
- Joined
- Apr 1, 2011
- Messages
- 15,828
- Helped
- 2,918
- Reputation
- 5,850
- Reaction score
- 3,077
- Trophy points
- 1,393
- Location
- Minneapolis, Minnesota, USA
- Activity points
- 118,535
I once overheard the following snippet of conversation between the Senior Engineer (SE) and the SMDE
SE: “Why did you do it like that?”
SMDE: “Because it means that the current more quickly builds up in the motor coils after they are switched in…engineering sense”
For a fleeting instant , I suspected that he was deliberately trying to lead the project astray, perhaps due to his grudge against the company…..he used to openly declare that he had a grudge against the company….he was often heard shouting at managers who asked him to write proving reports for some of his designs.
As I said before, regardless of which circuit you use the parallel LC is still there. Using an inner control loop like CMC can allow you to pretend the resonance is gone with respect to the outer control loop, but it is, in fact, still there. Also your own schematic shows the current sense after the DC link capacitor, so if that branch is really current regulated then it will effectively eliminate that capacitor as seen by the inverter, and the resonance will be "gone."..sorry but I disagree, -with the "normal" method of BLDC drive control, the coils are high frequency PWM'd within the commutations to control the coil current, and this effectively illiminates the inductance of the coil.....just like in a current mode full bridge converter, ...the fact that its a current mode full bridge means you do not get an LC resonance between the output inductor and the output capacitor of the full bridge....because the inductor current is controlled...it is not allowed to resonate with the full bridge output cap............it is the same principle with the "normal" method of control of bldc drives, you do not get a resonance between the dc link cap and the coil precisely because the coil current is being peak current controlled by high frequency pwm.
I'm also skeptical of whether it's worthwhile. At most, having the current biased inverter might mean you don't need individual current sensors on each leg. Also I think that it would only work well if you used FOC rather than trapezoidal control (so that current draw is truly smooth DC).I note that your call for a "citation" above has not been heeded, as no doubt I believe you expected.....one thing we can say for sure, is that the "bogus" method of bldc control described in the top post, certainly has no advantages whatsoever over the "normal" method of bldc drive control............so at best its a waste of time....and likely is totally bogus.
Right so if you use trapezoidal drive you might get sharper rise and fall on the current in each leg. I don't see how that is desirable, though. If anything it should make torque ripple worse.Speculation on my part...
He may have intended for constant current regulation to act during the time a coil is switched on. The coil would receive higher voltage immediately, in order to attract a distant magnet as strongly as possible.
Then while current builds in the coil, the magnet gets closer, and voltage is reduced by the current regulator.
A smps, operating at several kHz, would easily be able to do all this while a coil was energized, say 1/400 or 1/800 of a second.
He may have believed this would increase torque, as well as overall efficiency.
Also your own schematic shows the current sense after the DC link capacitor, so if that branch is really current regulated then it will effectively eliminate that capacitor as seen by the inverter, and the resonance will be "gone."
I don't think the capacitor is eliminated, ..it would be if the buckboost had infinite feedback loop bandwidth, but in reality, the buckboost has a bandwidth of a few kHz, and the capacitor is very much there...the buckboost bandwidth is not high enough to cancel out the LC circuit.
For the sake of clarity we should probably consider the issue of the current biased inverter separately from the design of the actual current bias. So for arguments regarding the inverter we presume we have a controllable current source with high bandwidth, and worry about how to actually implement that later.
...you are touching on something here that I was investigating some months ago whilst I was actually working at the place, -I was considering whether it would be possible if the buckboost bandwidth was high enough. -At the time, there was no reponse to this. (by the way, it was 200uF and 56uH and thus 1.5kHZ)...We couldn't fit 300uF onto the small PCB.Also didn't you say your DC capacitor was 300uF and the motor was 56uH? That gives a resonance at 1.23kHz. If your buckboost is operating at 150kHz then getting a bandwidth higher than that should not be a problem.
Thankyou, by the way, I am not sure what you meant by "fixed duty cycle", but the inverter was not giving a fixed duty cycle for the high frequency PWM'ing of the coils, as the IGBT's were not high frequency pwm'ing the coils, -once the igbts had commutated to the next coil, they just stayed on all the time until the next coil was due to be commutated in.You have 2. the idea of a hardware topology, a buckboost DC/DC and a (fixed duty cycle) BLDC driver. At this point current or voltage control loops aren't yet fixed.
Why?I know it is incorrect, but I need the official explanation to take to the boss so we can get the project scrapped.
...thanks, I was cutting a long story short, I want to be sure future companies I may (or may not!) work for don't go down the same road to nowhere.I don't understand why you want to go back to a company you used to work for and explain to the boss there why he should scrap a project that he's already scrapped. Seems like a waste of time.
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?