[RFI] What Does Variable Speed Mean?

[Jim Brown]

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.