Do i need to "warm up" batteries before going WOT?

[ord3r]

It isn't the motor. It's the inverter which is located in the motor assembly. The motor does not need to be warmed but as it is doubtless on the same coolant loop as the inverter it will get warmed too.

Based on CAN bus data, it's actually the coils in the stator that are used for heat generation. Looks like 3.5KW per motor, so AWD variants have about twice the heating power as RWD.

You can see that that motor power goes up, and the stators reach temps of nearly 90c!

Looks like it also affects the motors driving dynamics while this is running.

 

[ajdelange]

Based on CAN bus data, it's actually the coils in the stator that are used for heat generation. Looks like 3.5KW per motor, so AWD variants have about twice the heating power as RWD.

Let me again be perfectly clear that I am not privy to any Tesla design documentation so that the explanation I am offering is only based on common sense and what I know of modern electric vehicle design practices. This forum is a showcase for Dunning-Kruger and so I do want to make it clear that I am (I hope) smart enough to know how dumb I am when it comes to automotive engineering. Caveats out of the way lets get to the meat:

You can see that that motor power goes up,

I assume that the linked video is the evidence being cited for this conclusion. In this video I do NOT see the motor power going up. I see the DRIVE TRAIN power going up. If the car is rolling along it is delivering a fixed amount of power to the road. If without acceleration or going up a hill drive train power consumption is increased then the DRIVE TRAIN has some how become less efficient. That means either the motor itself has become less efficient, the gear box has become less efficient or the inverter has become less efficient. As we want to warm the battery we want something on the same coolant loop as the battery to be the source of the waste heat and that means either the motor or the inverter. Now I don't know how to electrically reduce the efficiency of a motor without mechanical redesign or by screwing up it's power factor which I suppose one could do with DQ control but I do know how to trivially ruin the efficiency of the inverter and that's by slowing the gating waveforms and/or by allowing some common mode current to flow as I described in a previous post. It may be possible to do something to motor power factor but using the transistors as resistors just seems so intuitively obvious and simple a thing to do that I am hypothesizing that this is what they do. Engineers hold KISS in high esteem. While I could be wrong in my surmise there is no evidence to contradict what I have proposed in the linked video,

and the stators reach temps of nearly 90c!

They are on the same loop as the transistors.

Looks like it also affects the motors driving dynamics while this is running.

Of course it does. You are modifying the transistor switching pattern.

 

[ZOMGVTEK]

The reason for the considerable stator temp rise is due to the cooling design. The motor is effectively being locked, and this results in 0% efficiency. This of course means the 3.5kw is essentially all ending up as heat, which is the intent. The trouble is for the motor 3.5kw is a fair bit of heat to sustain. Normally it probably wouldn't see that much heating unless it was operating at something like 60kW steady state. The motor cooling is a bit slow at getting the heat out, and so the temps climb until the heat rejection is matching the input. This does not mean the mosfets are subjected to 90C. They are on the glycol loop, the motor has its own oil system with a heat exchange to the glycol loop. The exit temp of the motor cooling loop is presumably quite similar to the pack inlet temp. I'm not sure where in the path the inverters are, but presumably they would be before the motor and closer to the pack exit temp. It's likely the mosfet die temp isnt all that high while the motor is being used as a heater.

 

[ajdelange]

The motor is effectively being locked, and this results in 0% efficiency.

How does one "effectively lock" a motor while the car is running down the freeway as depicted in the video?

This of course means the 3.5kw is essentially all ending up as heat, which is the intent. The trouble is for the motor 3.5kw is a fair bit of heat to sustain. Normally it probably wouldn't see that much heating unless it was operating at something like 60kW steady state.

If cruise power is 30 - 50 kW and the motor/inverter 90% efficient 3 - 5 kW waste heat would be produced. Normally this gets dumped through the radiator. When it is desired to warm the battery it is desired to retain that heat and so the radiator is taken out of the loop.

The motor cooling is a bit slow at getting the heat out, and so the temps climb until the heat rejection is matching the input.

In normal running the waste heat is disposed of by controlling the heat dunpingh portions of the coolant loop (radiator, pumps etc.) If it is desired to increase the temperature of the loop to warm the battery coolant bypasses the radiator or a radiator shutter is closed. The 3's cooling system is quite sophisticated. If extra waste heat is wanted it is easily obtained and controlled by gating the transistors that are normally off partially on.

