Investor Engineering Discussions

[bkp_duke]

Unless you are racing I'd think heat dissipation is minimal in a 90%+ system. How much power is required to keep a vehicle moving at say 65 mph? Low speed stop and go will not be creating that much heat either. And how does any of this change the small single digit potential efficiency improvements which could in no way magically allow a poor battery density chemistry to be applied?

You have to dissipate that heat regardless, racing or not. You discharge 100kwh from the pack, you dissipate as heat any of that which was not used for motion. The only difference in racing is that you have to dissipate it faster. Brake rotor heating aside, which is different in these scenarios, the same amount of heat is dissipated, just over a much shorter time during racing.

 

[bkp_duke]

Acceleration is a time limited event and thus energy (heat) limited. Why would that impact range?

All things equal, it would not. However during racing, it will affect range because the cooling system for the battery and for the motors is one system, and if you surpass the rate at which the radiators can dissipate the heat, the pack temperature goes up. Once the pack temp goes up, the range drops.

I've seen this first hand in my M3P on race days (and track mode even tracks it for you so you can plan accordingly). In these situations, heat from higher discharge rate from the pack and higher utilization through the motors is competing for limited dissipation through the cooling system. If you can reduce the motor heat production through increased efficiency you can race longer, or in something like the semi obtain more range because you keep the pack in the optimal thermal window for longer.

 

[JRP3]

You have to dissipate that heat regardless, racing or not. You discharge 100kwh from the pack, you dissipate as heat any of that which was not used for motion. The only difference in racing is that you have to dissipate it faster. Brake rotor heating aside, which is different in these scenarios, the same amount of heat is dissipated, just over a much shorter time during racing.

Efficiency is going to be much lower in racing than normal driving, you won't be spending much time in the peak motor efficiency range when racing.

 

[mongo]

All things equal, it would not. However during racing, it will affect range because the cooling system for the battery and for the motors is one system, and if you surpass the rate at which the radiators can dissipate the heat, the pack temperature goes up. Once the pack temp goes up, the range drops.

I've seen this first hand in my M3P on race days (and track mode even tracks it for you so you can plan accordingly). In these situations, heat from higher discharge rate from the pack and higher utilization through the motors is competing for limited dissipation through the cooling system. If you can reduce the motor heat production through increased efficiency you can race longer, or in something like the semi obtain more range because you keep the pack in the optimal thermal window for longer.

But who is racing semis?

I agree it's better to not waste power as heat, but semi's thermal mass and dissipation to the environment are massive. And that is assuming its pack is over the optimal temp.

In your racing scenario, is the issue that the pack is hot (less kWh), or that the car is expending more energy to cool it (higher Wh/mile or W in general)?
For cells with multiple cycles, 55C capacity is less than 45C, on par with 35C, and higher than 25C.
Cold cells are less efficient, but hot cells are more. https://www.greentechrenewables.com/article/howdoes-temperature-affect-battery-performance

 

[bkp_duke]

But who is racing semis?

I agree it's better to not waste power as heat, but semi's thermal mass and dissipation to the environment are massive. And that is assuming its pack is over the optimal temp.

In your racing scenario, is the issue that the pack is hot (less kWh), or that the car is expending more energy to cool it (higher Wh/mile or W in general)?
For cells with multiple cycles, 55C capacity is less than 45C, on par with 35C, and higher than 25C.
Cold cells are less efficient, but hot cells are more. https://www.greentechrenewables.com/article/howdoes-temperature-affect-battery-performance

Semi's use case is high-regen due to the load it carries, so the same principles apply as with racing (high heat dissipation, which is significantly reduced by more efficient motors).


When I have tracked, it's a mix of both - the ability of the car to dissipate heat (a problem on 100F days at Buttonwillow) and always that the pack is hot (hotter pack, lower power). Granted, this is a M3P. The S Plaid has much more cooling built into it for better heat dissipation.

 

[bkp_duke]

Efficiency is going to be much lower in racing than normal driving, you won't be spending much time in the peak motor efficiency range when racing.

What? The motor efficiency range for electric motors is VERY broad, unlike traditional ICE engines. You are never out of the power band, whether at 20 MPH or 150 MPH.

 

[mongo]

Semi's use case is high-regen due to the load it carries, so the same principles apply as with racing (high heat dissipation, which is significantly reduced by more efficient motors).


When I have tracked, it's a mix of both - the ability of the car to dissipate heat (a problem on 100F days at Buttonwillow) and always that the pack is hot (hotter pack, lower power). Granted, this is a M3P. The S Plaid has much more cooling built into it for better heat dissipation.

Sample numbers. 80k lbs at 55MPH is 3kWh, 80% efficiency = 600Wh of heat (1.8k BTU-h). If semi pack is 10x the size of a 3, that's a 0.7C temp rise.

Yah, I'd expect power limitation due to a hot pack and increased power to the thermal system, but not a direct reduction in available pack energy.

What? The motor efficiency range for electric motors is VERY broad, unlike traditional ICE engines. You are never out of the power band, whether at 20 MPH or 150 MPH.

The power band is wide, but efficiency has a sweet spot. 400V @ 10A = 4kW = 200V @ 20A, but different efficiencies.
Heat generation in the stator is proportional to the square of current which is proportional to torque, inverter loss is proportional to current.
Acceleration requires more torque -> more current -> more loss.

 

[bkp_duke]

Sample numbers. 80k lbs at 55MPH is 3kWh, 80% efficiency = 600Wh of heat (1.8k BTU-h). If semi pack is 10x the size of a 3, that's a 0.7C temp rise.

