Why is this a bogus three phase BLDC drive circuit?

[treez]

Hello,
Please can you explain why this three phase BLDC fuel pump motor drive regime is totally bogus and incorrect? (the diagram is attached)
I know it is incorrect, but I need the official explanation to take to the boss so we can get the project scrapped.

The attached pdf shows the arrangement….
It’s a current output regulated Buckboost converter, attempting to regulate the speed of a BLDC motor by regulating the output current of the Buckboost converter, the BLDC is switched by a three phase inverter, which is controlled by a Sensorless BLDC Motor control IC (ML4425), -the Sensorless BLDC Motor control IC is commutating the BLDC coils, but not high frequency PWM’ing the coils within the commutations. The Sensorless BLDC Motor control IC is a standard Voltage source inverter controller IC. The speed regulation circuitry inside the inverter control IC has been bypassed

…you can see that the Buckboost converter should not be regulating the current to the BLDC in order to control its speed. (in this circuit, the BLDC coils are only being ‘commutation switched’, they are not being ‘high frequency PWM’d’ within the commutations) As you know, instead, the speed regulation should be done by the BLDC inverter controller IC, and the Buckboost converter should simply provide the requisite bus voltage to the inverter. (-it was actually a “buck or boost converter”)

Can you say why it is a bogus drive method from an electrical point of view?

 

[FvM]

Is it a relaunch of this thread? https://www.edaboard.com/threads/311275/

 

[treez]

It is the same topic, but back then, I entertained the fact that the pdf shown in post 1 above might be OK....but now I am sure it is bogus. I know it is bogus because the entire department of engineers and managers was sacked because of wasting ages on it.
We don't ever want to see this happening again, so I am wondering, in "electric motor" terms, why the above method is bogus?.......that previous thread that you kindly highlight did not cover the bogus nature of the above drive scheme.

Specifically, trying to regulate the speed of a BLDC by supplying it from an output current regulated buckboost converter is bogus...but why? (from a technical viewpoint)

 

[mtwieg]

I still dont think there's any reason a current bias can't work, but will change the dynamics of the speed control. Similar to how current mode control works in smps.

Specifically what problems did you have in your attempt?

 

[FvM]

I still dont think there's any reason a current bias can't work, but will change the dynamics of the speed control. Similar to how current mode control works in smps.

That's also my general viewpoint. In terms of control theory it's well possible. The previous discussion was about detail problems like specific input current waveform of the BLDC inverter, size of bus capacitors etc. It might be that the current feed topology involves more problems for the controller design, but saying it can't work at all most likely doesn't hit the point.

 

[BradtheRad]

From what I understand about brushless drive...

RPM depends on the frequency at which you switch the stationary coils.

You can also change the volts-amperes, which changes the attraction/repulsion of the magnets to the coils. It changes how quickly the armature responds (or wants to respond) in its travel between coils.

That's the advantage of brushless drive, you can cut down volts-amperes, to the point where the magnets feel just enough pull, sufficient to make them move to the next coil.

If you apply too high voltage, that's wasteful. It won't increase rpm.

If you apply too little voltage, the armature won't move at all, no matter what frequency you switch the coils at.

----------------------------------

I would think position sensing is crucial. I have been expecting to see you talk about the position sensors in this motor. Pumping liquids is the sort of thing where load can change suddenly. You need complete control over both (a) switching speed, and (b) supply voltage to the coils. You need to know when to apply greater or lesser voltage, for best efficiency.

I think variable voltage is good to have. I don't believe it is used to change the speed. But a variable supply will make your job easier. It's easier than PWMing the coil waveform, because the simple pulse waveform is complicated enough already, is my impression.

 

[treez]

The (output current regulated) buckboost output feeds into into the dc link capacitance, then that feeds the motor coils, via the inverter..the inverter is not PWM'ing the coils at high frequency, but is just commutation switching from coil to coil.
This is surely not viable?.....it is trying to control the current in an LC circuit to a constant value, ..that can't be done, LC circuits are resonant in nature.
This technique of trying to control the bldc speed by thus controlling the output current of the buckboost is surely bogus?.....there is no literature on it anywhere on the web.

I am not speaking about a current source inverter...that indeed is not bogus, as you know.
The setup that I described is not like as in a current source inverter.

We never actually got it working.....when we got the motor to spin, the LT8705 buckboost controller had its error amp saturated , so it wasn't actually working in the intended output current regulation mode....it was just working in open loop. (I say its "error amp", but we actually used an external error amp and wired it in to the comp pin of LT8705).
We couldn't start the motor like that...we had to push the motor round with our hand to get it going.

 

[treez]

I've attached an LTspice simulation of an output current regulated buckboost converter here.
Its driving high power leds at 16 Amps. Surely you agree that this method could not be used to regulate the speed of a three phase BLDC? (ie remove the leds and put a 3 phase bldc there with its inverter)

The whole concept of regulating the speed of a 3 phase BLDC by regulating the output current of a buckboost converter is surely bogus?

Surely you agree that the correct way to regulate the speed of a BLDC is to use an inverter control IC to regulate the BLDC speed by suitably PWM'ing the inverter IGBTs.?.......A buckboost converter would simply be used to provide the constant regulated DC link voltage (in this case, 40V) to the inverter and BLDC.

Do you agree?

 

[FvM]

Surely I won't agree about "could not be used".

But that's only the principle point. You can say the topology in post #1 isn't advantageous and less easily to control than others. Claiming that a topology can't be implemented asks for refutation.

Specifically I don't see how the constant current LED power supply design is related to the question.

 

[treez]

Specifically I don't see how the constant current LED power supply design is related to the question.

....then please allow me to explain........that simulation of a current output regulated buckboost that I showed in post #8 is what they were proposing to use to regulate the speed of the BLDC....so take away the led load of that circuit, and put the BLDC and inverter there instead, and that's how they were going to do it. They were going to put in extra glue logic to make the current_output_regulated_buckboost regulate the speed of the BLDC by adjusting its output current.

That is just not viable....its viable to regulate current to a led string, but you cannot regulate a dc constant current in an LC circuit, because its resonant. (the LC circuit is formed from the 200uF output cap of the buckboost and the 56uH of the motor coils.
And as discussed, the BLDC inverter controller had had its speed regulation circuitry bypassed, and was simply just commutating the coils without high frequency pwm'ing them to control the current.

..Does that now sound bad to you? it did to the bosses to....who sacked the entire department virtually on the spot when they came over and realised they were doing it like this.

Incidentally, the (very very senior) engineer who devised this bogus method actually suddenly left the company by surprise.....he had no job to go to....his linkedin account just went blank......also, his deputy also left at exactly the same time...again without a job to go to...his linkedin account again went dead, even though he had previously been updating it throughtout his long career....did they realise that it was bogus and decide to make a run for it?

Surely you appreciate that one cannot regulate a BLDC's speed like that?

I have attached here the same simulation but now with a rough imitation of a bldc coil as its load...the buckboost is trying to regulate a constant 16A current in the load. But remember the coils are switching in and out, so every 500us or so, there is a step input into the LC circuit of the 200uF output cap and the respective motor coil....as you can see, there will just be massive resonantions.
(I haven't included the glue logic which senses the motor speed, and then converts it into a signal voltage for feeding back to the reference pin of the output current error amplifier of the buckboost, in order that the buckboost can regulate motor speed by adjusting its output current)

 

[FvM]

You are talking now about specific design details rather than the fundamental topology. Resonant LC circuits are found in many control processes, e.g. in any constant voltage switched mode power supply. They have to be considered in controller design and of course determine the system behaviour.

I'm facing a large variety of control processes with odd topology, either designed on purpose, inherited from previous designs or by simple mistake. Most of them can be stably controlled somehow.

From this viewpoint, I'm doubting any of your general statements in this thread, admitting at the same time that the empirical decision against this topolgy might be right. But don't make it a question of faith.

 

[chuckey]

I think the point that has been overlooked is the current taken by the pump is related to its drive and its load. At a light load the constant current supply will increase the voltage drive is a vain effort to get the current right.
Servo motors use a constant current drive, but this is to optimise their speed and to provide a holding current. I do not think a pump motor has the correct arrangements for this .
Frank

 

[treez]

Here is a simulation (LTspice) attached here ("Buckboost _output current regulated_bldc_2") that shows the equivalent situation that would happen with the buckboost trying to control the current into the combination of the inverter and drive. (as you can see, the "inverter" is only commutating from coil to coil, and is not high frequency PWM'ing the coils within the commutations)

You can see that it is nothing more than a ridiculous LC resonator...surely proving how bogus this method is?

Quite clearly, surely you agree that the buckboost should instead simply be providing a constant 40V output, the inverter control IC should then be switching the inverter on that 40v dc bus, to control the speed of the BLDC?

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Servo motors use a constant current drive,

..yes but that is from a "current source inverter". That is totally different than the current regulated buckboost shown here.
"Current source inverter" is a good, genuine technology. What I show here is totally bogus I am sure you agree?

