PMSM control strategy
PMSM control strategy
(OP)
Hi guys,
I have a question regarding the control of a PMSM via an inverter.
The idea is to control the motor's power according to the gaspedal-angle. So let's say it's a 85kW motor, pushing the gaspedal 50% would mean a power demand of 42.5 kW.
I've been looking at control strategies and I've bought the book "Permanent Magnet Synchronous and Brushless DC Motor Drives" by R. Krishnan. It discusses quite a variety of control strategies, like Vector Control (same as Field Oriented Control?) and Space Vector Modulation but I'm kind of lost on which one is the one I need.
I have also noticed that most practical examples are about controlling the speed of the motor, where I need to control the power of the motor. Can I instead control the speed of the motor, and calculate the necessary speed by using the motor's torque/rpm characteristic to determine what torque corresponds to a certain speed, multiplying those to give me the power? And what is the best control strategy to use in this case?
I have a question regarding the control of a PMSM via an inverter.
The idea is to control the motor's power according to the gaspedal-angle. So let's say it's a 85kW motor, pushing the gaspedal 50% would mean a power demand of 42.5 kW.
I've been looking at control strategies and I've bought the book "Permanent Magnet Synchronous and Brushless DC Motor Drives" by R. Krishnan. It discusses quite a variety of control strategies, like Vector Control (same as Field Oriented Control?) and Space Vector Modulation but I'm kind of lost on which one is the one I need.
I have also noticed that most practical examples are about controlling the speed of the motor, where I need to control the power of the motor. Can I instead control the speed of the motor, and calculate the necessary speed by using the motor's torque/rpm characteristic to determine what torque corresponds to a certain speed, multiplying those to give me the power? And what is the best control strategy to use in this case?





RE: PMSM control strategy
http://nl.tinypic.com/r/143dyx/8
The control block would need a few more inputs, and it would have to act on the difference between the desired power and the actual power.
The idea is to create the controller in Matlab Simulink first.
RE: PMSM control strategy
With a synchronous type motor you will be looking at a few degrees angular displacement, rather than 40 RPM.
Another approach may be to reduce the torque by reducing the effective voltage.
Possibly the easiest way would be to choose a controller with a torque limit function.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: PMSM control strategy
My question is about two things:
RE: PMSM control strategy
There's a reason that the control strategy usually is for constant rpm.
Benta.
RE: PMSM control strategy
Think of how your car works; gas pedal tells engine how much torque to put out, irregardless of speed or power.
Recall also what power is in a motor: power output is 0 at 0rpm and even with max torque output increases linearly as speed increases; hence, it is not normally associated with a gas pedal type input function.
What type of application is this that you want to command power from 0 to 100% linearly with a gas pedal?
You also as asking about which buzz word to use for the motor control; use any one of the 3 you mentioned. At the end of the day they will all do the same thing.
www.KilroyWasHere<dot>com
RE: PMSM control strategy
So you're saying it makes more sense to say that the angle of the gaspedal will determine the amount of torque that's needed?
So I can map 0%-100% of the gaspedal to 0-maximum torque of the motor? Is that how it's done?
And the required torque is an input of the vector controller, which generates the right pulses for the inverter? And what happens if we would use an inverter that takes the required torque as an input, would the vector controller be already implemented in the inverter itself?
BTW: I'm more familiar with electronics than with electric motors, thanks for your help so far :)
RE: PMSM control strategy
The needed torque or power is determined by the load, not by the motor or its electronics.
Again, if the EV is running downhill, it makes absolutely no sense to tell the motor to deliver more torque than needed. This will be a completely unstable situation where the car will accelerate in an uncontrolled manner, theoretically towards infinity.
Benta.
RE: PMSM control strategy
So it is the rpm of the motor that needs to be controlled between 0 and a certain maximum, but how is that maximum determined? Is it a specification of the motor?
RE: PMSM control strategy
Benta.
RE: PMSM control strategy
if the EV is running downhill, it makes absolutely no sense to tell the motor to deliver more torque than needed. This will be a completely unstable situation where the car will accelerate in an uncontrolled manner, theoretically towards infinity.
Right! But WHERE IS THE VELOCITY LOOP?? Hint: between your ears.
When was the last time you drove your car, began to go down a hill, and DIDNT BACK OFF ON THE GAS PEDAL? SEE? It works.
A PMSM motor, like most servos, needs THREE control loops to run, and an EV is no different. The torque loop is your gas pedal input compared to actual motor output torque (amps in motor) and done by the EV motor drive. The velocity loop is your brain; it decides what speed it wants to go (the command), and watches your speedometer for the feedback. Then your brain ALSO does the POSITION loop - the command is your idea of WheRE you want to go, the feedback is your eyes seeing how close you are to your destination.
It really is that simple.
