106 replies to this topic

[idare2bdul]

Motor theory is great and as has been pointed out there have been many motor builders in the past that have contributed to the development of our current generation (pun intended) of slot car motors. While most if not all those guys studied a book or two on electric motors much of the progress was good old fashioned trial and error. Brush size and length, brush compound, commutator size, magnets, length of race heats and the type of racing all enter into the design of a motor. There doesn't seem to be one optimum set up and every time it looks like we have reached another milestone we find new ways to go faster and in many cases with more durability.

While I have enormous respect for what Stu Koford has accomplished I seem to remember that his first product, an alignment bar was considered slightly the wrong size. What he and Dan Debella now turn out for the price they charge and the quality they offer shows what dedication and product development can accomplish.

While not trying to minimize Koford selling to the military, remember they tend to buy from the low bidder so that could be a factor as high torque, low (or high)rpm brushless motors are readily available and widely used in Radio Control cars, boats and aircraft.

The moral of this post is that if you are trying to make a motor for a particular class go faster try changing something and see what it does because you probably won't find the answer in a book. We are seeing a 300 amp spike when launching some drag slot cars, which means that they are almost effectively a dead short during their 4 tenths of a second run. I suspect that the books don't delve much into doing stuff like that with a 24 gauge armature.

[gotboostedvr6]

I felt compelled to bump this thread. I really hope everyone that adds their opinion to this reads every page first.

[Pablo]

The only experience I have on moveable endbells is AMSRA Speed FX 16D handout motors - usually when you messed with the timing, they cooked.

 

gotboostedvr6, on 01 Jan 2015 - 11:26 PM, said:

I really hope everyone that adds their opinion to this reads every page first.

Well said. I hope the habitual "posters who don't even know the subject of the thread" read and heed this advice wherever they may roam

[Bill from NH]

No, I didn't go back & re-read all 103 posts. When I was running a lot of Chinese 16d's in the late 90s & early 2000's most shops hadn't leagalized the use of the AT cans. We retimed arms by heating the comm, twisting it counterclockwise, then reglueing it to the arm shaft with super glue. The AT cans provided 5-7 degrees advance, the retimed arms usually provided an additional 20+ degrees of advance, depending upon how much slack there was in the comm wire connections. It was easy to retime an arm for 38-40 degrees of advance, sometimes you could get more.

[Pablo]

Sorry, Bill, I didn't mean to suggest posters re-read every single post

I'm talking about guys who post without even knowing what thread they are in.

Sorry for the thread drift - but it's all gotboostedvr6's fault

[Big Booty]

Wow this post is full of semantics.  I'll try and keep everyone happy by saying that the "effective" timing is advanced by rotating the endbell in the opposite direction to armature rotation.  Static armature timing remains the same.  That takes care of the original question.....I hope.

 

But why does timing affect rpm?  Three clever slot racers named Maxwell, Faraday and Lenz, about 200 years ago, developed Laws that govern electromotive interactions.  Lenz said that a electric motor is also a generator.  The faster the motor spins the greater the voltage generated.  When the generated voltage equals the applied voltage the motor speed has reach a maximum.  But why does changing the timing change the rpm?

 

Most text books give the following formula: V = B x l x v

 

Where V = voltage B = magnetic field strength l = length of coil v = velocity

 

They leave out the cosine of the angle in the formula because they give the simple example where the angle is zero so cosine0 = 1

 

By rotating the comm or the endbell the angle is changed from something other than zero and cosine (of the angle) is now a value less than one.

 

V  = B x l x v x cosine (angle > 0)

 

For example the cosine of 20 degrees is 0.94.  Therefore the voltage produced by the generator is now 6% less for the same speed.  The motor will now speed up by 6% at which point the voltage will once again match the applied voltage.  Yes I can appreciate that this is a simple text book answer but the principle is correct.  This is why you get an increase in speed by advancing the timing.

 

It comes at a cost though.  A reduction in low rpm torque.  Nothing is free, everything costs you.  As with most things there is a sweet spot where rpm x (stall torque) is maximized.  Find it and then gear the car accordingly, if the rules allow for changes in timing.   

 

And you thought that your physics teacher was boring!!  Should have paid closer attention all those years ago. 

[Koford fan]

I am trying to understand all beeing said, but here is my toughts: