theothertom
Posts: 38
Joined: Wed Sep 19, 2018 4:13 pm

Help me understand how regen works

Thu Jan 24, 2019 7:45 pm

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

SmokingRubber
Posts: 162
Joined: Fri Oct 13, 2017 9:32 pm
Location: Pismo Beach, California

Re: Help me understand how regen works

Thu Jan 24, 2019 8:52 pm

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.
2017 Chevrolet Bolt Premier (DD)
2016 Mazda CX-5 Gran Touring (Wife's DD)
2013 Toyota FJ Cruiser (4x4 beachmobile)
2016 Mazda 3 Touring

BarfOMatic
Posts: 59
Joined: Sat Feb 25, 2017 8:03 am
Location: Santa Clara, CA

Re: Help me understand how regen works

Thu Jan 24, 2019 9:29 pm

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.

SeanNelson
Posts: 1425
Joined: Tue Jul 05, 2016 2:43 am
Location: Vancouver, BC

Re: Help me understand how regen works

Fri Jan 25, 2019 1:52 am

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.

tomofNV
Posts: 11
Joined: Thu Sep 13, 2018 5:31 pm

Re: Help me understand how regen works

Sun Mar 03, 2019 1:18 am

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.

theothertom
Posts: 38
Joined: Wed Sep 19, 2018 4:13 pm

Re: Help me understand how regen works

Mon Mar 04, 2019 1:05 am

Thank you, tomofNV !! I've been searching for an explanation that makes sense, and you've provided it. Thanks again !

Telek
Posts: 12
Joined: Fri Apr 12, 2019 3:11 am

Re: Help me understand how regen works

Fri Apr 12, 2019 3:36 am

tomofNV wrote: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.


Although I agree with the technical aspects, that doesn't actually explain the different levels of regen since the motor is not connected directly to the battery, and also doesn't explain how it would vary that amount. That description is much more accurate for how the car controls the speed of the car, but not the amount of regen.

Somewhat less technically, you have this:

Motor/Generator <--> SPIM <--> Battery

(there's a distribution block in there but that's irrelevant to this discussion)

The SPIM (single power inverter module) switches power between DC (battery side) and 3-phase AC (motor side).

A generator, like any power source, provides a voltage. That voltage is generated quickly, but is meaningless unless you have something to draw current. As soon as you start drawing current you create resistance which requires force to maintain, otherwise the speed will reduce, which reduces the voltage.

You may have seen exhibits at your local science center, but if not imagine this: you have a hand crank flywheel, and a few old incandescent lights that you can turn on and off with some switches. Crank the wheel with no lights on and it takes a little bit to get up to speed, at which point it's easy to keep turning (voltage is created but no current draw = no additional resistance other than normal friction). Turn a light on and suddenly you can feel that it's harder to keep the wheel spinning. Turn 2 lights on and it's harder still. 3 lights on and it's really hard to keep the wheel turning, and it starts to slow down.

Or, look at it another way - get the wheel spinning as quickly as you can and stop. It may take 60 seconds to fully stop with no lights on, 20 seconds to stop with 1 light on, 10 seconds to stop with 2 lights on and 5 seconds to stop with all 3 lights on.

Increasing resistance due to added load = more force required = more slowdown. So if you can control the amount of load (current drawn) then you can control the amount of resistance, or force required, or slowdown. I can control the load based on the number of lights that I turn on, but what if I only have 1 light?

What if I put an automatic switch that turns the light on/off with a given frequency. If the switch has a 50% duty cycle (turns on and off say once per second) then the resistance will be halved, and will require half of the force, so the wheel will slow down half as quickly. If the duty cycle is 25%, then I require 1/4 the force. Unfortunately we're going to see the light flicker as it goes from off to full brightness once per second.

Instead of once per second, however, what if I do that 100 times per second, or 100,000 times per second? Instead of seeing it flicker, I'll just see it at about half brightness with a 50% duty cycle. At 25% duty cycle, the brightness will still appear constant, but at 1/4 the brightness as full.

This is what our SPIM does during regen through the use of MOSFETs (and a bunch of other complicated stuff, but that's the main relevant part).

The force is the kinetic energy of the vehicle. The car monitors the power output of regen, and tells the SPIM to control the duty cycle so that it maintains the exact amount of power (deceleration) that it wants.

Telek
Posts: 12
Joined: Fri Apr 12, 2019 3:11 am

Re: Help me understand how regen works

Fri Apr 12, 2019 3:43 am

SmokingRubber wrote: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.


That's mostly correct, but keep in mind that it's not exactly that the battery can't accept any more regen.

The battery is 360V nominal and would have a upper recharge limit of 403.2V, if not for the car's systems having an upper limit of 400V.

You recharge a battery by applying a voltage higher than its open circuit voltage. The higher the voltage difference, the more current the battery will draw.

But if the battery is near full (let's say 395V), the car will only go up to 400V - which is about 25kW of power drawn by the battery.

So although the car does know when the battery is full, it's not limited by that as much as it is limited by its own upper voltage limit.

theothertom
Posts: 38
Joined: Wed Sep 19, 2018 4:13 pm

Re: Help me understand how regen works

Fri Apr 12, 2019 4:57 am

Thanks, Telek!

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