Shaded Pole Fan Motor - Speed Control Question

[Tenaya]

Hi, I'm new here.

We have an oven with a shaded pole AC fan motor - 120VAC, 40W, 3K rpm (the inserted photo is very similar). Since the oven manufacturer cheaped out big time, the plastic fan wobbles badly, generating a very loud vibration noise. I'd like to slow the fan down. I figure that 50-75% speed will eliminate the noise while still providing the cooling the oven requires. I tried a ceiling fan speed control, presumably triac based, but it works too well, slowing down to about 25% speed. While messing with that, I discovered that the fan slows way down when my bench vise jaws clasp the magnetized stator. Further experimentation revealed that any sizable ferrous metal also slows it down. I'm sure I can find just the right size to bring the rpm down to the range I want.

My question is: Will the shaded pole motor be happy with this chunk of metal attached, or will it overheat or otherwise fail?

Thanks in advance,

Tenaya

 

[Bluejets]

Shaded pole motor speed is frequency dependant.
To answer your queries, any mods you suggest will probably damage the motor in some way, either heat destroying the coil , dry out the bearings and sieze.

 

[Minder]

You should be able to get full range control with a proper Triac 'Dimmer' control.
Do you mean it didn't go over 25%?
M.

 

[duke37]

Try a series resistance, experimentation will be needed.

 

[Tenaya]

Thank you for all the responses.

My original plan was to use a fan "dimmer" with a variable resistor, but I live in an electronics desert (smallish town) and the only one I could buy off the shelf was 3-speed: Off-Hi-Med-Lo-Off. I don't know for sure, it's riveted closed, but it probably uses a triac circuit. Hi is clearly full speed, Lo appears to be about 25% speed and Med seems to be the same as Hi for some reason (the low power draw of the fan?). My estimate is that 50-75% speed will provide sufficient cooling, AND decrease the vibration enough to reduce the noise to a tolerable level.

I'd still like to pursue the idea of placing two pieces of steel along the stator, one on each side. My uneducated guess is that adding steel in this way changes the magnetic characteristics of the stator, limiting the speed at which it can spin the rotor. Using cut-and-try, I can determine the sizes to achieve the desired speed reduction, but I don't know enough about how these motors work to understand the long term effects of doing this. Since I have a complete spare fan/motor assembly, I'm tempted to hack it all together to see what happens after running for several hours.

Does anyone have a better understanding of this variety of shaded pole motors and what my proposed modification might do?

Thanks,

Tenaya

 

[Alec_t]

My uneducated guess is that adding steel in this way changes the magnetic characteristics of the stator

The vice jaws acted as a magnetic shunt, so that less magnetic flux passed through the rotor. I don't think two separated steel additions would have the same effect.

 

[Minder]

I'd still like to pursue the idea of placing two pieces of steel along the stator, one on each side. My uneducated guess is that adding steel in this way changes the magnetic characteristics of the stator, limiting the speed at which it can spin the rotor.
Tenaya

That is not going to work.
Also the stator is designed to work with the shading ring for phase control so you also could upset this arrangement.
I would open the 'dimmer' and replace the switch with a pot.
M.

 

[Tenaya]

The vice jaws acted as a magnetic shunt, so that less magnetic flux passed through the rotor. I don't think two separated steel additions would have the same effect.

Hmm. In my ignorance, I stumbled onto the whole magnetic flux thing by clamping the stator between my bench vice jaws to hold the fan while I ran it with the "dimmer". When 120VAC was applied without the "dimmer", it ran really slow. At this point, I thought that the vice was somehow distorting the stator, which also forms the mounting frame for the bearings. When I re-clamped on the mounting sheet metal, the fan ran at full speed. While clamped this way, I laid a pair of pliers against one side of the stator, the pliers was magnetically attracted to the stator, and the fan slowed down noticeably. I'm extrapolating from that experience to using two pieces of steel, adjusted for size and crafted in such a way as to stay in place. My thinking is that if the steel can "bleed off" enough of the magnetic flux, I may be able to achieve the speed reduction I need.

Hacking the "dimmer" is certainly an option, but I'm trying to avoid it. The oven takes 220VAC from the wall and the 120VAC needed by the fan motor comes from, and is switched by, the control panel. It's not at all clear how the control does that (one hot to neutral, step down transformer or . . .) and how the controller will react to the "dimmer". The controller may well not care. In any case I'd really prefer not to release the magic smoke from the oven's most expensive single component. Even though the oven is loud, it does still cook.

Tenaya

PS: I really appreciate the responsiveness of this forum. Thank you all for that.

 

[duke37]

The fan is driven by AC and so there will be an altermating flux in the stator. It is unclear whether the solid steel will increase the inductance and so reduce the current or give a shorted turn needing higher primary current and heat in the shunt. To get over this, use laminated steel from an old transformer.