This is an Occam's Razor explanation but if ever there was a case for Occam's Razor this seems to be it.

 

[ZOMGVTEK]

You’re entirely misinterpreting what I was saying. Are you suggesting the heating is done by the inverters power devices?

 

[ajdelange]

Have you read any of the thread? Specifically Nos. 16 and 22.

 

[ZOMGVTEK]

The Model 3 uses switched reluctance and AC induction motors, both of which are electronically commutated. 3.5kw is an insane amount of heat for the inverter to dump, and it would be the worst available option for heating by a considerable margin. The drastic thermal stresses would dramatically accelerate wear and shorten inverter life likely orders of magnitude if they would tolerate it at all. SiC packages like used in the rear inverter are capable of carrying high currents, but only if they are efficient. They can’t dump considerable heat super effectively. High efficiency means low heat. An electronically commutated motor does not magically spin when power is applied, it requires knowledge of the rotor position and then it coordinates the waveforms applied to hit the desired motion. This is very complex, and not super easy to do efficiently. There’s tons of ways for this to be marginally less efficient, and the primary heating impact is going to be on the motors windings which are quite capable of tolerating heating, at least short term. The thermal cycling doesn’t typically cause as considerable fatigue as it does with a mosfet, as well as the dramatic increase in surface area and thermal mass. It would be the logical choice as a heater by a considerable gap.

 

[ajdelange]

The Model 3 uses switched reluctance and AC induction motors, both of which are electronically commutated.

They aren't commutated at all. They are three phase motors. They have 3 windings of 4 poles each connected in a wye connection.

3.5kw is an insane amount of heat for the inverter to dump

. Each percent inefficiency of the inverter (in normal operation) represents 500 watts at a 50 kW draw (which isn't unusual in my car). So in normal operation I would expect a couple of kW and going to 3500 would hardly be a stretch. I wonder if you have ever done a semiconductor thermal load calculation. If one has extra heat to dispose of he designs his system to have lower thermal impedance to the sink and thus prevent excessive rise. Are you aware that these inverters use silicon carbide transistors which are pretty tough when it comes to temperature.

and it would be the worst available option for heating by a considerable margin. The drastic thermal stresses would dramatically accelerate wear and shorten inverter life likely orders of magnitude if they would tolerate it at all.

I wonder why you say that? Do you have any relevant literature, experience or even reasoning to back that up? Reasoning is OK. At least it forms the basis of a discussion. And as Tesla isn't revealing design details that's all we have.

SiC packages like used in the rear inverter are capable of carrying high currents, but only if they are efficient. They can’t dump considerable heat super effectively.

This implies that you at least know about silicon carbide. Transistors are perfectly well capable of being operated inefficiently if they are in a low thermal impedance path to a sink.Interestingly enough the very first site I come up with typing "silicon carbide transistor" into the search bar offers:

"As an alternative to traditional silicon MOSFETs, silicon carbide MOSFETs offer the advantages of higher blocking voltage, lower on-state resistance, and higher thermal conductivity."

High efficiency means low heat.

Yes.

An electronically commutated motor does not magically spin when power is applied,

The ones in my house do. No magic involved.

it requires knowledge of the rotor position and then it coordinates the waveforms applied to hit the desired motion.

Rotor position is not required for the motor to spin. If you want to do vector field control it is and we do in this application.

This is very complex, and not super easy to do efficiently.

I grant you the intricacies of DQ control give me a headache but once you have the transforms sorted out and have DSP chips that will do them fast enough the basic concept is pretty simple. The inverter constructs sin waves at the proper frequency, amplitude and phase to give the desired torque and field. D and Q are like the field and armature currents in a DC motor and so can be controlled by PI controllers making it possible to do all the neat stuff the Tesla cars can do.

There’s tons of ways for this to be marginally less efficient,

Yes, as the simplest of them is to simply slow the rise time of the transistor gating waveform and/or allow some common mode current which is why I am quite confident that this is what they do. Besides that I have seen it mentioned in other places that this is the technique and as it makes so much sense I have come to accept that though I readily admit that Occams Razor is not proof.

