Can a Tesla really produce 500hp? Electronic questions about powertrain

[ssulb]

I am a student pursuing a career in electrical and computer engineering, so I am not an absolute expert with electronics. I am also a car enthusiast. Many have probably heard of Tesla producing stunning electric cars that can even smash many high end supercars. But I have always wondered how they worked. I would love to gain some more in-depth knowledge from fellow electronics enthusiast. I hope this topic will not only interest me, but many other members of the EEVBlog forum.

Some questions I have are:
Is the power made by a Tesla constant throughout its acceleration? I looked at some dynamometer runs and noticed lots of power at low speed, and then the power dies off. Would this be a result of the limitations of the battery or any other components. Or is it related to the gearing of the car (there is not transmission in this car because the motors directly drive the wheels).

How does a Tesla put out that much power from the battery? Is it using capacitors or other amplifying circuitry, or is it just directly from the battery?

Overall, I am really interested in the electronics side of things that happens with Tesla's powertrain.

I look forward to what fellow members have to say and I appreciate the information. Please feel free to add additional information I may have missed.  Thank you in advance for all the feedback and comments.

[james_s]

Electric motors produce peak torque at zero RPM and then it falls off from there. It's a complex topic but the simplest way I can think to put it is that the faster the motor turns, the higher the drive frequency must be. The windings in the motor act as inductors and the higher the frequency, the higher the effective impedance so it requires progressively higher voltage to push enough current through the coils. Torque is proportional to the current so for a given voltage, torque will fall off as RPM increases.

As far as I know, the power is direct from the battery. Tesla uses a very large battery with a series/parallel arrangement with a terminal voltage of several hundred Volts. The higher the voltage, the lower the current for the same amount of power.

[johansen]

if they are still using 6000, 18650 lithium ion batteries, then you need about 75 watts from each cell. at 3 volts that's 25 amps.

that is not an impossible current and power density, but it seems fairly high. their 500HP capable cars probably have more than 6000 18650 cells if they are still using that package.

[Rerouter]

As its a 3 phase AC motor with a lot of computing power behind it, they can cheat some more torque out of it than a normal DC or mains AC motor, as they can detect the rotor position and hold the input phase at exactly 90 degrees to the shaft to give the maximum turning moment in the magnetic field for maxiumum torque at a given RPM,

It will decrease as RPM increases, but it will have a curve that would describe the curve, i would imagine it would have a quite wide band where the torque is high. then at over legal speeds would reach the knee and decrease more dramatically.

[ssulb]

Electric motors produce peak torque at zero RPM and then it falls off from there. It's a complex topic but the simplest way I can think to put it is that the faster the motor turns, the higher the drive frequency must be. The windings in the motor act as inductors and the higher the frequency, the higher the effective impedance so it requires progressively higher voltage to push enough current through the coils. Torque is proportional to the current so for a given voltage, torque will fall off as RPM increases.

As far as I know, the power is direct from the battery. Tesla uses a very large battery with a series/parallel arrangement with a terminal voltage of several hundred Volts. The higher the voltage, the lower the current for the same amount of power.

Your explanation about peak torque at zero RPM makes a lot of sense. I understand the properties of an electric motors affects this. Thank you.

if they are still using 6000, 18650 lithium ion batteries, then you need about 75 watts from each cell. at 3 volts that's 25 amps.

that is not an impossible current and power density, but it seems fairly high. their 500HP capable cars probably have more than 6000 18650 cells if they are still using that package.

"A 90 kWh battery built with Panasonic 3.4 Ah 18650 cells will then require about 8650 cells." - Teslamotors.com I know Tesla recently started working with a newer version battery, the 2170 battery cell. It’s thicker and taller than the previous cell that Tesla developed with Panasonic, which was in an 18650 cell format. The CEO of Tesla said, "highest energy density cell in the world and also the cheapest”.

As its a 3 phase AC motor with a lot of computing power behind it, they can cheat some more torque out of it than a normal DC or mains AC motor, as they can detect the rotor position and hold the input phase at exactly 90 degrees to the shaft to give the maximum turning moment in the magnetic field for maxiumum torque at a given RPM,

It will decrease as RPM increases, but it will have a curve that would describe the curve, i would imagine it would have a quite wide band where the torque is high. then at over legal speeds would reach the knee and decrease more dramatically.

