Can you swap electric motor thermal protection devices?

[Rykk]

Hi, all! Ok, here's my problem. I have an electric motor - a Dayton 10J152, 1/7hp - that has a one-shot thermal protection fuse in it. I hate these! So, I took an automatic reset one from another Dayton motor I have (both motors rated at 40deg C) and swapped it in place of the one-shot thermistor. I did this on a previous motor of the same model, using this same auto reset thermistor and it worked great.

With this motor, I took a little more time and ran it on my bench for awhile. It seemed to me that (1) the motor was a little louder than I'd expected and (2) it gets a lot hotter to the touch (not near the bearings but on the side) than I think it ought to be. Especially since it's under no load whatsoever. I fixed the noise by tightening the screws holding the motor together so that they were completely and evenly seated.

Am I just freaking over nothing or have I created some kind of problem by doing this? I never ran the motor with its original thermal protector, so I don't know if it was always noisy and just runs this hot normally - or is defective. It gets hot enough that it's pretty darn uncomfortable within a few minutes. Or do these auto fuses/thermistors have a certain "polarity" and maybe I have to swap the wires on it? Both the one shot and the auto have 2 contacts. On the one-shot, the ends of the fuse body were different, almost like an axial electrolytic capacitor which has a positive and negative lead. Or might it me the "B" value/curve of this this auto fuse is such that the insertion loss is significantly higher or lower than the design of the motor allows?

I'm pretty sure that the motor shouldn't get so hot unloaded. That said, I did this to the 1st, exact same model, motor because it was overheating in the hot ambient the thing has to work in in the first place and didn't want to swap one-shots all the time so maybe it's just the way these "stack" motors are? This is the same auto fuse I used in the 1st motor. It burned out some windings after about 5 months of almost continuous use, albeit in a somewhat overloaded and dusty configuration.
Thanks,
Rick

 

[dArsonval]

How are you utilizing the motor or what is its application Rick?

The thermal protective device in the motor winding has nothing to do with the motor overheating.
According to the Grainger website, a Dayton 10J152 is a shaded pole "Air Over Motor". It's a fan motor.
It requires a fan mounted on the shaft to provide adequate cooling of the device.
Hence, running the motor without proper airflow can cause it to run hotter.

John

 

[jraef]

Honestly, I have NEVER heard of a "one shot" thermistor fuse. Motor thermistors in my world are resistive transducers typically used with an external device measuring the circuit voltage across them so that you can use an external controller to shut it down. But mico motors are not my thing, so I suppose that if you put a PTC thermistor in series with a very very small motor, it would increase the circuit resistance to the point where the motor would stop running, or in the case of a Shaded Pole motor it might actually work to slow it down by reducing voltage to the winding, thus reducing torque. In a centrifugal fan then (the intended use), slowing down the fan will reduce the load on it by the cube of the speed reduction, aleviating the overload. Ive never seen this, its strictly conjecture here, but in my mind plasible. But it sould then cool down and allow it to run again once the overload condition is removed. Small Shaded Pole motors like that are often inherently resistant to being overloaded because even if the rotor is locked, the current draw is not enough to damage the windings. But long term overloading will eventually cook insulation on anything, so designing a motor that inherently limits it's own overloading would be one way to protect it.

Given this possibility, I'd say that the problems you are having are most likely DIRECTLY related to the ambient heat; this motor is not the right type for this, unless you plan on replacing it often. The thermistor is not INTENDED to be sacraficial, it is just working out that way because even though it slows the motor down, which was SUPPOSED to take care of overloading, the ambient temperature remains too high and the current continues to be higher than design. Eventually, the weakest link fails, and that happens to be the thermistor.

You seem to have tried to circumvent the situation by replacing a PTC thermistor with a Klixon, but it may be that the motor was DESIGNED to have a certain amount of resistance in it from the thermistor, so by removing it the motor winding is getting too much voltage now and is over excited, making it vibrate more, and by the way, it will now run even HOTTER! My prediction is now a more rapid demise.

"Will work for (the memory of) salami"

 

[Rykk]

Ahhh! So, I take it that these auto-reset thermal fuses - or "Klixons" - are VERY different from the one shots in that they might not have any, or negligible, series resistance? THAT really changes things. I wish like heck I could find the original TP and measure its resistance - at least at ambient. I could then heat it and get an idea of its curve and add a resistor in series with it that is somewhere near the top 1/3 temp end of its curve. I *think* it's called the "b curve" but might not remember the name correctly, just its function. Might that be an option?

