What makes the traction motor sound?

Hi

This is something I've been thinking about for a while now and would like to know more details about: where does the distinctive "whine" or electrical traction motors come from? I'm mostly interested in GTO or IGBT controlled AC motors, but if you know about DC motors feel free to share them.

Specifically, I'd like to know how usually each series (e.g Electrostar, Networker, Desiro) has a sound different from all the others, and why are there sometimes differences in a series, e.g 357's have a lower, quieter tone alongside the usual Electrostar tone, which you can hear at starting from 0:05.

Also, as far as I know, IGBT's can "switch" much faster than GTO's, which is why trains equipped with these whine at a constant pitch, while GTO's sound like a car changing gears. However, how come some IGBT trains have one "gear change" sound, where the whine becomes significantly higher once the train reaches a certain speed (e.g 374, 1:09).

If someone with a technical background is willing to enlighten me, it will be much appreciated.

My vague understanding is that it's a lot to do with whether the traction current is DC or AC, and whether the motors are DC or AC. In DC they just hum because it's a constant current, and it's basically the windings in the motors and transformers vibrating. However, with AC it's a different sound as it's switching back and forth 50 times every second. I think rectifiers also play a part as they make their own noise when you have AC current supplying DC motors. In that case you've got the current switching direction, and the motor hum. Plus there's stuff going on the switchgear which controls the power to the motors and probably just enables more windings for more power.

I'm not an expert, and this probably incredibly obvious.

KingDaveRa said:

My vague understanding is that it's a lot to do with whether the traction current is DC or AC, and whether the motors are DC or AC. In DC they just hum because it's a constant current, and it's basically the windings in the motors and transformers vibrating. However, with AC it's a different sound as it's switching back and forth 50 times every second. I think rectifiers also play a part as they make their own noise when you have AC current supplying DC motors. In that case you've got the current switching direction, and the motor hum. Plus there's stuff going on the switchgear which controls the power to the motors and probably just enables more windings for more power.

I'm not an expert, and this probably incredibly obvious.

Click to expand...

Thanks, that makes sense. I was actually asking more about inverters in AC traction, but your reply basically confirms what I've thought before.

Hi there Straller
Pop me your email address via PM and I can send you a document about that.

straller said:

Also, as far as I know, IGBT's can "switch" much faster than GTO's, which is why trains equipped with these whine at a constant pitch, while GTO's sound like a car changing gears. However, how come some IGBT trains have one "gear change" sound, where the whine becomes significantly higher once the train reaches a certain speed (e.g 374,

1:09).

If someone with a technical background is willing to enlighten me, it will be much appreciated.

Click to expand...

I'm interested in this as well.

I suspect that the step change in sound you're referring to is some sort of torque limiting mode dropping out when the risk of wheel spin subsides.

I linked to a video demonstrating the “gear changing” noise of a 3 phase traction converter a few months ago in the 442 thread here:
https://www.railforums.co.uk/thread...-and-redeployment.127422/page-72#post-3423708

An electric traction system with ac synchronous motors has three or four main sources of sound:
1) the mechanical sound mainly from the pinion drive to the axle sometimes coloureswith resonances in the motor casing
2) the effects of magnetostriction which is the metal parts of the motor vibrating in sympathy with the magnetic currents in them. As you have already noticed, the frequency can rise and then gear change down to rise again. This variation is caused by the magnetic poles being switched (say from 8 to 4 to enable higher rotational speeds to be kept withing the range of the ac inverter. *
3) the inverter which takes DC and produces the variable ac to drive the motorat the correct rotational speed. This will have some inductive components that control spikes of current that would otherwise interfere with other equipment (e.g. signalling, comms etc.
4) if the train is powered from OLE, it will either produce a deep (50Hz) hum from its main transformer (magnetostriction again) or it may have an electronic transformer system with a higher pitch noise.
* note that this pitch can vary even at steady speeds as the motor stabilises with frequent rises and falls (Electrostars at speed - say 80 - 90 mph) as the load varies. It also has a similar 'gear change' sound when the train is decelerating as the motors feed power back to the inverter for conversion to 50Hz ac to be sent back to the OLE via the transformer and pantograph. The sound will usually stop at around 10-20 mph as the disc/wheel brakes take over.

I believe they are generally asynchronous AC motors, where the speed of the rotating magnetic field is slightly faster than the speed of the rotating component (or slightly slower when braking). A few trains use synchronous motors (TGV Atlantique springs to mind) and they may be more common in future as sufficiently powerful permanent magnets become available.

Regenerative braking is blended out at 10-20mph on DC motors but with AC motors it can continue virtually to zero.