Can you suggest a method for changing the inverter's gating in such a way that the motor itself becomes less efficient? I may be slow but other than locking the rotor and running DC I can't think of a way to do it. Actually something just came to mind: harmonic distortion and phase imbalance. This would lead to negative sequence currents which reduce inefficiency. Is that what you have in mind?

and the primary heating impact is going to be on the motors windings which are quite capable of tolerating heating, at least short term. The thermal cycling doesn’t typically cause as considerable fatigue as it does with a mosfet, as well as the dramatic increase in surface area and thermal mass. It would be the logical choice as a heater by a considerable gap.

Have you considered that in normal operation the inverter transistors are subject to constant thermal shock as inverter power demand can go from 0 to 150 kW (or more typically 100 kW) and then back to 30 kW in a matter of seconds? Have you considered that the transistors, heat sinks and motor steel are all part of the same thermal mass?

 

[ajdelange]

A little further poking around got me to a video where the guy pulled apart a model 3 inverter. It has 24 FETs. If we assume the car is stationary and set the gating on those FETS such that each dissipates 148.5 Watts that would give a total of 3.5 kW "waste" heat. A little cruising through SiC FET data sheets shows that pieces are available with thermal impedances of 0.31 °C/W die to case. Add 0.1 for case to heatsink and we'd have 0.41 °C/W. With 148.5 W dissipated per device that would give us a rise of 148.5*0.41 = 60.9 °C. In one of Bjorn's videos I saw a stator temperature of about 70 °C (IIRC) and as the stators are on the same loop as the inverter we might assume inverter heatsink temp as high as 80 °C which would imply Tj = 141 °C which is well within the spec for this particular device which can operate up to 175 °C. That's one of the advantages of SiC. Operating junction temperatures of up to 200 °C are advertised.

Thus there is even more evidence (hard, this time) that the inverters could be the source of battery heat. Again we must say that this does not mean that this is the approach Tesla has taken though it is rumored that they have.

We should also point out that when the battery is being warmed as when approaching an SC, while the car is in motion the motor(s) are running normally and as such are generating some waste heat which is being transferred to the battery. It just isn't enough and so the inverters are set to generate some more as described.

 

[vickh]

If the battery is too cold there's a little snowflake icon that shows up on the screen. I assume if that's not there you are free to drive the piss out of it and it will act accordingly.

how cold is is too cold? how does snowflake icon change driving?

 

[ord3r]

Well sure technically both inverter and stator will heat up during this heating cycle, but the bulk of the ~3kw of waste heat per drive train is being dumped into the stator. The inverter is controlling the current flow and essentially running the waste heat program, but isn't dumping the heat into itself. 3kw of heat on the inverter would cause significant thermal stresses if not melting the thing outright.

A little further poking around got me to a video where the guy pulled apart a model 3 inverter. It has 24 FETs. If we assume the car is stationary and set the gating on those FETS such that each dissipates 148.5 Watts that would give a total of 3.5 kW "waste" heat. A little cruising through SiC FET data sheets shows that pieces are available with thermal impedances of 0.31 °C/W die to case. Add 0.1 for case to heatsink and we'd have 0.41 °C/W. With 148.5 W dissipated per device that would give us a rise of 148.5*0.41 = 60.9 °C. In one of Bjorn's videos I saw a stator temperature of about 70 °C (IIRC) and as the stators are on the same loop as the inverter we might assume inverter heatsink temp as high as 80 °C which would imply Tj = 141 °C which is well within the spec for this particular device which can operate up to 175 °C. That's one of the advantages of SiC. Operating junction temperatures of up to 200 °C are advertised.

Huh? You sure you weren't looking at ingineerix teardown of the power conversion module? EDIT: nvm i see what vid your talking about, either way the front drive train can also do ~3kw of heat dumping with IGBT, don't see anyway 3kw of heat is landing in this thing:

Rotor position is not required for the motor to spin. If you want to do vector field control it is and we do in this application.

Ok now you are just spreading misinformation
EDIT: NVM misread this as Rotor position is not required for this application, sorry!

Let me again be perfectly clear that I am not privy to any Tesla design documentation so that the explanation I am offering is only based on common sense and what I know of modern electric vehicle design practices.

The explanations containing common sense is debatable ;p , but the missing Tesla documentation can be attributed to a lack of google fu

Hint: 'siteastebin.com model 3'

But anyway don't take technically sounding posts as fact from most people on this forum including myself, here is what tesla has to say about waste heat mode and stator heating: