series resistance is the limiting issue with this design.
Things helping this issue:
Larger slow, low RdsON devices can be used (compared to PWM optimized devices)
Things hurting this design:
Way to large a volt second bandwidth across "gears". So components will be large for the worst case.
also current bandwidth is large increasing component size and cost to be able to handle worst case.
Clever and interesting nun the less.
I agree that this design requires a larger number of power transistors. This increases the overall cost. But this replaces a mechanical gear box, plus differential, plus...
And I don't understand why power transistors should be larger than for other designs for similar power motors.
Because when you switch in and out transistors from series to // the voltage rating and the current ratings must be able to handle the worst case. So when in // voltage worst case...this sets the Rdson (higher voltage higher rdson).
Whne these are put in series then you will have excesive voltage drop compared to a series only design.When in sries the current is max (per xstr)and the die area must be larger.
But it may still be better considering all the benifits. Lots of semiconductor cost, again may be offset.
With brushless motors, you have to reverse the current as the rotor turns. The switch from series to parallel is done at the same time as this current inversion.
But you're right. As this design requires many transistors in series, each Rdson has to be as small as possible.
The subtlty is the rdson will be bigger for the worst case (parallel) requirement but will be an extra efficiency loss when in series times # in series.
Butt this idea has a bank account of efficiency and cost savings to draw down from in removing weight and mechanical transmission.
Just can't give up more than ~75% of these advsntages to be viable.
I think it has legs.
Even if this design did require a greater bandwidth, the frequency is low enough that almost any transistor of adequate ratings can handle the job. But the big advantage is that it is still cheaper than actual gears, and the switching function is both cheaper than gears and not requiring mechanical shifting. So really it is quite elegant. It needs a few more wires, but that is certainly cheaper than gears and mechanical shifting, which would still require wires and power transistors, plus, the powered shifting mechanism would use much more power.
I am not convinced that this requires more wires. The torque and the power of the motor are proportional to the length of the wires. So, for the same power, you need same length. The difference is that you have to split the same wire length into different pieces, in order to connect them in series or in parallel.
Refer to patent number: 7595574
When I investigated this in the past, the technology appeared to be quite sound and a great way to eliminate mechanical gear boxes. There are, of course, concerns over the cost of copper and magnets.
Also see the LaunchPoint motor (which uses Litz wire instead of PCB, but I think PCB is a good approach and there are others employing PCBs for stators and even rotors).
Incidentally, in reference to earlier comments related to unsprung mass, see the viewpoints from Protean Electric: