Featured Electric motor efficiency introduction

To be entirely fair, Tesla also runs one small motor on the Model 3, where they run one big, two small, or one small and one big motor on the Model S. And, the Model 3 is lighter, and possibly has better aerodynamics, too.

I actually wouldn't be surprised if the Model S gets an efficiency benefit from its induction motors, due to the sizing of the motors, though - induction motors are less efficient at full power, but a Model S spends very little time at full power, instead spending most of its time at a rather low percentage of possible power. And, in that regime, induction motors' lack of cogging torque (due to not having permanent magnets to generate the field, instead generating it electrically) can improve efficiency.

 

One part of the regime where induction motors may give benefit is at high-speed where permanent magnet motors will require field weakening, probably incurring some extra losses.

kevin

 

Induction benefits -> Lower costs because of materials and no dependency on importing rare earth elements, Larger efficient power range because magnetic field strength (B) can be varied while it is fixed by the Permanant Magnets (PM) in a DC brushless design.

DC Brushless benefits -> lighter, about 2% higher peak efficiency. (90% versus 88% when considering full scope from battery through inverter to output shaft).

Historically there have been other trade offs, like a induction motor is harder to drive precisely, but but modern electronics have taken care of this. I believe the gen III prius motors are a combination of the two, with a weak PM and induction. This cuts down on costs and the use of rare earths, and imports from china.

Think of it this way. The more powerful the motor and the more expensive the rare earth elements, the more the engineering trade-offs will lead to induction. The model 3 requires less powerful motor(s) than the model S and X, and the prices of rare earth elements have dropped significantly. I have no idea if the rotors in the model 3 motors have induction coils as well as PM. One thing is sure though, if rare earths have shortages or get expensive again like they did in 2011, tesla can switch to induction or a hybrid of both. The stator design is the same between dc brushless and AC.

For the efficiency difference, I don't think most people care. Say you need a 75 kwh battery instead of a 73 kwh one, and use an extra 1200 kwh over 200,000 miles. If the less efficient car with the same range costs $1000+ less they would probably pick the slightly less efficient one. That's a good thing. The ability to switch to use less rare earths from china, is what pushed china to lower the price again. Right now rare earth elements aren't nearly as expensive, so DC brushless makes sense.

Most of the efficiency gains between the S and 3 appear to have nothing to do with the motor type.

The biggest difference between the S and model 3 are weight and rolling resistance. The 310 mile version of the 3 with a 80 kwh pack is 16% lighter and with the lower weight and lower rolling resistance 18" aero wheels, it significantly improve mpge versus the 19" wheels/tires on the 75 kwh model S. The car accelerates quickly but they didn't put in motors that had nearly the power (258 hp versus the model S rwd 362 hp) which allows them to keep the peak efficiency band closer to the operating power making it much more efficient in low power city driving. I would not be surprised if the battery charging also became more efficient with the new battery design.

 

Most permanent magnet motors for EVs get a significant amount of their torque from switched reluctance in addition to that form the permanent magnets. This can save up to 30% of the amount of permanent magnet material required for a given torque.

For some mild-hybrid designs they are switched reluctance only with a wound rotor - like a conventional car alternator.

I don't see how you can have a hybrid of both induction and synchronous permanent magnet motor (except by having two motors) as there has to be some slip for current to be induced into the rotor for induction torque.

kevin

 

I freely admit to having no idea what toyota did with the gen 3 motors. Press releases were scant on technical details at the time, and most information is now poof. Rare earth prices have gone down, and toyota hasn't mentioned anything about reducing the amount in the gen IV prius or the camry hybrid variants.

I don't see your technical problems in combining permanent magnets and coils in the rotors in a single motor. Simplest thought experiment solution is to simply have two rotors on the same shaft - one induction, one magnet. Then again, using my googlefu, I don't see any technical papers talking about this or doing this on the same rotor, despite reports saying toyota was using induction and permanent magnets.

What I do see, which is more likely is that they were using hybrid magnets. That is 2 different magnetic materials combined on the same rotor. I think this is the most likely way toyota reduced use of rare earth elements in the gen III. That seems to be likely considering what you are saying. In 2011 toyota was talking about going to induction motors if they couldn't buy enough rare earths at a reasonable cost. The threat of going away from rare earth elements appears to have changed china's mind on pricing. Still if enough hybrids and plug-ins are built the cost materials for permanent magnets will rise. The gen II mg1 and mg2 used 1kg(2.2 lbs) of Neodymium, which cost around $500 at peak in 2011, but now sourced in australia and probably costs around $75. I don't see toyota releasing the quantity in later generations of the prius.

