R&M 3804 speed coil - I have the ohm readings but I don't know if they're good

[Noah Britt]

Hey yall I'm wondering if my phase shift windings are bad or not. What I've got is an R&M missing the motor tag, but I'm 95% sure it's a 3804 because it has the later adjustable osc wheel.

Anyway, the fan wouldn't start on its own, and I could get it started either direction by hand. The stator is good, and I had it wired up right also. It also ran on all three speeds. This led me to believe the phase shift in the speed coil was bad. (Could there be any other explanation?)

So I took the ohm measurements of the 5-tap coil, and I have them down in the wiring diagram below. The ohms between the speed taps seem good, which makes sense because the speeds worked. I have it all color coded, so for example if you want to read the ohms between taps hi and med, you'd see that they're coded green and brown, and you'd find the green to brown measurement on the left of the diagram, where it says 2.8 ohms.

What I want to know is whether the phase shift windings are good. Does anyone know what good readings would be for brown (hi) to yellow, and then yellow to red? I got 1.6 and 1.1 ohms, respectively.

 

(Note: disregard the labels "minor" and "major to motor". Those were to help myself with connecting the stator to speed coil.)

(Also note: the little numbers "1" or "2" next to the colored speed coil tap terminals indicate how many strands of magnet wire come from the coil and attach to the terminal there.)

 

Here's the coil. It doesn't look burnt to me.

 

Thanks!

[Mark Olson]

It looks burnt. And, those are a biatch to wind, because due to the #33 wire, it is damn near impossible to count the turns between taps. YMMV

[Noah Britt]

It looks a lot less burnt in person. The bare spot is where I peeled the friction tape off. What about #33 makes the turns hard to count?

[Mark Olson]

9 hours ago, Noah Britt said:

It looks a lot less burnt in person. The bare spot is where I peeled the friction tape off. What about #33 makes the turns hard to count?

First, 33 AWG is about 0.007", not too much thicker than hair.

Second, if the wire breaks, very likely, finding the end is almost impossible.

Third, there can be close to 2000 turns in that little bitty coil.

 

The turns can be counted without too much difficulty by cutting the coil, but then one has no reference to where the taps are. If one were to unspool a good, unburnt coil, then the chances of getting a good autopsy are dramatically improved, but who is going to unspool a good coil? I suppose a good estimate could be made, if there are no turn to turn shorts, by the D.C. resistance.

[Noah Britt]

Well this particular coil is 26 or 27 AWG. As long as my phase shift readings are good I won't even need to rewind it.

Does anyone have coil readings for an R&M 3804 or comparable Western Electric? Or maybe even a 3704? Or if someone has an idea whether my readings are in the ballpark that'd be great also.

[Edward Stokes]

Hi Noah,

Take a look at the thread R and M speed coil help early 1920 steel version lost tag  http://www.afcaforum.com/view_topic.php?id=61451&forum_id=1&jump_to=517832#p517832  on the old website.  That thread refers to an earlier thread that might be useful also. They contain a lot of information which may answer some of your questions.  Don't try too hard to understand them.  They jump into the middle of trying to solve a particular problem without much insight.  I don't have access to that fan at the moment and may need to refer to them if you have questions.  I have a busy week, so I may be slow responding.

The numbers from your speed coil are in the ballpark.  It might be OK, but I think you should check the stator also.  Can you post the stator's ohm readings between the 3 head wires?

Good luck.

[Noah Britt]

Unfortunately I only have the speed coil with me right now. I'm trying to make the fan work for my cousin, and a few weeks ago I rushed to get it working the day before going to visit him. I got it to the point where it would start with a push, so I left most of the fan with him so he could fix it up, and I took the speed coil home to rewind myself. That was my plan until I tested the coil extensively the other day and now I'm not so sure it's bad.

Before, when I was wiring up the fan (the headwire was cut when I got it) I used the link below and some reasoning to hook up the motor right. If I recall correctly, I think my stator readings were similar to Levi's in the post below, which are 3.8, 20.8, and 24.3 ohms. Though I seem to remember the lowest reading being a little lower than 3.8 ohms. If only I had written down the measurements for the stator! I do remember that after giving the fan a push start, it was pulling almost 1 amp, which seemed a little high.

https://www.afcaforum.com/view_topic.php?id=60304&forum_id=1&highlight=R%26amp%3BM+3804+wiring+diagram

It'll probably be Thanksgiving before I have the whole fan in hand to mess with.