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Please now find attached a ltspice simulation ("Buckboost _output current regulated_bldc_startup") showing the current_output_regualted buckboost trying to regulate a constant current into the bldc_and_inverter at startup, when the back emf is zero as the motor is not yet spinning at all fast......just look at the ridiculously peaky coil current....nobody would choose to regulate a bldc's speed like this...surely you agree?.

A company I worked at did try and do this, and now all 12 managers and engineers no longer work at this place...they all went at the same time....despite having been there for many years.

 

[mtwieg]

Your simulation show you driving the equivalent of three solenoids, not a BLDC. It won't behave anything like reality. Like I said in the other thread, you should actually make a motor model based on its equations of motion to get sensible behavior out of it. Without that it's basically worthless.

 

[treez]

when the motor is stationary, just before it gets started up, the motor coils will look just like a plain inductance...as shown in the "...startup" ltspice simulation above.
I realise the other simulation doesn't include the back emf, which as you know varies dynamically as the motor spins, but in so far as needing to represent why this method is bogus, it does the job.
You would surely agree that the method of controlling BLDC speed by controlling the output current of an upstream buckboost converter is a method with absolutely no documentation about it anywhere on the web, or in any book. .....because it is a bogus method which absolutely nobody uses, or has ever used.

There are two ways to drive a BLDC, as you know, these are the Voltage source inverter and the Current source inverter, the "bogus" method depicted here is neither.
The ltspice simulation shown with this post ("3ph BLDC & inverter") shows how a BLDC should have its speed controlled. (again I admit it doesn't show the back emf, but it is just to remind us of the general switching pattern and modus operandi.
..all the .txt files must be in the same folder as the simulation for the simulation to run.)
...and please remember that only the "3ph BLDC & inverter.txt" file is the actual simulation, -the one that should be changed to "3ph BLDC & inverter.asc" so that ltspice can run it.

192 people have viewed this thread in less than 24hrs.....not one of them has come out in favour of the bogus method depicted in this thread. Not once in the entire history of edaboard has any edaboard forum member ever told of the "bogus" method hereby depicted, because of course, it is bogus, and nobody does it like that, apart from my old department, who tried to do it, and all lost their jobs because of it.

And by the way , my previous threads on this bogus method (back in about mid 2014) were seen by my workmates at this company, who worked out that it was me, and complained bitterly to the managers that I was using a forum to discuss what they thought was company business...what they failed to appreciate, was that this is basic electric drive theory, and not the IP of that company or anyone else. We aren't workmates any more, we all lost our jobs.

 

[mtwieg]

when the motor is stationary, just before it gets started up, the motor coils will look just like a plain inductance...as shown in the "...startup" ltspice simulation above.

Okay, so your simulation shows the driving of a stalled motor. What do you expect to see? How would a non-"bogus" drive respond?

I realise the other simulation doesn't include the back emf, which as you know varies dynamically as the motor spins, but in so far as needing to represent why this method is bogus, it does the job.

No, it absolutely does not, because there's no proper way to drive a stalled motor.

You would surely agree that the method of controlling BLDC speed by controlling the output current of an upstream buckboost converter is a method with absolutely no documentation about it anywhere on the web, or in any book. .....because it is a bogus method which absolutely nobody uses, or has ever used.

[citation needed]

192 people have viewed this thread in less than 24hrs.....not one of them has come out in favour of the bogus method depicted in this thread. Not once in the entire history of edaboard has any edaboard forum member ever told of the "bogus" method hereby depicted, because of course, it is bogus, and nobody does it like that, apart from my old department, who tried to do it, and all lost their jobs because of it.

And by the way , my previous threads on this bogus method (back in about mid 2014) were seen by my workmates at this company, who worked out that it was me, and complained bitterly to the managers that I was using a forum to discuss what they thought was company business...what they failed to appreciate, was that this is basic electric drive theory, and not the IP of that company or anyone else. We aren't workmates any more, we all lost our jobs.

So you think just because no one has published a cookbook approach to a control method that you can copy and paste into LTspice, it's bogus? You can't be serious.

Also why were you trying to use a current biased inverter to begin with?

 

[treez]

Also why were you trying to use a current biased inverter to begin with?

That is a very very good question.
I kept on asking that question to group members, but nobody really knew. One Engineer hinted to me that the senior-most design engineer (SMDE) had designed it. This man was considered to be the be-all-and-end-all of electronics design at the company. Everybody was too afraid to question anything that he ever did. But I questioned it, wrote a report on why I thought it was wrong, and thus the SMDE became very agitated with me.

