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[RFI] What Does Variable Speed Mean?

To: rfi@contesting.com
Subject: [RFI] What Does Variable Speed Mean?
From: Jim Brown <jim@audiosystemsgroup.com>
Reply-to: jim@audiosystemsgroup.com
Date: Fri, 11 May 2018 12:47:59 -0700
List-post: <mailto:rfi@contesting.com>
You have a SWITCHABLE speed motor -- it has two speeds, fast and slow.  Think of it as an DPDT switch. No electronics involved.  Think of a variable speed motor as being more like a volume control, so its speed can be varied over any value from zero to high. Except that the speed is NOT set by a pot, it's set by sending power to the motor in a train of DC pulses (square/rectangular waves) whose frequency and/or ratio between on and off is varied. As I hope we all have learned (by studying electronics and/or math) any fast transition between on and off produces an infinite series of harmonics, the strength of which depends primarily on the speed of the transition.

Switching power on and off is a fast transition -- we often hear it in our radios as a short pulse (even a crackle) of noise. That variable speed waveform is doing it thousands of times a second, produces a continuous train of crackles.

73, Jim K9YC

On 5/11/2018 12:36 PM, Dale Johnson wrote:
Question, what is considered a variable speed motor or furnace blower/fan?  I 
have a furnace that has variable speeds, one higher speed for when the air 
conditioner is on then there is a couple speeds for when the furnace is on, one 
lower, one higher when demand is greater, is this considered a variable speed 
blower or fan?  The furnace that I have produces no RFI that I can detect.

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