RPC buzzing when machines first start. Is this normal / OK?

[dustin wassner]

The idler motor is 7.5hp. Powering a 5hp bandsaw and 3hp jointer. When I start the bandsaw the idler is making a 1.5 second long buzz, like the sound you would hear on a gameshow when someone gets the question wrong. Surprisingly, it does the same thing for slightly longer when I start the jointer. I would have thought the opposite since the jointer has a smaller motor (idler is 2.5x the size of it).

measuring between two legs at the converter control panel I am getting 240. When I start the bandsaw it drops to 219 and almost immediately returns to 240. Measuring at the bandsaw it drops to 216 when I start it.

I am running 10 ga 2ft from the main panel to the converter control box and from there split to each machine and the idler using 12 ga. Distance between the idler and each machine is roughly 40 ft since each machine and the iderl are about 20ft from the control panel.

What I am wondering is
1. what is actually making the buzzing noise
2. will it eventually mess something up?

thanks a bunch
DW

[Charles Lent]

If it's for that short a time, I don't think you have a problem. When high current is being drawn, such as the starting period of one of your tool the high current flow will greatly increase the magnetic field inside the RPC motor and the metal laminations of the rotor will vibrate against each other. A large power transformer will also do this when high current is being drawn through it. If you think the buzz is bad on 60 cycle power, you should hear it in Europe on 50 cycle power (something close to the sound of a mad swarm of bees).

Charley

[Bradley Gray]

I have a homemade converter, 7hp also that has buzzed like that for 35 years.

[dustin wassner]

good to know this. I will continue to use as is then. thanks