# 4-Wheeled Regenerative Braking



## SPYK30 (Nov 19, 2012)

First post around here so go easy on me. Sorry about the blocks of text ill try to organize it.*

Intro*
Hello I am a member of a team that is assisting in the development of an student designed electric racecar. More specifically we are assisting in the design and prototyping of the motor control and regenerative braking. 

*Overview*
A couple of the design requirements are that the car will be 4-wheeled drive with 20kw BLDC (200V) motors on each wheel and there is to be regenerative braking from each of them. The car will not have regen-braking active until the brake pedal is pressed and the car is above a specific speed. The system will be powered by 4x 50V lithium ion batteries in series to provide 200V and the motors are connected in parallel to batteries. Now I don't need a crash course on how regenerative braking works as I have done plenty of research. The situation I'm hoping someone has some experience is as follows.

*My Question/Problem*
At the moment we have 4 motors that are going to be supplying 4 different back EMF to be charging 1 energy storage device. Anyone who has taken any basic circuits class knows that when you connect voltage sources in parallel you get trouble with the sources becoming loads of each other. Therefore something needs to change or be included in the design. 

*My Best Solutions So Far*


Add a super-capacitor for each wheel to Regen into. Then use this capacitor to either charge the battery or provide a boost to the motor during heavy acceleration, it is a racecar after all.
Build some sort of isolation network. The overview of this idea is we have 4 50v batteries in series supply 4 motors in parallel. We incorporate a system of high power relays to change the batteries from being in series to having each motor connected to each battery. This was proposed during a meeting but could pose to be problematic due to excess weight and the size of the latches/relays required. Then there could be problems reconnecting the batteries in series to run the car again.
*Conclusion*
Personally I think the capacitor route is the way to go. Though there is some resistance with some of the team. I have researched regenerative braking but have yet to find a project or layout that uses 4 wheels in parallel such as ours. I know this forum has quite an experienced group of people who browse it and I was wondering if anyone has any pointers or ideas. 



Thanks


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## cpct (May 31, 2012)

So if I understand it right you are using the 4 BLDC motors in parallel on 1 controller then  ?


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## frodus (Apr 12, 2008)

You won't be able to use one controller for 4 motors on 4 wheels, it won't work.

If you use 4 controllers, they each handle their own regen and regen at pack voltage, so they control the regen and the different back EMF themselves.


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## major (Apr 4, 2008)

SPYK30 said:


> First post around here so go easy on me. Sorry about the blocks of text ill try to organize it.*
> 
> Intro*
> Hello I am a member of a team that is assisting in the development of an student designed electric racecar. More specifically we are assisting in the design and prototyping of the motor control and regenerative braking.
> ...


You're making a problem where there is none. Four motors, each needs its own controller, 4 controllers from one source (battery), run in torque control, same or different command to each controller, doesn't matter, works without a problem.


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## SPYK30 (Nov 19, 2012)

Sorry I was a little vauge. We have 4 BLDC motors in parallel, each with their own controller. We want to regen brake the 4 motors into one battery. 

The problem I am seeing is that the back emf of each motor will not be the same and the motors with the weaker back emf will become the load rather than the battery. 

I am wondering if anyone has had experience doing regenerative braking on a 4x4 electric vehicle and knew more about this. 

I am thinking we are going to either have to put super capacitors on each wheel to Regen-brake into or somehow throttle the back emf, possibly using the controllers and software, to equalize the back emfs.


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## major (Apr 4, 2008)

SPYK30 said:


> Sorry I was a little vauge. We have 4 BLDC motors in parallel, each with their own controller. We want to regen brake the 4 motors into one battery.
> 
> The problem I am seeing is that the back emf of each motor will not be the same and the motors with the weaker back emf will become the load rather than the battery.
> 
> ...


Yes, we understand. You have no problem if you use torque control, which you need for vehicle propulsion anyway. Multiple drives connected to a common bus with independent regeneration is commonplace in industry and no different on a vehicle using a single battery.


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## SPYK30 (Nov 19, 2012)

So if I am understanding this correctly because the motors are being controlled to meet the desired torque if the motor's power is higher than desired power, i.e. when it's regenerative braking, the controller will already stabilize the back emfs with no modifications to the preexisting torque control? 

That's not entirely how I envisioned it working. I'm assuming that's using some modulation in the motor driver. It's fairly obvious I'm going to need to run some low power tests to actually understand exactly how some of this works. I am having some trouble simulating some of this due to the frequencies involved and the computing power available. 

Thanks for the replies! My experience with motor control was practically nothing before I started this project and I would like to figure this out.


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## major (Apr 4, 2008)

SPYK30 said:


> So if I am understanding this correctly because the motors are being controlled to meet the desired torque if the motor's power is higher than desired power, i.e. when it's regenerative braking, the controller will already stabilize the back emfs with no modifications to the preexisting torque control?
> 
> That's not entirely how I envisioned it working. I'm assuming that's using some modulation in the motor driver. It's fairly obvious I'm going to need to run some low power tests to actually understand exactly how some of this works. I am having some trouble simulating some of this due to the frequencies involved and the computing power available.
> 
> Thanks for the replies! My experience with motor control was practically nothing before I started this project and I would like to figure this out.


Your first paragraph above doesn't make much sense to me. But at least it appears you accept the fact the motor controller will essentially decouple the generated voltage (Eg) in the armature from the DC bus (battery voltage, Vb). Consider the motor voltage (Vm). 

When in motoring mode: Vb > Vm > Eg. So the controller is in buck mode making Vm (load) lower than Vb (source). 

When in braking mode: Eg > Vm < Vb. Now the controller becomes a boost converter using Vm as the source and Vb as the load.

The converter uses modulation in both cases and that is controlled to yield the proper armature current (magnitude and direction) to satisfy the torque command. 

Multiple converters can be connected such that Vb is common and have complete independence of respective Eg due to individual control of Vm.

That is just a very basic view but I hope explains theory.


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