Penjelasan soal 5th gen electric drivetrain yang disebut tanpa rare earth oleh BMW


BMW bilang akan menginvestasikan 200 juta euro untuk membangun pusat pengembangan baterai biar lebih maju dari sekarang.
BMW Group invests 200 million euros in Battery Cell Competence Centre

Selain itu BMW juga bakal mengembangkan sistem penggerak listrik generasi ke-5 yang menggabungkan dinamo, transmisi dan sistem elektronik jadi satu. Dinamo nantinya nggak bakalan pakai bahan langka:

future electric drive is that the electric motor, transmission and power electronics are combined in a new and separate electric-drive component. A further highlight is that the new electric motor does not require the use of rare earths, making the BMW Group no longer dependent on their availability.

Seperti apa itu? Bahan langka yang dimaksud sepertinya adalah untuk magnet.


Menurut penulis BMW bakal meniru seperti yang dilakukan oleh Tesla, yaitu menggunakan dinamo induksi yang digerakkan dengan listrik AC, ciptaan Nikola Tesla. Ini dijelaskan sama teknisinya pembuat dinamo untuk Tesla sebagai berikut:
Induction Versus DC Brushless Motors, Wally Rippel, Principal Power Electronics Engineer January 9, 2007

With brushless machines, the rotor includes two or more permanent magnets that generate a DC magnetic field (as seen from the vantage point of the rotor). Unlike the DC brushless rotor, the induction rotor has no magnets – just stacked steel laminations with buried peripheral conductors that form a “shorted structure.”

Dinamo DC butuh dua atau lebih magnet untuk menghasilkan medan magnet. Dinamo induksi nggak butuh magnet.

Keuntungan dan kerugian masing masing dijelaskan berikut ini:

One of the main differences is that much less rotor heat is generated with the DC brushless drive. Rotor cooling is easier and peak point efficiency is generally higher for this drive.

With DC brushless, as machine size grows, the magnetic losses increase proportionately and part load efficiency drops. With induction, as machine size grows, losses do not necessarily grow. Thus, induction drives may be the favored approach where high-performance is desired; peak efficiency will be a little less than with DC brushless, but average efficiency may actually be better.

Permanent magnets are expensive – something like $50 per kilogram. Permanent magnet (PM) rotors are also difficult to handle due to very large forces that come into play when anything ferromagnetic gets close to them. This means that induction motors will likely retain a cost advantage over PM machines. Achieving stability over the entire torque-speed range and over temperature is more difficult with induction than with DC brushless. This means added development costs, but likely little or no recurring costs.

Disebutkan bahwa bagian berputar dari dinamo DC brushless akan jauh lebih dingin saat beroperasi. Pendinginan lebih mudah dan efisiensi puncak lebih tinggi. Namun saat ukuran diperbesar, efisiensi akan berkurang. Sementara itu di dinamo AC efisiensi tidak selalu berkurang. Jadi untuk pemakaian yang membutuhkan performa, efisiensi dinamo AC akan lebih tinggi dari dinamo DC.

Magnet itu mahal. Magnet juga susah dihandel. Oleh karena itu dinamo AC bakal lebih murah daripada dinamo DC. Namun membuat torsi konsisten di rentang rpm bakal susah dan butuh pengembangan yang lebih banyak.

Kesulitan itu dijelaskan oleh kepala teknisi Tesla sebagai berikut:
Q&A with Tesla’s lead motor engineer (Full Interview)

A large portion of the time people spend driving is in low-torque highway situations. However, there are a lot of motors that offer great 0-60 MPH performance but are very inefficient in the low-torque highway-speed regions. So the question is, can I have everything – both high efficiency and high performance? The answer, unfortunately, is no. But you can make intelligent choices between things that are competing with each other.

This is the beauty of optimization. You can pick among all the options to get the best motor for the constraints. If we model everything properly, you can find the motor with the high performance 0-60 MPH constraint and the best possible highway efficiency.

Another example is the overall motor efficiency versus its cost. There are cases where making a motor in more expensive ways could potentially increase efficiency and buy off multiple times the cost difference by saving money on the battery, or other aspects of the car. So, if you are able to model the motor efficiency and costs accurately, you can plot it against battery cost savings. Now you can see that the optimal motor for total cost minimization is often different from the cheapest motor.

Intinya mendesain motor itu harus mempertimbangkan apa motor mau dipakai kencang atau pelan. Bisa kencang itu bakal mengurangi efisiensi saat mengendarai di saat pelan. Masih seperti kendaraan bensin, kencang jadi boros. Tapi susahnya yang begini ini nggak bakal ketahuan sama kita. Mau menentukan mana motor irit sama mana motor boros pasti jadi tantangan. Mau ngukur pakai full to full susah karena ukuran baterai pasti beda beda.

 

Kemungkinan BMW memilih dinamo tanpa magnet itu karena butuhnya memproduksi kendaraan yang fokus pada performa.

Videonya BMW:

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