# Humming Coming From Pm932



## 1cjk (Dec 21, 2015)

last night when I was working in my shop with the mill I noticed a humming coming from the control box on the mill when it was turned off. I tripped the E button also to see if it would kill the power to the electronics and the humming is still there. The only way to get it to stop was to unplug the mill. I unscrewed the cover and listened again and it sounds like it is coming from the transformer. I did try to call Presion Mathews but they did not answer. In the mean time I was checking to see if anyone else has experienced this before I try to call PM this afternoon.


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## mksj (Dec 21, 2015)

That would be normal. Power transformers hum when powered, some more than others depending on how they are potted, quality, lamination build, application, load, etc.... When your machine is plugged into power, the 24VAC transformer is hot, as it provides power for the low voltage switches which includes the ON/OFF switch which operates the main power contactor.  So, just being powered, a power transformer will hum.  I would not worry about it unless it smells like it is burning.


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## pdentrem (Dec 21, 2015)

The humming equals your power bill going up! I would put a shut off before the machine or unplug it.
Pierre


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## mksj (Dec 21, 2015)

A small night light draws more power, leave it be.  Unloaded transformers have about a 5% or less power drain of their rated VA when unloaded, and this is just the primary inductance which generates heat. This is not an issue with smaller transformers. With larger ones it can be an issue, and as they get older they can have more  inductance leakage.


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## tmarks11 (Dec 21, 2015)

My lathe does the same thing.  I always flip the circuit breaker off when I am not using it, mostly because it is annoying.


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## 1cjk (Dec 22, 2015)

Thanks everybody for the replies. I thought when you turned the power off it killed all power. I will just put a switch in line on the power cable to kill the power coming in. I don't use it all the time so that should make me sleep better at night. ;-)


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## Fitter Bill (Dec 29, 2015)

Sorry for the late post....When I switch my PM 932 M PDF to OFF it's absolutely quiet, no noise at all. Machine was new April 2015.


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## tmarks11 (Dec 29, 2015)

Well you are lucky because your control transformer is quiet.  The transformer is powered straight off the plug and always has power.

Give it time, it will start humming eventually...


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## Muskt (Dec 29, 2015)

I have a 932PDF from April or May 2015--Went out to the shop about an hour ago--Plugged it in and ran it a bit, then turned it off to listen--quiet as a rabbit patting its foot in the snow.
Jerry in Delaware


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## Fitter Bill (Dec 29, 2015)

tmarks11 said:


> Well you are lucky because your control transformer is quiet.  The transformer is powered straight off the plug and always has power.
> 
> Give it time, it will start humming eventually...



Yes, it hums, but only while the power button switched on....


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## 1cjk (Jan 20, 2016)

I heard mine basically while my ear was up against the control box. I was reaching for something on the wall behind the mill and had my head up against it. I did talk to Matt at PM and he did confirm that there is constant power to the transformer while the mill is plugged in. I just put a two pole switch in line just for ease of not having to unplug it each time at the end each milling session.


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## 65Cobra427SC (Jan 20, 2016)

My PM932 doesn't make any noise when it's Off, but I'll probably do the same with it as I did with my PM1236. I installed a Manual Motor Switch from McMaster #7657K31 which isn't anything fancy, but it's durable and can easily handle the load.


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## hermetic (Jan 21, 2016)

Turn it off or unplug when not in use! the hum is usually caused by the transformer laminations coming loose, and moving in time with the supply frequency,which is caused by heat, which is caused by leaving it on. That is why some do it and some don't, if you leave them with the transformer powered up, eventually they will ALL do it, and they are known as "watt theives" more loosenes in the laminations=more noise=more heat=more current. Pull the plug or switch it off.


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## John Hasler (Jan 21, 2016)

hermetic said:


> Turn it off or unplug when not in use! the hum is usually caused by the transformer laminations coming loose, and moving in time with the supply frequency,which is caused by heat, which is caused by leaving it on. That is why some do it and some don't, if you leave them with the transformer powered up, eventually they will ALL do it, and they are known as "watt theives" more loosenes in the laminations=more noise=more heat=more current. Pull the plug or switch it off.


