> Cogging in BLDC/PMSMs is because of square wave drive currents > Not exactly. There are 2 things at play here.
1) Cogging is from the variable reluctance based on rotor angle. It has nothing to do w/ the drive waveform. It exists even when the leads are open circuited. In simple terms, the magnet is getting closer/farther from a pole that forms a magnetic short (low reluctance, similar to low resistance). It wasn't to 'roll downhill' and sit in a valley and get as close to the metal as possible. 2) Torque ripple is torque (and thus power) ripple from a multitude of factors. Cogging may or may not affect torque ripple (it definitely adds to it, but it may be deliberately cancelling a ripple from the drive waveform giving something closer to a net constant.) Sinusoidal vs trapezoidal drive doesn't necessarily fix things either. A motor designed for trapezoidal drive will often have trapezoidal back-emf. For a surface magnet rotor, the magnets can be left flat on their face and the magnetic field varies approximately trapezoidaly in the airgap (the airgap is obviously varying) . IIRC, Trapezoidal drive w/a trapezoidal back-emf can produce a motor w/ fairly low torque ripple. Similarly, a surface magnet rotor thats been machined smooth will give more sinusoidal back emf and when driven w/ sinusoidal drive will give very low torque ripple. "Negligible" is a matter of opinion. There are many research papers & methods on how to reduce the remaining torque ripple even w/ sinusoidal drive. Some distort the flux distribution in the motor, some deliberately distort the drive waveform. High inertia loads are fairly insensitive. It can be a big problem with low inertia loads, backlash, systems w/ resonances, etc however. There are lots of ways to shape the flux distribution in the airgap other than just flat vs curved magnet surfaces. There are buried magnet rotors, saturatable pole tips, etc. The thing to take away is that torque ripple is complex and one type of drive waveform or the other isn't necessarily better. Its a system problem and a system solution is required. Stephen ------------------------------------------------------------------------------ For Developers, A Lot Can Happen In A Second. Boundary is the first to Know...and Tell You. Monitor Your Applications in Ultra-Fine Resolution. Try it FREE! http://p.sf.net/sfu/Boundary-d2dvs2 _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
