Field Weakening Control of PMSMs

welcome everyone so this is another session on permanent magnet synchronous machine control and today we are going to look at field weakening control of permanent magnet synchronous machines now in one of the previous videos we looked at DC machines that had a wound field and we tried to study how the field we can in control works and what does it mean to achieve here we can in control what kind of advantages do we get in being able to control and we saw that we can extend the operating speeds to to higher operating speeds by using field wave in control and we looked at the constant torque region and the constant power region for a DC machine now here today in this life we will look at similar concepts but applied to a permanent magnet synchronous machine and see how we can achieve that achieve the same capability of extend the capability of synchronous machines through field weakening control okay all right so just to refresh if you if you recall from the earlier video in a permanent magnet machine in case we if we are to do field weakening control we need to have two sets of windings typically if you are looking at a permanent magnet DC motor you will have magnets on the stator and your armature which is the rotor has a set of windings right you see that there are windings on the rotor and those rotor windings are connected to a commutator which maintains the current direction of it which adjusts the applied voltage polarity so that the magnetic field generated in the stator in the in the roca is 90 degrees out of phase with the state okay we know that for torque generation in any machine we try to maintain the phase difference between the rotor and the stator flux okay if you recall this from our earlier videos if this Evo alignment or if this is the direction your rotor flux if your rotor flux is pointing this way we try to orient our state of flux to be 90 degrees out of phase in this way so that we generate optimal top right now in a DC machine this is mechanically achieved to the commutator because the commutator keeps changing the current at a certain point along the rotor it will maintain that phase shift continuous you are almost close to 90 degrees okay now in permanent magnet synchronous machines let's we need to study that a little bit because now we don't have control over the over the the rotor Magnus right we don't have control however because it's a three-phase machine and once you transform that three-phase machine into rotor reference frame we have two voltage equations if you recall okay and let's actually go into those equations all right so if we if we look at the equations for the steady state equations for a permanent magnet synchronous machine we see the following relationships okay now salience e we talked about salience e in our prenup previous discussion a salience is basically the difference between lq + LD the inductance are different because the rotor architecture is designed in a certain way the material is different therefore the inductance tend to change now given given this whether it's a lien on until a machine if we look at this at the steady state voltage equations we can see that the y axis which means it's directly along the flux ok the D axis is basically directly along the flux if we are to let's try to draw a simple motor model so if we have let's say an SMP MSM right so you have a north and a south so in this case if this is the rotor this is my rotor flux axis lock rotor bearing axis D axis now when we apply or when we generate currents using the stator we try to generate the stator flux vector to be 90 degrees out of phase okay which means we try to align the stator flux along the Q axis of the rotor okay Q axis quadrature axis of the rotor now this way we guarantee this 90 degrees phase shift now in this sense if if you look at these two equations we have a voltage equation for D axis we have a voltage equation for Q axis alright now when we say we are doing flux weakening control that essentially means we are let's say we are reducing the effective amount of flux along the d axis okay we are we are technically diminishing the amount of flux in the DX is now practically thinking of a machine if it has permanent magnets right if you had firmness maintenance it's not clear how can we control that direct axis flux right so let me propose a couple of different control knobs that you can control okay so we have two options the goal is if you remember from the DC machine discussion we were able to reach higher speeds with by by controlling by reducing the amount of flux because if you can reduce the back EMF constant that means you have more voltage left to go to higher speed now trying to follow the same concept here changing the direct act in order to diminish lambda M in this case so in order to change the M what we need to do is we need to introduce some form of negative right so if we can somehow generate some negative flux in this direction so the total cumulative flux are all along the left axis will be lower and which means our back EMF conscious or lambda n prime the flux linkage constant will be smaller okay so that means we can we are able to apply or use the same voltage and reach higher speeds because now you can increase Omega R because your lambda M prime R decrease okay all right so say you in order to achieve that what we need to do is we need to generate negative D axis current okay because this is the D axis this is where the flux is oriented if we can generate negative DX's instead of two axes so if you have if you have Q axis current it will result in a Q axis flux if will generate D axis current will have color more flux along the D axis hopefully we are not saturated if we have negative D axis we basically have we are basically reducing the amount of flux that's contributed along the D axis which is influencing lambda M prime R okay so for the same IQ command for the same IQ command by by introducing negative P axis current what we can do is we can reduce the effective lambda M prime R and be able to reach higher speeds now that's that's so we talked about the D axis current now let's look at the voltage equation so this is just I'm rewriting the Q axis voltage equation here and then we said okay by changing by introducing negative V axis current we are able to change lambda M prime R right be able to lambda J influence lambda M prime are a little bit not a whole lot we can we can change it a little bit which gets us the opportunity to increase speed without changing the total voltage however we need to see another phenomena with negative D axis current depending on the speed that we are operating at we are also able to contribute a little bit of voltage to the total voltage that is available with a higher speed so there are two effects that we can see and with negative D axis current we are increasing we are adding voltage since this is a negative site negative negative becomes positive therefore this total voltage will increase as well as the flux linkage constant will decrease enabling us to reach higher speeds for the same amount of voltage that we applied assuming IQ is constant okay so this is and this is an interesting topic this is something that you can investigate with a practical setup if you have a machine that you can experiment with and depending on if you want to extend the operating speed for that for you know brief period of time you can use this technique to achieve higher speeds now in order to do that you are sacrificing current okay so be careful when you are specially operating closer to the voltage limit this might be difficult and also you are sacrificing voltage which means you know in order to generate current you need to apply voltage and that means you are you're kind of clicks on voltage which limits your total torque capability in order to achieve that highest speed so you're still going to be looking at a torque speed curve that has this kind of a shape okay so you have a certain point that you can operate at constant though and beyond that if you want so this might this is a rate at speed and beyond that if you want to reach higher speeds you will have to weaken the field weaken the flux which will be which will allow you to reach higher speeds but at the same time your talk is dropping the highest or maximum poke you are able to reach is dropping because you have sacrificed voltage to we can defeat okay so again if this is not very straightforward I recommend you to go look at the DC machine feel we can in control and then come back and revisit and look at the two equations that here we that we have here who we kind of feel and reach higher speeds okay alright so that's the end of the field weakening control part let me know if you have questions concerns or you know any clarifications needed on this topic thank you

Channel: Sandun Kuruppu

Views: 2,670

Keywords: motor control, AC machines

Id: kJwRDxSGBrA

Length: 14min 4sec (844 seconds)

Published: Wed Jul 15 2020