All transformers hum at least a bit (though you may not always be able to hear it) because iron is magnetostrictive.  Loose laminations buzz.  Leaving a quality transformer powered will not cause the laminations to work loose unless they are defective.  They do consume some power, though.


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## 65Cobra427SC (Jan 21, 2016)

Although unplugging the unit is an option, I would think it's inconvenient for most. My outlets are located directly behind the machines and even though I have a lot of room back there, the first time I smacked my head on the bar holding the DRO, I said that's it. Also, my workshop has 4-wire outlets and the plugs are the locking type... somewhat a pain to lock/unlock to install/remove. On the PM1236 I installed the McMaster motor switch on the left side... and directly to the left of the removable cover so it's actually mounted on that side of the electrical box. Haven't figured where to mount it on the 932 yet, but it's on the list.


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## hermetic (Jan 22, 2016)

ok, lets get technical, which i was trying to avoid too much, the magnetostriction movement and heat eventually loosens the laminations, and causes more heat, and more noise, and more current consumption. It seems that in th US many outlets are not fitted with switches, whereas in the UK unswitched outlets are now extremely rare. and on 3 phase are forbidden. There is no advantage whatsoever in leaving it on, and it wastes energy. I do not know where you would go today to buy a quality transformer, PM is made in Taiwan, but that doesnt mean the transformer and electronics is not chinese.


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## mksj (Jan 22, 2016)

Lets be realistic about this, the power control transformer used in these machines are small (100vA) and draws maybe a few watts (less than 5% of rated power) if that when not in use. The rated life of a transformer is 20 years under full load. Just about every piece of machinery with a contractor has a VAC transformer to supply voltage to the contctors, these often remain on all the time. Historically almost all  control wall wart transformers  and system transformers remained plugged in the wall "on" and had an operational life of decades, I have never seen one fail. Look at older electrical equipment that was run 24/7 for decades, almost all had transformers. Not going to happen in our lifetimes, there is a higher chance of you burning up your motor or a system board failure, then a transformer going south because it was plugged into a wall. Leave it be and stop worrying about it.

When a Transformer is operated under ANSI / IEEE basic loading conditions (ANSI C57.96), its normal life expectancy is about 20 years. The ANSI / IEEE basic loading conditions for Transformer are:
i. The Transformer is continuously loaded at rated kVA (kilo Volt Ampere) and rated Voltages (Transformer must be operated at the rated Voltage and kVA)
ii. The average temperature of the ambient air during any 24-hour period is equal to 30°C (86 °F) and at no time exceeds 40°C (104 °F).
iii. The height where the transformer is installed, is not above 3300 feet  or 1000 meters


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## John Hasler (Jan 22, 2016)

mksj said:


> When a Transformer is operated under ANSI / IEEE basic loading conditions (ANSI C57.96), its normal life expectancy is about 20 years.


Which is conservative as a transformer has no inherent wear-out mechanism.


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## JimDawson (Jan 22, 2016)

From a power use perspective the control transformer stand by power usage is pretty much in the noise.  For instance, in my shop right now, there are 2 control transformers, 5 tool battery chargers, 2 computers, and 2 VFDs all powered up.  Total shop power usage right now is 0.122 KW (@87% PF, 0.143 KVA) , so at my power rate of $0.125 / KWH, that's $0.36 in 24 hours.  The control transformers are probably less than 5% of that total.


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## hermetic (Jan 22, 2016)

"The rated life of a transformer is 20 years under full load." Hope you have informed the chinese of this!  I have just scrapped a Taiwanese dehumidifier with a burnt out psu transformer, no parts available, about 14 months old with a 12 month warranty What planet do you people live on?  $0.36 a day=$131.00 a year......................for nothing.  But hey, forget it, leave it on, be my guest!


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