Is this a 'bogus' motor speed regulation method?

[Flyback]

Is the attached block diagram showing a bogus way of regulating the speed of a BLDC?

sorry I already have an old thread on this, but I edited it and its too far down the list for people to see the edits..
https://www.electro-tech-online.com/threads/is-this-a-bogus-kind-of-motor-drive.140374/

I suspect that our method (as shown) is bogus, and that we are all just totally wasting our time here in our Dutch Engineering company?
The inverter switches at 500Hz with 200us dead time.

ML4425 inverter datasheet.
http://pdf.datasheetcatalog.com/datasheet/fairchild/ML4425.pdf

 

[ronv]

What makes you think it doesn't work?

 

[Flyback]

i just think its far from optimal, and an overly expensive way forward.
For a start, the load of the buckboost suddenly goes open circuit every 2ms for 200us when the dead time happens at the inverter commutation intervals.
So the vout of the buckboost will go sky-rocketing upwards in this 200us, and a voltage clamp will be needed, as we have no room for a significant capacitor output bank.
I think its cheaper and easier to just do a vout regulated smps, which is current limited to 20A output, would you agree?

 

[ronv]

Sounds like you have it figured out.

 

[Flyback]

May I ask another question about our water pump drive (its for a Dutch tulip field irrigation system)?
The PDF below again shows the block diagram of our setup.
....it goes like this....VIN...Variable output buckboost SMPS....Inverter.....BLDC.
The point is that our DC link capacitor is only 300uF, and we have no room for more capacitance than this.
The problem is(?), that the resonant period of the DC link capacitor, and the motor coil inductance is just 814us....and this is less than than the inverter commutation period of 1.05ms.........
therefore, do you believe that we will suffer overly high resonant currents in our BLDC motor coils?
Please remember that usually, the inverter would switch the motor coil current at high frequency, such that the motor coil current is in control at all times, and the resonant LC situation doesn't emerge....but we are using the buckboost to effectively control the motor current, and the problem is that the inverter switchs a motor coil in for a time period that is longer than the LC resonant period of the dc link capacitor and the motor coil inductance.
(This is an analogous situation to a coupled sepic converter's capacitor, whereby the switching period in a sepic should be much shorter than the resonant period of the sepic transformer leakage and the sepic capacitor.)



When the Inverter's BLDC is permanently on 100% duty cycle like in our system, then shouldn't the DC Link capacitor be big enough such that the motor coil commutation period is much less than the resonant period of the DC link capacitor and the motor coil inductance?

 

[tvtech]

May I ask another question about our water pump drive (its for a Dutch tulip field irrigation system)?
The PDF below again shows the block diagram of our setup.
....it goes like this....VIN...Variable output buckboost SMPS....Inverter.....BLDC.
The point is that our DC link capacitor is only 300uF, and we have no room for more capacitance than this.
The problem is(?), that the resonant period of the DC link capacitor, and the motor coil inductance is just 814us....and this is less than than the inverter commutation period of 2.5ms.........
therefore, do you believe that we will suffer overly high resonant currents in our BLDC motor coils?
When the Inverter's BLDC is permanently on 100% duty cycle like in our system, then shouldn't the DC Link capacitor be big enough such that the motor coil commutation period is much less than the resonant period of the DC link capacitor and the motor coil inductance?

Time for me to sleep.

tvtech

 

[Flyback]

though this is potentially a revolutionary method to drive BLDC's.....that is , you simply use a "dumb" inverter which just switches at maximum duty cycle all the time, and then you vary the speed by varying the output voltage of the power supply that's upstream of the inverter.

But do you believe that this can only be done if the LC resonant frequency of the DC link capacitor with the motor coil inductance, is much, much less than the commutation frequency in the inverter?

Do you believe that our modus operandi is , from a practical viewpoint..bogus?
To summarise, we are using a buckboost converter based current source to regulate the speed of a BLDC.
The output current of the buckboost regulates the BLDC speed.
The Inverter that feeds the BLDC literally just alternately switches the coils of the BLDC to the output of thebuckboost......the inverter does not limit its duty cycle to achieve the required speed...the inverter just simply switches at max duty cycle all the time.
It is the first of April today.
Do you think that this system that I have described here is bogus?
...I mean..... to prevent LC oscillation in the BLDC coils, the buckboost feedback loop bandwidth would need to be 15KHz plus
This is because the motor coil inductance is 56uH and the buckboost output capacitor is just 200uF.....due to room constraints.
A 650W , 15KHz buckboost with 18-32Vin is not practically possible in a short time frame
I see we are on a road to nowhere here...do you agree?