Question about a fan motor that vibrates ONLY at low speed.

[Jeff7]

Yet another pellet stove issue here - the thing has a convection blower, which has recently started to vibrate quite hard. The stove is on a concrete floor, and you can feel the vibration in your feet just standing near it. Really low frequency too, probably around 40-60Hz, probably 60Hz, as it's a 120VAC motor.
Anyway, the oddity about this problem: it ONLY vibrates at its lower speeds. I had the blower out and ran it full-out, right off of 120volts from the wall, and it did not vibrate at all. The bearings are in excellent shape, and nothing seems to be off balance.
Is it possible that it simply has a broken winding somewhere, something akin to what happens when a spark plug goes bad?

Specs:
Englander stove, model 55-SHP10
Parts link. It's the PU-4C442 blower.

 

[arcas]

Another possibility is that this is a case of harmonics. Perhaps the blower is ever so slightly out-of-balance and that only at low speed are the vibrations "in phase" so that they don't cancel out.

 

[WW]

could be resonanting...how is it connected? just bolted on? hanging or supported somewhere other than the duct?

maybe try a rubber washer under the bolts to isolate it?

 

[Jeff7]

Originally posted by: WW
could be resonanting...how is it connected? just bolted on? hanging or supported somewhere other than the duct?

maybe try a rubber washer under the bolts to isolate it?

It's bolted in. There is a braided fiberglass gasket between the blower and the rest of the stove.
Rubber washers would be a problem, as they'd melt pretty quickly.


Concerning balance, I still would have expected the thing to vibrate like crazy when it was sitting freely on the floor, spinning at maximum speed when I fed it full 120V current.

 

[WW]

Originally posted by: Jeff7

Originally posted by: WW
could be resonanting...how is it connected? just bolted on? hanging or supported somewhere other than the duct?

maybe try a rubber washer under the bolts to isolate it?

It's bolted in. There is a braided fiberglass gasket between the blower and the rest of the stove.
Rubber washers would be a problem, as they'd melt pretty quickly.


Concerning balance, I still would have expected the thing to vibrate like crazy when it was sitting freely on the floor, spinning at maximum speed when I fed it full 120V current.

melt? silicone rubber is good up to what, 500F? how hot is the fan get? does it vibrate when cold or only when it gets hot?

how about how the oven mounts to the floor / wall? maybe a pad inbetween would help quiet it

 

[imported_Tick]

Use springs if it's a resonate issue.

 

[Jeff7]

Well I guess I might specify - pellet "furnace." This thing is to heat the entire house. I never cook using a range/oven, so I call it a stove.

And when you say "rubber," I think "natural rubber." If you mean silicone, say "silicone."

We have a temporary solution - I called Englander Stoves. I can now adjust the speed of the blower independently of the heat setting. So the blower is now set to level 5, regardless of what the heat setting may be, unless the heat is set to 6 or higher, then the blower is set higher as well.
I don't think it's resonance either - I can feel the blower itself vibrating heavily and only at these low speeds. If it would be resonance, I think it might have manifested itself a lot sooner. The stove has been in the house for almost 3 years now and this only just started. That's why I'm thinking it might be a bad winding in the motor, but I just don't know if that would cause vibration like this.

Catch 22 on the temperature versus vibration - the blower only comes on when the stove reaches a certain temperature, as determined by the control circuitry.

Mounting - no mounting. It's a free-standing stove. Some kind of cushioning might dampen the vibration into the house's foundation, but it'd do nothing for the sound radiating right into the air.