However, the SMDE, and his deputy, the senior engineer, both suddenly then left the company together, without apparently having jobs to go to. The SMDE was often heard saying how the company had mis-treated him. But it seemed strange why they both suddenly left before we’d got his “bogus” design prototyped and working.


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”


When I first started at the company, we held a meeting over how we would control the buckboost as a current regulator…the SMDE strangely told me to leave the FB pin of the LT8705 controller open…..something that one never does with a high impedance input pin…..he also advised me that it was perfectly fine to use ceramic capacitors above their maximum voltage rating. 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.

The current controlled buckboost (bogus) method that I speak of was very strange, particularly due to the abundance of extra external circuitry that it needed outside of the buckboost controller and the ML4425 inverter controller.

But as I said, everyone was too afraid to question the SMDE. And the managers were just interested in getting something ready for the deadline, almost not even seeming to care what it was.

There were some other strange things that I noticed happening on the periphery of this “bogus” project, and I am actually highly suspicious that the top management actually realised that we were all stupidly working on a bogus project, and deliberately gave us the freedom to keep going on and on, wasting money on it, -before pulling the rug from underneath us all, and declaring that we were all a bunch of idiots, and sacking us all on the spot. (I actually left before the sackings happened, indeed I left because I thought it was a bogus method)

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So you think just because no one has published a cookbook approach to a control method that you can copy and paste into LTspice, it's bogus? You can't be serious.

...I am sure you appreciate that you cannot control a constant current output from a buckboost converter if your output impedance is not constant. In the case of the Inverter/BLDC load, its not only not a constant impedance, but the motor coils are inductive, and will form an LC resonator with the output capacitor of the buckboost. Remember the inverter controller is just commutating from coil to coil, and not high frequency pwm'ing the coils to regulate the current, the coil current is supposed to get regulated by the upstream buckboost...but it would be bogus.

 

[treez]

Also, if his bogus technique was actually genuine, then we would see current regulated buckboost control IC's with motor speed error signal feedback inputs...and of course, we do not see such a thing off-the-shelf, because it is bogus.

admitting at the same time that the empirical decision against this topolgy might be right

...this shows that FvM appears to be suggesting that it is indeed a bogus drive method?

[citation needed]

..this by Mtwieg appears to also show that Mtwieg has serious doubts about the "bogus" method.?

 

[mtwieg]

...I am sure you appreciate that you cannot control a constant current output from a buckboost converter if your output impedance is not constant.

Why not? It's done all the time for LEDs, which have a very nonlinear characteristic.

In the case of the Inverter/BLDC load, its not only not a constant impedance

I don't think so. The emf in the windings will change, but to my knowledge that does not affect the impedance of the windings. I presume that you know that when a 3 phase load is driven by an inverter with SPWM, the power draw from the DC link should be constant (neglecting the very high frequency ripple from SPWM). So if you use SPWM, the DC-DC stage should not struggle with that load, regardless of whether it's regulating DC voltage or DC current.

but the motor coils are inductive, and will form an LC resonator with the output capacitor of the buckboost. Remember the inverter controller is just commutating from coil to coil, and not high frequency pwm'ing the coils to regulate the current, the coil current is supposed to get regulated by the upstream buckboost...but it would be bogus.

If such a resonance exists, it would also exist with a voltage-regulated loop as well, so I presume it's a problem that can be dealt with. Again, my intuition is that the difference between using a voltage bias and a current bias for the inverter shouldn't make or break the design. It's basically the exact same circuit, just different feedback paths.

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Also, if his bogus technique was actually genuine, then we would see current regulated buckboost control IC's with motor speed error signal feedback inputs...and of course, we do not see such a thing off-the-shelf, because it is bogus.

....Or because it just doesn't offer any real advantages. Doesn't mean it's bogus.

..this by Mtwieg appears to also show that Mtwieg has serious doubts about the "bogus" method.?

No I have serious doubts that your anecdotal experience proves or disproves anything.

 

[treez]

If such a resonance exists, it would also exist with a voltage-regulated loop as well

..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.

...(so by "normal" method of bldc control, I mean having a voltage regulated dc bus for the bldc inverter to switch off.)

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.

But I am certainly grateful for Mtwieg and FvM to discuss this, as few others would, since the world of BLDC drives is a world of secrets.....as my old department (now disbanded) found to our detriment. Certainly this has evoked great interest, since 395 people have viewed this thread in 3 days...(I just hope none are lawyers looking for payback for the time that my old dept wasted working on this bogus method.)