 

[Edward Stokes]

Ok.  An electric fan is definitely harder to work on when it's not all in the same room.

Based on your comments above your stator is probably good.  I think you would have noticed if your numbers were much different than Levi's.  However, I think that fan should be drawing about 0.5 amps when running normally and a current of 1 amp is too high indicating there is a problem somewhere.  (For your education, your assumption in your 1st post that the stator is good because you could get it started in either direction by giving the blade a push is not correct.  That usually means the start windings are not functioning properly since their primary purpose is to get the motor started in a certain direction.  That could be due to a bad stator, bad switch, bad wires/wiring, moon particles, etc.)

 I'm also not sure if your speed coil needs rewinding since your ohm readings are in the ballpark.  However,  I am concerned that the reading of 1.6 ohms in the transformer's primary (yellow to brown on your diagram) is too low.  It should be between 3 and 3.5 according to posts from the old website.  This could help explain why the start windings in the stator aren't working properly and for the increase in current.  

I see two problems that should be investigated - the low ohm reading and the excessive current draw.  It may not be obvious how to investigate them since the design of this switch makes it diffuclt to understand how it works.  I have figured most of it out and try to work methodically rather that rely on trial, error & luck.  I would be happy to give you some guidance when you get started back up. 

Good luck, Edward.

[Noah Britt]

Thanks for your help! I guess there could've even been a bad connection somewhere like a corroded terminal or something. I also think I noticed the blade move just a hair in the right direction one of the times I plugged it in.

Just curious, but if my fan has a good start winding in the motor paired with a bad phase shift in the speed coil, would the stator allow the blade to be hand-started in either direction?

[Edward Stokes]

Noah, to answer your question: yes, there are situations where your fan could be hand-started in either direction if the stator is OK but the switch assembly is bad.  For example, that might be what's happening when I mentioned in my post above that one of the your coils has a low ohm reading.  It might be partially shorted out and everything else is OK.  You are also correct in that the problem could be caused by a bad connection, corroded terminal, or something else.  I just hope it's easy to figure out.  It might not be so easy if there are multiple problems or something is borderline operational, like your switch coil might be.

You mentioned above that you may have noticed the blade move on it's own in the right direction when you plugged the fan in.  That could be important.  If that can reproduced then there's a weak current flowing through the start windings in the motor.  If you can't reproduce it then try hand-starting the fan in both directions and see if you can determine if it's "easier" to start in one of the directions and/or it picks up speed faster.  That too can help determine if there's some current flowing.  If it's easier to start in clockwise (correct) direction then look for loose connections, corroded terminals and things like that.  Or, If you think it's trying to run in the counterclockwise direction then you probably have a wiring error to look for.  

Finally, you need to be aware you're woking with a fan that was developed in the late 19-teens with a radically different design that made it more efficient to run, able to run at much lower speeds and has more starting torque, which helps prevent "start coil burnout".  The motor still has start (auxiliary) windings, but the choke coil in the switch that helped with the phase shift has been replaced by an autotransformer.  Don't get frustrated or fall into any traps if the switch doesn't work the way you think it should.  This also affects the way we discuss the fan.  Talking about a pahse-shift or choke winding is not correct and can be confusing since it doesn't have one.  It may also be confusing to you when I talk about the primary (the line side) and secondary (the motor side) of the transformer, so please ask questions.

Good luck, Edward.

[Noah Britt]

Thanks so much for your help and advice!

I have not dissected a speed coil with an autotransformer, so I don't know how that would look. I assume there are four coils between the five taps, all wound the same direction and in series, right? Based on the position of the wires coming out of the coil and the number of strands of magnet wire on each tap (1 or 2), I was thinking the low speed (purple) tap is the start of the coil on the outside, then med (green), hi (brown), followed by the yellow tap, and lastly the red tap. (Starting from the outside of the coil and working to the inside.) Please correct me if I'm wrong on this. 

I also do not know how an autotransformer works. (I was having enough trouble trying to think of it as a phase shift winding!) Hopefully that knowledge is not necessary for whatever I have to get into.

