Tesla regen regulation/charging

[mcan]

Hi everyone,

I understand how regenerative braking generally works, but I'm seeking more detailed information on the specific processes involved.

Are both motors working as generators for Model Y?
How is the battery charging controlled during this process?
Does this conversion and control happen within the motor inverter, or is there a separate circuit acting as a DC-DC converter that handles charging the battery? And what is the input and output voltage range for that circuit to charge the battery.
I'm not looking for theoretical explanations but rather detailed insights into the actual electronic and control mechanisms involved.

[tom91]

Can bus.

Battery limits (voltage + current) from BMS (battery management) to Rear motor.

Rear motor is the "master" and commands the front.

As simple as that on a stock Tesla or really any EV.

If you need more info look at youtube videos on how EV motors operate, synchronous or asynchronous. This really just feels like you have not tried doing any learning or research yourself.

[jrbe]

The easiest way to think about it for me is with a battery drill.

Try to turn it by hand and it's difficult. Depending on the drill you could charge the drill battery if you turned it fast enough. Puting energy into the system is like decelerating / regen. This moves energy into the battery if fast enough.
This analogy is ignoring inductive energy an ev can recover from the inverter pulsing the motor so that it's not running at 100% regen. Inverter is outputting very short chopped sine waves. As those each collapse they let out a burst of energy that's higher than the input voltage. This goes to the battery.

Give the drill a little trigger and turn it at the same speed and the drill motor feels free, this is like coasting in an ev. This isn't using much power as the work (also viewed as battery power) is overcoming drag and light electrical losses. The motor controller is pulsing the motor lightly to be right in between regen and acceleration with partiall length chopped sine waves.

Acceleration is requesting more power or speed. The motor controller is outputting a larger pulse (more of a full sine wave for each phase) which pulls more energy out of the battery.
This is like trying to slow down the drill with your hand. The inverter is outputting fuller length chopped sine waves.

There are diodes in the power stage of the inverter that also work to rectify the AC to DC for the battery.

An ev will get it's regen info from a few sources to negotiate the regen level.
If the battery is completely full it will basically say 0 regen energy please.
With a partially full battery it will change the allowed regen energy as the state of charge in the battery drops from full or near full.
As regen is allowed by the battery it will look at what the driver has selected for regen level and calculate that from throttle position and maybe some from brake pedal position as well as stability control, abs, etc.

Both the front and rear motors will regen if it's AWD. This helps balance the vehicle. The front will likely regen 60-75% more than the rear depending on weight distribution and suspension design.
No extra hardware required, just inputs, info, communication, and good code.

*Edit* torque vectoring can regen one or more corner of the vehicle if it has individual motors. Torque vectoring will request + or - torque from each wheel. You can have one wheel accelerating and one wheel decelerating / regening.

[mcan]

Thank you , for the regen voltage it can be really high or lower than battery's current voltage level so how does motor's inverter works as DC DC converter at all? Let's say battery is at 380v and regen generated 360v (open or closed circuit is also depends right?) for DC DC converter I can get the idea alternator's field coil can adjust the voltage of the alternator and DC DC will charge the battery.. but for Tesla , which coil used as field coil and also pm motor is another unknown to me. I wonder how the inverter switch between motor controlling and battery charging modes.

[jrbe]

Thank you , for the regen voltage it can be really high or lower than battery's current voltage level so how does motor's inverter works as DC DC converter at all? Let's say battery is at 380v and regen generated 360v (open or closed circuit is also depends right?) for DC DC converter I can get the idea alternator's field coil can adjust the voltage of the alternator and DC DC will charge the battery.. but for Tesla , which coil used as field coil and also pm motor is another unknown to me. I wonder how the inverter switch between motor controlling and battery charging modes.

There's no field coil in a Tesla. There are some new style ac motors that use one but try to get the idea of a field coil out of your head, it will only confuse you while trying to understand a 3 phase permanent magnet motor.

These questions are explained in my drill analogy. I used a drill because it's small and fits in your hand so it's a smaller, easier to understand, and wrap your head around example. I broke it down into regen, coast, and acceleration. Read this, then the drill example and all specific questions where you are stuck.

The inverter is controlling the amount of power from the motor / generator with how full the sine wave feeding the motor is. Say at 100kph / 60mph there is energy in the moving vehicle, so you might have -50kw of regen and 50kw of acceleration. The inverter is controlling this range by how full the sine wave is to drive the power stage.

