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Discussion Starter · #1 ·
I sat and watched my 1991 Le Mans vid' again last night - smiling at the demise of Schumi's Merc - and then got out my various 787Bs to give 'em a fettle and drop 'em in me box for Tuesday's sojourn at the track.

So my question to you is; what is the difference between the black can motor and the silver? Or is it as with the wrapper and no wrapper NC-1s, i.e. no difference at all.
 

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Pretty sure we were talking about this on another thread somewhere else the other day. Some say there is a difference and the early motor is quicker?
 

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Discussion Starter · #3 ·
Ah, but which is the earlier can?
 

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Discussion Starter · #5 ·
Fighting the urge to delete your post, Fergy!
 

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Hi

I stayed out of this because when the SRSs were new, they were uncommon in the US. I have a few, some picked up new, some used, some recently when Rad Trax had them really cheap.
They came with a variety of motors. ALL are FT130s much like the current SCX RX 41 and Turbo. SOME had shorter prop shafts as the mount was closer to the rear axle. Some had chrome cans, some had black. Inintially, I THOUGHT the black cans had the hotter arm, but with later examples found out this was not true!

Then I aquired a chrome can with the hot arm.

OK, SOME of the arms from the SRS period are identical to the current RX41 in wind. SOME of the arms are similar to the current turbo. and SOME are about 25% hotter still. About the only way I can actually tell is to use a ohm meter!

Fate
 

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Discussion Starter · #8 ·


Er, thanks for that, Prof.

Gee, sometimes I wish I'd never asked the question - the answers can be complicated and you can set yourself up royally.

Okay, so the thing to do is either: get a meter to measure...er,... something; or look out for cans with fatter windings? Have I got that last bit right, or will it not be that obvious to a simpleton such as I?
 

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Not to confuse things more but I believe there were also variations in the armature stack length that changed the performance also. Somewhere on my hard disk I had a msg from a guy in Spain that had a pretty good explanation on different motors that came out of SCX during different times and different owners. If memory serves correct it was a real mess. Maybe I'll find it someday.

Jimmy
 

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QUOTE Not to confuse things more but...

Ya, thanks, Jim!


Sheesh, it couldn't just be: the fast one is silver and the faster one is black, could it.

Thanks to all the usual names up above for chipping in.

I've decided I'm going to take my four 787s, put the same tyres on 'em, take 'em down to the track and let the clock decide which is the faster. Then I'm going to paint that motor pink!
 

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Hi

Basic tool: "Volt/Ohm" meter. Your local electronics place will sell one that is digital, reads accurately to tenths of ohm for under 15pounds. Folds up and fits in your pocket.

Basic tool? Well, when looking at these told SCX motors, for instnace, you pull the brushes and springs and shove the probes in the holes and read the meter. the lower the number the faster the arm.

Meter the arms. If they slow suddenly, you can read the arm. If the arm hasnt changed, then you have something mechanical in the motor to deal with but usually fixable.
Kinda your choice. The cheap Scaley S cans are "throw aways". They slow, dump em. The SCX style motor is easily rebuildable. That means a good one can be massauged to run for decades.

Fate
 

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Discussion Starter · #14 ·
QUOTE Your local electronics place will sell one that is digital,...

Ah, gotcha! Nice try. Get me to buy something digital indeed!


Actually, I could be tempted to get one of these meters and have a play- could tack it on to the order for some conducting paint I'm about to place.

Only thing is I'm not sure I understand your post, Prof. Let me see if I have you right here: pull the springs and bushes, fine, then let the probes brush on the commutator?

QUOTE Meter the arms. If they slow suddenly, you can read the arm.

Qué?
 

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Brian Ferguson
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Wankel, what you are trying to do is measure the resistance of the armature coils. This can help you determine the relative potential of one armature to another and can also help detect faults in a previously good arm. All readings are done in "static" form - no power applied. Care should be used to avoid damaging the comm surface with the probe tips. Keep a record of the ohm readings for each motor or armature.

On a motor that you can remove the brushes and springs from, you can do this without further dissassembly. With the meter set to read "ohms", insert the probes into the brush holes to contact two of the three commutator plates. The meter will read the resistance of one pole of the arm in parallel with the other two poles.

Obviously, it is even easier to do this with an arm that has been removed entirely from a motor.

The number you get is a handy way to compare armatures. In theory, the lower the ohm reading, the greater the current that the arm will draw and thus the faster it will be. However, an extremely low reading can indicate a short within the armature.

If you know an armature's reading, and it subsequently slows down when racing, a further check with the meter can tell you if the arm has gone bad or if there may be another reason for the loss of performance.

Meter readings should normally be done with the armature at room temperature - this will make comparative readings more valid as temperature will affect the resistance. On a motor that slows when it gets hot however, a warm reading may help you determine if there is a short that only occurs when the arm is hot - the resistance should increase on a hot armature, but if the reading gets lower you know you have a short.

Does any of this help... or have I compounded the confusion...
 

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Discussion Starter · #16 ·
Say, hand me that spade, I need to dig in deeper here!

Okay, so if I get the little reader fellow I'll make sure I ain't trying to stick the probes in whilst giving the motor full chat.


Thanks Fergy.

QUOTE ...a further check with the meter can tell you if the arm has gone bad...

How day do dat den? Gone bad? Broken wires?
 

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Brian Ferguson
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QUOTE Gone bad? Broken wires?

A broken wire will produce noticeably higher readings. Instead of reading the ohms of one pole in parallel with two others, you will be reading the resistance of just one pole or just two others - in either case the reading will be higher than when the arm was healthy. But a broken wire will usually result in a motor that is very slow and won't always self-start - it is running on just "2 cylinders" instead of 3. This situation is usually quite obvious.

