Nigel Goodwin said:
Have you ever seen a brushless DC motor that does that?.
Oh I don't know abt 1/2 of the perminant magnet machine I have around hte lab.
There really isn't such a thing as a brushless DC machine.
A machine with a trapizoidal flux distrobution will quite happily run with a sinusoidal induced airgap flux, just like a sinusoidally wound PM-machine will happily drive with a trapizoidal flux distrobution.
Calling a machine Brushless-DC really isn't correct (like saying someone is having a heart attack). a PM-machine is either sinusoidally exited or trapisoidally exited, both of which are AC (difference being one is easier to generate)
THERE are alot of advantages to sinusoidally exciting a PM-machine, the main one is torque ripple. In a trapizoidally excited PM you can ONLY get zero torque ripple IF and only IF you can inject rated current into the winding (to produce the required flux distrobution) instantaniously, which as you know is impossible due to a small thing called inductance.
This results in commutation dips in the current which manifests themselves as torque-dips at commutation.
Sinusoidally excited PM (with optimum mag setup) only need to generate sinusoidal flux ==> sinusoidal current which is a damb sight easier then trapizoidal blocks of current.
A Sinusoidally distrobuted machine when excited with a trapizoidal control will actually give a worse torque ripple then a trap+trap
BUT a trapizoidally distrobuted machine exited with a sinusoidal control will give a vast improvement in performance, BUT you run the risk of saturating the stator iron and thus to get the best out of it it becomes a "lazy" machine