting cogging torque include magnetic wave shapes, air-gap length, slot opening, number of stator slots and rotor poles, skewing, copper fill, pole pitch, flux distribution or density, magnet volume, and material weight.  Relationships between some of these factors, including electrical degrees/cycle, and cycles/rev vary among multiple-pole motors.  

Analyzing the ripple torque for each type leads to a set of guidelines for new designs.  For example, the maximum number of cycles in one electrical cycle for a stator with an even number of slots can equal the number of slots itself.  But for a motor with an odd number of slots, the number of cycles can be twice the number of slots.

Moreover, for a given frame size and type of lamination, slot and pole combinations as well as different pole arc to pitch ratios and magnetization, can produce different cogging torques.  Keeping the number of poles on both rotor and stator ID and slot openings constant, and varying the number of slots, shows how cogging torque behaves for different slot and pole combinations.

Skewing the magnets or the stator core often can lower cogging a bit more.  When a design without skewing already shows minimal cogging, the skew angle required to reduce cogging below a particular value will be much smaller.  Also, designing for a trapezoidal or nearly sinusoidal air-gap wave-form (made by varying the pole arc to pole-pitch ratio) is a common practice that often reduces cogging torque even further.

Click on the link below for information on the Servo Motors available from Servo2Go.com –

Servo2Go’s Servo Motor Product Family

For more information, please contact:

Warren Osak
Toll Free Phone:   877-378-0240
Toll Free Fax:       877-378-0249


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