I found a Dyno chart for a Tesla P85D here: http://www.dragtimes.com/blog/wp-content/uploads/2015/05/tesla-p85d-dyno.jpg

The computing behind Tesla's motors make sense and I think it defiantly plays an important role.

[rstofer]

My Chevy Spark EV is no Tesla but it moves right along.  It's only 105 HP  but over 400 ft-lbs of torque.  That torque figure puts it pretty well up with the muscle cars.  Really nice performance from a dead stop and mid range passing is a lot of fun.  The thing just hauls...

[Gregg]

It isn’t too difficult to get tremendous power out of a relatively small electric motor these days.  A few years ago I had a project to install new water chillers in a data center.  These had close to 400 HP and were about the size of a normal 7 ½ HP 3 phase motor or for comparison about the size of an automatic transmission in a car.  They were 3 phase variable frequency pulse width modulated high frequency to commutate the stators; permanent magnet rotors that were magnetically levitated (no bearings to wear out) and turned something like 20K RPM. 
Similar to this: http://www.synchrony.com/products/high-speed-motors-generators/index.php
To make a 3 phase motor turn slower for a given input frequency, more poles are added and there is a lot of efficiency to be gained by increasing the frequency.  That is why older military aircraft used 400 HZ motors and transformers to save weight. Tesla does seem to have some good battery technology and can supply tremendous current for short duration at the cost of reducing the charge distance significantly vs a long slow battery drain.
Improvements in both motors and batteries are an exciting field for a future career, I’d say go for it.

[rstofer]

At my last house, we had an 8 kW solar array so providing power for my Spark EV cost $0.15/kWh.  Without the heater or AC, it's pretty east to get 5 miles/kWh so my cost was about $0.03/mile.

[Phoenix]

Electric motors produce peak torque at zero RPM and then it falls off from there. It's a complex topic but the simplest way I can think to put it is that the faster the motor turns, the higher the drive frequency must be. The windings in the motor act as inductors and the higher the frequency, the higher the effective impedance so it requires progressively higher voltage to push enough current through the coils. Torque is proportional to the current so for a given voltage, torque will fall off as RPM increases.

As far as I know, the power is direct from the battery. Tesla uses a very large battery with a series/parallel arrangement with a terminal voltage of several hundred Volts. The higher the voltage, the lower the current for the same amount of power.

Your explanation about peak torque at zero RPM makes a lot of sense. I understand the properties of an electric motors affects this. Thank you.

Thats only true for a DC motor with a constant terminal voltage. An electric car will run a DC/BLDC motor drive with current regulated control scheme (or something more sophisticated like FOC/DTC for an induction motor), effectively giving a flat torque-speed curve up to its rating. That means full torque from 0rpm all the way up!

For a little more detail this is an exemplary torque-speed curve for any well driven motor:

Note
- The constant torque region below rating (where you'd generally expect a car to operate under acceleration)
- The constant power region above rating
- The last part is high speed operation and field weakening.

Yes, electric motors are just that good...

[max_torque]

The biggest limitations are thermal, not electrical.

Practical electric traction systems can't actual do full torque at zero rpm (because at zero rpm, you're only using two legs of the inverter, not all three, so thermal limits are significantly reduced). However, once turning, at anything more than about 100rpm, full torque is available, if the motor and inverter are cool enough.

At low speed and cold (speed = power, because of the fixed drive ratio as no multispeed gearbox is fitted), the inverter output current limit is the, er limit, and motor torque will be maximum (Torque is proportional to phase current). As speed increases, and the voltage ratio between the DClink (battery supply voltage) and Phase voltage falls, battery current increases, and at some point, the battery power limit is hit.  In conjunction with this, the motor will reach a speed where it's back emf equals the supply voltage, and in order to keep turning faster, a technique called field weakening is used (this uses a deliberately miss aligned (ie  offset from the quadrature axis) phase voltage application to squash some of the magnetic field in the air gap, which reduces KE, and hence allows the motor to spin faster for a given DClink voltage.  However, as you are now using some of your phase current to reduce KE, you loose some torque capability, so the motors power is also limited.  In this field weakening region, (typically 2x basic DClink/KE region) motor power falls slowly with speed increase.  (demag limitations must be observed).  Ultimately, the motors phase inductance finally limits the power output (assuming battery power >  motor power) as the motor is turning too fast for the applied voltage to push enough current through the windings in the time available.

