Has anyone come across any good information comparing 6 step brushless drive versus 3 phase sine wave? My interest is with efficiency, not torque ripple. I would think that peak torque, rms torque, and efficiency would be better with sine. My one test case on a particular motor indicated that over all efficiency was worse due to the fact that the 6 step was running at 100% PWM so the losses by the sine wave heated my electronics more. I appreciate your thoughts, experience, and references on this!

Because all switches in inverter switch at all times in sine drive mode, inverter efficiency will be bad compared to six steps bldc where only 2 switches conduct at a given time and only one of them switches. All other aspects are in favor of sine drive.

Motor efficiency is ususally higher for 3 phase sine fed but electronics efficiency is usually lower despite the fact that the motor RMS current is slightly lower. In my experience this is not a straightforward call and depends on many things such as DC bus voltage, power and also electrical frequency. In one notable system we did for F1 KERS, weight was the critical factor and that steered us towards 6 step as it lowered electronics losses that were harder to get rid of than the motor losses. Sorry Bill, in my experience this has to be judged on a case by case basis and as ever it is important to design inverter and motor as a system to reach the optimal solution.

The motor phase current goes down 42% so you would think that this would be a really big gain for the electronics and the motor.

What is the size and type of the motor? What application? Note that any unbalance or other imperfection in the 3-phase power supply may cause more losses in the motor than the inverter (which corrects for any unbalance and most other "imperfections").

Bill, how did you get the 42% figure? I am sorry but I don't see how it can be any where near that figure.

Bill, I agreed with what Charlie said. Motor like sine voltage/current, but inverter will prefer lower switching frequency. We tested on some small BL motor (Wout<500W). for most load range, the overall system efficiency between these two control methods is similar.
The 6 step control is heavily related to the advanced angle, if there is an easy way to adjust the advanced angle dynamically like the DQ control, 6 step would probably provide a ever higher system efficiency.
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Bill, I had this discussion with man who designed 1st digital sine drive in one big country (not USA). Hard to say....
everything depending on your application: power, PWM, load changes, motor R and L, DC to AC ratio etc, etc...

I was shocked when I saw 1st time sine drive spinnig loaded motor with 0.5 rpm!!!!

don't think can do servo drive in few months, no way U can do it in your place.

on paper torque ripple looks 100000 better for sine, in real life U may be surpriced.

Adam, I had this discussion with man who designed 1st digital sine drive in one small country (not USA), at the end of years '70 :-)
I agree with you, if you consider only electronics (not motor), efficiency is depending from many factors, also ambient temperature, production lot of power semiconductors, SMPS, IR emissivity factor of heatsink and case, etc.

The 42% came from (1-1/sqrt(3)).
My application is all high speed - I don't care about torque ripple.
I think you are all correct - in my case I think its how much power I loose in the current ripple vs. how much I gain by reduced IR losses. I'll get into the lab here in January and do some more testing.
Thank you, everyone.

Mario, motor can have 80-95% efficiency, electronic box can be 90-97%....
it pays to "squeeze" % from motor and control box.

70s and digital? using what? 74hc00?