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Why does a train sound like it is changing speed when it starts?

Why does a train sound like it is changing speed when it starts?

Silent traffic is coming. Okay, electric cars hum a little, and there are electric cars that hum through the speaker, but you’ll hear less and less of the gasoline engine revving and shifting. Except for the Porsche Taycan or Hyundai Ioniq 5N. For more shifting noise you can go to the train platform. The NS train doesn’t have a gearbox, but you can clearly hear it moving. What’s up with that?

You probably know the sound we mean, often aided by the very loud ears of a fellow traveler. If not: Just listen to the audio in the video below. Do you hear it? The note the train makes rises, then falls, then steadily climbs — like a car pulling into a limiter and then changing gears.

We asked Ricardo to help with this problem. This is not a model train assembly company or a Top Gear HQ, but an innovative company that provides technical services to NS, among other things. An employee of the company explains the source of the switching noise.

The sound really comes from the switch.

There is direct voltage on the overhead line, but the electric motors of the train require alternating voltage. Therefore, the direct voltage must be converted to alternating voltage. This is done by a transformer. This device turns the DC voltage on and off 16,000 times per second. By rapidly switching the DC voltage on and off, the current is changed to AC voltage.

The process of starting and stopping is called pulsing, and the series of pulses is called a pulse pattern. A certain speed of the train corresponds to a certain pulse pattern. This varies by manufacturer. So it is possible for one train to never switch between 0 and 5 km/h and another to only switch after 10 km/h. As the train accelerates, it switches between different pulse patterns. If the pulse pattern is too fast or too slow, parts will heat up and power will be lost. In addition, electrical disturbances can occur if the pattern does not match the speed.

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“When the pulse pattern changes, you can hear it through a different sound from the drive: it sounds like the car is shifting into higher gears,” a Ricardo Rail expert tells TopGear Netherlands. So the converter acts as a kind of automatic gearbox. Unfortunately, the operator cannot shift gears or move towards the selector himself.

Why don’t you hear most EVs switching on?

According to Ricardo Rael, this same system is also present in electric cars, but you don’t hear it. “The fact that an electric car doesn’t produce the same noise as a train is partly due to the power of the motor. Because of the lower power, the car’s inverter can switch at a higher frequency, which makes the motor sound less audible. This also means that there is no need to change the pulse pattern,” says Ricardo Rael. In other words, all electric cars make a switching noise, but they make much, much less noise.

The sound of a train changing gears

Electric cars with hybrid gearbox Alfa Romeo Tonale