For an EV without a multispeed transmission (most of them, for cost reasons) there is a trade off in the gear ratio chosen between the traction motor and the wheels, and hence the lowest road speed at which the system can make peak power.  In something like a Tesla, which is expensive, they use a very over-specified inverter to be able to push massive current through the motor (when it's cold) at low speed, to make up for the lack of gears.  Cheaper EVs don't have that luxury, and performance from zero speed is typically below that of an ICE powertrain (something like a Leaf, or I3 really only accelerates well above about 20mph, rather than from 0mph)

Typically, peak power falls at around 50 to 60mph, and above that, power is constant, as limited by the battery and field weakening capability of the Emachine.

Take a look at the Tesla inverter:



that is NOT a cheap device!!


But as i said, it's the battery, inverter and motor thermal performance that sets the limits really.  This is especially true of the peak power condition, where rotor heating becomes accute, due to the addition of iron losses (eddy current driven) to the copper losses (P=I^2.R) in the stator.  On the Emachine i developed for the Mclaren P1 (~24kg / 200kW in FormulaE spec) we used a internal water cooled rotor, and had to get a stream of cooling water into, and back out of, the spinning rotor at up to 20krpm!  I have quite a few grey hairs to show for it too......   

[viperidae]

The batteries in a Tesla are pushed very hard. One of the reasons it doesn't have a good lap time on the nurburgring is because the battery over heats before it can finish the 21km lap.

Car goes into low power mode after 3 minutes into a lap:
https://www.google.co.nz/amp/jalopnik.com/heres-what-a-tesla-model-s-can-do-around-the-nurburgrin-1600644908

[MasterBuilder]

@max_torque, nice photo and good insights
Some information from my world...

Container cranes typically use a pair of 500kW (~700hp for ~1400hp total) motors for the hoist. Theses are rated for constant operation and have a separate fan motor of about 6kW for air cooling alone. The mains supply to these cranes would be between 1 and 2mW.

The VFD (variable frequency drive)for these hoist motors would be about the size of a PC tower. The motor would be about the size of a typical 4 cylinder engine. My point is it is certainly possible to make this kind of power and shrink it into a car engine bay.

With this kind of power cooling is a challenge whether it be forced air or liquid cooled. In the case of a motor and VFD that has to fit into a car engine bay I imagine it would be a major challenge.

[eugenenine]

So its a triangle which is the opposite of a Y configuration.  The flux capacitor was a Y config so this must be the anti-flux capacitor.

[james_s]

@max_torque, nice photo and good insights
Some information from my world...

Container cranes typically use a pair of 500kW (~700hp for ~1400hp total) motors for the hoist. Theses are rated for constant operation and have a separate fan motor of about 6kW for air cooling alone. The mains supply to these cranes would be between 1 and 2mW.

The VFD (variable frequency drive)for these hoist motors would be about the size of a PC tower. The motor would be about the size of a typical 4 cylinder engine. My point is it is certainly possible to make this kind of power and shrink it into a car engine bay.

With this kind of power cooling is a challenge whether it be forced air or liquid cooled. In the case of a motor and VFD that has to fit into a car engine bay I imagine it would be a major challenge.

Crazy how much electronics have shrunk. Back in the 80s my dad did (mechanical) design of equipment in pulp & paper mills, I remember seeing some pictures of VFDs in roughly that power range that were the size of large refrigerators.

[eKretz]

On the Emachine i developed for the Mclaren P1 (~24kg / 200kW in FormulaE spec) we used a internal water cooled rotor, and had to get a stream of cooling water into, and back out of, the spinning rotor at up to 20krpm!  I have quite a few grey hairs to show for it too......   

Would love to hear more about how you accomplished that! The sealing aspect alone must have been a bear.

[FlyingHacker]

Don't forget that the Ludicrous mode uses a bank of super capacitors. It needs time to recharge. I believe it is a few minutes. EDIT: I was wrong here. Move along.

So it is not going to perform well racing form stoplight to stoplight. It also craps out above about 60MPH, where other cars with gasoline engines pass it. Keep in mind this is less than 3sec into a "race."