Tesla has always emphasized efficiency, it had to, as the industry claimed stuffing a battery capable of going 300 miles old EPA, would be too expensive and heavy to ever sell. Its pretty amazing that the 85 kwh model S in 2012 was able to achieve that (265 mile current epa is more than 300 on the old epa which was CARBs requirement for the most useful, and therefore creditable car) with 90 mpge on the highway test. The greatest reduction in efficiency was from plug to battery at about 80% at 120VAC and 85% at 240 VAC. Improvements on the battery, charger, inverter, motors raised that to 105 mpge on the highway test in the current model S 75D. Charging efficiency wasn't that important when the car came out, and with not enough resources they didn't worry about it. It was more about reliability, the charging didn't change the pack size. If you chunk the numbers, and we don't know SOC limits, the original S 85 RWD used 100 kwh in the epa test or 18% more than its rated capacity, the S 75D 13% more, and the model 3 only 3% more. Either tesla is getting more conservative on SOC, or they have drastically increased charging efficiency.

Compare Side-by-Side



The model 3 with approximately 80.5 kwh battery gets 120 mpge on the highway, only 2 less than the ioniq electric with less than half the range. I would expect the 210 mile standard battery to exceed the ioniq on the highway in dual motor version which is due out in about a year. That is major efficiency gain.

The model S came out when rare earth magnets were expensive. Definitely now that they are reasonably priced, they give a slight efficiency boost and lower weight for the same range in the model 3. Tesla is also going to be building motors in nevada, so they can change quickly when economics change. When tesla went from single to dual motors suppliers couldn't keep up. I believe they are going to possibly with increased costs, make it easier to make the right motors for the model 3 as technology and material prices change.

Tesla to Produce Model 3 Electric Motor at Nevada Gigafactory | Fortune.com

 

Although you could put an induction rotor on the same shaft as a permanent magnet rotor it would need a different stator, so it would be in essence two separate motors on the same shaft.

In order to create torque an induction motor requires that the stator magnetic field rotate faster than than the rotor itself. This slip is to create the currents in the rotor to create torque.

In the permanent magnet motor the stator magnetic field rotates synchronously with the rotation of the rotor.

kevin

 

Kevin,

I do believe I was wrong about my car (2010) having a less rare earths because toyota was using other material. I would like to know.

I think part of the discussion is semantic. I would consider a device with a single shaft and a single plug (whether it had dc, or 1 phase, or 3 or 6 or 8 or X) a motor, no matter how many rotors a motor. I have specified motors in equipment with 6 phases before, so to me having two rotors and two stators and a single plug and control algorithm driving a complicated inverter with N phases driving the beast would still be a motor.

I probably am wrong in terminology also. In my mental experiment 3 phases could nicely drive the two roters, one of permanent magnet one of metal only. I don't know what the best configuration of that (induction/reluctance) roter, but I was thinking copper squirrel cage. If B of the magnet is high enough then it will rotate synchronously and not create torque in the second rotor. When it slips (during acceleration too high for the pm rotor or from insufficient torque from B) then the second rotor will provide power. Maybe it wont work. Algorithms for driving the inverters are definitely more complicated but fairly straightforward.

Currently with reasonable prices for rare earth elements we don't need to worry about it. I'm sure toyota and tesla and the motor companies are all figuring out whether to go back to induction if prices spike again. Two motors as tesla has on their D configuration seem to yeild more efficiency, and a PM for efficiency at higher speeds on the front axle, and a induction motor with more power on the rear axle might be a good way to distribute the 2 types.

 

The problem I see with that is that when the PM part is not running synchronously it may not produce any torque at all - if the magnetic field is not in the correct position relative to the rotor the torque can be negative and will average to zero. There will also be a lot of torque ripple that will make it noisy and rough running.

There may be some advantage to that - the control might not necessarily be speed related but power demand related. A small efficient PM motor (with only small amounts of rare-earths) could provide propulsion when demand is low (eg cruising) while the induction motor could cheaply provide the power needed for acceleration or hill-climbing.

The two motors could be on different axles as in the Tesla or could be on the same axle but they would need their own stators and inverters.

kevin