DC-AC traction questions

[David Collins]

This is a super duper technical question here, so bear with me. On General Electric built locomotives with AC traction motors, they seem to make a roaring noise that gets higher pitched as the speed increases on the locomotive, DC traction GE engines, and EMD engines (both DC and AC traction) do NOT make this noise. Where does the sound come from on the AC current locomotives? It’s intrigued me for awhile but have never gotten a good awnser.

[CM44AC]

Are you referring to the roaring (or a whirling sound) that appears to come from the traction motors themselves and gets louder with speed?

[David Collins]

Are you referring to the roaring (or a whirling sound) that appears to come from the traction motors themselves and gets louder with speed?

I'm talking about the low roar/moaning sound coming from the engines as they pass in this video (go to 1:36) https://www.youtube.com/watch?v=Vp6wU2OBBkM

It does seem to get louder with speed though. So yes.

[amessmann]

I know the sound you are thinking of, one of my favorites on the railroad actually. CN's Ex-CitiRail units are particularly noisy in that regard.

I had thought it was possible to hear the magnetic field moving in AC motors sometimes, or I suppose the vibrations caused by them (and not the actual rotation of the rotor). When using a three phase variable frequency drive (inverter), it is pretty common actually. This is usually a very high pitched sound though, like a mosquito flying nearby. I think this is how drones make the warning beeps on startup, by just sending a tone to the motors, different from the normal three phase supply from the ESCs.

Part of me wonders if that noise is just the traction motor blowers, I presume those speed up when the locomotive is under increased load.

Either way, the noise is not heard during load testing.
I too would love a definitive answer from those who actually work on locomotives.

[SD80MAC]

That’s just traction motor whine.

[PatAzo]

Where does the sound come from on the AC current locomotives?

The noise is primarily due to movement and vibrations within the motor caused by the mechanical forces the magnetic fields create. When we think of electric motors in simple terms it is easy to envision the mechanical forces as rotating in a circle with the armature. The forces are actually a combination of varying radial and tangential forces. The tangential components turn the armature contributing to traction and the radial components which can be quite high contribute to vibration. The vibration can be in the structure itself and movement of the armature in its bearings.

The AC power arriving at your home is a 60hz sine wave. Think of it as an analog AC. AC traction pulses a DC on and off varying the pulse to simulate an AC sine wave. The driver electronics noise is often perceived as a singing sound that varies in pitch. The electronic noise is there as well but not as prominent as everything else.

A transformer makes noise due to the core vibrating. Just an example of an AC device the makes noise with no moving parts.

DC traction motors make noise too.