50 amp transfer switch hums ?

[Sean]

I disagree on hum being normal. ...

That depends on the loudness.

All 120vac contactors hum.  Good engineering and manufacturing can make the hum as quiet as you'd like, even as to be inaudible to most people with normal hearing.  However, the contactors typically used for RV ATS applications are not this quiet.  These are reversing three-phase motor starters, so the design goals at the contactor manufacturer do not include "inaudible."

The ATS manufacturer can quiet these down a bit further by using vibration-isolating mounts for the contactors, non-metallic enclosures, and the like.  But here again, most manufacturers do none of  these things.

Without knowing how loud John's hum is, it's impossible to say whether he might have a coil with loose laminations.  Moreover, even a barely audible hum can be amplified by the environment, for example by mounting the device to a resonant surface.

... I worked in a big power plant for many years. Noises were tracked to the source and repaired.

I would submit that any hum that could be heard in a power plant would be very loud indeed.  I have to assume that is not what we are discussing here; my impression is that this is the first time John has ever used such a contactor, and in such a case even the normal amount of 60hz hum can be unsettling.  I would guess that if he removed his ATS and mounted it in a power plant, no one would hear it there.

These ATS units are in all the high end units and tolerance for loud noise is very low.

Without knowing what make and model switch he has, it's hard to say it is "high end."  RV transfer switches are commodity items, and many of them are made with inexpensive contactors.  I include in that the two most popular brands, Iota and Progressive.  The inexpensive contactors have a great degree of variability in manufacturing tolerances, and I've seen units that were barely audible and ones that would rattle your teeth out from the same batch.  I've also seen units where the two contactors in the same enclosure had different sound levels.  I don't think you'd have much success sending one back because the hum was too loud.  High-end RV manufacturers sometimes use more expensive switches; more often they take care to mount the ATS where it will not be heard, such as next to the (also humming) inverter.

Can you make the noise stop by pressing on the coil? Does the noise stop if you apply pressure to the assembly internals? Obviously use a non conductive tool to do this check.

Only a few large ATS contactors have coils you could even access while they were operating to press on them.  At best you might be able to press on the contact carrier.  50-amp and larger contactors often have fully enclosed coils.

One way to help resolve the question, though, would be this:

John, does the switch hum at the same volume whichever side you connect to power, or does one input hum louder than the other?

Also, can you tell us the make and model of the ATS?  I can see if I can find out what contactors they are using.

-Sean
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