Tweakers' Asylum

Interesting. I would not expect a switch closure to cause a pop.

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Al Sekela

In Reply to: RE: Line noise and then agian. posted by jking67 on January 11, 2008 at 13:57:01

Electrical noise usually comes from interrupting current that is already flowing, because of the circuit inductance. If you get a pop in your audio system when the blower starts, then perhaps the plenum switch is faulty.

Does your furnace have a digital control that operates the blower? Have you tried a line filter on the AC supply to the furnace?

Follow Ups:

RE: Interesting. I would not expect a switch closure to cause a pop. - jea48 09:54:48 01/13/08 (2)
Al what's your thoughts on the switch closing with an inductive/capacitive load connected to it. Been to many years since I studied LC circuits. Could the inrush of current, to the motor, cause the capacitor to send a transient spike back out on the ac line? Is it just the nature of the beast?
==================================
See link below.
Quote from link.
In all motors the windings of a motor are highly inductive so the voltage
always arrives after the current. The capacitor changes the relative
impedance of the circuit on one winding causing the shift in the
relationship between the voltage and current on one winding and the
voltage on current on another winding. This difference in the time the
electrical energy is dispersed in a winding allows the motor to rotate.
In summary, the capacitor provides a delay in the energy given to
one of the windings. This delay causes the forces of the motor to
be unbalanced and the motor then starts.
- http://www.madsci.org/posts/archives/1999-03/920525652.Eg.r.html (Open in New Window)
- It may well be the starting switch opening. - Al Sekela 15:36:31 01/15/08 (1)
  If the motor is direct-drive but has a capacitor, then it still has a centrifugal switch that opens when the motor reaches 75% of running speed. The starting winding draws about five times the running current, so there will be a considerable arc and RF pulse when the switch opens.
  
  I was misled by the OP's comment that the blower was direct-drive. I assumed the blower was similar to the variable-speed blowers in high-efficiency furnaces, and would not have a starting capacitor.
  
  Closing the switch to a series L-C circuit should not cause a current in-rush.
  - No switch on the motor.... This type of motor does not use one - jea48 20:24:11 01/15/08 (0)
    I believe the motor is called a shaded pole motor.
    Trade off is slower starting torque.
    I think the capacitor is a run capacitor on the direct drive motor.
RE: Interesting. I would not expect a switch closure to cause a pop. - jking67 05:44:19 01/12/08 (1)
I pretty sure its happening on a start up.I have a mobile home and can watch it and hear it start at the exact time the snap or pop goes through my speakers.My furnace is oil fired and has no circuit bored or the such.I did take the switch out but like most things these days you can not service it.I'm going to see if the blower motor has a capacitor on it and change it if it does.Thanks ,Joe.
- The oil burner... - Al Sekela 16:48:37 01/12/08 (0)
  ...as I undertand it, uses a high pressure pump and a high-voltage spark igniter. Either of these items could cause an electrical disturbance. The main evidence, though, is your observation of exactly when the pop occurs, and when the blower starts.

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