Hi, I'm new here but have lurked around for answers before.

My question is with regards to a BLDC motor I plan to use and how I could implement both regenerative braking and the ability to use it as a full on generator to charge the power supply. Correct me if I'm wrong but I believe they are different (the regenerative braking of a brushed motor from my understanding uses the PWM switching as part of a boost converter, to be used as a generator I believe it would have to be spun faster than the power supply would make it spin in order to generate a voltage high enough to recharge it.

I've had ideas (mostly on paper, circuit diagrams as such) but don't have the things to test them right now or they seem to be flawed. Originally I thought of hooking the terminals of the coils on the motor up to rectifiers but I think this would effectively give me a short. I drew a little diagram:



Description:
-L1 is the motor coil, T1 is the transistor/mosfet ill use to switch it on/off.
-d1/d2 are the rectifier diodes and C1 is the capacitor to smooth out the voltage signal (not sure if C1 is necessary, please tell me if you happen to know)
-L2, T2 and d3 make up the boost converter to increase the voltage to let me recharging the power supply.

I know its missing some diodes across the transistors etc but this is more of a concept than anything. Anyways the problem I see is that wouldnt current just flow through the loop indicated by the arrow and ignore the motor? I've been googling for answers to BLDC motor regenerative braking and havent really found an answer I can understand, this is what I've managed to piece together from that research.

My main questions are: am I horribly off and on the wrong track? Do I need the boost converter there (some circuits I've seen don't include one)? Would the motor coil by bypassed like I think it will?

Thanks in advance for any help.

 

If you use regenerative braking mode of a motor controller the controller is only allowing current to go one way, into the battery, the motor coils act like boost inductors and the voltage will naturally rise up to above battery voltage. So basically you don't need a boost converter to push current into the battery, having one might help you increase the regenerative power but it is much easier to achieve this by switching the coil configuration if you have a motor with 6/9/12 etc coils.

Another way of thinking about it is if you disconnect any motor which is rotating at high speed without leaving a flyback current path then voltage in motor coil will go very high and arc in order to complete the circuit and discharge the magnetic flux energy in the motor coils (hence why DC motors usually have Schottky diode across poles).

Two things you have to be careful about though are:
a) You need to make sure all your components are rated for the higher voltages under regenerative braking and e.g. you don't go over the reverse voltage rating of your drive quadrant transistors etc. This is even more of an issue if you are using a boost convertor or chaning coil configuration to increase the regenerative power.

b) Battery charging. If you start with a fully charged battery and then go straight into regeneration then you'll overcharge/explode your batteries. You might want something that shuts off regenerative braking or diverts to an alternative current path e.g. super high power resistor when battery voltage reaches a certain level.

 

So there isnt a need for the boost converter? I was under the impression that the motor would only generate a high enough voltage to recharge the battery if and only if it was being spun faster than it would be running when connected to the battery.

Anyways thanks for the answer, so really I only need the motor and the rectifier? no need for a boost converter (where d2 and C1 meet can just go to the positive rail?)

Another thing to note I guess is that I plan to use a capacitor with a voltage regulator rather than a battery, will this affect the outcome? I'm aware that I'd need to charge the capacitor to a higher voltage than the motor will run on due to the exponential decay of the voltage across the capacitor. So in this case should I keep the boost converter so I am able to charge the capacitor with the higher voltage required? If I do keep it how do I stop my original problem of the short circuit? Or is there no short circuit and I've misunderstood something.

Also, you mention regenerative braking mode on a motor controller. My question wasn't really to do with a motor controller chip it was to do with the circuit I posted above. I don't have a "regenerative braking mode" I can just choose to switch on I was hoping to design/make one

Thanks a lot for your help so far.

 

I'm not using a motor control chip so I don't have a 'regenerative braking mode' I can select, I was hoping to make one myself.

So basically I can remove the boost converter and just connect the point where d2 and C1 meet straight to the positive rail?

Another thing is I'm planning to use a capacitor with a voltage regulator rather than a battery, because of this I'll have to charge the capacitor to a voltage exceeding that at which the motor runs due to the decay in the voltage across the capacitor as it discharges so that I have a sufficient voltage for a decent amount of time.
Because of needing the capacitor to be charged to a higher voltage should I leave the boost converter in? If so how do I get around the issue of the short circuit I mentioned before (is this an issue or have I misunderstood how the circuit would work?)?

Thanks

 

Your design looks like a textbook two way boost converter (although it has been a long time since I looked at any textbooks) which isn't really suitable for motor drive.
Driving brushless DC motors is complicated, they almost always multi phase, you need current limiting, sensing or hall sensor feedback for starting, etc.
Start by looking up the 4 quadrants of motor control and respective brushless motor control topologies.
Personally I wouldn't seek to make a brushless motor controller myself unless I really wanted the learning experience, 3 phase brushless controllers are readily available and cheap.

 

I would like the learning experience and I want to find a way to implement regenerative braking which I don't think comes standard in the motor controllers

 

There are loads of motor controllers which come standard with regenerative braking, it is not very well documented for RC stuff but it is for e-car/bike controllers. Any 4 quadrant motor controller includes regenerative braking by definition, I suggest reading up on how current implementations of these work first before you decide to do something new.