criterion II humming on start up

[bobj0694]

I have a 2003 criterion with a new blower fan motor and capacitor. Intermittently, when the blower comes on (both heat and cool) I hear a humming noise for a short period of time ( 2 secs up to 15 secs). The blower is working during this time.
Then I hear a fairly loud click or clunk and the humming stops and everything seems normal.. Do these things have relays that go bad???
Any help would be appreciated and I apologize if this is a repeat question but I didn't see it in the forum.
thanks
bob

[paul52446m]

I have a 2003 criterion with a new blower fan motor and capacitor. Intermittently, when the blower comes on (both heat and cool) I hear a humming noise for a short period of time ( 2 secs up to 15 secs). The blower is working during this time.
Then I hear a fairly loud click or clunk and the humming stops and everything seems normal.. Do these things have relays that go bad???
Any help would be appreciated and I apologize if this is a repeat question but I didn't see it in the forum.
thanks
bob

I don't know what kind of motor you have, but some motors have a end switch in them so when they start it puts a extra kick to one winding to get the motor started in the right direction. When the motor get up to RPM the end switch will
click out and you can hear that happen. After it kicks out the humming would stop. Not all motors have that type of start switch so some hum and some don't. Later Paul

[HayZee518]

most motors in a large unit have a magnetic contactor to handle the larger current. the humming occurs because the contactor's movable pole piece hasn't made up completely with the immovable pole piece. these pole pieces are made up of laminated punched iron and rivetted in place. one or more immovable pole pieces may have a copper wire "shunt" embedded in the laminations. This is to limit something called "eddy currents" which is an induced current within the laminations. When you hear the snap, it is this movable pole piece that has made up intimately with the immovable pole. It may also be the centrifugal switch on the motor as Paul pointed out.

[bobj0694]

Thanks, that is really useful information. If it is what you or Paul suggest (and both seem plausible) then I shouldn't be worrying right?
I just get nervous when it seems to hum for a long time before I hear the "clunk" and then it gets real quiet.... By the way, I think I mentioned that a lot of the time it starts up very quietly with no hum or click/clunk. The motor I put in is an A.O. Smith 794 and the capacitor that came with it
bob

[HayZee518]

normally a magnetic contactor works quietly in the background, but sometimes "junk" gets between the pole pieces and cause a hum. if you listen to a transformer you'll hear a dull hum from the 60 cycle frequency current passing through the coils. in a magnetic contactor, the immovable pole piece has a coil wound around it. when operating it acts as an electromagnet same like in a transformer so it hums because of the frequency. the frequency of 60 cycle is rising from zero to 90 degrees, then down to the zero reference at 180 degrees, then below this reference to 270 degrees and then back up to the zero reference to 360 degrees completing a complete cycle of 360 degrees or a circle which in theory is a complete cycle. it is changing polarity 120 times a second. while its doing this it is inducing a magnetic field in the iron laminations making it a magnet.

[paul52446m]

Thanks, that is really useful information. If it is what you or Paul suggest (and both seem plausible) then I shouldn't be worrying right?
I just get nervous when it seems to hum for a long time before I hear the "clunk" and then it gets real quiet.... By the way, I think I mentioned that a lot of the time it starts up very quietly with no hum or click/clunk. The motor I put in is an A.O. Smith 794 and the capacitor that came with it
bob

If i sold you that motor i would not chance it, while its under warranty i would
change it out. Later paul