I have a 250w M58 lowbay in my chicken coop running a /DX MV lamp. I have noticed that, sometimes the fixture hums loudly (60hz) and is sometimes so loud you can almost feel it. Sometimes it gets louder and softer and repeats, sometimes it will abruptly stop and go back to quietly humming. Is the ballast on it's last legs?

It could be but also it might be some DC voltage on your mains supply this can make some transformers hum louder than normal

How is the ballast mounted in it's housing ? I have a couple of high bays from my former university and they are noisy but not to the point of being excessive. The ballast is held in the housing by the compression of the cover on it when screwed down. There is no other mounting for the ballast. They are 250W M58. When they were in the university's gym, there were about 25 of them (a mix of 250W and 400W) and when they were all lit it was quite noisy (especially during exams).

In case you're wondering, I am now using them for up lighters at church.

I've noticed that my fixtures do that occasionally. It may be due to slight fluctuations in voltage or maybe HF pulses along the lines from the utility company (I forget the purpose of that). My crouse hinds and other bucket fixtures does that too, and it quiets down when I push lightly on the lens.

How is the ballast mounted in it's housing ? I have a couple of high bays from my former university and they are noisy but not to the point of being excessive. The ballast is held in the housing by the compression of the cover on it when screwed down. There is no other mounting for the ballast. They are 250W M58. When they were in the university's gym, there were about 25 of them (a mix of 250W and 400W) and when they were all lit it was quite noisy (especially during exams).

In case you're wondering, I am now using them for up lighters at church.

I checked the ballast, it was screwed in tightly. hopefully the ballast is okay.

Magnetic flux acts to some extent like electrical current - It takes path according to its resistance. Most will flow where there is least resistance, less will flow elsewhere

The resistance for magnetic flux is Reluctance. The iron core is the intended path of least reluctance by design. But there allways will be some flux that leaks to other paths - metal brackets, metal gear trays, box enclosure and such which are near the ballast. Air have relatively big reluctance, so little flux is going "completely" through long air paths, but when it is just a small air gap on the way to another path going for the most part through metal, some noticable flux will still go there

When flux goes through metal parts, they are magnetized and will pull other metal parts - With force that changes 120 times/sec. For example the ballast core pulls the gear tray, or the tray pulls the sheet metal walls of the enclosure. If everything is bolted together tightly it will be pretty silent. But if they are lightly touching at some point (in the corner not under a screw, under the center of the ballast, ...) the force may be exactly what it takes to make t hit and part 120 times/sec - And there is a buzz



"Magnetic conductors" - The iron core mainly - have limited capacity for magnetic flux. As the flux gets closer to the max that the core can handle, it saturates - Its reluctance goes up, which makes the leakage paths become more attractive and catch more flux. And the magnetic forces there go up too. Most ballasts work not far from saturation. In the smaller ones for the same wattage (yard blaster ballasts, ...) the size of core is allready barely sufficient to handle the flux, so they are right on the verge of saturation

That means, that any relatively small increase in the current in the core, can be exactly the small difference that starts getting the core into saturation or not, so affect pretty big time the amount of flux that escapes from it to other paths, and related noises

What can get the current up :

 - Any DC components in the current. If the lamp electrodes are not giving the exact same performance (that can happen allways, but i'd guess a vertically burning lamp is just asking for that as the upper electrode is hotter than the other), one will act better than the other as a cathode - Lamp will lightly rectify, and DC current component will appear. Now that means, that the sine wave of the current is shifted up or down from its normal position perfectly around zero. Lets say down. Now on the positive half cycle the peak current is less than normal, but on the negative it is even higher - so 60 times in second there is a peak in current, to a value that does not happen with a "normal" lamp

 - Other lamp characteristics changing as it warms up... Some lamps do slowly fluctate up and down in their voltage/current all the time they are burning, not only at startup

 - Changing line voltage

In some ballasts (ex. all chokes, so pretty much all 240V/50Hz HID gear) The effect of saturation is self increasing : The saturated ballast's reluctance goes up, impedance goes down, so the ballast lets more current to the lamp, so it saturates more, and so on

With intact lamps and ballasts those effects are not big enough to cause damage or downright problems with the working of the lamp, but they are sufficient when it comes to making noises - or the 60 Hz flicker that comes and goes with some lamps...



One more thing : The ballast itself heats up, and heats the sheet metal trays, box walls and such. They may flex a bit and start or stop touching in the spot where they hit each other. So the lantern may buzz or not when it is cold, then start or stop buzzing as it heated to a higher temperature. That in addition to the effects of saturation and lamp behavior

Something else I should probably mention is that when I got the fixture, it had a very well used 175w MH lamp in it. The ballast is 250w, so that means it was probably overheating. Is it possible that this has something to do with it?

That probably has something to do with it. I heard (and tried) that 250w mercs and 175w mercs can be used on each others ballasts, but 250 on 175 is the best and the converse of that isn't as great because of higher loading of the 175w arctube on a 250w ballast.

But how do the arc voltages compare ?

If the 175W arc voltage is not (significantly enough) lower than the 250W, then it will be getting about the same 250W power. Then the heat on the ballast will be same as with the proper lamp, and heat from the lamp will be same as with the proper lamp (250W on an 175W lamp is overload for the lamp, but on the outside it will heat as any other lamp using 250W)

Actually, overloading an arc lamp may push the arc voltage up (as it does to extreme levels with HPS..) and that would only drop the current down

So would that mean it's like the same principal as putting 34w lamps on a 40w ballast and having it overheat and all that good stuff? I also noted that the arctube lengths between two good old boy 175 and 250 lamps are the same, but with a tad difference due to the pinchseals.

So would that mean it's like the same principal as putting 34w lamps on a 40w ballast and having it overheat and all that good stuff? I also noted that the arctube lengths between two good old boy 175 and 250 lamps are the same, but with a tad difference due to the pinchseals.

Not really. The 34W are designed to operate on the same ballast, so same current as the 40W ones, so there the arc voltage differs.
However the lower arc voltage of the F40 lamp may cause some ballast to operate at a bit higher current. The current difference is negligible from the lamp perspective, but as the ballast power losses are linked to the square of the current, it may cause extra temperature. And as the speed of many of the degradation mechanisms are very steeply linked to the temperature and as many ballasts are already designed just on the edge regarding the losses and operating temperature, even such slight difference may already have strong impact on the operating life of the ballast.

The 175 vs 250W MV are of the same arc voltages, so on the same ballast they will both run at the same power, as Ash mentioned no difference seen by the ballast at all. That means one arc tube is loaded according to it's spec, the other one is then either over, or underloaded.
The arctubes are a bit more tolerant, but on the other hand the difference here is 30%, so not that small anymore. The 250W lamp will likely handle the 175W operation well, although with lower efficacy. The 175W lamp operated at 250W will likely feature higher efficacy at the start of it's life, but it will degrade faster, so the output level will reach the end of life threshold way sooner than the formally rated lamp life.
When the lamps would be normally manufactured and the installation means it is easy to replace, it would be rather advantageous to sacrify some life for the extra efficacy. But with the MV technology nearing it's extinction from the market (if not by the government bans, for sure by the natural market evolution), I would not recommend to overpower the lamps. And/or use some higher efficacy technology instead (e.g. 150W CMH).