[GregK]

Sorry if this seems like a simple question but I am puzzled with the behaviour of a couple of motors and I wonder what is it about a motor that determines it's characteristics? For example, what determines how much voltage it can safely handle? What determines it's RPM?? What about the windings and magnets???

Any help would be appreciated.

 

[rick1776]

Some general rules of thumb.

For a given diameter wire, if you reduce the number of turns (windings on the armature) the RPM goes up and vice versa

For a given number of turns, if you increase the diameter of the wire you increase the torque and vice versa.

The torque is a function of the current flowing through the wire, the number of turns and the magnetic strength of the magnets.

Advancing the comm timing with respect to the armature stack will generally increase RPM (up to a point) at a slight cost of torque.

What is the max voltage?? Its a number of factors that determise this. Insulation break down in the wires, max design RPM before things start to fly apart, ability to dissipate power (heat generated). etc.

hope that helps.

cheers
rick1776

 

[300SLR]

Rick has given a good set of general rules of thumb.

These are rules of thumb for a given size of motor.

Smaller diameter armature motors (like the slim can motors) are likely to produce less torque.
Longer armature stacks produce more torque (other things being equal)

Armature balance is important with higher revving motors, a properly balanced armature in a high performance motor will rev much higher than an unbalanced one. The more basic motors don't rev high enough for this to be significant.

Advancing the comm timing with respect to the armature stack will generally increase RPM (up to a point) and torque at high rpm at the cost of less torque at low rpm.

 

[the shrew]

TBH im new to slot cars but i have a lot of experience with RC cars the principles are the same.

Ive spent a lot of time messing with motors over the years.

You have to remeber with motors even if you buy 20 of the same motors they will all be different ( quite a big difference or can be ) this is due to the magnets have different strengh and the windings are slightly different.

If you have hight revs you will have less tourque and visa versa. You advance the timing to give you more revs - but you reach a limit the performance of the motor goes downhill very fast when you go over it all you end up with is a very hot motor.

Some RC motors are made more for tourque some for revs you choose what you want for what track layout etc maybe the same with slot cars ?

Now this is where the strange bit comes in - I used to Dyno all my motors some times they would look great figures id think to myself id have a cracking motor but sometimes id put it in the car it would be rubbish..... go figure

My experience is the best motos are normally a compromise of revs and tourque.

 

[300SLR]

There are production tolerances in motors, as the shrew says, this can lead to quite big differance in performance in supposedly identical motors.
Big differences are more common at the low cost end of the spectrum. In classes where competitors strip and rebuild motors the variation is smaller, because most of the things that suffer from production variation in low cost production are aligned or adjusted correctly when the motor is rebuilt. Rebuilding doesn't make the best of the batch that much different, but it can often make the bad ones in a batch much closer to the best.

I agree with the shrew that Dyno measurements are not a very reliable guide to how a motor will work in a car.
Motor characteristics can make a big difference to how easy it is to drive a car round corners. "Easy to drive" in the sense that it allows the good drivers to exploit the limits of the car's cornering more consistently - thus giving lower lap times and less offs. One factor is that slow motors tend to be easier to drive than quick ones, obviously that looses straight line performance. There's a lot more to getting a combination of good straight line performance and good cornering. Where as a Dyno might help sort out the slow motors, they are not telling you much about other factors.

 

[the shrew]

I wasnt aware you could strip Slot motors

If thats the case having a good clean comm, well seated brushes goes a long way to improve performance. Messing about with brush shape can also help. Checking the aligment of the can bushes well give better performance. On RC motors these were never put in the cans correctly also try to polish them if they are brass.

One thing that made a big difference was how you ran the motor in when new or had new brushes.
Different motors we did different things to them. Some i used to run in in a cup of soapy water others just at low volts with no load.

 

[300SLR]

Some sorts of slot car motor (e.g. the ones normally supplied in home sets) are not really intended to be stripped down and many of the sets of rules for racing that sort of motor prohibit taking them apart.
Some sorts of slot car motor are intended to be taken apart for maintenance etc. and in most places where these are raced it standard practice to do so. Some sets of rules for this sort of racing explicitly list all the things you can do, other organisers take the view that it is so obvious this has always been allowed that there is no point using extra paper saying so.

Yes correct brush gear alignment and a clean comm make a differance along with so many other things such as bearing alignment, magnet gap, lengthways armature spacing, armature balance, the commutator running true. How much is to gained from each one depends on how far out it was before it was corrected.

 

[Ichabod]

Is there a diagram somewhere which shows the various parts of the motor? My Google searching skills have proven inadequate.

 

[the shrew]

If you cant find anything slot related have a search for Big Jims Motor Black Book or something like that its called. Hes a RC car guy that tuned motors im sure you will find some pictures. It will be pretty much the same principle as slot cars.

If you get stuck give me a shout and i will try to explain the different parts for you

 

[Ember]

Ichabod, try "How stuff works". One of my favourite websites when I'm having a blonde day.