When the first 1/7hp tripped its one-shot after only 30 minutes, I added a high cfm Boxer fan to the chassis to cool the motor windings along with installing the klixon from a 1/15hp model of the same make (Dayton) and the same temp rating (40deg C). I live in Florida where it's brutally hot day and night for 335 days a year, so ambient is always high. 40C is only 104F, so with it 95+ degrees in the shade, that leaves very little margin.

The 1/7hp is the strongest I can fit into the space on this machine without very major redesign and modification. The motor is indeed turning a significantly higher load than a fan blade. It is on a vibrating lapping machine that I've modified to have a quasi-reciprocal motion using an offset shaft. It has a pulley that spins another one attached to the offset shaft. That, in turn, is inserted into a 4-bolt bearing on the bottom of a flat bottomed steel pan that is fastened atop three 2.25"x1.5" rubber shock mounts. Turning by hand, the load doesn't seem very heavy but certainly more than a fan blade. Might I lower the perceived load on the motor by using larger diameter pulleys, with the load pulley a bit smaller? I need to speed up the vibration somewhat anyway.
Thanks a TON!
Rick

 

[ScottyUK]

Is it definitely a thermistor or could it be a thermal fuse? The latter are common and cheap one-shot devices, but they're not the same as a polyswitch or similar device which are the self-resetting variety.

 

[Compositepro]

small fan motors often have a thermal cut-off (thermal fuse) in the windings to prevent the motor from burning-up if something stalls the motor. A stalled fan motor doesn't draw much more current than a fully loaded fan but has no cooling, so over-current protection does not work.

 

[davidbeach]

If, as dArsonval says, this is a shaded pole motor then you have to accept that they're sort of good for one thing and one thing only. That one thing is powering a fan that blows air across the motor. Nothing more. If you're trying to use a shaded pole motor for a vibrating lapping machine you'll never be satisfied with the performance/life of the motor. Find a single phase induction motor or a universal motor that will be satisfactory. They may be larger, and they may turn too fast, but they'll last.

 

[electricpete]

If, as dArsonval says, this is a shaded pole motor then you have to accept that they're sort of good for one thing and one thing only. That one thing is powering a fan that blows air across the motor

I assume this was a typo, but I'll clarify for op. It has nothing to do with "shaded pole" and everything to do with "air over". "Air over" means the motor relies on the driven fan to cool itself.


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(2B)+(2B)' ?

 

[davidbeach]

Quibble quibble. Shaded pole motors are air-over. I focused on the cause rather than the symptom.

 

[jraef]

Learned something new today. There is such a thing as a "one shot thermal fuse" used in motors. It is not however a thermistor in the traditional sense of the term. It seems more like a eutectic melting alloy employed into a device that changes shape (like a Klixon), but the alloy solidifies before the shape returns to normal so it cannot be reset when it cools. Cleaver little trick to force you into buying another one.

http://www.devale.com/thermal-fuse/one-shot-thermal-fuses.html

Like I said, tiny motors for appliances is not my thing.

As to changing pulley size, that will only work if doing so somehow changes the LOAD on the motor. Pulleys just trade torque for speed, they don't chage load per se, unless the load is a centrifugal machine like a pump or fan where the Affinity Laws apply. Could be just that your 1/17th HP is too much for the load. You can often get away with higher ambient heat if you de-rate a motor, so if you cut the load an equivalent amount (since you say you can't fit anything bigger), that might work.

Or search around in something other than Grainger, their offerings are decidedly limited to very high volume movers. Try someone like Oriental Motor or Bodine and talk to a human being about what you need, I'm sure there are better choices.

Good luck.

"Will work for (the memory of) salami"

 

[Rykk]

The motor is 1/7 horsepower and it is cooled by a high velocity, 120Vac Boxer Fan. It is turning a belt that turns a shaft with an offset of 3mm at the opposite end. I don't know the x axis specs of the 3 rubber shock mounts to try to figure out the load on the rig. The motor is definitely not too much for the application. As it it, it has so little torgue at start-up that I have to push in a spring loaded tensioner pulley to allow it to get up a head of steam before letting the tension pulley tighten the belt back up.

A 1/4 hp would definitely be much better but it seems that 1/7 or 1/6hp may be the cutoff for thin, "stack" motors. Redoing the chassis for a bigger, Y48 NEMA frame size motor would require a LOT of redesign and cutting of steel or building a steel fram external to the unit for the motor. I see what y'all say about these air-over motors needing air moving thru them. Even the original 1/15hp this lap came with would shut down within 30 minutes, even at night here in FL. Once I added the fan, that problem went away. So maybe it is that running in my hands on the bench with no cooling, the motor gets hot and this is to be expected. I went ahead and installed it into the machine and it is working. It runs sometimes for over 36 hours straight before I power it down. So far, so good!
Rick