Definitely more improvement to come in the future.

[tooki]

I am a student pursuing a career in electrical and computer engineering, so I am not an absolute expert with electronics. I am also a car enthusiast. Many have probably heard of Tesla producing stunning electric cars that can even smash many high end supercars. But I have always wondered how they worked. I would love to gain some more in-depth knowledge from fellow electronics enthusiast. I hope this topic will not only interest me, but many other members of the EEVBlog forum.

Some questions I have are:
Is the power made by a Tesla constant throughout its acceleration? I looked at some dynamometer runs and noticed lots of power at low speed, and then the power dies off. Would this be a result of the limitations of the battery or any other components. Or is it related to the gearing of the car (there is not transmission in this car because the motors directly drive the wheels).

How does a Tesla put out that much power from the battery? Is it using capacitors or other amplifying circuitry, or is it just directly from the battery?

Overall, I am really interested in the electronics side of things that happens with Tesla's powertrain.

I look forward to what fellow members have to say and I appreciate the information. Please feel free to add additional information I may have missed.  Thank you in advance for all the feedback and comments.

Another thing you might find interesting, then: Train locomotives. For going on a hundred years, they've all been electric, because electric motors are so much better at providing low-end torque than internal combustion engines. "But wait!" you say, "locomotives have exhaust pipes belching smoke!" Indeed, they do! (Unless it's a pure electric system with third-rail or overhead catenary power.) A "diesel" locomotive is actually a diesel power plant generating electricity to run electric motors — diesel electric. It's also better for the environment, insofar as the diesel generators can be optimized in ways they couldn't if they drove the drive wheels directly.

[Psi]

Tesla do a lot with temperature monitoring.
They can push the inverter a huge amount over its continuous power rating if it's for a short burst and the temp stays within the safe range. This is how they get things like ludicrous mode.

So when looking at dyno curves it would be normal to see crazy high torque initially which then falls off as the inverter scales back the output to something more sustainable.


On the battery front, getting huge power out of a battery bank of size X is a simple matter of combining series/parallel strings correctly to maximise current delivery. Two cells in parallel can usually supply more current than a single cell of the same weight. You just have to deal with the space/weight overhead from having multiple cells.
You can also push them hard for a short time as long as you watch the temp of each cell

[ovnr]

Don't forget that the Ludicrous mode uses a bank of super capacitors. It needs time to recharge. I believe it is a few minutes.

Based on what? I've never heard that the Tesla runs a supercap bank.

On the P90D in Ludicrous mode: It's rated for 568 kW (762 hp) peak, which is "just" 6.3 C. Assuming 3.4A 18650s, you're looking at 21-22 amps/cell, which is not unreasonable for a short-term load. There are some cells around that are spec'd at 35A (albeit with lower energy density), which would happily cough up 1250 hp in the same pack size.

[sca]

So its a triangle which is the opposite of a Y configuration.  The flux capacitor was a Y config so this must be the anti-flux capacitor.

<trainspotter>No, it's definitely a Deltic.</trainspotter>

Sca

[FlyingHacker]

Don't forget that the Ludicrous mode uses a bank of super capacitors. It needs time to recharge. I believe it is a few minutes.

Based on what? I've never heard that the Tesla runs a supercap bank.

On the P90D in Ludicrous mode: It's rated for 568 kW (762 hp) peak, which is "just" 6.3 C. Assuming 3.4A 18650s, you're looking at 21-22 amps/cell, which is not unreasonable for a short-term load. There are some cells around that are spec'd at 35A (albeit with lower energy density), which would happily cough up 1250 hp in the same pack size.

Hmmm... It seems my source (a Tesla owner who does NOT have the Ludicrous speed option) was wrong. I cannot find any info about it using Supercaps...

I did find this:

"The boost in acceleration doesn’t actually come from the extra battery power, it’s the upside of what Musk calls “fairly advanced and exotic electronics.” To date, the car’s acceleration has been capped by how much current can be safely drawn from the battery, about 1,300 amps. Go much higher than that, and the fuse can melt. Tesla’s developed a new fuse that uses electronics and its own tiny lithium ion battery to detect when it will melt, and cut power if necessary. That lets you build in less of a margin of error, and push the amps higher."