SlotForum banner
1 - 13 of 13 Posts

·
Registered
Joined
·
522 Posts
Discussion Starter · #1 ·
When the throttle is released, I have read the motor is shorted which provides additional resistance for the armature. I have experimented with and understand it.

What I dont understand is how partial throttle ALSO acts as braking? When I go to partial throttle, a car with braking will quickly adjust to the new speed, yet a car without braking will "coast" to the new speed. BUT, at partial throttle, the motor is not shorted!

So for instance, if my FLY car is approaching a tight turn, I will release the throttle to 15%, the car quickly brakes and loses most of its speed before entering the turn. Its providing braking yet is also providing output.

Can someone please explain. Ultimately I'm trying to figure out if there is a technical limitation that explains why carrera digital cars do not provide braking.
 

·
Registered
Joined
·
4,634 Posts
my understanding is that this partial braking should occur on tracks with dynamic brakes and also on tracks without.

Obviously there is an elictrical circuit when the trigger is pulled a bit, if the speed of the car means that the motor is spinning faster than than the applied voltage, then once again there is a back emf situation and there is a braking effect.

Rather like engine braking in 1:1 cars i guess
 

·
Brian Ferguson
Joined
·
3,652 Posts
Dynamic braking can only occur when the throttle is fully released. In other words, the wiper must be on the controller's lowest band, or activating the braking switch, or contacting whatever the particular controller uses for brake activation. There is no "partial" activation, such as 15% throttle.

Dynamic braking is, indeed, the shorting of the power rails together, which provides a heavy load to the decelerating motor (acting as a generator) and slows it quickly, and thus cannot occur when any power is being delivered to the rails since that would also short-circuit the power supply.
 

·
Premium Member
Joined
·
5,553 Posts
I think you will find it all works via voodoo.
 

·
Registered
Joined
·
398 Posts
Any braking you notice at partial throtlle is a combined effect of the magnetic field of the cars motor magnets on the armature and if running a magnet traction car it is the attraction of the chassis magnets to the metal track rails that helps slow it.

Also to an extent the gear ratio can help or hinder such braking effects. A 5-1 ratio will stop a car quicker than a 2-1 ratio as well as let it accelerate quicker.

This all does not involve dynamic brakes if not used.

Even with dynamic brakes some motors with weak magnets and high (2-1) gear ratios will make some cars less effective with the dynamic braking effect of shorting the motor out.

Astro:

The basic law of thermo dynamics (that energy will always seeks a lower level) would rule out the slot motor under dynamic braking would exceed the present power supply output. That would be not possible without a secondary power source feeding it (no batteries allowed in cars or controllers
). That would be like saying a solar power panel could produce more power than the sunlight source if a cloud covered up the sun.


Lots of factors involved in slot car racing.
 

·
Brian Ferguson
Joined
·
3,652 Posts
Astro, Larry pretty much covered it. The effect at part throttle is not dynamic braking and will not be noticeable on cars of different characteristics. Yes, some cars will brake better than others at part throttle but they will on any track, with any controller, whether or not the track is wired for brakes. Back EMF has its greatest effect at high RPM, under power, where it actually becomes the limiting factor to a particular motor's top speed. Under deceleration, back EMF plays no part, as the motor is acting as a power generator and there is no opposing force except for friction, and preferably, track wiring that takes dynamic braking into account.
 

·
Registered
Joined
·
4,634 Posts
QUOTE (Larry LS @ 15 Nov 2004, 21:58)The basic law of thermo dynamics (that energy will always seeks a lower level) would rule out the slot motor under dynamic braking would exceed the present power supply output. That would be not possible without a secondary power source feeding it
<{POST_SNAPBACK}>

If you accelerate a car to its top speed, and then drop the voltage from 12 volts to .1 volts, the speed of the wheels and momentum of the car WILL generate a back emf greater than the input voltage. This is from the energy stored in the car, and its momentum, not magiced out of no-where.

I maintain that this back emf must have a braking effect, similar to that in dynamic braking.

ie if you accelerated 2 cars to full velocity, and on one you reduced the input voltage to 0.01 volts, on the other you severed the wires to the motor, the one with 0,01 volts would stop quicker.

I haven't tested it, and will accept scientific arguement to the contrary, but with respect Larry's reasoning does not cover the hypothesis suggested!
 

·
Registered
Joined
·
522 Posts
Discussion Starter · #9 ·
I appreciate the response but I think you are missing something.

Braking DOES occur at partial throttle. Remove the brake line. Then reconnect it. With it connected, you will see that cars brake better even if you never completely release the throttle.
 

·
Brian Ferguson
Joined
·
3,652 Posts
QUOTE Remove the brake line. Then reconnect it. With it connected, you will see that cars brake better even if you never completely release the throttle.

The brake line (red wire on most controllers) is connected to NOTHING unless the controller is in the fully released position! In the fully released position, it does, by way of the wiper, connect the two power rails together, thus implementing dynamic braking. At ALL other times, that brake wire, as I said before, is connected to NOTHING, and hence can have no effect. Sorry guys, but it's true!
 

·
Registered
Joined
·
522 Posts
Discussion Starter · #11 ·
QUOTE (astro @ 15 Nov 2004, 23:24)QUOTE (Larry LS @ 15 Nov 2004, 21:58)
The basic law of thermo dynamics (that energy will always seeks a lower level) would rule out the slot motor under dynamic braking would exceed the present power supply output. That would be not possible without a secondary power source feeding it
<{POST_SNAPBACK}>

If you accelerate a car to its top speed, and then drop the voltage from 12 volts to .1 volts, the speed of the wheels and momentum of the car WILL generate a back emf greater than the input voltage. This is from the energy stored in the car, and its momentum, not magiced out of no-where.

I maintain that this back emf must have a braking effect, similar to that in dynamic braking.

ie if you accelerated 2 cars to full velocity, and on one you reduced the input voltage to 0.01 volts, on the other you severed the wires to the motor, the one with 0,01 volts would stop quicker.

I haven't tested it, and will accept scientific arguement to the contrary, but with respect Larry's reasoning does not cover the hypothesis suggested!
<{POST_SNAPBACK}>


Yes, I think you are right.

This would explain why carrera digital cars dont have braking. They draw current from the full DC voltage on rails. At this point I am doubtful they will be able to add braking. Wanna buy some pro-x?
 

·
Nobby Berkshire
Joined
·
2,015 Posts
Fly in-line cars brake well as they make lighter cars (not in all cases!) with gear ratios (8:27) and centrally concentrated magnets that enhance braking WITHOUT electrical braking at the controller. The sudden 'letting go' of the trigger merely doubles their performance to take the cars to a total stop.

Scalex and others make heavier cars and slightly weaker braking effective gear ratios for their in-line motors (9:27) along with wider magnetic range. You have to rely upon the controller more to brake their cars suddenly.

Types of motor also have an influence on braking.

Everyone, it seems, make the same sidewinder gear ratios but Fly's magnet design and lighter cars still make them more responsive.

Levels of braking aren't always down to the controller!
 
1 - 13 of 13 Posts
Top