SynRM | A new giant in the electrical world

did you know that synchronous reluctance motors which were invented in the late 1900s are now considered superior to induction motors these motors have advanced electronic controls which makes their efficiency and torque output far superior to any other motors many industries and even the company named after the inventor of the induction motor tesla have started switching to syn rm motors tesla uses an advanced version of the synarm motor let's discuss the physics and design features of this new giant in the electrical world the physics of this motor are quite simple you might have observed this interesting phenomenon when a magnet comes within range of iron nails the nails are attracted to it to comprehend the reason behind this response we have to understand two things first the magnetic field chooses the path with the least resistance and second the structure of iron let's explore the resistance the magnetic field has to face more specifically this resistance is called reluctance magnetic flux always has a tendency to flow through the path of least reluctance maximum magnetic flux passes through the iron instead of the air because iron's reluctance value is much lower than the reluctance value of air now let's learn about the structure of iron iron has a domain based structure domains are small areas with individual magnetic poles however as you can see these poles are naturally arranged in a random direction so if you sum up the total magnetic field it cancels out a typical domain area is the result of atoms with unpaired electrons spinning in the same direction as shown in this visual as the permanent magnet's magnetic flux flows through the iron nail its domains are aligned in a single direction once that alignment occurs the entire iron nail will have a resultant magnetic field and it acts like another permanent magnet thus an attractive force is generated between the nail and the permanent magnet however for the domain to be aligned the existence of an external magnetic field is mandatory so it's more accurate to call the iron nail a temporary magnet let's do an experiment and generate a torque using the reluctance force concept we have just learned a solid iron bar which is free to rotate is positioned as shown now let's keep an electromagnetic at an offset to the iron bar the iron bar will definitely be attracted to the electromagnet due to the reluctance force and it will rotate however after being aligned with the magnetic field the torque on the iron bar becomes zero this is a crucial concept to note when the iron bar and the magnetic field are perfectly aligned the torque on the rotor will be zero now let's design a simple syn rm using the fundamentals we've developed so far here a three-phase coil arrangement replaces the electromagnet when a three-phase alternating current passes through this coil it will produce a magnetic field which is rotating can you tell what will happen to the iron bar under the influence of this rotating magnetic field a straightforward answer is that the rotor will align with the magnetic field as we learned in the previous experiment and it will rotate at the same speed as the magnetic field this answer seems quite logical however when you apply a rotating magnetic field to a still rotor the results might surprise you the rotor actually resists rotation let's learn why as the end pole approaches above the rotor the iron bars domains start to align as shown and the opposite poles will have attractive force between them next the rotor should rotate the rotor does rotate but the villain here is the rotor's inertia which causes it to achieve a very low speed compared to the rmf by this time the succeeding s-pole will come upon the rotor which causes a repulsive action well you might think that because there are no permanent magnets the induced poles on the rotor can change as the rmf changes however they don't here's the catch the magnetic domains that we learned earlier actually take time to spin this is a well-known phenomenon called hysteresis thus as the south pole approaches the rotor poles are unable to change quickly resulting in a repulsive force on account of rotor inertia and hysteresis a still rotor is subjected to alternating attractive and repulsive forces this is why the syn rms are inherently not self-starting therefore the highly adopted way to make this motor self start is to reduce the speed of rmf during start and then gradually speed it up let's test this method we can easily control the rmf speed by varying the frequency of input current initially the rmf speed is almost zero an opposite polarity magnetic pole is induced in the rotor and it becomes attracted to the rmf and slowly starts the controller device detects the position of the rotor depending upon this position the controller adjusts the rmf speed so that there will always be an attractive force between the rotor and the rmf as the rotor speeds up the controller increases the rmf speed as well thus the rotor runs in synchronism apart from starting the motor the controller plays a crucial role in the synrms normal operation as well as we've seen if the iron bar is aligned with a magnetic field the torque produced will be zero a hypothetical working with zero load on the rotor is shown here adding a load on the motor results in the rotor rotating behind the field at an angle this angle is known as load angle now suppose the load on the motor were to increase abruptly obviously the load angle will also increase however if the load angle crosses a critical limit the rotor will slip out of synchronism and come to a halt the controller comes to the rescue in such situations it continuously measures the rotor's position efficiently adjusts the angle and magnitude of alternating current and makes sure that the load angle is always below the critical limit clearly syn rms are software powered just by adding one more iron bar perpendicularly we can produce double the amount of torque please note such a rotor has to use a 4-pole rotating magnetic field now let's discuss another crucial concept related to syn rm you can observe that the interaction between the magnetic field and the rotor area is high at this angle and low at this angle the difference of flux interactions results in reluctance torque production which means that in order to increase the reluctance torque further a maximum flux interaction in the left-hand side alignment in a very low flux interaction on the right-hand side alignment is necessary a perfectly round rotor design will result in maximum magnetic flux interaction however such a design does not have a minimum flux interaction position the flux is constant in all the angles to understand how to produce a minimum flux interaction design let's use fea simulation the fea software em works 2d by solidworks accurately simulates the shape of the rotating magnetic field now let's arrange alternate layers of a ferromagnetic material and a non-magnetic material along the magnetic field lines this design will obviously produce a high flux interaction at this angle the interesting thing is that when the rotor is offset 45 degrees the flux interaction becomes extremely low due to the non-magnetic barriers the rotor experiences higher reluctance at this position this is a perfect design from an electrical engineering perspective however when engineers tested this design they observed that the rotor fails mechanically at higher speeds since the bond between different layers fails to provide necessary centripetal force moreover manufacturing of such radially stacked rotors is quite rigorous for this reason the modern syn rms use a slightly different design a rotor design based on thin laminations manufacturing of this design is quite easy and at the same time it maintains the same good electrical qualities here curved cavities are punched on the thin laminations also the natural air between these cavities acts like a magnetic insulating material synrms have started replacing induction motors in most of the industries due to its remarkable performance in the induction motors torque is produced due to the interaction between the rmf and current flowing through the rotor bars this current flow results in considerable amounts of i squared r losses losses in the form of heat this is why the induction motors have a lesser efficiency compared to the syn rms due to the absence of i-squared r-loss and synrms they run cooler for the same current input synrms are able to produce 10 to 15 percent greater torque than induction motors finally the most obvious advantage syn rms always run at synchronous speed the speed of rmf in an induction motor the rotor speed will be slightly less than the synchronous speed and this speed varies according to the load we hope you now have a good understanding of how synchronous reluctance motors work next time when you attach a magnet to your refrigerator you'll know it attracts due to the same reluctance force we thank em works for their fea support in this video please don't forget to press the support button thank you

