# Regenerative Brake Project

#### Ahmed Shaker

Joined Mar 22, 2016
6
I need to design a simple regen brake for a light electric go-kart I plan on entering into a race this year.

However I do not know what exactly to do.

I am using a brushless motor operating at twelve volts.

I have multiple options, and have identified a few.

Because I am using a brushless motor, during regen, it acts as an alternator, creating an alternating current. I can use many AC voltage stabilizing methods.

The simplest would be a servo transformer. But the high frequency of the motor's output (~800 hz) might make building a transformer difficult.

A more complex, but more lightweight method would be to use a PWM static voltage regulator, but I am not sure how this works and would like an explanation on how this works.

I can also rectify the AC to DC and use a boost converter.

Can you guys help me with finding a good method?

#### Dodgydave

Joined Jun 22, 2012
11,301
Why can't you just use a Bridge Rectifier...

#### ronv

Joined Nov 12, 2008
3,770
I need to design a simple regen brake for a light electric go-kart I plan on entering into a race this year.

However I do not know what exactly to do.

I am using a brushless motor operating at twelve volts.

I have multiple options, and have identified a few.

Because I am using a brushless motor, during regen, it acts as an alternator, creating an alternating current. I can use many AC voltage stabilizing methods.

The simplest would be a servo transformer. But the high frequency of the motor's output (~800 hz) might make building a transformer difficult.

A more complex, but more lightweight method would be to use a PWM static voltage regulator, but I am not sure how this works and would like an explanation on how this works.

I can also rectify the AC to DC and use a boost converter.

Can you guys help me with finding a good method?
It's almost hard to keep a motor controller from regen, but it is hard to explain (at least for me). Here is a nice write up. The trick is to use a FET as the flyback and not just a diode.
http://www.utc.edu/college-engineering-computer-science/research-centers/cete/electric.php

Last edited:

#### BR-549

Joined Sep 22, 2013
4,928
Methods......

If you have a free end of shaft, you could couple an alternator.

If no free end, you could use stationary stretch of axle, to belt or gear couple alternator.

Also....regenerative wheel hubs.

#### ronv

Joined Nov 12, 2008
3,770
So, how is a constant output voltage maintained?
It's not really. The battery keeps the voltage from rising much above the battery voltage.

#### Ahmed Shaker

Joined Mar 22, 2016
6
But how is the battery charged when the motor's output voltage goes below battery voltage?

#### Ahmed Shaker

Joined Mar 22, 2016
6
My original idea was to use a rectifier and then a boost controller.

I wanted to use a circuit like https://learn.adafruit.com/diy-boost-calc/the-calculator.

But I do not know if this circuit is able to output constant output voltage throughout a range of input voltages.

If anyone could tell me how to modify it to handle constant voltage output, or show me another circuit which can, it would be very appreciated, thanks.

#### ronv

Joined Nov 12, 2008
3,770
My original idea was to use a rectifier and then a boost controller.

I wanted to use a circuit like https://learn.adafruit.com/diy-boost-calc/the-calculator.

But I do not know if this circuit is able to output constant output voltage throughout a range of input voltages.

If anyone could tell me how to modify it to handle constant voltage output, or show me another circuit which can, it would be very appreciated, thanks.
I have a hard time explaining it. Did you read the article about it? It is very similar to a boost converter with the motor being the inductor. So the voltage is higher than the motor voltage due to the inductive "kick"

#### ronv

Joined Nov 12, 2008
3,770
Another:

One of the well kept secrets of motor control seems to be regenerative braking. Yet it really is no secret: the circuits that give regen braking are not uncommon, yet few people seem to realize what happens. So here goes.

In the first circuit (above) is shown the output pair of MOSFETs, with the motor being driven. It has also been pointed out that, to the motor, the output from the controller is a pure d.c. voltage (because the motor's inductance keeps the current essentially constant during the switching cycle). Now the motor will generate a back e.m.f. which is proportional to its speed of rotation. At zero load this back e.m.f. will rise to be equal to the output from the controller.

We have already seen that the MOSFET is a bi-directional switch which conducts resistively (when it is turned on) for both directions of current. So consider the situation when the current is zero and the controller's output is now reduced. The motor's back e.m.f. is now higher than the controller's output voltage - so the motor will try and feed current back into the controller. If it succeeds in so doing the motor will be braked - we will have regenerative braking.

This type of circuit (where hi-side is turned on when the loside is off) is capable of sourcing current or sinking it. The way this works is that the reversed motor current is now a forward current to the flywheel MOSFET so when this is on it shorts out the motor - whose braking current rises during this period (arrow B, reversed). The Flywheel MOSFET now turns off, but this current must keep flowing - because of the motor's inductance. So it flows as reverse current through the drive MOSFET, recharging the battery as is does so. The extra voltage for this is derived from the energy stored in the motor's inductance. The process of switching from drive to braking is entirely automatic. Moreover it is done entirely by the motor's speed exceeding the drive voltage and without any change of state or switching within the controller. The regen braking is, if you like, a by-product of the design of the controller and almost a complete accident.

#### Ahmed Shaker

Joined Mar 22, 2016
6
Another:

One of the well kept secrets of motor control seems to be regenerative braking. Yet it really is no secret: the circuits that give regen braking are not uncommon, yet few people seem to realize what happens. So here goes.

In the first circuit (above) is shown the output pair of MOSFETs, with the motor being driven. It has also been pointed out that, to the motor, the output from the controller is a pure d.c. voltage (because the motor's inductance keeps the current essentially constant during the switching cycle). Now the motor will generate a back e.m.f. which is proportional to its speed of rotation. At zero load this back e.m.f. will rise to be equal to the output from the controller.

We have already seen that the MOSFET is a bi-directional switch which conducts resistively (when it is turned on) for both directions of current. So consider the situation when the current is zero and the controller's output is now reduced. The motor's back e.m.f. is now higher than the controller's output voltage - so the motor will try and feed current back into the controller. If it succeeds in so doing the motor will be braked - we will have regenerative braking.

This type of circuit (where hi-side is turned on when the loside is off) is capable of sourcing current or sinking it. The way this works is that the reversed motor current is now a forward current to the flywheel MOSFET so when this is on it shorts out the motor - whose braking current rises during this period (arrow B, reversed). The Flywheel MOSFET now turns off, but this current must keep flowing - because of the motor's inductance. So it flows as reverse current through the drive MOSFET, recharging the battery as is does so. The extra voltage for this is derived from the energy stored in the motor's inductance. The process of switching from drive to braking is entirely automatic. Moreover it is done entirely by the motor's speed exceeding the drive voltage and without any change of state or switching within the controller. The regen braking is, if you like, a by-product of the design of the controller and almost a complete accident.
Sorry if I'm pestering you, but as I understand it, as long as the drive voltage is below motor output, the battery is charged at a constant voltage?

This would require for the drive to actually be switching, right?

But would it work for a brushless motor?

#### ronv

Joined Nov 12, 2008
3,770
Sorry if I'm pestering you, but as I understand it, as long as the drive voltage is below motor output, the battery is charged at a constant voltage?

This would require for the drive to actually be switching, right?

But would it work for a brushless motor?
Yes, it assumes a pwm speed control. The regen occurs when you take your foot off the gas so to speak. My golf cart has a brushless motor and regen. You have to actually drive it down hill or it will stop when you take your foot off the "gas".
The voltage would go very high if the battery wasn't there. The current is limited by the motor resistance. So unless you start out downhill with a fully charged battery you can't overcharge it.