So, to continue in line with these thought patterns: Will a long can motor always have a higher torque rating than an S can with the same RPM value/rating? Is this why the long cans are more favoured for performance?

 

[300SLR]

QUOTE (Ember @ 5 Dec 2011, 01:57) <{POST_SNAPBACK}>Will a long can motor always have a higher torque rating than an S can with the same RPM value/rating?
No, not in always.
There are many examples where the long can does have more torque than a S can for the same RPM.

QUOTE (Ember @ 5 Dec 2011, 01:57) <{POST_SNAPBACK}>Is this why the long cans are more favoured for performance?
For some sorts of cars S cans give better performance than long can, for other sorts of car long cans give better performance than S cans.
S cans are lighter which can be an advantage.
The magnetic field from the two are different, sometimes this gives the long can an advantage. (Obviously this is irrelevant on non magnetic tracks)
For sure torque / RPM characteristics make a difference to performance.

Power is proportional to RPM multiplied by torque, more RPM and less torque (and different gear ratios) may or may not be an advantage.

 

[Ember]

Gah! Every time I think I'm getting the hang of reasoning all this out I ask a question and finish up confused and back to square one again

 

[the shrew]

What is it your stuck with ?

You can tell if your motor revs well because you can hear it. The more revs you have genrally the less tourque you will have.

You will be able to tell in the car if you have a bad motor = it will be slow.

To get the most out of any motor you will need to gear the car correctly. This may change from motor to motor. Also if you have the car geared correctly the motor will run coooler and they give better performance when cooler.

 

[Ember]

Narh. More simply a selection thing. Choosing which will be right for the job at hand.

Not something that matters greatly on my wee small track, but always good to understand.

 

[the shrew]

You just answered the question yourself then mate.

If its only a small track then you need tourque rather than revs

 

[300SLR]

QUOTE (the shrew @ 5 Dec 2011, 14:26) <{POST_SNAPBACK}>The more revs you have genrally the less tourque you will have.
Generally a motor will produce maximum torque at zero revs, the torque decreases as the revs rise and at maximum revs the torque drops to zero.
If that's what you are saying, we agree.

Motors with a higher rev rating may or may not have more torque than motors with lower rev ratings. Quite often higher rev motors have less turns of thicker wire which gives more RPM and more torque.

QUOTE (the shrew @ 5 Dec 2011, 23:19) <{POST_SNAPBACK}>You just answered the question yourself then mate.

If its only a small track then you need tourque rather than revs
The same motor with a different gear ratio is an option that can work well.

 

[the shrew]

QUOTE (300SLR @ 6 Dec 2011, 09:42) <{POST_SNAPBACK}>Generally a motor will produce maximum torque at zero revs, the torque decreases as the revs rise and at maximum revs the torque drops to zero.
If that's what you are saying, we agree.

Motors with a higher rev rating may or may not have more torque than motors with lower rev ratings. Quite often higher rev motors have less turns of thicker wire which gives more RPM and more torque.

Yeah i agree i ment if you had 2 of the same motor

The same motor with a different gear ratio is an option that can work well.

Yes it could but the genral rule of thumb that i always used when racing RC if the bigger faster flowing track id use a higher rev motor. For a smaller more technical lot of on off power type track then one with more tourque would be better. Obv you still have to gear them correctly. Why because on a fast flowing track your already going at a fair speed and dont need the tourque as much to pull you out of the corners.

 

[Ichabod]

Big Jim's book must be good, based on the price of it. . .the free info on howstuffworks has helped me immensely though.

So, here's my next basic question. Does a motor with a higher torque brake (dynamic braking I mean) more rapidly than one with lower torque (by which I mean slow more quickly, if you'll pardon the tortured phrasing), or is that too much of a leap in thinking?

 

[300SLR]

QUOTE (Ichabod @ 7 Dec 2011, 19:31) <{POST_SNAPBACK}>So, here's my next basic question. Does a motor with a higher torque brake (dynamic braking I mean) more rapidly than one with lower torque (by which I mean slow more quickly, if you'll pardon the tortured phrasing).
Yes in general it does.
(There are reasons why that's not precisely correct, but I guess we won't get too worried about that detail?)

QUOTE (the shrew @ 6 Dec 2011, 10:04) <{POST_SNAPBACK}>Yes it could but the genral rule of thumb that i always used when racing RC if the bigger faster flowing track id use a higher rev motor. For a smaller more technical lot of on off power type track then one with more tourque would be better. Obv you still have to gear them correctly. Why because on a fast flowing track your already going at a fair speed and dont need the tourque as much to pull you out of the corners.
With slot racing motors, higher rev motors and higher torque often go together and often it works best to use motors with both more revs and more torque on a fast flowing track than a tight one.
Although there are similarities between slot racing and RC, not everything reads across from one to the other.