August 24, 2017
|By: Jim Harrison
Guest Blogger, Lincoln Technology Communications
After great success with their home-built AC induction asynchronous drive motor used in all three of their first vehicles, Tesla has reportedly changed to a synchronous permanent magnet AC (PMAC) motor for their just introduced Model 3.
The reports are that Tesla disclosed the motor type in documents for its EPA certification, where it stated: Drive Motor - AC 3-Phase PM, 192Kw, 258hp. The base model motor may be sized differently than the long-range one.
As a bit of an aside, these EPA documents also said that the long-range Model 3 battery pack has a voltage of 350V and a capacity of 230Ah, which results in 80.5kWh. The base model (50 kWh) offers 220 miles of range and a 0 to 60mph time of 5.6 seconds. 0 to 60 for the long-range model, which starts at $44,000, is 5.1 seconds.
Also, the Chinese magnetic material company Beijing Zhong Ke San Huan Hi-Tech Company (ZKSH) has announced the signing of an agreement to supply Tesla Motors with neodymium iron boron (NdFeB) magnets for the next three years. These could be used in places other than the motor, of course.
PMAC motors are a bit more complicated than induction types, but still quite simple and reliable. They are smaller, lighter, and somewhat more efficient than induction type, especially at low and high loads. Almost all other EV manufacturers use this type of motor.
Figure. 1. Tesla Model 3
I would think that the synchronous operation of the PMAC motor means that Tesla had to significantly change their motor control algorithms, and those algorithms are probably a bit more complex than they were. From the early short test drives people have reported on, it would seem they did a good job. It will be very interesting to see if the improved motor efficiency yields significantly more miles/Kwh and if there is any change in drivability. One report said Tesla is using neodymium and praseodymium (Nd/Pr) rare earths for their magnets – not confirmed.
The efficiency of both PMAC and induction motors is greater at full load. Large (>100hp) PMAC synchronous motors typically offer 98% full load efficiency, while premium induction motors are more like 92% to 95%. Induction motor efficiency falls to around 80% at 20% load, while PMAC motors are still 88% at this light load.
One big reason not to use a PMAC motor is material availability. Especially neodymium. Its price skyrocketed in 2011 from $35k per metric ton to $250k. The price then settled back down to about $35. And in mid-July, neodymium hit about $65 a ton, up 30% from the most recent low last November.
Mines in China produce most of the neodymium material. While there are a number of deposit sites of neodymium around the world, including the California desert, it is difficult to extract and process profitably.
Molycorp Inc. was the only American mining company producing rare earth materials. Headquartered in Greenwood Village, Colorado, they owned the Mountain Pass rare earth mine in California. The corporation filed for bankruptcy in June 2015 after technical difficulties and "changing competitive circumstances" (read China) led to their spectacular downfall. They reportedly spent more than $1B on the mine. The company was eventually purchased by its largest creditor, Oaktree Capital Management, and has been reorganized as Neo Performance Material. That company now includes a rare earth and rare metals processing facility in Estonia.
The Critical Materials Institute, a U.S. Department of Energy Innovation Hub, has fabricated magnets made entirely of domestically sourced and refined rare-earth metals. A number of research labs are working on alternative materials for high-strength magnets. A company called Peak Resources is developing the Ngualla Rare Earths Project in Tanzania. The company Rare Earth Salts is providing rare earth materials from a plant in Nebraska. RES' initial production feedstock is from recycled fluorescent light bulbs, making its production a 100% domestic solution.