BLDC (brushless DC) motor as power generator

[rascupanamuha]

Maybe some one can help me here. Here is my problem: i am building electric scooter, and motor will be chained direclty to rear wheel. I dont have freewheel. Motor will be connected to electric speed controler, and to arduino for PWM. Everything should work fine when motor is powered, and everything is spinning, but when wheel gets faster than motor (when i am braking and slowing down), then i think it will break ESC, is that right?
I think the simplest think to do is to put diodes on those 3 wires between motor and ESC, but how, what direction?

And as i have motor connected to the wheel direclty, when slowing down, i could generate power from it and charge batteries..


Does someone know how to do it?

 

[kubeek]

Look at this pdf https://www.electro-tech-online.com/custompdfs/2013/05/Regenerative20Braking20of20BLDC20Motors.pdf Basically everything depends on the design of your ESC and your motor.
But anyway regenerative or resistive braking is certainly possible.

 

[rascupanamuha]

Wow ! Very nice, thank you !
Here is what ESC i will be using: **broken link removed** , and
MOTOR: **broken link removed**

so what configuration do you think i should be using? And what are the pink and green things stands for (page4)?

 

[kubeek]

Pink and brown things are measurement points for voltage and current, green thing is an oscilloscope.
It seems the ESC is capable of applying brake, BUT I think it does in the way that the braking energy is dissipated in the ESC, so it is not suitable for braking a loaded scooter from speed to standstill, more like a parking brake or something.
Another thing is that your motor doesn´t have a position sensor, so you need a sensor-less controller solution. Here I found an application note from atmel https://www.electro-tech-online.com/custompdfs/2013/05/doc8012.pdf

It may be possible to use the ESC you have and add another circuit to it which applies the brake, but you´d have to test what the esc does when you set the pwm to off without the braking mode, it will probably output some voltage back to the supply rail, so you would then use some switch to go into the braking mode and load this voltage.

 

[rascupanamuha]

Yes, ESC has break option but you are right, it is made for stopping propeler to rotate on RC planes and not to stop around 100kg from 30km/h to 0.. Although i have mechanical brakes, but the main point of this is not to break ESC due to not having freewheel
What do you think about this i have drew?



And... if this WOULD work, how good would that be? What breaking torque would it have, how can i know? And if i have 24V (6S) battery, haw good would it charge my battery?

 

[kubeek]

If you mean that as regenerative braking to recharge the battery, the problem is that the motor will never put out higher voltage than the battery voltage, so the battery would not charge. So you either need a single boost converter to get higher voltage out, or use an esc to do that like they show on pages 11-13 of the microchip pdf.

 

[rascupanamuha]

Okay then, but please help me with understanding the pdf

I normally put signal of ESC to arduino, ABC from ESC to motor. and then those ABC wires that connect motor and ESC also connect to mosfets (and which ones) and where then?

 

[kubeek]

Those six mosfets are inside the ESC, so you can´t really control them the way you´d want to (unless the esc has that function).
You could try to measure the output of the ESC power supply wire when the bldc is spinning and you change the control signal, it might be that if you set it low the esc will generate more voltage then when you set the control high, it just might work. Just crank the motor say with a drill to get some speed with the battery disconnected and see what you get out when you change the control signal.

 

[rascupanamuha]

Hmmm it is very interesting... I dont have motor nor ESC yet. But i have tons of smaller motor and esc (30A). Adn i have connected just motor to ESC and + and - to voltmeter and it says 2.4V when spinning at full speed of the drill. So this is not bad for ESC? now how do i get it to 24+V to charge my battery?

 

[kubeek]

First thing is that actually a drill will hardly reach the full speed of the motor. But anyway if the esc can´t do it for you, then you will need to use the schematic you have drawn, but between the top of the diode bridge and the battery you will need to add a boost switch mode converter.
Or just get an ESC that can do it all

 

[tcmtech]

The easier way to do it would be to have a three phase rectifier set up running parallel to your ESC and have that dumping into a simple boost converter that only comes on when the ESC is not putting out any power to the motor.

Most ESC's do not like having their outputs being connected and disconnected by relays so I would leave it permanently connected and deal with the regen braking as an independent unit.

 

[rascupanamuha]

I have bought ESC
So DC-DC booster. Lets say i got 5V out, i need to boost it to 24V, but current drops then, so if it was lets say 5V, 5A, it would be 24V, 1A, is that right?

 

[tcmtech]

Yep. something like that.

 

[rascupanamuha]

Ok then, my schematics but without relays.
But is that ok? Because i will spinn my motor with power from battery, and this motor would create lets say 5V trough 6 diodes and this 5V 5A apmlified to 24V,1A would be power loss od 24W

 

[kubeek]

I am not too sure what you mean, the drop on the diodes could be say 1.5V times the 5A current is about 7.5W loss distributed over all the diodes.
When you spin the motor from the battery the boost converter needs to be off. Then when you brake you turn the esc off and turn the boost converter on to get regenerative braking.

 

[rascupanamuha]

If i have around 500W output power (20A), i dont know how much current would it pull, but if this is my situation, and if i dont get 5 but 10 or 15V, 10A, and then charghe battery with 6.25A, 24V, i first lose 150W, then get it back. So my output power wouldn be 500 but 650W. Am i right? Why do you think ESC dont like relays?

 

[rascupanamuha]

Tcmtech said that i should rather connect wires without relays. But now i realised i have something messed up.

Here is new schematic:


1) Motor is working, current goes from battery to ESC and motor and from ESC through diodes out with loss of around 10W
2) Motor is still spinning, but ESC is not powered, through diodes i recieve power of 150W

Am i right?

 

[kubeek]

What? How? In number 1, where would the current go when the esc is on? The diodes will be reverse biased for sure (the voltage at the motor cannot be higher that the voltage of the battery), so there will be no current through them.
In number 2, esc is still powered from the remaining voltage, but the control should be set to off. Then you will get whatever power the circuit can handle according to the load applied, for as long as the inertia lasts.
Also, there si no "getting back" of losses, once you lose somthing you dont get it back, it just becomes heat. Also you will never get more energy out than you put in.

 

[rascupanamuha]

I have wrongly express myself about power loss and getting power back, forget about it

1)
Battery is powering motor, throttle is not 0, no power loss

2)
Motor is powering battery through diodes with small power loss on them, throttle is 0

Is this ok?

 

[kubeek]

Yes that is the way it should work, but you still need to include the step up converter in the part 2. Such that the top side goes to the converter and then the output of the converter goes to the battery +.