Problems with class 4f Fowler

[Moonglum]

Right.  Now I think I am really confused.  From your two answers you seem to be saying that if there is a zero reading that is a short circuit (bad).

Now my meter displays a 1 on the screen when a setting is selected.  When I put the probes in that number does not change on the screen with any combination for the Fowler.  I have tried the same tests on 2 other locomotives with the same results apart from when the probes are in 1 and 5.  Then I get readings.  The meter is set at 20 and the readings are around 40 on one loco and around 55 on the other (should I expect them to be different on different locomotives?) However all other combinations do not give a reading which indicates short circuits, yet both those locomotives run.

Now before you write me off as a complete idiot, my brain is wired for words, music, history etc.  As soon as maths or physics is encountered my brain 'short circuits'.

[Chrissaf]

However all other combinations do not give a reading which indicates short circuits, yet both those locomotives run.

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The 1 display is confusing you....read this post which clarifies your observations.

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Before you apply the probes to any part of the loco do this.

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With the probes in your hands NOT TOUCHING ANYTHING you say you get a numerical value 1 on the display. If it is assumed that that you have got the switch setting set correctly and the probes plugged into the correct holes on the meter, then for your particular meter the value 1 is the equivalent of my example OL or --- on the display. In other words 'Infinity Resistance' which is also termed an 'Open Circuit'.

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Now touch the probes together (still keeping them well away from the loco). You should now get a zero numerical reading (or close to zero). This is the indication of a 'short circuit' or zero resistance.

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Here are two of my many meters. With the probes not touching (probes are 'open circuit' infinity resistance). The meter on the left displays 1 whilst the meter on the right displays OL. Both of these displays are reading the same 'open circuit' infinity resistance value.

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/media/tinymce_upload/3b1cf29bcd3fc0d97c75171160caf3d0.JPG

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With the probes touching (loosely because I needed my hands free to hold and operate the camera), the meter on the left (2,000 ohm range) now reads 0.002 ohms whilst the auto-ranging meter on the right now reads 0.19 ohms. For all intent and purpose a 'short circuit' reading on both meters.

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/media/tinymce_upload/42fdcc1c490f4d82454460d034b9495c.JPG

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The 40 & 55 ohm readings across Pins 1 & 5 that you have observed are the motor windings. I thought that they might have been a little higher, but although a little on the low side they are more than likely correct for your particular locos.

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So in conclusion, your 1 meter readings are not 'short circuits' they are 'open circuits'.

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[Moonglum]

Thank you that lifts some of the mist.  So I am getting the right results mainly but I should be expecting readings from 1 and 5 combined.  I am assuming it is not a short circuit there because it is an open circuit.  As the readings should be for the motor windings (whatever that is) does that indicate that the motor is the problem?

[Chrissaf]

The 40 & 55 ohm reading between Pins 1 and Pin 5 is probably correct. In other words, the motor is not the problem. I know you said that Physics was a mystical artform, but I did not think that I would have to explain absolutely everything from 'first principles'.

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An electric motor consists of a magnet in which a coil of wire rotates. When you physically rotate the coil inside the magnet a current is generated in the coil.....that is called a Dynamo (like you used to have on old 1960's cars to charge the battery). However, if you pass a current through the coil instead, the opposite happens and the electrical current creates a mechanical force that rotates the coil inside the magnet. This is called a 'motor'. The wire that is used to make the coil is called a winding. Because the wire is wound around a former.

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Wire has resistance, and there is a lot of wire (because it is very thin) in the motor windings. It is the resistance of the wire in the windings inside the motor that you are measuring with your 40 & 55 ohm values.

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This online animation demostrates the principle of my wordy description above.

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Each coil (loop) shown in the animation is a 'winding'. I suggest only watching the first 3 minutes, as after 3 minutes the animation starts getting more in depth.

[Moonglum]

The two motors that give the 40 and 55 ohms readings both work.  I tried the meter readings on them to try and understand the differences between them and the dead Fowler.  That has no readings on 1 and 5 but shows the 1 on display.  That's why I asked if it could be the motor at fault because there didn't seem to be any shorts at the pin end.

