High Pitch Sound from the PF Motor

[Zeon]

Dear Train Tech readers!

As suggested on this forum, I just bought my first Lego train for a starter set (7939, PF motor), but whenever the engine starts to work (pulling) it generates a high pitched faint noise. My son doesn't seem to be bothered by it, but it gives me a headache (literally).

Is this noise normal? If the engine is on standby (turned on, standing) it's silent. Should I return it to the vendor?

thank you

[Hrw-Amen]

All of my LEGO Train motors give out this noise as soon as there is power to them. It is a little headache inducing, however once the train starts rolling I find it is drowned out by the actual running sound so I don't have to put up with it for too long.

However I do not get the same noise, (Or any noise.) from my PF XL motors that I am using for the Emerald Night and other steam trains, so it seems it is just that particular one.

Not sure why? Maybe if we have a tech-priest on the forum they could enlighten us, as I have wondered myself?

[JopieK]

That is quite typical for DC motors I guess. I know the reason as well. They are controlled using PWM:

So you send a digital signal to it. Since a digital signal is either 1 or 0 that is a problem (0 means 0 power, 1 means full power so e.g. 9V). By switching very quickly between 0 and 1 one can control the power output. The example (from arduino.cc) gives 0 = 0V and 1 = 5V, but of course with a 9V motor (and a 9V supply, the lipo actually is 7.8 V or so) a 1 will be 9V.So if the duty cycle is too low, you hear the pulses, but the train would not move (or just move a little bit).

[Zeon]

That is quite typical for DC motors I guess. I know the reason as well. They are controlled using PWM:

So you send a digital signal to it. Since a digital signal is either 1 or 0 that is a problem (0 means 0 power, 1 means full power so e.g. 9V). By switching very quickly between 0 and 1 one can control the power output. The example (from arduino.cc) gives 0 = 0V and 1 = 5V, but of course with a 9V motor (and a 9V supply, the lipo actually is 7.8 V or so) a 1 will be 9V.So if the duty cycle is too low, you hear the pulses, but the train would not move (or just move a little bit).

So boiled down to my level: the DC motor (tries to) rotate at full speed (at the 1/9th of the time with 1st speed then stops for 8/9th unit of time, then goes for 1/9th) and the motor rotating makes this noise?

Thank you for the detailed answer. Considering my son's happines factor, seems like I'll have to get used to it

[JopieK]

So boiled down to my level: the DC motor (tries to) rotate at full speed (at the 1/9th of the time with 1st speed then stops for 8/9th unit of time, then goes for 1/9th) and the motor rotating makes this noise?

Thank you for the detailed answer. Considering my son's happines factor, seems like I'll have to get used to it

Yups, that's totally correct :) well, good luck with it then. Fortunately if one ages one is less sensitive to high pitch tones (although my ears still pick them up quite well, sorry for my students that try those high pitched ringtones on me ;)).

Actually the signal is divided in (probably internally 255) eight steps, the specs say seven steps, but if one looks at the documentation they mention 8 steps (0..7). So seven is 100% on, 3/7 of the time on, 4 is 4/7 of the time on, etc.

[lightningtiger]

Actually all electric motors can have this problem, especially if the motor is close to stalling it will make odd sounds as it strains to preform it's task. Try and run the engine on it's own, then add each piece of rolling stock to see when the noise starts. Plus check the drivetrain mechanics to make sure they run freely/smoothly - less friction the better.

Good luck 'Zeon'.

[bricks n bolts]

That is quite typical for DC motors I guess. I know the reason as well. They are controlled using PWM

Makes sense, I don't hear anything like that on mine as they are controlled via a 12v transformer which just lowers the voltage for slower speeds.