# Variable Current Draw from motor for torque control [closed]

I am building an electric go-kart. I would like to use regenerative braking. I would like to have a brake pedal that controls the amount of torque (slowing the motor and wheels down) that the back emf from the motor creates.

In a hypothetical situation, the motor (90amp 36volts permanent magnet dc motor) will be spun up when the car accelerates. It will then spin freely once up to speed. This part of the circuit I have figured out this part of the circuit already.

(in this image the important switch is the pedal microswitch which closes every time the throttle is engaged Also, the Main, Foward (F) and Reverse(R) notations refer to contractors used to provide power to and switch the polarity of the motors respectively)

If we now treat the motor as a generator, from what I understand the amount of current that we draw from that generator the more torque will be applied to slowing the motor down. If we take the extreme and fully connect the motor leads together with zero resistance the motor will lock up.

Where I am confused is how to draw variable current from the motor and control it with the brake pedal. For safety purposes, I plan on installing a mechanical hand brake as well.

Any help on this would be appreciated, I am sure that I do not have a full understanding of this mechanism.

Note this diagram is from a Curtis motor controller manual. Here is a picture of the cover.

## closed as too broad by Ale..chenski, Bruce Abbott, Elliot Alderson, Edgar Brown, Sparky256Feb 6 at 4:40

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

• I am trying to understand...You are trying a go-cart, and need a heavy ampage motor control. And you mention the Curtis Instruments as an example. Plus you want to add regenerative braking. Did you notice that Curtis was in this business since 1960, and a lot of R&D engineering was put into the problem? Yet you need a simple advice how to master this project... Doesn't is sound a bit too ambitious? curtisinstruments.com/company/research-and-development – Ale..chenski Feb 2 at 7:48
• I am also a little alarmed to see both contactors labelled "F" on the same side of the bridge. I hope I am simply misunderstanding your notation. – Brian Drummond Feb 2 at 12:21
• To clarify, I am not trying to design a motor controller myself. I just did not understand how to limit current draw from a motor. I will look to buy some pre-designed parts to actually build it! – Frederick King Cunningham Feb 2 at 18:03

I don't think you realize how much is involved in building such a powerful motor controller. I will explain some basic concepts to hopefully give you and idea of what you are trying to achieve, but I suggest you purchase a speed controller with appropriate ratings. Building a safe and reliable speed controller for 90A is a big deal.

To a first approximation, motor current = motor torque. If the current is flowing from motor into battery (regen) then the torque is acting in the "braking" direction. If the current is flowing from battery into motor, then it is acting to push the vehicle in the desired direction. To produce a regulated braking torque, you will need to produce regulated motor current flowing from motor into battery. This can be done using PWM control in conjunction with a full bridge motor drive circuit. Please note that regen must be limited to avoid charging the battery at too high of a current, and also to avoid overcharging a battery that is already fully charged. Your solution must accommodate these constraints.

In order to control the motor current, you will need some kind of control system which can sense current and adjust PWM duty cycle to produce the desired current. For example it could be a micro controller with features included to facilitate PWM control.

Here is an image of an H bridge which I found online here.

When Q4 is on, the duty cycle of Q1 can be adjusted to apply variable forward voltage to the motor. When this voltage is greater than the back EMF, there will be forward current and the motor will push the vehicle forward. When the applied voltage is less than the back EMF, this will produce regen current and braking.

By adding shunt current sense in the motor path, you can use a microprocessor to control PWM duty cycle to achieve the desired motor current.

• Yep, this must be that simple. 10-20 years of experimentation, like at Curtis, and you are done. – Ale..chenski Feb 2 at 7:53
• @Ale..chenski, I was trying to provide a simple explanation that the OP could maybe grasp. Do you think my answer is so unhelpful that I should delete it, or should I just add a bunch of caveats that it is just an explication of the concept? Of course, what the OP should do is buy a speed controller for EV's. But that is not what the question asked about. – mkeith Feb 2 at 8:32
• No, no, your answer is fine, and reflects all most important sides. I am just dumbfounded by the level of disregard for complexity of the EV technology from OP. I also may want to make Tesla Model S in my garage..or a jiggaflop-level supercomuter... but it can't be serious, and this kind of inquiries must be terminated quickly, IMO. – Ale..chenski Feb 2 at 8:44
• This is very helpful! Obv my expertise is limited... do you have any suggestions for a module that I could buy and attach to my existing setup to achieve the current limiting effect. I already own the Curtis controller... – Frederick King Cunningham Feb 2 at 18:07
• Also if the batteries are fully charged could I divert the motor current to a resistor? If Yes, what would the ratings on that resistor have to be? – Frederick King Cunningham Feb 2 at 18:08

You can use field oriented control to control the torque of the motor.The field oriented control does the torque control by regulating the current .first you have to select a proper motor as per the torque requirements .You can select PMSM/Induction motor/Bldc motor for this purpose.Then you have to design the motor controller for the motor.If you are not familiar with circuit design you can buy ready made curtis/sevcon controller.

how to get regenerative breaking: Our aim is to activate the regenerative breaking as soon as the throttle/accelerator is released.Suppose if the motor is running in clockwise direction with a positive torque to extract the energy from the motor back to battery you have to apply a negative torque to the motor from the controller.The amount of negative torque applied will decide the stopping distance of the motor,also the energy dumped back to the battery.

if you wish to design your own motor controller hardware and software you can refer to texas instrument reference design. Here is the link :http://www.ti.com/tool/TIDA-00364

V=IR + E. For a given speed, thus a given electromotive force (E) you will need to vary voltage in order to vary current. You will need to include a DC converter which is able to absorb current from the motor. Maybe a full bridge with optional filtering between it and the motor. Then, you will need to design the control and connect it somehow to the pedals. It can't be done just as it is, because you only have on-off speed control.

P.S. The full bridge is indeed implemented with mechanical switches, thus your gokart would suddenly start and suddenly brake. You need to switch faster than it can respond and thus vary the average value of V and I you apply. That is the purpose of the converter.

• What is a "fem (E)"? Use the edit link below your question ... – Transistor Feb 2 at 9:51