PMAC vs induction motor for model 3

[Rashomon]

Add me to the list of those confused by this subject.

Are you saying that a PMAC is torque limited (or at least not competitive with IM) without gears ?

No, you can get all the torque you want out PMAC -- but you might pay at cruise efficiency. The only place where a PMAC motor is less efficient than an inductive is at loads that are a very small percentage of peak power. A smallish PMAC motor will be as efficient as an bigger inductive motor at those highway-cruise loads, because instead of 3-4 percent of peak power, it will be running at, say, 12 percent, moving up on the efficiency map.The big inductive is least efficient at high power loads -- but you don't really care because you never run those for more than a few seconds unless you're on an autobahn or racetrack. You always have to think about the duty cycle, and optimize for that. Where the inductive is great is that you can shut it down completely with no magnetic drag. So best of both worlds: you have a smallish PMAC for efficiency, a big inductive for those occasional bursts of acceleration, and a significant improvement in efficiency.

BTW, I was just in Stuttgart, and I can say with assurance that you will not see the same solutions in high-performance German EVs as in current Tesla's. The autobahn has a lot to do with that. Very different duty cycle than US driving.

 

[Cosmacelf]

Yes, re:autobahn, Elon messed up when he made an early prediction that Germany was going to be a big market for Model S. he did not realize that spirited Autobahn driving pushes the Model S into overheat protection...

 

[SageBrush]

No, you can get all the torque you want out PMAC -- but you might pay at cruise efficiency. The only place where a PMAC motor is less efficient than an inductive is at loads that are a very small percentage of peak power. A smallish PMAC motor will be as efficient as an bigger inductive motor at those highway-cruise loads, because instead of 3-4 percent of peak power, it will be running at, say, 12 percent, moving up on the efficiency map.The big inductive is least efficient at high power loads -- but you don't really care because you never run those for more than a few seconds unless you're on an autobahn or racetrack. You always have to think about the duty cycle, and optimize for that. Where the inductive is great is that you can shut it down completely with no magnetic drag. So best of both worlds: you have a smallish PMAC for efficiency, a big inductive for those occasional bursts of acceleration, and a significant improvement in efficiency.

BTW, I was just in Stuttgart, and I can say with assurance that you will not see the same solutions in high-performance German EVs as in current Tesla's. The autobahn has a lot to do with that. Very different duty cycle than US driving.

Thanks for the reply.

How much parasitic loss are we talking about for a large PM motor, meaning that part that cannot be 'shut down' ?

 

[TEG]

Yes, re:autobahn, Elon messed up when he made an early prediction that Germany was going to be a big market for Model S. he did not realize that spirited Autobahn driving pushes the Model S into overheat protection...

Plus driving at such speeds is wasteful of energy, so you need to store a lot to get useful range. BEV battery just cannot compete with the energy storage of a large gasoline tank.

 

[Rashomon]

Plus driving at such speeds is wasteful of energy, so you need to store a lot to get useful range. BEV battery just cannot compete with the energy storage of a large gasoline tank.

It may be wasteful, but most Germans who could afford an expensive car would feel like like an idiot if they bought a car that couldn't cruise at a reasonable speed on one of the still unlimited autobahns -- say 230kph or more. You'll get passed by VW Golfs if you go slower than that. There really aren't that many opportunities to go that fast -- most of the autobahns now are both congested and variably limited -- but it's part of the car culture there. I was told by a local friend that you see Tesla Model S's in the slow lane with the trucks -- not because of thermal limits, but because of range. By the time you get to 200 kph, even a S100D won't go that far. It's surely been one of several reasons why Germany has been a tough market for Tesla.

 

[McRat]

German 'EV's tend to be either urban-only, or a small electric motor and battery with a far larger gasoline engine.

It's not that EVs are inefficient at speed, it's that gasoline cars don't care. You can drain a 20 gallon tank in under 20 minutes if you want in a gas car. Or at least I have. Well, perhaps a slight exaggeration, you run into fuel starvation issues at 20 minutes.

 

[JonathanD]

Plus driving at such speeds is wasteful of energy, so you need to store a lot to get useful range. BEV battery just cannot compete with the energy storage of a large gasoline tank.

True but I'm sure they could increase the efficiency at high speeds by using gears. Since that doesn't apply to the majority of potential owners though, it's really not worth the complexity. IIRC Tesla experimented with a 2 speed gearbox at one point and had nothing but problems.

 

[R.S]

German 'EV's tend to be either urban-only, or a small electric motor and battery with a far larger gasoline engine.

I think he was talking about the upcoming ones. Jaguar will also use a PMAC.