 

[Minder]

At this point, I thought that the vice was somehow distorting the stator, which also forms the mounting frame for the bearings. When I re-clamped on the mounting sheet metal, the fan ran at full speed. While clamped this way, I laid a pair of pliers against one side of the stator, the pliers was magnetically attracted to the stator, and the fan slowed down noticeably. .

You could we be affecting the optimum phase shift caused by the shading ring, if so you could cause extra heating.
M.

 

[Bluejets]

The 3 speed control is likely to be a switched capacitor arrangement, very common these day. Designed for 60w split phase fans, not shaded pole motors.

 

[Tenaya]

The 3 speed control is likely to be a switched capacitor arrangement, very common these day. Designed for 60w split phase fans, not shaded pole motors.

Short of disassembly, is there an easy way to tell?

Tenaya

 

[Bluejets]

Does the supplier not know?
Usual indication is how you described the unit....being 3 switching positions.
A triac controller is usually an isolator switch and a pot style control(turns 270 degrees) together on a single plate or perhaps just the latter.

 

[Tenaya]

All I know about the dimmer is that it's Lutron Model FSQ-2FH-DK and is marked PADDLE FAN ONLY.

A quick search on www.lutron.com yielded no additional information. Something may be there, but I couldn't find it.

I actually wanted the fully variable type, but this is all I could find locally (Lowe's and Home Depot). In any case, I missed a store that may carry what I need (memory is the second thing to go). The next time I go into town, I'll try again.

Thus far I've limited myself to controls that specifically say they're for speed control on a fan. Will a "light dimmer" work? That is, are light dimmers with a knob triac- based and, therefore, will safely speed control a shaded pole motor? Fully variable light dimmers are easily available.

All that said, I'm still leaning toward the magnetic flux approach. It's a more elegant installation and while there's a potential for overheating the motor, there's significantly less chance of killing the (expensive) control panel.

Tenaya

 

[Minder]

The light dimmers they have at H.D. are just about the same as the fan motor variety, the latter usually has a suppression choke or other noise suppression features.
The Triac is usually rated for something higher than a simple lamp.
M.

 

[Bluejets]

The fan is there for a reason. You may end up doing damage elsewhere if it is not operating as designed. I'd say leave it as it is and maybe get some earplugs. I have a Bosch oven with a cooling fan and without it I would think there could be a good chance of a fire under extreme conditions given the ovens designed enclosed nature.

 

[Tenaya]

The fan is there for a reason.

You're right, it wouldn't be there at all if they could do without it. The company has been using what I consider to be substandard manufactured parts to hold to a price point. I'm on my fourth fan from two different sub-assembly suppliers. The plastic fan blades wobble visibly and are way out of round. From what little I can extract from their technical support people (it's hard to have a two-way conversation with someone who will only repeat what their script says), this is an ongoing problem that they can't solve, or, more likely, decline to solve (not every customer complains). I used to run product management for a company whose plastic parts were made in China and I know first hand that the only way to fix manufacturing tolerance problems like this is to pay more for each part, i.e. better suppliers who charge more for molds, material, production and experience.

I wish there was some way to field-balance the fan blade, but it's design and material make that hard/impossible. My only hope is that I can slow the fan down enough to minimize the vibration while still providing sufficient cooling.

I'm headed to town tomorrow to find a fully variable one. I plan to run it on the bench to see how speed correlates with vibration. If I can find a happy medium, I'll try it in the oven.

I'd say leave it as it is and maybe get some earplugs.

That was our first approach. Unfortunately, the vibration and noise is so bad that we can't ignore it. Sometimes the oven actually shakes the wall its cabinet is mounted on. FYI, this was a warranty claim until they replaced a bunch of parts, declared the oven non-repairable, and refused to do anything more because they had ran out the clock. I'm trying VERY hard not to get lawyers involved, but . . .

Tenaya

 

[Alec_t]

If the motor shaft is a standard size perhaps you could find a non-OEM fan blade of better quality?

 

[Minder]

If you have any friends that are RC aficionado's, you could check to see if they have a prop balancer, if so, this could be used for a balancing attempt of the fan.
M.

 

[Tenaya]

If you have any friends that are RC aficionado's, you could check to see if they have a prop balancer, if so, this could be used for a balancing attempt of the fan.

I thought of that, but rejected it, perhaps too soon.

The fan blade is a flat circle of plastic with curved blades radially on one side of the flat. This contrivance seems to be unbalanced in two dimensions. Not only is the shaft off-center and/or the flat out of round, the circle isn't really flat. Thus, it vibrates parallel to the shaft AND perpendicular to the shaft. Since under load the fan runs at about 2,500 RPM , it doesn't take much to cause substantial vibration. It might be possible to fix the out-of-center/round vibration with a prop balancer, but I don't know how to deal with the out-of-flat condition.

Now that I think about it more, maybe fixing the out-of-center and/or out-of-round conditions would reduce the vibration enough that the out-of-flat vibration won't matter within the actual installation.

I'll look into that more.

Tenaya