BLDC controller /w seperate PWM & commutator

[circuitsmith]

I've started designing a controller with separate FETs for the PWM throttling and 3 phase commutation.
This will replace the controller on the Currie/Kollmorgen motors.
My reasoning for the separate commutator bridge is that each arm will switch no higher than about 1000hz, given a 20 pole motor at 3000rpm.
The demands for switching speed and dealing with gate capacitance will be greatly reduced compared to PWM at ultrasonic frequencies.
Robust, heavy duty FETs can be used with a modest gate drive circuit.
The PWM will need only one (or a parallel group of) N-channel FET to feed the commutator, essentially a brushed motor controller.
Has this been done before? Thoughts?

 

[dnmun]

you can buy DC pwm current controllers on ebay for real cheap so why reinvent the wheel?

i was thinking about buying one to use for a fan speed control on the heater in my car because they were cheaper than buying the old resistor elements used to adjust the heater speed. amazing what ebay has done.

 

[IanFiTheDwarf]

Yes I have thought about this also. My idea was to build the commutation fets right on the back of the motor with a basic logic circuit to control the commutation, or an off the shelf commutation ic with advance and dead time. This way the motor can be used in direct replacement for a brushed dc motor.
In a way you are just moving the problem to the single fet but it’s a lot cheaper and easier to build a robust dc controller than a 3 phase bridge. The more I looked at it though, the more I found it was missing a lot of the precision of a well-designed brushless controller. I think it would end up being quite crude.

In the end I abandoned the idea especially with the brilliant work lebowski has done on his controller making it very worthwhile spending the time to build a robust output stage.
As my motor is going to be pretty big and it’s going in a car, space isn’t going to be an issue so I’m intending on using intelligent power modules which are basically IGBT’s with the gate drivers built in.

 

[circuitsmith]

you can buy DC pwm current controllers on ebay for real cheap so why reinvent the wheel?.

That by itself won't run a brushless motor. The separate commutator I'm proposing could go between an existing PWM controller and a brushless motor.

 

[circuitsmith]

In the end I abandoned the idea especially with the brilliant work lebowski has done on his controller making it very worthwhile spending the time to build a robust output stage.

I'm hoping to come up with something smaller and simpler, but still "good enough" to run this particular motor as well as the original Kollmorgen controller.

 

[circuitsmith]

Riding to work it dawned on me that I can still do some sine wave shaping with the PWM FET for quieter and smoother operation.