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Discussion Starter · #1 ·
Hi,

I have just suffered a 7005 chip failure.

The offending cars motor is now drawing over 600mA disconnected from the axle, and at least 500 when warmed up (using a 12v source), whilst three similar motors only show around 100mA.

The only change I have made since using the car successfully is presumed over oiling. Is this a likely cause please?

Needless to say I am keen to establish if this is the problem before I re-chip it.

Thanks.

Keith
 

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Before the motor gurus stop by...

If you have a digital meter that reads down to 1ohm... I would suggest...

With any faulty digital chips removed measure the resistance across the motor terminals (with no power applied!).

Use the car wheels to rotate the pinion by a few degrees at a time. When stationary, wait a second or so for the reading to stabilise and then look to see if the reading is in thr region of a few ohms e.g. 4-5 ohms for a scalextric 18k mabuchi. The reading will transition up (or down) during movement of the pinion due to induced EMF (and direction of rotation) hence the need to measure when stationary.

If you see significantly lower readings at some or all points of rotation suspect a short across the commutator or a shorted winding.

Perhaps the oiling exercise moved some conductive debris onto the commutator?

c
 

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Rich Dumas
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C8197, what is that? When it is free running a typical 1/32nd motor will pull 0.1-0.25 amps at 12 volts. I believe that your motor has a partial short across the commutator. Ohm readings taken across the motor terminals are often not very useful, you really need to measure across all three commutator segments, which means taking the motor apart. If there are holes in the motor case or in the endbell first try spraying contact cleaner on to the commutator with the motor running. Often that will remove dust from the slots in the commutator that cause the short. If that works the amp draw of the motor will drop. Avoid getting oil on the commutator, that will greatly increase the chances that dust will accumulate in the commutator slots.
 

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Discussion Starter · #4 ·
Thank you. This is fascinating.

I have done as you say.

The readings are in the region of 7 to 15 ohms 'all around', except for at one point in the rotation cycle where it reaches around 70 ohms! This is repeatable every time I try it.

I have tested several similar motors with more or less the same results - excepting the 'spike'.

With the apparent anomaly manifesting itself with respect to both impedance and current draw 'one' (that's me ) draws the conclusion that something is / must surely be wrong now with this motor.

I would be very interested to learn exactly what's amiss, and the likely cause - further comments welcome.

Whatever, I don't think I will risk using it with another chip.

Cheers.

Keith
 

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Discussion Starter · #5 ·
C8197, what is that? When it is free running a typical 1/32nd motor will pull 0.1-0.25 amps at 12 volts. I believe that your motor has a partial short across the commutator. Ohm readings taken across the motor terminals are often not very useful, you really need to measure across all three commutator segments, which means taking the motor apart. If there are holes in the motor case or in the endbell first try spraying contact cleaner on to the commutator with the motor running. Often that will remove dust from the slots in the commutator that cause the short. If that works the amp draw of the motor will drop. Avoid getting oil on the commutator, that will greatly increase the chances that dust will accumulate in the commutator slots.
Rich.

The C8197 is the standard Scalextric Mabuchi motor - re. Scalextric and Google.

I 'lubricated' the motor by spraying the ends and through the holes with - ugh hum... Inox .. as I couldn't be bothered to go out to the shed to get some 3in1.

The current draw of all my other similar motors match your spec. Thanks for clarifying that. I meant of course 0.600A and 0.1A.

I have clearly, well and truly k nackered the 'greedy' one!
 

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Discussion Starter · #6 ·
[quote name="slotterkeith" post="2378413" timestamp="1599826416"]Hi,
I have just suffered a 7005 chip failure.
The offending cars motor is now drawing over 600mA disconnected from the axle, and at least 500 when warmed up (using a 12v source), whilst three similar motors only show around 100mA.

Excuse me, I meant of course 0.600A, 0.500A and 0.100A .. time for an afternoon nap!
 

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Digital Guru
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I don't think the oiling has caused the issue, easy way to check is to flush the motor out with some contact cleaner, just spray it into the motor and let it dry and recheck using the above method..

Sounds like you have a duff pole on the motor, when this happens it just needs the motor to stop on the duff pole and when you pull the trigger the smoke gets released.

If you test it in analogue mode the car will just not move if you happen to land on the offending pole..

After all the upgrades that have been made to digital the weakest link in the chain is now the motor, if someone would release a ball raced quality motor that didn't rev above 13,000 I'm sure it would sell, would also need quality suppression..
 

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Just to check, does the meter read less than 1Ohm with the proble leads connected together? If so, the 7-15 Ohm readings sound valid - but that is too high for a standard mabuchi... new decoder and new motor probably the best solution.

c
 

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And if you want to be really careful with your new cars/motor then run them in Analogue form for a while before placing the chip in, it's very rare occurrence to get a motor with a duff pole, I think out of about 200 ish cars I've seen 1 with a duff pole that fried a chip...
 

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Rich Dumas
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Oiling the motor can contribute to the problem when oil gets on the commutator because the odds that dust will get caught in the commutator slots is greatly increased. As much as I like Inox I would not use it to lubricate a motor and I would never spray any lubricant inside of a motor.

When you take ohm readings be aware that you will always be reading all three poles because the poles are connected in a delta series configuration. If the motor had an open pole they would no longer be in series. If a motor draws too much current there are several possible causes. The armature shaft bearings could be misaligned causing excess friction. The brush spring tension could be excessive. A foreign object, like a body screw, could have been ingested. A motor winding may be shorted or, as I suggested the commutator could be shorted. If the commutator is shorted most of the current will not be going through the windings and the commutator can get very hot if you have a big power supply. I just checked the armature of a good Scalextric motor and it read 4.9 ohms across all three poles. If it is working properly a digital ohm meter will read 0.1 ohm with the leads shorted.

Because shorted commutators are a common problem I recommend using spray contact cleaner on a regular basis, especially with digital cars.
 

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Discussion Starter · #12 ·
Just to check, does the meter read less than 1Ohm with the proble leads connected together? If so, the 7-15 Ohm readings sound valid - but that is too high for a standard mabuchi... new decoder and new motor probably the best solution.
c
Hi.
Please excuse my delayed reply. I felt I needed a slight break before re-posting to take everything (several posts) in. The impedance between the probes measures only 1.54Ohms. I have 'binned' the motor. The decoder joins three others, hopefully destined for the attention for our 'in house' forum chip- restorer!
Kind regards and a great deal learned from everyone once again.
Keith
 

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Rich Dumas
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Before I posted my last reply I checked several Scalextric motors and one of those had a short across one commutator slot. I race my cars and the rules call for unopened motors, so if the spray contact cleaner does not work I have to toss the motor. In that case the slots were very narrow and the dust was not visible. To be certain that the short was not across the windings I cleaned out the slots with a scalpel and after that the ohm readings were normal.
 

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Discussion Starter · #14 ·
Before I posted my last reply I checked several Scalextric motors and one of those had a short across one commutator slot. I race my cars and the rules call for unopened motors, so if the spray contact cleaner does not work I have to toss the motor. In that case the slots were very narrow and the dust was not visible. To be certain that the short was not across the windings I cleaned out the slots with a scalpel and after that the ohm readings were normal.
I still have the offending motor and will try soaking the whole thing in meths (denatured alcohol) as I don't have any carb spray. I will then try blasting it out with the air compressor. Better than that, I may put the engine and the meths in a small, sealed container and bung the whole thing in the ultrasonic cleaner - to hopefully fully release offending detritus.
 
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