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Discussion Starter · #1 ·
Can someone explain how the back emf works?
does the motor create an ac or dc reverse voltage ?
one of our club members was saying that 'electronic' controllers as opposed to resistor ones make the back emf braking better by taking part of the section of the ac current and creating better braking...?
wonder if someone could clarify the differences and state the technology used.
 

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Rich Dumas
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Most controllers have dynamic brakes which simply short out the motor when the brakes are on. Any time a DC motor is turning it generates a voltage, even when it is under power. The voltage is opposite in polarity to the applied voltage and is referred to as the back EMF. When the motor is unpowered, but still turning, shorting it out causes it to stop faster. In order to have dynamic brakes there must be current flowing through the motor windings so that they generate a magnetic field. Higher end controllers have a potentiometer in the brake circuit that can reduce the amount of brakes. Some controllers have MOSFET brakes. With that type of brakes you get full braking as soon as you back off on the trigger and the brake control lets you adjust how long the brakes stay on.
 

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Back emf is a different concept to what is happening to the motor during braking, but I think this is just the terminology you are using.

A DC motor is also a DC generator. If you apply a voltage it will act as a motor. If you mechanically turn the armature it will act as a generator and produce a voltage. So once the voltage is removed from the motor via your controller that car is still moving, and hence the armature is moving and generating an output voltage. The amount of "braking power" the motor produces and hence the rate at which it slows down will depend on the electricl load you apply to the motor. Dead short circuit on the brake circuit (zero ohms on a pot for example) = full brakes. In this mode the motor will deliver the maximum possible current it can and hence produce the maximum braking power. Open circuit ie no resistor of very high resistance, and the motor will deliver very little or no current and hence very little braking power. Free coasting.

You can vary the braking power with a simple resistor typically in the range 0 - 10 ohms. Or you can have an "electronic" version which usually has a FET transistor which switches on and off at a particular on/off ratio, varying from 0% on to 100% on. When on it provides a dead short, when off it is open circuit. Which is better? Personally I dont think there is much difference but I prefer the idea of a simple resistor.

cheers
rick1776
 

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Discussion Starter · #4 ·
Rick & Rich many thanks for your replies.

so the 'electronic' controllers can enhance the braking as long as there is power to the track?

If power to the track and controllers is cut I assume that the braking effect is zero. Sometimes at the club we cut the power to the track/controllers if there is a problem and then the cars run on on their own so whatever the controller there is not braking or is there if there is a capacitor effect in the controller? I assume you do not want any braking in a power cut as you would want the car to travel it's max distance!
 

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One petunia in a field of onions
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QUOTE (slot32 @ 3 Aug 2011, 08:20) <{POST_SNAPBACK}>so the 'electronic' controllers can enhance the braking as long as there is power to the track?
By my limited understanding the above statement is incorrect in that the electronic controller (with variable braking) can not 'enhance braking' it can only 'decrease braking' as the braking is a function not of the controller but of the motor itself.

Am I finally getting it Rick(s)?

Embs
 

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Yes Embs you have it correct. Slot32 I think you still dont have it right in your mind what is happening. Granted back in the 60s some setups would actually apply a small reverse bias voltage to the track to slow the cars up, as the motors lacked any brakes. Im 99.9% sure that all current controllers and indeed the tracks no longer apply a reverse bias voltage. I know that in the slotit championship it stipulates that the controllers can not enhance the nominal track voltage. You can easily boost the track voltage with "smart" controller but I suspect most official clubs and championships frown upon such practices.

OK pretend that youre not even using a controller. You just apply 12V across the two rails on the track. The car is getting 12V and zooming along. You remove the two wires supplying the 12V. The car comes to a slow slop as it free wheels. The motor is turning as it slows down, it is generating (initially) 12V but there is NO CURRENT flowing. For a current to flow it needs a closed circuit to flow around which you dont have. So the braking effect will be minimal as no braking power is being generated by the motor (voltage x current = 12V x 0A = 0W). Now repeat the above but now when you remove the 12V we short circuit the rails by connecting a wire across the left and right rails on the track. We now have a closed circuit. The voltage being generated by the motor induces a maximum current to flow as we have a short circuit across the rails. We have maximum braking effect (voltage x current = 12V x 1A = 12Watts of braking). Note that the current that flows is a function of the motor and I just picked 1A as it was a convenient value. It could be more it could be less.