Yah, I'd expect power limitation due to a hot pack and increased power to the thermal system, but not a direct reduction in available pack energy.



The power band is wide, but efficiency has a sweet spot. 400V @ 10A = 4kW = 200V @ 20A, but different efficiencies.
Heat generation in the stator is proportional to the square of current which is proportional to torque, inverter loss is proportional to current.
Acceleration requires more torque -> more current -> more loss.

Correct, as you point out, efficiency is determined by power level, which is proportional to SoC in most cases. We see this when tracking and line racing, it's why we love having superchargers at/near the track (Buttonwillow).

 

[mongo]

Correct, as you point out, efficiency is determined by power level, which is proportional to SoC in most cases. We see this when tracking and line racing, it's why we love having superchargers at/near the track (Buttonwillow).

My 400V 200V example was at the motor post inverter, but yeah, lower SOC means lower pack voltage means more current for same gross pack power and more loss (lower net power).

 

[JRP3]

What? The motor efficiency range for electric motors is VERY broad, unlike traditional ICE engines. You are never out of the power band, whether at 20 MPH or 150 MPH.

As mongo pointed out efficiency range is different than power range.

 

[bkp_duke]

As mongo pointed out efficiency range is different than power range.

Only a little bit. Unlike a ICE engine, you don't see efficiency drop off by more than a few percent even at high RPM. The curves are pretty flat.

This is an illustrative example, but power and energy consumption track together in parallel, and efficiency over that range is relatively flat. This is also for a motor less efficient that what Tesla builds (i.e. the carbon wrapped Plaid / Semi motors), which should be better on all those across the board:

https://www.researchgate.net/figure/Power-and-efficiency-of-a-10-kW-electric-motor-as-a-function-of-motor-shaft-speed_fig4_341509170

 

[mongo]

Only a little bit. Unlike a ICE engine, you don't see efficiency drop off by more than a few percent even at high RPM. The curves are pretty flat.

This is an illustrative example, but power and energy consumption track together in parallel, and efficiency over that range is relatively flat. This is also for a motor less efficient that what Tesla builds (i.e. the carbon wrapped Plaid / Semi motors), which should be better on all those across the board:

https://www.researchgate.net/figure/Power-and-efficiency-of-a-10-kW-electric-motor-as-a-function-of-motor-shaft-speed_fig4_341509170

View attachment 917446

View attachment 917448

It's not the speed, it's the torque. Though low speed is also inefficient. I think your first plot is efficency vs constant torque.

Also from your link: efficiency vs constant power

Tesla was showing reduction in RPM (BackEMF) based power roll off, not efficiency.

Speed vs torque vs efficiency map:
Brushless Motor Power and Efficiency Analysis

 

[JRP3]

This is an illustrative example, but power and energy consumption track together in parallel, and efficiency over that range is relatively flat.

I've never seen a motor efficiency curve that flat. Maybe because that chart doesn't start at 0.

The Impact of AC Motor Efficiency

In 2011, I wrote a five-part series on alternating current (AC) motors and motor efficiency. This column will go into more detail about the impact of efficiency.

www.pumpsandsystems.com

https://www.researchgate.net/figure/Torque-speed-efficiency-map_fig5_334994872

Ninja'd by mongo.

 

[bkp_duke]

I've never seen a motor efficiency curve that flat. Maybe because that chart doesn't start at 0.

The Impact of AC Motor Efficiency

In 2011, I wrote a five-part series on alternating current (AC) motors and motor efficiency. This column will go into more detail about the impact of efficiency.

www.pumpsandsystems.com

https://www.researchgate.net/figure/Torque-speed-efficiency-map_fig5_334994872

Ninja'd by mongo.

Those are some pretty darned flat efficiency curves. You get to 30% load, you are already at 80+% efficiency, with a gain of only 10% more possible at best per that data. Most driving is above that 30% point, simply due to the weight of the car causing that kind of load on the motor(s).

 

[JRP3]

A single 200kW motor is putting out 40kW at 20%, that's over 50HP. Maybe 20HP is required to drive at 60MPH.

 

[mongo]

A single 200kW motor is putting out 40kW at 20%, that's over 50HP. Maybe 20HP is required to drive at 60MPH.

Semi at <2kWh/mile @ 60 MPH <= 120kW <= 161 hp steady state.

 

[JRP3]

Semi at <2kWh/mile @ 60 MPH <= 120kW <= 161 hp steady state.

To be clear I was talking about a single motor Model 3 type vehicle.

 

[BioSehnsucht]

TL;DR: Maybe everyone is right about efficiency, just depending on the particulars being compared

IIRC AC induction and PM motors have different efficiency curves in general to begin with, and in practice vary by specific motor design as well.

For AC induction motors, low load conditions are worse since it still requires some amount of power to maintain the induced magnetic flux (at higher load conditions, this power usage is dwarfed by that power used to provide useful torque against the load). I want to say that for the PM (non-induction) motors such as on the 3, Y, Semi, etc, the permanent magnets provide the flux and so that particular source of inefficiency is not present. However, PM motors can't be totally turned off in the same fashion as AC motors if I recall correctly, so AC motors are better for motors that may be only powered some of the time if a significant portion of usage will be steady state and not need all motors functioning, so the reduced at-speed losses may outweigh the increased low-load losses.

 

[JRP3]

Older chart from 2018 for reference

Tesla Model 3 = Most Efficient Electric Car On Highways

The Tesla Model 3 efficiency is outstanding, making it a great EV for highway trips

cleantechnica.com

 

[nativewolf]

What a fun conversation