You CAN try to complicate it by making the EV drive a velocity loop drive BUT THAT SELDOM WILL WORK IN AN EV FOR GOOD REASON: control loop stability. There is a little rule of thumb that helps explain some limits of a velocity loop: it will be stable over about a 5:1 MAX inertia change. How heavy is your EV? Even if you cheat and put in a little planetary gearing, the relfected car inertia is WAY WAY (1,000's) higher than that little 85kw PMSM motor; hence you can TUNE the vel loop to be stable and control the speed - WHEN THE CAR IS SOLIDLY ATTACHED TO THE MOTOR. Unfortunately, in THIS world, that never happens. A load ALWAYs has some amount of compliance or backlash. So what inertia does your motor see when in the middle of the compliance or backlash? Hint: 0 inertia. Your loop tuned to 3,000 times higher inertia is instantly unstable. Your motor will go backwards and forwards in an uncontrolled oscillation - not very nice for a car! I could go on and on why a velocity loop, and very much so power loop, is NOT going to be used at the end of your day.
(It may not be obvious, but this is right up my alley as a servo engineer who happens to also drive a 100% electric car of which I have intimate knowledge of the controls)
www.KilroyWasHere<dot>com
RE: PMSM control strategy
Of course there is a torque loop as well, this is where the design gets a bit more detailed.
That does not alter the fact that you cannot force a motor to output torque (for instance if there is no load). The load is the determining factor for _needed_ torque, which the motor then delivers.
Cheers,
Benta.
RE: PMSM control strategy
This is how I interpret things right now, based on this schematic: http://www.mathworks.nl/help/releases/R2013b/physm...
My input is the gaspedal, which has to be translated into a desired speed of the motor (0-100%), which, by comparison to the actual speed of the motor, will result in a specific amount of torque that is required to achieve that speed. That number is then used as an input for the vector controller (among with other inputs), that will generate the appropriate pulses to achieve exactly that amount of torque.
Is this about right?
RE: PMSM control strategy
No.
Your gas pedal commands torque. period. YOU decide when you have given enough or not enough and push harder or less hard. It really is that simple.
www.KilroyWasHere<dot>com
RE: PMSM control strategy
https://www.box.com/shared/0n8use8vl78c65zdo2cy
www.KilroyWasHere<dot>com
RE: PMSM control strategy
RE: PMSM control strategy
www.KilroyWasHere<dot>com
RE: PMSM control strategy
Is there a way of knowing the actual motor torque, or isn't that necessary? I mean so you can subtract the actual torque from the desired torque and build a control loop around it.
RE: PMSM control strategy
Also, if you request negative torque you get braking and the braking energy is returned to the battery.
RE: PMSM control strategy
Why do you want to subtract the actual torque from the commanded torque? That is what the drive you purchase does. Or are you saying you want to reinvent the wheel and build your own drive from component parts? There are a LOT of good drives out there that are dirt cheap - you cannot build one for the price you can buy one I think. Also, in an EV you have to contend with IP69 sealing for the drive - that has already been done too for you in the purchased drive. For instance the PM100 is ready to run your motor:
http://www.rinehartmotion.com/
www.KilroyWasHere<dot>com
RE: PMSM control strategy
Kind of a problem with this question. Those two things are not really an "either and/or" scenario.
PWM is just about firing DC in pulses to change some sort of property in the DC to attain a desired output result. PWM can apply to AC or DC outcomes.
SVM is the switching method of the output transistors to make a circuit that the motor thinks is AC. It's the think that makes a VFD an "Inverter" drive.
Once you have SVM, then the pseudo AC output can be tweaked to get different levels of performance from the motor. That "tweaking" is referred to as the "control algorithm", which can be Scalar (aka V/hz) where there is no feedback from the motor as to how it is performing against the demand, or it can be "Vector Control" where the motor performance is monitored via a feedback loop so that errors can be corrected.
Vector control can then be further broken down into different levels of performance. SVC (Sensorless Vector Control) is simple but not as accurate, FVC (Flux Vector Control) is more complex and more accurate, then FOC (Field oriented Control) is generally considered the most accurate form of FVC (although the ABB crowd will want to argue for DTC (Direct Torque Control) as being equal or better). But since I do not claim to understand why DTC is supposed to be so much better (I have never seen it out perform FOC), I'll leave that to others. FOC can then itself be broken down into needing an encoder feedback, which is where you can attain the holy grail of AC motor control, Full Torque at Zero Speed; or "Sensorless" FVC, where you can get ALMOST there. Since it would be highly unlikely that you would need FT@ZS on an EV, let's assume you will use the Sensorless version.
