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Re: [TowerTalk] Station Noise

Subject: Re: [TowerTalk] Station Noise
From: jimlux <>
Date: Sat, 17 Oct 2009 07:01:12 -0700
List-post: <">> wrote:
> Mike,
> The earlier TT suggestion to call the A/C dealer/installer, to complain about 
> EMI noise, is very good. Tell them the noise is unacceptable, and their 
> equipment interferes with your electronics. The manufacturer may have a fix 
> available. However, they may not, and you've probably paid for the thing by 
> now. 
> So, here are some ideas from a non-expert in modern A/C equipment, but one 
> who has had very good luck in finding RF and microwave noise sources and 
> suppressing the hell out of them.  These are based on trying to stop the 
> noise at its source.  BTW, to prevent confusion, the term "AC" used here 
> means alternating current, and "A/C" means air conditioner.
> The continuous noise (hash) is likely generated by a variable-speed drive for 
> the A/C air-mover motor. The noise is probably coming from the motor 
> speed-control drive circuit (as the output transistors/triacs/whatever switch 
> on and off). Variable speed motors are often used to reduce electrical power 
> consumption to just what power is needed, at the expense of the switching 
> noise.  If you change fan speed you can probably detect that change in your 
> receiver (a pitch change).  A multiple-speed AC motor (synchronous) of 
> course, would be much quieter, and may even be offered by the A/C 
> manufacturer as an alternate to the VFD (Variable Frequency Drive).

A lot of HVAC fans are PSC (split phase capacitor) type, which are 
basically induction motors, and will change speed nicely with a phase 
control SCR type controller (e.g. a light dimmer).  Fans have a very 
nonlinear load vs speed characteristic (somewhere between square and 
cube, depending on the fan design and motor windings), so the "low 
torque at low speed" problem with phase control speed controls 
(discussed in connection with drills recently) isn't as big a problem.

The big problem with SCR type controls on fans is the noticeable line 
frequency buzz transmitted mechanically from the motor to the housing, 
but in a HVAC application that might not be an issue.

Point here is that it might not be a VFD, so the frequency of the hash 
will stay constant (but it will have strong 60Hz harmonic content).

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