The things to know is that the motor could make say 700v or more in regen without a battery connected. It will only push current into the battery above the battery voltage. This is either by motor RPM or inductive collapse energy (somewhat similar to an ignition coil but no where near the voltage) after the pulse is switched off. Also the battery will limit the motor voltage by acting kind of like a huge energy mass while it's connected.

[mcan]

There's no field coil in a Tesla. ....

Let me see if I understand your analogy. Let's connect your pm drill to a constant RPM source, so the drill is mechanically driven by a larger motor at a constant RPM. The drill's battery voltage is at 480V, but the drill is generating 580V even while fully connected to the battery. How does the Tesla motor regulate this? From what I understand based on your explanation, it regulates the current, so as long as the current remains within a safe range, everything is fine.

But what happens if the drill generates only 400V? How does Tesla utilize that energy to charge the battery? Are you suggesting that the motor coils are stimulated in a way that they act as a step-up converter to charge the battery? It's easier to understand when the regen voltage is higher than the battery voltage and the battery can handle it, but I'm confused about how the system works when the motor generates a lower voltage than the battery's nominal voltage. You're saying the inverter somehow functions like a step-up converter, which is fascinating.

Another point I'm unclear on is how the maximum generated voltage is limited to keep the components safe. For example, if the battery pack's maximum charging speed is 2C and the motor generates energy at 3C, there must be some safety mechanisms in place to prevent damage, you mean they regulated this with pwm ? But how they dissipate the excessive energy on motors coils? Because of it is asynchronous motor without pm it is easy to limit voltage by using field coil but if it is pm and generates much more energy than battery can accept than this confuses me.

To simplify my question and understanding of Tesla's PM regen system:

If the motor generates a lower voltage than the battery's nominal voltage, the inverter somehow acts like a step-up converter to increase the motor's voltage above the battery's nominal voltage to enable charging.

If the motor's generated voltage is higher than the battery's nominal voltage and the charge speed is within the battery pack's maximum C-rate, the motor is fully connected to the battery with 100% PWM.

If the motor's voltage is higher and the charging rate exceeds the battery pack's maximum C-rate, the inverter reduces the PWM to ensure the charging current remains within safe limits.

Thank you for answers anyway. I appreciate a lot.

[tom91]

You clearly lack understanding of how a brushless motor works (induction motors work similarly but very different in some ways but wont create voltage on their own).

Here is info on controlling them





Point on regen


So yes the way to control the current that is allowed out of the coils/magnetic field into the battery pack the pwm duty is adjusted. Note the speed thus frequency is locked together.

This is key, You NEED emf (motor produced voltage) higher then the battery to push current back to the battery. Current always flows from low to you, you PWM it to control how much current is allowed through.

[mcan]

This is key, You NEED emf (motor produced voltage) higher then the battery to push current back to the battery. Current always flows from low to you, you PWM it to control how much current is allowed through.

Funny but it seems you clearly lack knowledge of the techniques that allow a BLDC inverter to function as a boost converter to charge the battery even when the EMF is lower than the battery voltage. Here is one technique that utilizes the motor's internal coils to boost the regen voltage:

https://www.slideshare.net/slideshow/re ... s/54859643

Additionally, from your previous replies, it seems you are only considering the freewheeling diodes for regen, which is a primitive and inefficient method. This is why you keep stating, 'You NEED emf (motor-produced voltage) higher than the battery to push current back.' Yes, with just freewheeling diodes, the EMF needs to be higher than Vbat + Vdiodes. However, this is not the only method we are discussing.

another way is i am not sure if this is used in any commercial inverter but another approach can be like this

This method clearly demonstrates that you can still charge the battery even if the EMF is lower than the battery voltage, thanks to the external boost converter.

another approach is bidirectional dc dc converter
https://www.mdpi.com/2032-6653/15/1/12
and there are more....
i am asking which topology tesla using for its inverters....

[tom91]

i am asking which topology tesla using for its inverters....

A "simple" 3 phase mosfet based full bridge, like everyone else. Nothing unique or special there, just the fact they have done this with IGBTs on the Model S in small packages and on the Model 3 now with also small package Mosfets.

Okay where did all this knowledge magically come from all of sudden? Guess ChatGPT "made" you smart.

[tom91]

Point on regen

Did you watch this...

You can use the motor to generate a higher EMF by having is short its coil to build charge to raise the voltage. Then connect it to the battery to let the energy flow there. It would be useful to actually look at my responses in detail.