With Sensorless FVC, the mP in the drive, using highly accurate current sensors, is able to fully separate the output current signature into the current that produces flux in the motor (what makes the motor a motor at all), from the current that produces torque in the motor (the current that does the work). They actually occur at slightly different times in the sine wave, so by using a very high speed powerful mP, you are slicing that "pie" of current into very fine pieces and tweaking the vectors of those two elements faster (compared to other algorithms) to affect a greater degree of accuracy and at the same time decrease the response time to a step-change in load, so that the motor corrects any error faster. WITHIN that FOC algorithm, there is a velocity loop AND a torque loop so that you can essentially control either or BOTH things virtually simultaneously. So you can do torque control WITHIN a velocity control loop, or velocity control WITHIN a torque control loop.
So how that applies to your question is, this gives you the best of both worlds. In a vehicle, you are deciding the speed you want to go, which is essentially a velocity loop between your sense perception and motor skills on the gas pedal. While you are performing that velocity loop, the FOC algorithm is going to AUTOMATICALLY deliver whatever torque is necessary (within the limits of the motor) to maintain that velocity.
As mentioned, that process is essentially "canned" in an off-the-shelf VFD (of any decent quality). Creating it from whole cloth on your own is likely going to be a daunting task. People spend entire careers developing these things (or lots of people with short pieces of their careers), funded by deep pockets of companies expecting to sell bajillions of them. If you can find one already made, it's likely the best thing for a small project.
"Will work for (the memory of) salami"
RE: PMSM control strategy
RE: PMSM control strategy
I have no clue what the subject of this sentence is; and I am curious.
WHAT is the big loop closed by driver?
www.KilroyWasHere<dot>com
RE: PMSM control strategy
RE: PMSM control strategy
But first let me say there is but one control loop on drives for these PMSM motors. All the other things mentioned are just SWITCHES. current limits, regen braking, etc. OK,OK, chargers, antislip etc are loops, but not to control locomation so I don't count them in the drive control of the motor while rotating. Even the commutation is not a loop or any kind - it is just logic in typically an FPGA IC.
So why EVs do not use velocity loops.... again, as I mentioned previously, it is done due to gain and how people drive cars.
When you go down a hill in an ICE mobile, if it were velocity control your speed would not increase; ditto going up a hill you would not slow down. But instead you apply more or less gas pedal pressure to modulate the current (torque) going into the motor. Simple. It works. Same in an EV.
Now back to gain again. The car weighs 3-4-5-6,000#. This weight is reflected back to the direct driven wheel motor and it is 1,000's of times more than the motor inertia itself. You CAN put gearing in to reduce the reflected inertia, but the available ratios are from 1 to about 8 max (with normal diam tires and 8:1 gearbox, motor will be spinning at about 8,000rpm @ 80mph! So forget reducing the inertia down to within 1-5x the motor inertia. Since the inertia will be VERY much higher than the motor, if there is ANY compliance (spring) or backlash (there is always) then you cannot tune the velocity loop for the required bandwidth - or it becomes very very unstable when the compliance acts or backlash is seen. I will go over what bandwidth is required later, just suffice it here to say it can't be high.
Then there is the over 100 years of perception how a car operates. If it WERE possible, then people would have to relearn how to drive - going up hill would not require pushing more, going down not backing off the pedal. We tried a new scheme on my car to see if folks could get used to something similar in order to try to not have to modify the stock brake pedal, which requires more certifications and rules: we tried making the pedal apply more torque when pushed as normal, but then automatically regen brake when released from whatever the present torque was. So running down the road, if one let go of the pedal, the car tried to stop fast! If one shook their foot some, the car would jerk forward and back! This experiment did not last to day 2! SAME thing would happen with a velocity loop controlled by the pedal.
Now to bandwidth requirements.... when you step on the pedal, you expect action. So you will need to be able to saturate the VL output to command full torque in a reasonable time. What is that?
It is about 100 mseconds or 10hz. So you better have a 20-30hz BW in your VL. I guarantee with springs on the car between the wheels and 5000 load, THIS WILL NOT HAPPEN WITH STABILITY. Guaranteed.
You can probably get stable control around 1hz. So who is going to be willing to floor the pedal and wait 2-3 seconds for it to go? I am not standing in line to buy that.
But cars have cruise controls and they are stable you say! Sure! What is their BW? they respond VERY SLOW - in order to be stable. It can do this low BW since it only turns on at a given speed, and only has to control +/- a couple mph variance. THAT is the response you would get if you try to close a velocity loop around the drive's current loop.
www.KilroyWasHere<dot>com
RE: PMSM control strategy
Mike, what do you see as the emerging choice in EV motors? Induction or Synchronous? Should I be changing my preferences?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: PMSM control strategy
Unfortunately I think if used, the marketing used against it combined with the loss of mileage would reduce the potential sales by a factor of 10.
