Dynamic Breaking

Dynamic braking is any system that captures kinetic energy at the wheel and converts it to some form of potential energy that can later be used to re-accelerate the car. Most likely, in your situation, when it comes time to brake, you would allow your electric motor to become a generator driven by the car's axle and re-charge the battery. Savvy?

In this case when the motor will re-charge the battery does it metter if i will use AC or DC electric motor ? What is the basic different between those two ?

Thanx

Unless you're going to carry an inverter on your car (a rather heavy and unnecessary device, so I would not recommend it), you will have a DC motor to go with your DC battery.

I would recommend you 3 phase permanent magnet motor for the following reasons: 1/ both generator and motor 2/ Compactness and ligthness for a given power 3/ Very efficient energetically 4/ It is now easy to find built-in PWM micro-controllers

DC is probably easiest and cheapest to work with for the fractional HP I'm assuming you'll be using. In small sizes most (probably) DC motors are permanent magnet type. The rotor for these is wound with coils, the ends of which are connected to a commutator. Current is fed to these commutators through brushes, which wear due to ordinary friction accelerated by tiny arcs. Were it not for brush wear, these motors would be more popular than they are -- although they are already very popular.

Brushless DC motors are very similar to an AC motor. Occasionally they are called PMAC (Permanent magnet AC). These commutate (in other words turn on the right coil to repel or attract a magnet advantageously) electronically, without the need for brushes: the rotor has permanent magnets, and the appropriate poles of the stator are fed current through the motor control circuits. Some of these are a little more efficient than a brushed motor, but the electronics add cost.

The Tesla electric car uses an AC motor. If you've already decided to do away with brushes, then the choice of AC vs DC brushless (or PMAC) is not simple -- in other words there is no clear leader in efficiency, etc. If you go to the Tesla motors site, and poke around, you'll find an article or white paper, I think, re the advantages of AC. I am not 100% convinced, but they make a good argument. If I remember, the efficiency of their motor is about 90% -- the same as that of the brushed motor I am using on a prototype electric vehicle. But it is possible that if one looked at efficiency under a wide range of running conditions, that the AC motor might be advantageous. AC motors are induction motors, meaning that the current in the rotor is induced by the current in the stator. The rotor is usually just laminations of steel with bars of copper or aluminum inserted, so the rotor is simple and rugged.

http://en.wikipedia.org/wiki/Induction_motor

You beat me to the correct answer(s), and you probably hit the button better than I would have had anyway......

Blink,

Nice work... where were you when I was 15 yrs. old? we need more people like you to steer curious minds in the right direction.

I just went thru interviewing 12 young applicants - for a pretty decent job... Only 2 seemed to have the curiosity and drive to learn "how it works" - Alas, they accepted the first offer from other companies.

the other applicants seemed they were more interested in "how fast can I become Vice president of blah blah blah..."

I know, off topic, but - thank you sir!

John

Thanks for the very kind words.

I've done some teaching and coaching of kids (in things like robotics teams), and when you are lucky enough to have a group of engaged youth or adults, it can be really gratifying to share knowledge and experience.

I'd like to think that with the right educational opportunities and experiences, upcoming generations could solve many our fundamental challenges with double, triple or (who knows?) ten times the efficiency seen now. Shouldn't the norm be the 2 out of 12? Sadly, our educational system doesn't support self-directed and self-paced learning.

I suppose there are some 5-year-olds who have no dreams of the great things they could do -- but I think most do -- you need to be taught that your dreams are unrealistic. It's a rare kindergartner whose eyes don't light up when they talk about the things they'd like to do as an adult. It's a rare college grad whose eyes do light up.

If only we were better at helping those kindergarten dreams take flight.

.......apart from dynamic braking you should also utilise a conventional friction braking for safety and better control ........

It seems every one has gone overboard.

What scale is your RC?

Dynamic breaking systems go from basic to the regenerative as discussed in this thread.

Basic systems will dump the energy off to a resistor to covert energy to heat and dissapate it.

I doubt that a scale RC will have the rolling mass to do much good with a regernerative system, and it will be very costly in terms of complexity and cost.

why not just stick with a friction break on the the axle, or just put a heat sink on your motor control to handle the heat generated using the DC motor?

you can place with wire diameter and windings to maximize efficiency.

For an RC car, I would go for a flywheel system. The electrical regenerative braking (BTW, it's more practical to charge/discharge a supercapacitor instead of a battery) implies too many energy conversions to be feasible at such a scale. If you want to learn about this subject, google "flywheel regenerative braking" or read this:

http://www.awesomelibrary.org/Classroom/Technology/Machines/Flywheels.html

All you need is to take a flywheel from a toy (those cars you need to push then release) and a servo-clutch in order to connect/disconnect the flywheel to/from an axle.

For real vehicles, another possibility to achieve regenerative braking is to build/release hydraulic pressure. If eager to learn, just google "hydraulic regenerative braking". Ford tested such a system.

"For real vehicles, another possibility to achieve regenerative braking is to build/release hydraulic pressure. If eager to learn, just google "hydraulic regenerative braking". Ford tested such a system."

Bosch/Rexroth is currently testing such a system on transit buses on the East Coast (Boston, maybe).

In practice, regen braking with a flywheel requires a pretty sophisticated infinitely variable transmission to enable storing of any useful energy. Without an infinitely variable transition, then when the clutch couples up, the wheels have to skid -- the flywheel cannot simply jump to a speed to match wheel speed. If you use the clutch to slowly and smoothly engage the flywheel, then you are simply using the clutch as a friction brake.

Without such a transmission, then a speed of (let's say) 50 mph (1000 wheel rpm in full scale with smallish wheels) corresponds to a flywheel rpm of 50,000 rpm (so a 50:1 transmission is required). If the car is going 20 mph and you engaged the clutch, expecting to brake, you'd find yourself brutally accelerating instead. So at twenty, you need something like a 150:1 step-up ratio to be able to speed up the flywheel. At 10 mph you'd need 300:1 , 600:1 at 5 and so forth.

This same sort of infinitely variable transmission is necessary to drive the vehicle as well. The alternative is to use an electric motor to speed up the flywheel, using the same electronics that you would use to charge batteries (a 2Q or 4Q controller). You might just as well put the energy back into the batteries, which you can do with 90% efficiency if the battery charge level is managed well.

The only place where flywheels have hope of being competitive is in stationary supplies like a UPS, and even then there need to be special requirements to justify the cost. This is purely a guess, but I'd say a flywheel system comparable in utility to a $50 dollar battery might be $5000.

Regen braking is about as simple as can be. Without any electronics at all, if you hard wire a motor to a battery so that a car goes 10 mph on the level, when that car reaches a downhill section it will regeneratively brake to slightly more than 10 mph as it goes downhill. It won't simply accelerate like a sled or runaway truck, and no control electronics are involved. Gaining control over the rate of braking is pretty simple electronically.