Image credit: wordlesstech.com Fig.1 Tesla model 3 motor
We’ve truly entered the EV age and even though most of us will still go for the conventional Petrol or Diesel car if we were to buy a new car today, we know in the back of our minds that soon we would opt for the EV instead in the foreseeable future. Most believe that by 2025 Electric cars will almost totally replace conventional internal combustion engine cars as the first buying choice.
I spoke about where we stand on the EV battery technology last week in the last newsletter of 1st June 2022. If you would like to refer to this, it is here.
Moving on, lets dive into the EV motors today. In the next issue, we shall talk about the EV chargers.
In a 4-wheel drive vehicle, the motors would be at the front as well as at the back as depicted in the picture below in Fig.2.
Image credit: electrec.com Fig.2
The most popular EV motor is basically an induction motor. The story of the induction motor goes back to the year 1887 when the first induction motor was invented by the famous inventor, Nikola Tesla at his workshop on 89 Liberty Street, New York and it looked as in the picture below in Fig.3.
Image credit: zmescience.com Fig. 3
The induction motor (IM) is the most widely used electric motor till date in all electric applications. Of course, there have been improvements in size, efficiency, torque, speed, efficiency, versatility over the years and interesting variants have also emerged.
The Tesla Model S and Model X use conventional IMs.
Today’s IM construction looks like the depiction below:
Image credit: oriental motor Fig. 4
An AC rotating field applied to the stator winding produces a rotating magnetic field in the stator. This magnetic field produces an induced current in the rotor’s embedded squirrel cage copper conductors which itself develops a magnetism and tries to follow the rotating magnetic field of the stator. Note that there are no brushes or commutator in this beautiful motor design and so it is simple, reliable, efficient and maintenance free. It also has regenerative capability that enables braking energy to be returned to the batteries. Further, speed control can easily be exercised via the AC frequency applied to the stator coils.
The picture below shows the electronics for generating the stator frequency and overall control for the Tesla model S motor:
Image credit: teslarati Fig. 5
The EV motor can be characterized by its torque-speed and power-speed characteristics as shown below in Fig. 6.
Image credit: engineering.com Fig. 6
The desired motor performance requires high torque at low speed, enabling proper starting and acceleration. The EV motor needs to have high power at high speed and a wide speed range in the constant power region as shown in Fig.5. The constant torque operating region is important at low speed to provide a good start and up-hill drive. The constant power region determines the maximum EV speed on flat surfaces.
In summary, these are the features we desire in an EV motor:
· High efficiency
· High instant power
· Fast torque response
· High power density
· Low cost
· High acceleration
· Robustness
These are the types of motors used in EVs:
· DC motors (DC)
· Induction motors (IM)
· Permanent Magnet Brushless DC motors (PM)
· Switched Reluctance Motors (SRM)
The features of these motor types are compared in Table-1 below:
It is noticeable that the IM motor type has all the characteristics suitable for EVs and is the standard best choice for most EV applications. However, depending on the features to be highlighted in a particular application, the variants are used, or a mixture of variants are also used. Summarized below is a table of EV motor type used in various popular brand and models of EVs:
As we can see, the SRM or Switched Reluctance Motor is also a serious contender in many EVs.
Image credit: linquip.com Fig. 7
That’s all in this newsletter; more next week.
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Arun Bhatia
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Thanks for your appreciation and encouragement. The starting torque is an issue with the Induction Motor but it can be handled by auxiliary windings quite well. Remember that with the Induction Motor, once you have the inverter electronics there is so much more you can achieve. As for the regeneration feature, also, as we know that when the rotor speed exceeds the rotating magnetic synchronous speed the motor goes into regeneration mode. So, when we are in the deceleration or brake mode we just have to decrease the frequency of the rotor field. This is exactly how Tesla solved these issues in models S and X. The trick is the inverter electronics and the algorithms it uses.
The BLDC motors…
Another excellent article articulated in very simple terms.
However I have some observations regarding normal Induction motors. Firstly normal squirrel cage induction motor has very low starting torque and their speeds are totally dependent on the frequency. Moreover it is not regenerative since the rotating field is generated by the input supply. I am not sure how these limitations have been overcome in Tesla Model S and X.
A permanent magnet is a must to have regenerative feature in any motor. Do throw some light on this and the low starting torque issues.
There have been a good progress in use of Brushless DC motors in the past few years. Are these also being used as in traction? Although series…