There is something called 'range anxiety' that all EV drivers have: it means we are to the point of paranoid watching the charge gauge; not having the option to pop into a gas station for a range extention, we are at the mercy of our batteries..... I have actually SWEATED in the past a few times wondering if my pushing it would leave me stranded. One time I could feel the motor shake due to dipping the available voltage below the required BEMF of the motor and the flat topped sine waves to the motor caused vibration if I stepped on it - in the last mile coming home.
With this need for more range, ANY loss is devastating. I'll make a guess that if I had the best designed induction motor possible I would loose about 10 of my 80 avg mile range.
My PMSM reaches 94% efficiency max; so we know an ac induction motor can match that, so it is not an efficiency issue.
It is a size & weight issue I think. There is no way to make the induction motor as small and lightweight as the equiv size PMSM motor with Neodymium magnets. In the past when we have compared both for specific applications where these two items were important, IIRC we could do the PMSM motor in less than 1/2 the size and weight of the induction. If that gets the driver with his range anxiety another 10 miles range, it is well worth it I think.
www.KilroyWasHere<dot>com
RE: PMSM control strategy
There is no substitute for information by someone who both designs and drives an EV.
That's the Eng-Tips advantage!
I'll work on changing my prejudices Re: induction Vs PMSM.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: PMSM control strategy
Electric car is a system that need a driver, it's not autonomus. Driver impose a speed (desired/reference) and driver also sense car speed and adjust pedal to reach desired speed. That is big loop and don't treat it as a standard velocity loop. No need to tune this loop, it's indirectly "tuned" by driver (and internal loops/logicals that avoid unsafe situations). Inside it there are more or less loops that cover various internal blocks.
This big loop need low bandwidth (10-15Hz, or maybe faster for high-class Formula 1 driver ...) as you tell.
Some info about EV control and motors - http://www.intechopen.com/download/get/type/pdfs/i...
RE: PMSM control strategy
www.KilroyWasHere<dot>com
RE: PMSM control strategy
One of the key reasons that AC induction motors are popular for pure electric vehicles is that you can vary their rotor field strength widely and dynamically, thereby changing the torque/speed tradeoff, and eliminating the need for a mechanical transmission. The prototype Teslas had a 2-speed mechanical transmission that was failing quickly, and they got rid of that in favor of more aggressive field control.
Interior permanent magnet synchronous motors permit some additional variation of rotor field strength compared to surface magnet motors, but not as much as induction motors. A proper comparison of induction to synchronous motors must include the size and efficiency of the gear box/transmission.
RE: PMSM control strategy
but know folks are now taking the ipm's reluctance field to new extremes!
ck out the speed torque curve i attached a few posts back. i asked the delphi design engineer of that motor how he extended our typical +30% 'field weakening' ipm to induction motor levels but he did not answer. that 230v 3ph 2500rpm motor reaches 230v @ 2500 yet continues upto 11,000rpm max!
i just had to tell a good customer that, although we could run their siemens 1FE5093 (100 or so hp iirc) pmsm spindle motor rated 480v @ 2400rpm with top speed 7000rpm on our supplied test stand, we did not supply the required output crowbar voltage clamp so if they hit e-stop at max speed the motor would self destruct due to the Kb generated 1300v rms!
www.KilroyWasHere<dot>com
RE: PMSM control strategy
You bring up a good point. I don't know if I would want to sell or buy a car with that kind of failure mechanism, even with a good crowbar voltage clamp!
RE: PMSM control strategy
RE: PMSM control strategy
You are heading down a good path if you're looking at SVM. Keep in mind that most all textbooks and papers describe SVM as a voltage-mode (VM) controller, i.e. the switch sequence results in an integral of V*t over each sample cycle equal to the voltage vector which you want to place onto the windings of the PMSM. An average current mode control (ACMC) algorithm is typically used to satisfy the torque command (remember torque is proportional to current). As you probably know, current (torque) lags voltage due to the series inductance seen by your inverter. The ACMC loop would basically attempt to follow the "gas pedal position" as a torque, as mikekilroy explained. A properly designed ACMC loop tells the SVM to put the voltage vector onto the motor which leads the back EMF voltage vector by an amount which will keep the current vector at the same angle as the field, for high power-factor and efficiency.
Darrell Hambley P.E.
SENTEK Engineering, LLC
RE: PMSM control strategy
www.KilroyWasHere<dot>com
RE: PMSM control strategy
I've been following SR motors for 30 years now, and there are definitely some intriguing aspects to them. As Mike notes, there are some "chicken and egg" issues in getting them and their drives into volume production compared to induction and PMSM motors.
They do have a key technical drawback compared to these other motors. Because they operate on a "vernier" principle - with the stator field counter-rotating with respect to the rotor and at a several times higher frequency, the iron losses in the motor, which are proportional to the square of this frequency are far higher than for the more traditional motors. The higher frequency also puts additional demands on the drive.
RE: PMSM control strategy