I also do think the next Model S will get PMAC motors. It's just much easier to develop and produce the same motors for the whole lineup. Two of the current M3 motors would surely be enough for a 100D and the performance version could just use 3 motors.

I also don't really buy the induction+PMAC AWD talk. It would only make sense if they had another small PMAC and a bigger IM in back. So 3 motors just for the Model 3? I thought they wanted less complexity.

I mean it could still be, they could use the small induction motor from the Model S. And I would really like the idea. But I don't think so.

 

[Rashomon]

Thanks for the reply.

How much parasitic loss are we talking about for a large PM motor, meaning that part that cannot be 'shut down' ?

Hard question to answer without data for a specific motor. I saw one 100kW motor where they were claiming the magnetic drag was 1.5kW if you just backed off at speed. I'm not sure if that was right, but when you're talking about something like 15-17kW (delivered) required to maintain 75 mph in a M3, it would be significant.

 

[TEG]

True but I'm sure they could increase the efficiency at high speeds by using gears. Since that doesn't apply to the majority of potential owners though, it's really not worth the complexity. IIRC Tesla experimented with a 2 speed gearbox at one point and had nothing but problems.

Adding gears wouldn't "fix" it to be competitive for Autobahn use. More efficiency isn't what is need, more energy storage is. A Tesla battery pack stores the energy equivalent of ~3 gallons of liquid fuel, so it isn't going to give you a lot of range when you are using so much energy fighting wind resistance.

I don't think ultra high speed driving is a place for EVs to dominate. Leave that for the "gas hog" fuel wasting conspicuous consumption dinosaur vehicles.

 

[JonathanD]

I don't think ultra high speed driving is a place for EVs to dominate

Unless of course it's in a vacuum tube

 

[R.S]

Adding gears wouldn't "fix" it to be competitive for Autobahn use. More efficiency isn't what is need, more energy storage is. A Tesla battery pack stores the energy equivalent of ~3 gallons of liquid fuel, so it isn't going to give you a lot of range when you are using so much energy fighting wind resistance.

I don't think ultra high speed driving is a place for EVs to dominate. Leave that for the "gas hog" fuel wasting conspicuous consumption dinosaur vehicles.

Energy storage isn't the problem, recharging is. If you could recharge in 5 minutes anywhere people wouldn't really care. The 3 gallons equivalent of fuel are enough to get 200 km even at rather high speeds. If you could recharge that in 5-10 minutes, anywhere, nobody would really care anymore.

 

[R.S]

Unless of course it's in a vacuum tube

Or just a regular train. An electric car probably wouldn't be a good idea in a vacuum tube, though. At least not like they are built today.

 

[az erik]

It may be wasteful, but most Germans who could afford an expensive car would feel like like an idiot if they bought a car that couldn't cruise at a reasonable speed on one of the still unlimited autobahns -- say 230kph or more. You'll get passed by VW Golfs if you go slower than that. There really aren't that many opportunities to go that fast -- most of the autobahns now are both congested and variably limited -- but it's part of the car culture there. I was told by a local friend that you see Tesla Model S's in the slow lane with the trucks -- not because of thermal limits, but because of range. By the time you get to 200 kph, even a S100D won't go that far. It's surely been one of several reasons why Germany has been a tough market for Tesla.

This is totally the case. I rented a Porsche for the bahn when I was there last year at this time. Got to actually open up a couple times for less than 30 seconds. Over crowded and oblivious drivers.

 

[Ulmo]

That's why a small bank of super-capacitors makes sense to me. It would dump road speed to the supercaps, and dump the supercaps back to acceleration. Battery use would be minimized. I would call it a Hybrid. Like a RAM cache, or disk cache, in computers.

Who marked my off-topic post "informative"? I wasn't informing; I was daydreaming, urging, suggesting, promoting. Entirely different.

 

[Ulmo]

BTW, I was just in Stuttgart, and I can say with assurance that you will not see the same solutions in high-performance German EVs as in current Tesla's. The autobahn has a lot to do with that. Very different duty cycle than US driving.

Is that why my Mercedes E500 was smooth as silk and comfortable as a cat at 120MPH for hours on end, but my Model S 60D was bumpy, wimpy and struggled to maintain 110MPH for more than 15 minutes? (Not on any local streets or freeways, of course!)

But the reason I'm really posting is this: how does Autobahn compare to driving up 1,800 feet in 10 minutes at around 60MPH? That's a rise of about 180 feet per minute.

 

[SageBrush]

how does Autobahn compare to driving up 1,800 feet in 10 minutes at around 60MPH? That's a rise of about 180 feet per minute.