I can see that you are frustrated with me but, just think, if you were to suddenly look into my specialities, history, anatomy and sports injury, music I may well have to explain,  what are to me, very easy things.  I am totally new to railway modelling and have never had any interest in electrics.  I don't understand them but I am trying to. 

[walkingthedog]

Trouble is that people who have a deep knowledge about a subject sometimes don't seem to understand that others might have no clue at all, absolutely nothing. Keep asking if you don't understand. You are not alone. I have a fairly good knowledge of most things electric but some of the stuff Chris discusses leaves me amazed. Very clever man. 

[Chrissaf]

I can see that you are frustrated with me.....

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Not at all.....if you knew my question answering posting history you would know that I have the patience of a saint. If I was frustrated, I would have stopped trying to answer your questions long ago. Knowing what your limits are, makes it easier for me to pitch my answers at the right level.....hopefully at a level that you can understand.

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Part of my working career in IT was training others....... of all abilities. My biggest issue is trying to convert what you write into a form that makes sense to me. I sometimes misunderstand what it is you are trying to explain and get the wrong end of the stick. Below is an example, as I was under the impression that all your tests were focusing purely on two locos....the ones with the 40 & 55 ohm motor readings. But it now appears that there is a third one that actually has the fault that you are trying to diagnose.

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That has no readings on 1 and 5 but shows the 1 on display.  That's why I asked if it could be the motor at fault.....

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If the comment above relates to a third loco....that is to say not the locos that have the 40 & 55 ohm readings. Then yes....for that specific loco the 1 on the meter display is an 'Open Circuit' and there is something wrong between the location where you are applying the meter probes and the motor OR the motor itself.

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Try placing the meter probes directly on the motor itself where the wires are soldered and see if you still get a 1 display. If you do, then it looks like a motor fault. A motor fault could either be the carbon brushes not contacting with the rotating armature commutator (refer back to the animation I gave the link for) OR a broken or burnt out motor winding.

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Another thing you can do...is source a 9 volt battery (the type with the press stud terminals). Attach a couple of wires to the battery and then touch the other (non battery end) of the wires directly onto the motor terminals. If the motor is good and the battery is a brand new fully charged one, then the motor should spin. It doesn't matter which way round the wires go on the motor. Reversing the wires should just make the motor spin in the other direction.

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If the 'connect direct' to motor tests work, then the open circuit is somewhere between the 8 pin socket and the motor.

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[Moonglum]

Thank you.  I have tried the probes and the 9 volt battery on the connections to the motor (what a game getting that body off).  Absolutely no response to either so it looks like a dead motor.

[Chrissaf]

You'll now know what to do next time with regard the use of a meter. Diagnostic testing is really just a case of applying logic to the problem and taking a methodical 'step by step' testing approach that sooner or later will hone in on the cause of the issue.

[Fishmanoz]

Murphy has been hard at work on this little problem.  First he gives you a fault that is hard to diagnose without the equipment you don't have, then the equipment results are confusing when you are using it for the first time and not too sure whether you are using it correctly.   He seems even to have stopped our terribly logical helpers from saying clearly and concisely at the start that all your pin-to-pin readings should all be open circuit except across motor terminal pins 1 and 5.

Glad you've sorted it though.

 

 

PS.  To the com mods, here's another instance where my original post had 2 carriage returns between first and second paras at first, then I edited it to put a third and then again with a fourth. Each time the forum software stripped the lot leaving the 2nd para immediately below the first.  I'm interested to see what it now does with this 3rd para PS as I've given it 4 returns too.  And lo and behold, this time it's left at least 2 in while still stripping all between 1 and 2.  Explain that!!

[Moonglum]

Murphy and I are great mates.  He likes my company.  Anyway I have learnt a few more things, one being I've got another duff loco.  I did replace a motor in the Flying Scotsman (not bad for a novice that was quaking about attempting it) but this one looks very different and a lot more complicated.