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Help me understand how regen works

Posted: Thu Jan 24, 2019 7:45 pm
by theothertom
According to my limited understanding, when you lift your foot off of the accelerator, the electric motor goes into generator mode (I assume there is some software that controls this?) and starts generating electricity, which goes back to the battery. This works because the rotor (magnet) is being turned by the wheels, and since it's inside a coil (stator) electricity is generated. I assume (?) this process creates some sort of "negative" torque on the rotor to slow the wheels.
The "amount" of regen is least when the selector is in "D", more when in "L" and even more if the paddle behind the steering wheel is pulled. How is the amount of regen (and "negative" torque) controlled so that it's different for each mode. Seems like the amount of electricity generated would be the same regardless since the rotor is spinning inside the stator.
I have just enough knowledge to be ignorant. Please educate me. thanks

Re: Help me understand how regen works

Posted: Thu Jan 24, 2019 8:52 pm
by SmokingRubber
theothertom wrote:According to my limited understanding, when you lift your foot off of the accelerator, the electric motor goes into generator mode (I assume there is some software that controls this?) and starts generating electricity, which goes back to the battery. This works because the rotor (magnet) is being turned by the wheels, and since it's inside a coil (stator) electricity is generated. I assume (?) this process creates some sort of "negative" torque on the rotor to slow the wheels.
The "amount" of regen is least when the selector is in "D", more when in "L" and even more if the paddle behind the steering wheel is pulled. How is the amount of regen (and "negative" torque) controlled so that it's different for each mode. Seems like the amount of electricity generated would be the same regardless since the rotor is spinning inside the stator.
I have just enough knowledge to be ignorant. Please educate me. thanks


Good question. I suppose you'd have to be a Chevy Engineer to know exactly how different levels of resistance are created. It's definitely software based, because it knows when the battery is full and unable to accept any regen. That probably doesn't answer your question, but since I don't work for Chevy, that's all I got.

Re: Help me understand how regen works

Posted: Thu Jan 24, 2019 9:29 pm
by BarfOMatic
If I remember my high school physics from a few years ago, the generation of electricity when "running a motor backwards" is the opposite of "running the motor forwards". By that, I mean that to accelerate (increase torque) using the motor one increases the current flowing in the coils. When using as a braking force (using motor as a generator), to increase braking, one also increases the current flowing in the coils (and it brakes more).

The 'D' and 'L' is in software (?) and will change how much current runs through the coils to control how much breaking happens.

Re: Help me understand how regen works

Posted: Fri Jan 25, 2019 1:52 am
by SeanNelson
theothertom wrote:How is the amount of regen (and "negative" torque) controlled so that it's different for each mode.

I'm certainly not thoroughly knowledgeable on this, but it's essentially the same principle that feeds varying amounts of power FROM the battery depending on how far you depress the accelerator pedal. Just because the battery is able to produce 200kW of power doesn't mean that it's "all or nothing". It's a question of how much power the control electronics choose to pass through. The same idea applies to power generated by the motor in regen mode.

Re: Help me understand how regen works

Posted: Sun Mar 03, 2019 1:18 am
by tomofNV
theothertom wrote:According to my limited understanding, when you lift your foot off of the accelerator, the electric motor goes into generator mode (I assume there is some software that controls this?) and starts generating electricity, which goes back to the battery. This works because the rotor (magnet) is being turned by the wheels, and since it's inside a coil (stator) electricity is generated. I assume (?) this process creates some sort of "negative" torque on the rotor to slow the wheels.
The "amount" of regen is least when the selector is in "D", more when in "L" and even more if the paddle behind the steering wheel is pulled. How is the amount of regen (and "negative" torque) controlled so that it's different for each mode. Seems like the amount of electricity generated would be the same regardless since the rotor is spinning inside the stator.
I have just enough knowledge to be ignorant. Please educate me. thanks


Your basic description of regen is correct.

The operating mechanism differs in details between AC induction motors, variable reluctance motors, and permanent magnet motors but all of these motors have a stator with poles, each pole having its own windings. Voltage is applied across these pole windings sequentially, sinusoidally in time, to create a rotating magnetic flux. Magnetic flux then varies sinusoidally in time across the air gap between a given stator pole and the rotor.

There are two main parameters of importance: The magnetic flux density through the air gap between the rotor and stator, and the frequency of the rotating magnetic field of the stator. The flux density is proportional to the voltage applied to the stator poles and inversely proportional to the frequency.

Glossing over differences in details of operation, when the stator frequency is greater than the rotor frequency of rotation the stator field results in a torque on the rotor. When the rotor has the greater rotational frequency it induces an emf (electromotive force, which is a voltage) in the stator, driving current to the batteries.

To accelerate the vehicle when the accelerator pedal is pressed, the frequency and voltage are increased to increase the rotational frequency of the stator magnetic field while maintaining the required air gap magnetic flux.

When you back off on the pedal the stator frequency is reduced. If it is reduced such that it is lower than the rotor frequency the time varying magnetic flux of the rotor then induces an emf in the stator windings (described by Faraday's law) and current flow into the batteries - regenerative braking. The more you "relax" (back off) the pedal, the greater the emf and work done in slowing the vehicle.

The frequency and voltage of the motor controller are varied differently in D and L mode when the accelerator pedal is relaxed, resulting in different amounts of regenerative braking. Basically, the "slip", the difference in rotor and stator frequencies, is not permitted to become as great in D mode when regeneratively braking so a smaller emf is induced in the stator and less work is done to decelerate the vehicle.

Re: Help me understand how regen works

Posted: Mon Mar 04, 2019 1:05 am
by theothertom
Thank you, tomofNV !! I've been searching for an explanation that makes sense, and you've provided it. Thanks again !