Wow this is a great site, wish I had found it sooner.

As a fun project I took apart my older treadmill hoping to figure out a way to make it quieter at low speeds as it hums fairly loudly under load. It has a DC motor which is supplied by a choke which runs off a PC board power supply, not sure what kind of supply it is exactly. I don't see a rectifier or any filter capacitors on the board, just a wire directly to the motor from the board and then one from the board to the choke, then to the motor.

I did check the brushes and cleaned up the commutator but that didn't make any difference. Researching this, it seems like the hum might be from the power to the motor not being pure DC. I am wondering if there is something I could do, like adding a rectifier or filter capacitor to the output of the supply to clean it up and make the motor quieter. The voltage supplied to the motor appears to peak at around 37, under load. I was not able to check the current.

Thanks for any input!

 

Is the hum 60 Hz or 120 Hz?

 

I guess I'm not sure. I think it's 60hz.

 

I'm really not sure too..

 

Wall transformers put out 60 Hz if you press your ear against them [and even if you don't]; flourescents put out 120 Hz.

 

Ok it's definitely a 60hz hum.....

 

So, I = C ΔV/ΔT, with ΔT being 1/(60x2) seconds.
If you know the motor V [let's say 12v], and you want a < 1 volt ripple on this then ΔV = 1, then if you know I in amps then you can calculate the min value of C in farads.
You ought to post a partial schematic or put a meter or a scope on this circuit first before you hook up some huge capacitor. Or, I guess you could put the cap in series with a fast blow fuse.

 

Willbe, thanks for your reply. There's a schematic glued to the inside cover but I can't remove it without damaging it and I haven't found one on the web for this model.

I will get a current measurement one way or the other. I'm planning on using a capacitor rated at 4 times the max voltage in the circuit. Please excuse the basic questions but this cap would go across the + and - wires, not in series with one, right? Is this sort of cap generally polarized?

 

Can you take a close-up photo of the power supply section of the schematic?
2x max V is big enough; if you have the bucks I'd spend it on capacity or ripple current rating.
The cap shunts the DC motor.
A cap however large this turns out to be will probably be polarized; aluminum electrolytics are cheaper than tantalums.

 

Actually they put out 60Hz and many harmonics 120Hz, 180Hz, 240Hz etc. so do fluroscents and AC motors.

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Flourescents with magnetic, non-electronic ballasts do this?

 

Magnetic ballasts of course.

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Can you show waveforms? The mag. ballast is all passive components.

Maybe the plasma discharge in the tube does this?

 

Hopefully this will be readable, it's 75kb, never posted a pic here before.

Attached Images

	Schematic 008b.jpg (75.0 KB, 7 views)
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