Are you asking about power demand ? Napkin arithmetic:

1 meter per second rise in a 2500 kg car is 25 kW extra over level driving
If 60 mph on level ground is ~ 7.5 kW for Aero, 120 mph is 30 kW

 

[scaesare]

Energy storage isn't the problem, recharging is. If you could recharge in 5 minutes anywhere people wouldn't really care. The 3 gallons equivalent of fuel are enough to get 200 km even at rather high speeds. If you could recharge that in 5-10 minutes, anywhere, nobody would really care anymore.

People keep repeating this, but I don't think this is true.

200KM (125 mi) at "rather high speeds" might only represent a bit more than an hour's worth of driving.... .

And even if the actual recharging event takes the time you mention, the total time for a charge session represents more than that... how much more depends on how close the supercharger is to the roadway. I've used some that were on rest stops immediately adjacent to the road, and you could get to them and park in a couple of minutes. Others are 5-10 minutes off the highway.

So a supercharging session looks like:

- Reduce speed, exit roadway: 1 min
- Navigate to supercharger: 1-10 mins
- Park, exit car, insert cable, wait for charger to negotiate with car and ramp up to power: 2 mins
- Charge: 5-10 mins
- Disconnect, get back in car, exit parking space: 1 min
- Navigate back to roadway: 1-10 mins
- Enter roadway, get back up to speed, resume trip: 1 min

So on average I'm going to guess that a "5-10" supercharging session is on average going to be 20+ minutes. If things stack up the wrong way, you are looking at more than half an hour.

I don't think most people are going to want to stop every hour and fifteen minutes for a 20 minute charge session. Even if you assume all autobahn chargers are immediately off the roadway, at those speeds, you are still talking about stopping for 10 minutes or more every 75 minutes. I can tell you when I've driven the autobahn, I sure didn't want to have to do that.

 

[R.S]

Are you asking about power demand ? Napkin arithmetic:

1 meter per second rise in a 2500 kg car is 25 kW extra over level driving
If 60 mph on level ground is ~ 7.5 kW for Aero, 120 mph is 30 kW

Aerodynamic drag is a force
Fd = 0.5*p*Cd*A*v^2
So at twice the speed, the Force is 4 times as high.

Power is Force times velocity
Pd = Fd * v = 0.5*p*Cd*A*v^3
So at twice the speed, the Power required to maintain speed is 8 times as high.

Energy consumption in cars is given as energy over distance. Since E = P * t, energy consumption is:
E/d = (P * t)/d = P/v = F
(interestingly energy consumption is the same as Force, the "right" unit would actually be Newton)

So if we say the power used to overcome drag at 60 mph is 7.5 kW, the Energy consumption just because of aerodynamic drag is 125 Wh/mile, or 279 N. At 120 mph the Power needed to overcome aerodynamic drag is 60 kW, Energy consumption is 500 Wh/mile, or 1116 N.

 

[R.S]

People keep repeating this, but I don't think this is true.

200KM (125 mi) at "rather high speeds" might only represent a bit more than an hour's worth of driving.... .

And even if the actual recharging event takes the time you mention, the total time for a charge session represents more than that... how much more depends on how close the supercharger is to the roadway. I've used some that were on rest stops immediately adjacent to the road, and you could get to them and park in a couple of minutes. Others are 5-10 minutes off the highway.

So a supercharging session looks like:

- Reduce speed, exit roadway: 1 min
- Navigate to supercharger: 1-10 mins
- Park, exit car, insert cable, wait for charger to negotiate with car and ramp up to power: 2 mins
- Charge: 5-10 mins
- Disconnect, get back in car, exit parking space: 1 min
- Navigate back to roadway: 1-10 mins
- Enter roadway, get back up to speed, resume trip: 1 min

So on average I'm going to guess that a "5-10" supercharging session is on average going to be 20+ minutes. If things stack up the wrong way, you are looking at more than half an hour.

I don't think most people are going to want to stop every hour and fifteen minutes for a 20 minute charge session. Even if you assume all autobahn chargers are immediately off the roadway, at those speeds, you are still talking about stopping for 10 minutes or more every 75 minutes. I can tell you when I've driven the autobahn, I sure didn't want to have to do that.

You grossly overestimate the distances where you can really drive 200 km/h. From Munich to Hamburg, which is a distance I wouldn't take the car for, 750 km, if you could really drive 200 km/h continuously, it would only take 3h and 45 minutes, just the Autobahn portion. Google maps says it takes 7h right now, based on traffic and speed limits.

So if I had to take a 20 minute break three times, I'd still be much, much quicker than Google maps says.

And no petrol car could do that trip anyways, maybe a diesel with a large tank, but range drops, too. My wife's diesel is really efficient, but I'd have to refuel at least once on that trip. And since the car is halve empty on average, I'd have to refuel twice. So in the end it's one single stop more with the EV. Not sure if that's really a big problem.