Motor-Generated Radio: Investigation of a Strange Phenomenon

[Ivan Seeking]

A buddy of mine called me with this so I thought that I would throw it out for comments. The problem is that he can hear a local AM radio station coming from at least two motors in his house. If the fridge fan or compressor [not sure which], or the space heater fan motor is running, he can hear the station; disconnect the appliance from power and the sound goes away. And he can hear it coming directly from the motor, with the audio clear enough that he can even recognize which DJ is on at the time.

At first I assumed that this is related to something like an amplifier with a bad ground in the neighborhood, but I guess a motor could resonate electrically at just the right frequency, and then convert this to sound? Anyone who works with VFDs can verify that motors do convert electrical sine waves having a frequency ranging from 1-20Khz, to sound, very nicely. Also, it occurred to me that the wiring in buildings tends to resonate at about 100Khz - very near the bottom end of AM radio.

 

[russ_watters]

Not to mention, an iron core with wire wrapped around it is in both a motor and an antenna...

 

[DaveC426913]

"I think I can hear it better... wait, wait it's coming from ... closer ... closer ..."

"Hey what's that zizzing sound? What's that burning-hair smell?"

"AAAAAaaaaaiiiieeeeee!"

 

[Ivan Seeking]

I once took 25,000 volts up the nose that way.

 

[pallidin]

This is one of the oddest things I've heard, but interesting.

 

[Ouabache]

strange RF

Recieving RF through a motor sounds intriguing. I've heard some complex sinusoids coming through my home stereo speakersa few times. An LP was playing at the time plus very faint CW signal.

Nearest I could figure, someone may have been operating that mode from our university radio club and my speaker wires were resonant at their xmtr frequency. I was several blocks from the club at the time

 

[Ivan Seeking]

Perhaps with a properly sized variable capacitor tied across the motor leads, we could even tune to the desired station.

 

[Ivan Seeking]

I called my buddy, who happens to have been my high school physics teacher, and told him what I think might be happening. In the course of the conversation he mentioned that another one of his former students was Don Buchla.

Don Buchla (1937— ) is a pioneer in the field of music synthesizers, releasing his first units months after Robert Moog's first synthesizers. Buchla was born in Southgate, California, and studied physics, physiology, and music. [1][continued]

http://en.wikipedia.org/wiki/Don_Buchla

 

[Ouabache]

Perhaps with a properly sized variable capacitor tied across the motor leads, we could even tune to the desired station.

I believe you're on to something.. let's try one of http://demo.physics.uiuc.edu/LectDemo/descript/710/PIC00024a.jpg ?

 

[Ivan Seeking]

If this really is what's happening, I guess the number of windings around the core is what effectively yields amplification? I can't see how else this would translate into audible levels of vibration. In the case of VFDs, we impose a 1-20Khz carrier wave, but that also carries large currents, so it's easy to see why VFDs cause motors to ring, but in the case of a radio signal...?

 

[Paulanddiw]

Ivan,
How far away is the AM radio station?

 

[nik282000]

I had a simmilar situation last summer. Between about 10pm and 1am most of the electrical devices in my house would make the faintest buzzing noise but it wasn't a constant buzz it was pullsed like some one talking. It came from the filliment of lamps and speakers the loudest. It turns out that it was some one on my streed who had a huge (several kW) ham radio setup and they were pretty much brute forcing their signal into everything.

Paulanddiw, is spot on. If you are close to where it is being transmited then there is a good chance that the signal is just so strong that it is more apparent to you then to some one farther away.

 

[wolram]

We used to have a battery powered clock that picked up radio signals, it
became a focus for vistors, this was before we had local broadcast stations,
at times you could hear words clearly, but most of the time it was interferance riddled music, as i was only a kid at the time i can not remember
stations that were picked up, but i think they were long wave.

 

[waht]

This effect you are describing might sound weird, but it's not. There is a simple explanation.

AM radios just like FM use a common technique known as heterodying or mixing. Meaning they convert the incoming radio wave from antenna to a lower frequency where they can be processed easily.

The motor generates lots of noise of different frequency. When some it reaches the antenna, it will then reach the mixer. Now, the mixer itself is a nonlinear device capable of generating harmonics. So now you have the signal of your station mixing with the noise, and the local oscillator of the radio. Then the garbage that comes out of the mixer will carry signal from other station or even FM, or local TV broadcasts.

If the radio had better filtering, and selectivity that would reduce the effect. But if the interference is strong, then there is nothing you can do.