RadioBanter - View Single Post - RFI: Odd consequences of variable speed AC motors.

Telamon · December 3rd 07, 12:52 AM posted to rec.radio.shortwave

In article
,
wrote:

Robert Blum and I discussed this last summer and I had forgotten all
about it until I got a call last night from a fellow SWL who was
experiencing some sudden onset RFI. He had been bragging about his
new Panasonic variable speed heat pump and how much it had saved him
this summer. I asked him to take a MW transistor radio near his
fancy new heat pump and to let me know the results.

Surprise surprise. The noise was most intense at the exterior heat
pump. . We had a very odd situation at work in that the bearings in
the vari- speed motor that drives the ventilation system, a very big
motor and fan, kept going through bearings at a frightful rate. The
bearings are supposed to be good for 10K hours. we were getting as
little as 6 weeks.

In one of my searches I had found a reference to the longitudinal
currents induced in the rotating shaft. This current wants to flow
from end to end and must punch it's way through the lubrication. This
destroys the lube and eats tiny wholes in the bearing surface. A very
bad situation. I gained permission to go into the service area, an
area off limits to even engineers with my trusting DX398 and the 19uH
noise probe. There was an intense amount of static coming from the
motor. Our electrician didn't call me an idiot, but he stiffly agreed
to check the voltage from end to end on the shaft. It was well over
100 volts. After he pried the dust caps off, and when we turned out
the lights, you could see a million little arcs at each end in the
bearing!

The solution was to use ceramic coated steel bearings. Noise gone,
And no more bearing replacements.

The only HF reception gear where I work is a WWV clokc backup, 3rd
level redundancy, that I monitor at my bench. I had noticed periods
of noise strong enough to unlock the WWV derived clock, but hadn't
been able to locate the noise source.

The audio guys were very happy because their long AES digital audio
liines were experiencing serious, as in unusable, noise issues.

I wonder how many of the new high efficiency heat pumps and air
handlers are going to have some rather odd problems.

My friend is trying to figure how to explain what is happening to the
warranty people. I don't envy him. If work was a valid example, his
bearings ought to fail pretty soon.

And before you ask, reducing the RFI from this unit was a nightmare.
It took several hundred dollars of heavy duty RFI filters, a lot of
ferrite and a lot of bypass caps with a local grounding rod and 4AWG
bond back to the home's NEC ground.

You post about the strangest stuff. I have never heard of this kind of
problem. One or more of the rotor windings must be some how shorted to
the shaft.

Ceramic bearings are not the solution as the current can go through
whatever gear train and mechanical load to which it is connected. The
motor shaft should not be electrically hot. That would be a shock
hazard. That motor is either built wrong or designed wrong.

--
Telamon
Ventura, California