Now what would happen if instead of placing the wire across the track I rapidly connected and disconnected the wire? Lets say I was able to rapidly connect/disconnect the wire so that it was connected 50% of the time and disconnected 50% of the time. On average I would then be producing 50% of my maximum braking effort. This is what most electronic braking controllers do. An electronic switch connects and disconnects the wire that shorts out the track.

What about a simple resistor? Well the current that flows from the motor depends on the resistance in the circuit. If you have Zero ohms (no resistor) the maximum possible current will flow out from the motor. If I dial up a value of resistance "XX" such that the motor only produces a current that is 50% of maximum then I will have 50% of my maximum braking power.

I hope that helps???? After this your on your own.

cheers
rick1776
 

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QUOTE (slot32 @ 2 Aug 2011, 23:20) <{POST_SNAPBACK}>If power to the track and controllers is cut I assume that the braking effect is zero. Sometimes at the club we cut the power to the track/controllers if there is a problem and then the cars run on on their own so whatever the controller there is not braking or is there if there is a capacitor effect in the controller? I assume you do not want any braking in a power cut as you would want the car to travel it's max distance!
If power to the track and controllers is cut, full braking is available with simple brake contacts, simple resistor adjustment and some types of electronic brake adjustment.
If power to the track and controllers is cut, no braking is available with some types of controller with FET type electronic brakes.

By available I mean you get brakes if the controller trigger / plunger is in the brake position, otherwise you don't.
That is what you want.
If no brakes are available, when the power is turned off any cars that near the end of straights crash into the next corner. (The crashing does happen with some types of controller with FET type electronic brakes.)
 

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QUOTE (slot32 @ 3 Aug 2011, 08:20) <{POST_SNAPBACK}>If power to the track and controllers is cut I assume that the braking effect is zero. Sometimes at the club we cut the power to the track/controllers if there is a problem and then the cars run on on their own so whatever the controller there is not braking or is there if there is a capacitor effect in the controller? I assume you do not want any braking in a power cut as you would want the car to travel it's max distance!

Not really, it can cause your car to crash if your at the end of a straight. Not sure why clubs operate this system. It would be far better to dial the voltage down slowly because at a recent meet I heard a few nice cars were slammed into the fence.
 

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Discussion Starter · #9 ·
great excellent replies.

let's hope those out there now understand a bit more!

Sorry to hear about the slamming at the end of the straight! I've had this with my turbo parma as the 'earthing 'did not make contact when braking, so I have now added an extra wire from the earth end of the resistor to the contact wiper of the trigger. Hope that makes sense!

braking is now 100% and no slamming.

I guess I could add a variable resistor on the brake line to reduce the max braking capability but I quite like braking late and some motors do not generate as much back emf as others.

thanks again for the replies..you deserve a freebie from me next time we talk.
 

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Rich Dumas
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Shutting off the track power should have no effect on brakes. If the track has a power relay that is in the wrong place the brakes would not work when the power was off. I have tried several controllers that had MOSFET brakes and those feel different than ordinary dynamic brakes. The car will slow rapidly, then it seems to coast. 3rdEye controllers have that sort of brakes. Back in the mid '60s cars really wanted to coast, even with dynamic brakes. I wired up a battery and a variable resistor in the brake circuit and that worked well, however that scheme was outlawed before I could enter a single race. With PWM controllers it is possible for the cars to see a voltage that is higher than the track voltage and it is also possible to build one that applies reverse voltage braking. Some people view PWM controllers with considerable suspicion for that reason.
 

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Turing off power at the end of a race is standard where races are run for a fixed length of time.
It was done that way because the result depends on measuring the stopping point.
Also it's needed for segmented racing to give the starting place on the next lane (segmented racing is normal at so many top level meetings these days).
These days there are several of computerised race management systems that can estimate the finishing position, although that's not practical in segmented racing. Even in single lane racing there are anomalies like what happens when a car deslots or is overtaken on the last lap.

Some controllers with FET type electronic brakes do continue to work after the power is turned off. I don't know why the others don't do it that way.
 
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