If the arm has developed an internal short, the resistance will be lower than it was previously - often very noticeably so. A very slightly lower reading might indicate a short in the winds nearer to one of the comm leads and can often be ignored - though further degradation is likely to occur. In fact, any short is likely to produce more problems because the short usually involves wire-to-stack contact and thus the short can affect windings on other poles. A shorted winding develops less magnetic field and thus the motor performance will suffer. When an arm develops a short, it is usually only a question of time until total failure occurs.

The big benefits in metering armatures are: comparison between arms for evaluation purposes, checking the health of an arm for which you already have a reading, and, as described below, checking "electrical balance" of an arm.

To properly "meter" an armature, you should take readings in 3 orientations to determine the "electrical balance" of the arm. Assuming we call the 3 poles "A", "B", and "C", you should measure and record the readings with the probes on "A" and "B", "A" and "C", and "B" and "C". The best armature will have not only a low average reading but should also produce 3 nearly identical readings. A low average won't indicate a great arm if the 3 readings vary widely. On the other hand, an armature with a higher average, but 3 equal readings will often be a better performer. Electrical balance is just as important as physical balance.

Good grief... I'm writing a text book...
Stop me!
 

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I'm not going to stop you Fergy!!
This is all quite interesting, so keep the science coming. Whether I'll actually get around to measuring the armatures in my motors or not is another kettle of fish entirely


Mark.
 

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If anyone finds a black can with a green wind, they are rubbish, send it to me...
Email me for my address.

Seiously though, they rule, fastest can of that type I believe. You can still send them to me though...

Never really understood the whole reisitance thing. I just temporarily stick it into the car, if it's good, it's glued permanently, if it's bad then change it. Because in my experience, you may have the best motor in the world but it could be crap on the track. Conversly, I have had a motor that by all scientific methods has been proven to be crap. Though it's still the fastest thing at our track. I can't explain it and I don't think my brain could take an explaination for it. All I know is I thinks it's best to try it.

However, if you have as many motors as Fate and Fergy seem to have, I'd come up with any idea to same all that effort! lol
 

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Brian Ferguson
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More motor theory.... skip if bored!
Okay then.... one person is still reading...


The entire reason our little motors spin is because the windings on the armature produce a magnetic field when energized with power and they then rotate in an attempt to align with, or repel from, the stationary magnets in the motor can. But as the armature rotates, the motor brushes deliver power to different segments on the commutator and thus different windings (poles) on the armature become charged, and the motor continues to spin. Yes, it's more complex than that, but you get the idea and it suffices for the purposes of this discussion.

Obviously then, we want the armature coils to be energized to the maximum level possible. Increasing the voltage will do this, but we usually don't have that option and, if the voltage is raised too high... well, smoke replaces RPM! At a given voltage, the maximum field strength of an armature coil is determined by two things - the amount of current (amps) passing through the coil, and the number of windings (or more accurately, the amount of wire) in the coil. We can't measure the latter on a factory wind, but we CAN determine the amp carrying capacity of an armature coil. We do this by measuring electrical resistance, since the amp draw is directly proportional to the resistance.

As a general rule of thumb when comparing factory winds on non-exotic motors, the lower the resistance, the faster the armature. As mentioned before, you can measure the resistance of the armature windings with a VOM (Volt/Ohm Meter), by touching the probes to the commutator segments with the meter set to read Ohms. Because an armature has three poles (coils), you are actually reading the resistance of one pole in parallel with the resistance of the other two. This will give a reading that is lower than the resistance of a single pole. For example, if each pole on an armature has a resistance of 10 ohms, the meter will tell you that the resistance is 6.67 ohms. There is a formula for this, but breathe a sigh of relief because I'm not going to mention it here! However, no two poles on an arm will ever have exactly the same resistance and it DOES matter that you meter the arm in all three possible combinations - probes on poles A & B, A & C, and B & C. The readings will differ. For example (and these numbers are not necessarily representative of any 1/32 arm), if pole A is 9.8 ohms, pole B is 10 ohms, and pole C is 10.1 ohms, you will get three readings that range from about 6.56 to 6.66, with an average somewhere around 6.60. Another arm may produce a lower average (say, 6.57), but have much more inconsistency on individual readings (say, a range of 6.41 to 6.78). The first arm may well be the faster one because it has better electrical balance even though its average pole resistance is marginally higher. It will draw more consistent current and produce nearly identical magnetic fields as it revolves, unlike the second arm which will be causing high-frequency variations in its power demand and in its magnetic field strength - this is akin to an armature with a bad physical balance (poles which physically weigh more than other poles).

Will metering tell you everything? Absolutely not! Sometimes an arm that meters poorly will run strong. But this is usually indicative that other factors are ideal - dynamic armature balance, commutator concentricity, shaft straightness, etc. And attempting to compare arms from different motor designs is usually pointless - for example, arms with different stack lengths, varying commutator advance, magnet strengths, etc.

Arm metering is merely one more tool that a serious racer can use in an attempt to perfect his craft. It's also one of the few tools that we can all utilize. It doesn't eliminate every case of "try it and see" but it can help you to understand why some motors are better performers, and aid in spotting armatures that "should" be good ones.

And, don't forget, it can also help you spot arms that have gone sour - dead shorts, broken winds, etc. - or determine that the arm in a "bad" motor is okay and that other factors are involved (brushes, springs, etc.). And if you have prior data, you can also spot arms that have permanent heat damage too.

It's nothing magical - just another tool that you can choose to use or lose! Wow... almost poetic...


And now.... I promise.... no more blathering! Well... not today anyway.....
 
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