Channel: Lesics

Views: 2,232,577

Rating: 4.9149041 out of 5

Keywords: SynRM, reluctance motor, direct axis, quadrature axis

Id: vvw6k4ppUZU

Length: 10min 39sec (639 seconds)

Published: Wed Sep 30 2020

Reddit Comments

[u/Chromosomaur]

Why are the thin laminations less susceptible to failure at high rpm?

Edit: The thin laminations do not have any non-magnetic material but rather use air. This would imply that the adhesive forces (between the non-magnetic material and the iron used in the disks susceptible to failure) are lower than the centripetal forces. Is this really the failure mechanism? And are there no better ways of generating greater adhesive connections between the non-magnetic material and the iron? It would seem to me if the adhesive forces were greater than the centripetal forces at any rpm, then the disks (with non-magnetic materials) would be less prone to failure than the thin laminations.

[u/gusmeowmeow]

but wouldn't the condition that the rotating EMF must be started slowly to avoid a seized rotor mean that there is some limit on how quickly you can get these up to a certain RPM? if I want to go from 0 to n RPM with maximum torque and as little time as possible (aka floor it on my Tesla) wouldn't this condition require the controller to mitigate that by a significant degree so as not to sieze the rotor?

[u/aFewPotatoes]

https://www.quora.com/Why-did-Tesla-decide-on-a-switched-reluctance-motor-over-a-conventional-permanent-magnet-motor?top_ans=174345986

A very good explanation of what the difference in motors actually are.

[u/Pitaqueiro]

Very low torque at lower speeds!

[u/KDE_Fan]

I'm not sure how these motors can always maintain the same speed under load where induction motors will slow down. They said it's "software controlled" but I'm assuming that if the rotor is lagging (or trying to lag/fall behind) it's because a load is being applied and at some point the load has to be too much.

Since the speed is related to the frequency, how does the motor correct for the increased load, where the induction motor would slow down, what allows this to maintain the speed? Does it just dump more current into it or increase the voltage some how - maybe the controller uses PWM to maintain the speed.

I just can't see how these motors don't get overly hot if they maintain the speed under load - that extra power has to come from somewhere and some of it's going to be lost as heat.

[u/Studio_Top]

short answer is, this motor will not replace induction motors or any other motor. It will replace a motor where the applications says only use SynRM.

Its not cheap to mass produce, awkward lamination design,...,PMSM, BLDC and IM are much better options at any cost point.

[u/3Quarksfor]

I don't recall the good professor, I do know that this was at least 15 years ago. I was in the leadership of the Industry Applications Society of IEEE at the time. I was an engineer working in industry most of my professional life. Even then switched reluctance synchronous machines were available though power ratings were limited. As I recall the paper presented high power (>100 kW) machines used in traction applications.

[u/3Quarksfor]

Many years ago (retired engineer) I was at a conference where a paper was presented on switched reluctance machines. At the break I was approached by an earnest professor and asked when I thought industry would adopt the switched reluctance machines and abandon the old fashion induction machine. I responded "Not in my lifetime ". He was a bit perplexed and proceeded to explain all the advantages of switched reluctance machines. I'm still right and I foresee no change in the near future with regard to the SymRM machine. Any motor used in industry must be able to start across the line and be driven by power electronics ( ASD's). Induction machines fill the bill and operate at about 95% efficiency.