You describe two problems. The door holders are hot and the door holders buzz.
Two types of electrical power could be used for the door holders, AC and DC. Some of them are AC only, some of them are DC only, and some of them can be either AC or DC. Usually the rating is stamped on the door holder assembly somewhere.
- AC is usually utility power, either 50 cycles or 60 cycles a second. During each cycle, the electricity has changed from positive to negative and back again. This can be stepped down using a transformer, but even though the voltage may be changed, the power is still AC.
- DC is converted in a power supply. It is somewhat constant plus or minus voltage, but may have a little AC riding on the DC.
Starting with the heat, a door holder is an electromagnet - it's a piece of wire wrapped around an iron core. The wire carries current, and if the wire is too hot, then the only cause is too much electrical current is passing through the wire.
There are a couple of problems that can cause too much current.
- Using DC current rather than AC current, if the door holder is rated to use AC only.
- Using a higher than rated voltage to activate the magnet in the door holder.
Both of these could cause the heating problem you describe.
The other problem is the buzz. A DC door holding electromagnet will either not buzz at all (if it is powered with true DC) or only a little bit (when there is some AC ripple mixed in with the DC). An AC door holding electromagnet doesn't usually buzz more than a DC system because the AC is changed to DC in the door holder assembly, or something is done with the iron core to smooth out the AC magnetism.
The main cause of a door holder buzzing, if it is loud, is that AC is being used when the door holder is rated to be used with DC.
From what you describe, you are using utility power AC when the door holders are rated at a much lower DC voltage. This will make the wire too hot and the magnet to buzz. Because it is an electromagnet, it will still hold the door open, but the door holder will get too hot and burn out (possibly causing a fire), and it will buzz. Of course, that's really just a guess, but that is something that needs to be looked at.
You are going to have to measure the amount of voltage and whether it is AC or DC, and then compare what you measured to the label on the door holders themselves to find out what is causing the problem.