Tesla Motor’s engineers made a stunning design choice when they developed the Tesla Model 3. They abandoned the conventionally-used and well-proven induction motors and replaced them with a new kind of motor, called the IPMSynRM motor. These motors have a totally different design, making use of both magnetic and reluctance action.
Tesla Motors has started replacing the induction motors in their Model S and X with this new motor as well! In this article I will go through the journey of Tesla model-3’s motor and What makes IPMSynRM motors so special? Before exploring these new motors lets see why the induction motors are not up to the mark.
Every motor has a rotor and stator arrangement. Clearly, as you can see in the Fig:1 below, the rotating part of an induction motor is an arrangement of conducting bars. Once Stator creates a rotating magnetic field. This fluctuating field interacts with the rotor bars and generates currents in the rotor bars.
The Induced currents and the RMF interact, imposing force on the rotor bars, and the rotor starts to spin as shown in the Fig:2.
But there is an issue, suppose for long drives at cruise speed, losing 3% to 4% of energy to generate currents in the rotor bars is definitely not efficient. That’s why to overcome this issue Tesla motors is replacing this motor in RWD and AWD cars with IPM-SynRM motors for better efficiency. I will explain to you in detail about the IPM-SynRM motors in the section below, let's move to it.
The IPM-SynRM is a combination of Permanent magnet and SynRM motors, and if we integrate it by placing the permanent magnets into the slotted cuts of the SynRM motor, deep within the iron core. To understand it in a simple manner, I will explain both types of motors here, first is permanent magnet.
The PM motors work on the basis of the attraction between two magnetic fields(Fig:3A). They produce a good starting torque from the same motor volume. This is more efficient when you operate them using a controller, and they do not experience energy loss in the rotor. But the PM motors have an issue, when the car cruises down the road at high speeds, permanent magnet motors have terrible performance because of the generation of back EMF in stator coils.
The back EMF is induced when magnetic field lines produced by the permanent magnets link with the stator windings. This is clearly a reverse voltage to the stator’s supply voltage. The higher the rotor speed, the more it produces back EMF(Fig:4).
One more disadvantage is that magnets result in magnetic eddy current losses, which increases the heat in the machine. And is not favourable. Keep this explanation in mind, now I will explain the second type of motor.
Now we will understand how Tesla motor has overcome these issues by using SynRM motors. For high speed operation, Tesla engineers made use of iron’s reluctance property. Here iron gives less opposition to magnetic fields than air. Using this phenomenon SynRM motors generate rotation. To get this rotational motion, we cut slots in the rotor. At this rotor position, the rotor is in a high-reluctance state as shown in the Fig:5 left side of image. However, if the rotor is turned by 45 degrees, it will face a very low reluctance as shown in the Fig:5 right side of image.
The rotor always has a tendency to attain a low-reluctance state. Therefore, if the magnetic field rotates, the rotor will rotate along with it so that the rotor can always be in a low-reluctance state. The rotor’s rotation speed will be the same as the RMF speed. The torque produced by this phenomenon is known as ‘reluctance torque,’ and such motors are called synchronous reluctance motors. You can check our previous Article also it is based on SynRM motors, you will get extra information there.
You have to keep this in your mind that SynRMs can enhance high speed operations, they don't have back EMF issues and permanent magnet motors can be desinged to perform good at low speeds.
Then it is good to integrate the SynRM technology into the permanent magnet motor(Fig:6). If we do it, this motors can work efficiently
The interesting thing about this design is that the permanent magnet and reluctance parts of this motor have totally different behaviour in regard to the position of the RMF.
Now, let’s analyse the Tesla model 3’s motor. It's clear from the total torque graph that if the RMF angle is around 50 degrees, this is illustrated in the Fig:8 below, we’ll get maximum torque from the motor. So Tesla Motors engineers made sure that when you start the car, the RMF angle is around 50 degrees, which will guarantee maximum torque production. Using both reluctance and permanent magnet torque. But after a certain high speed the back emf shows up. To overcome this Field weakening is done. I will explain it below in short.
The permanent magnets in the IPMsynRM motor still have some effect on the stator producing back EMF at higher speeds. To overcome this issue, we can align the RMF opposite to the permanent magnetic field, as shown in the Fig:9 below at left side.
The RMF weakens or almost cancels the permanent magnet field, as shown in the Fig:9 below at right side. This way, even at high speeds, such motors won’t produce much back EMF. Obviously, at this stage the torque production will mostly come from the reluctance effect.
You have to know the difference between previous motors and Tesla model-3. In the previous motors use solid magnets, whereas each magnet in Model 3 motors is segmented into four parts.
That’s all about Tesla model 3’s motor. I hope you now have a good understanding of Tesla Model 3's IPM-SynRM electric motor.