[Edward Stokes]

Noah, your transformer is wound using one piece of wire and all segments between taps are wound in the same direction.  Based on how it looks in the above post, I think your assumption the transformer's secondary winding (red) was wound first, looped at yellow and continued to finish the purple lead at the top.  (It could have been wound "inside-out" putting the secondary winding on top, but it's always best to follow the original design.  I don't have one to play with at the moment, so I'll trust your guess.)

You don't need to know anything about how autotransformers work, just  that they don't work the same way the previous generation of choke (aka, phase shift) coils did.  Unexpected results you get during testing might be due to these differences and not that something is wrong.  But you do need to know enough so we can talk about them.  Your switch has 4 coils, which one is the "phase shift" coil?  None of them, it doesn't have one per se.  (Most of the phase shift is caused by the windings in the motor.). So, when you say "This led me to believe the phase shift in the speed coil was bad" I don't know which one you're talking about, and when I mention the primary and secondary windings of the (auto)transformer you probably don't know what I'm talking about.  Let's not turn your fan into an antique flash bulb, they're not worth much.  Just be aware, cautious and ask questions.

Good luck & have fun! -Edward

[Noah Britt]

I had assumed that either the red/yellow coil (1.1 ohms) or yellow/brown (1.6 ohms) coil (or both) caused the phase shift. You are correct; I don't know what you mean by primary and secondary windings.

[Noah Britt]

On 8/31/2022 at 4:52 PM, Edward Stokes said:

However,  I am concerned that the reading of 1.6 ohms in the transformer's primary (yellow to brown on your diagram) is too low.  It should be between 3 and 3.5 according to posts from the old website.  This could help explain why the start windings in the stator aren't working properly and for the increase in current.  

 

Well I think I'm on track to fixing the problem! I decided to go ahead and rewind the speed coil in order to rule that out of my possible problems. As I unwound the coil, I found that two turns which were 68 turns apart had become melted together at some point, shorting out 68 turns of the yellow-to-brown segment of the coil. These 68 turns are about .8 ohms of resistance, which fits into your thinking that the yellow-to-brown coil was suspiciously low in resistance. 

[Edward Stokes]

Congratulations!  I think you've found the problem and you're on the way to getting the fan running just in time to cool down the turkey when it comes out of the oven on Thanksgiving!  Thanks for the additional information.

A relatively large number of members have reported seeing this particular coil being "burnt".  If you haven't started your rewind yet then you may consider using the thickest wire that will fit into the magnetic core laminates.  In the link cited above (R and M speed coil help early 1920 steel version lost tag) Chris Heinis says he could barely get 25 gauge wire to fit.  If the wire used by R&M was 26 gauge then using a 25 gauge wire with similar properties would result in about 20% less heat being generated, which might extend the life of the coil,  This is a guess and this theory would have to be proven in practice, but I do think it would improve what I believe was a design flaw. 

Please keep us informed of your progress and good luck,  -Edward

Edited November 14, 2022 by Edward Stokes

[Noah Britt]

Good news! Me and my cousin stuck my rewound coil into the fan over Thanksgiving and it works great! Unfortunately I was pressed for time to get the coil done before Thanksgiving, so I went with 26 gauge just to be safe. Had I used 25 AWG and it not fit, I wouldn't have had time to redo it.

Now for the stats. I didn't write down the stator readings, but I remember they were around 4, 20, and 24 ohms.

Stupid me forgot to measure the resistance between all of the taps on the new coil to compare with the burnt one, but I did measure the two readings between the three speed taps and they were about the same as the old coil. However, neglecting the fact that the turns are wider towards the outside of the coil, I can use the ratio of the number of turns in a section of the coil to the resistance of that section to estimate the resistance of the two sections I didn't measure. Doing this, I get: brown-to-yellow, 4.3 ohms; yellow-to-red, 1.2 ohms. I realize this is probably a pretty rough estimate due to the variation in the turn sizes, but compared to the old measurements of these sections (1.6 and 1.1 ohms, respectively), this is further evidence of the brown-to-yellow section (and only that section) being bad on the old coil.

The amp draw after:

Hi .79

Med .70

Lo .67

 

Thanks so much again for all your help! My cousin is really happy with the fan.