Motor noise - why are some motors silent?

[benjamin84]

This may be a silly question, but why do some e-bike motors make a lot of noise, while others are perfectly silent? Are there fundamental differences between the way these motors work, or are some motors simply tuned/built/designed more quietly with casings and bearings that cause less noise?

There is a large difference between hub motors and mid drive motors for example, where something like a QS205 hub motor like I have on my Qulbix (admittedly mid mounted) is completely silent, compared to the motor in most mid-drive kits, or even the Sur-Ron, where you hear a very audible whine every time any power is applied.

I've tried to google this and apart from finding out that some controllers (sine wave vs square wave) are quieter than others then I can't really find a clear answer to this. Please enlighten an EV-noob

 

[bionicon]

Its all in the rpm of the motors, a hub motor sees maxx arround 1100rpm thats why he is silent.
The middrives are much higher in rpm ,and then be geared down. The surron is arround 4000 to 6000rpm, when rpms raise the wining begins. I have a 138 qs middrive on my senda and he's compleetly silent under 1000 rpm, but the higher it goes the higher the wining. That why i choose to make a surron middrive with a qs 205 just for silence :lol:

 

[cal3thousand]

This may be a silly question, but why do some e-bike motors make a lot of noise, while others are perfectly silent? Are there fundamental differences between the way these motors work, or are some motors simply tuned/built/designed more quietly with casings and bearings that cause less noise?

There is a large difference between hub motors and mid drive motors for example, where something like a QS205 hub motor like I have on my Qulbix (admittedly mid mounted) is completely silent, compared to the motor in most mid-drive kits, or even the Sur-Ron, where you hear a very audible whine every time any power is applied.

I've tried to google this and apart from finding out that some controllers (sine wave vs square wave) are quieter than others then I can't really find a clear answer to this. Please enlighten an EV-noob

There are a couple things going on there.

Firstly, like you said, commutation type makes a difference. With the same motor, sine wave commutation is going to be much quieter than square wave.

Then, with the differences between mid-motors (some) and large hubs is RPM and gearing. Mid-drives generally have some type of reduction gearing to change the rpms down to bring up torque. The motors run faster as well as transmitting the drive through gears that make noise as rpms rise.

 

[dustNbone]

Also the composition and cut of the gears, if there are any, makes a huge difference in noise. Nylon gears are quieter than metal ones, helical cuts are quieter than straight cuts.

 

[benjamin84]

Thanks for the replies chaps I believe the gearing is probably the main contributor to the noise, and now that you mention it the noise from a Sur-Ron is pretty close to what you get from a straight cut racing gearbox under load!

Not the best video as he barely gives it any gas, but that whine is just like the whine from bikes like the Sur-Ron, Alta Redshift, or the KTM Freeride-E. And probably due to the gear reduction being noisy.

[youtube]a-k3YAjIQzI[/youtube]

 

[Micha(Simson)]

One source of noise is the jerky operation of an electric motor. I can feel the discrete steps of the magnetic field when I push my bike with a QS138 motor. These small jerks cause the chain to whine and the drive to emit a shrill screech. The teeth of the sprocket strike the rollers of the chain.

 

[Apartunis]

In my observation direct drive motor + sine wave controller - [gears, chain, belt] = quiet.

Chain adds the most noise. Square wave is noticeable. Gear reduction varies a lot and can be fairly quiet.

 

[Apartunis]

One source of noise is the jerky operation of an electric motor. I can feel the discrete steps of the magnetic field when I push my bike with a QS138 motor. These small jerks cause the chain to whine and the drive to emit a shrill screech. The teeth of the sprocket strike the rollers of the chain.

Some motors have a helically twisted stator to mask cogging torque. They still buzz or whistle when powered by a square wave controller. Any chain connected to one will still make a racket from the series of meshing and unmeshing teeth.

Small sprockets 13t and smaller especially can make noise by both meshing and varying chain speed around the polygon described by the chain pins.

 

[Micha(Simson)]

And I have a 10-tooth sprocket installed. I once had an 11-tooth one installed to get a little more speed, and it was a bit quieter right away. An odd number of teeth is also an advantage because then the top and bottom teeth aren’t pulling on the chain at the same time. But I’m back to 10 teeth because of the better acceleration and because that’s what’s specified in the registration. I’ve now made up for those few km/h with the Votol controller. It also has a noise reduction mode that helps a little with that annoying scraping noise at top speed.

 

[yooper2001]

Nobody mentioned Helical or slanted teeth are made to be quieter due to gradual engagement, but they do it at a cost of being slightly less efficient.

 

[99t4]

An odd number of teeth is also an advantage because then the top and bottom teeth aren’t pulling on the chain at the same time.

What is the benefit of that? And isn't it dependent on the amount of chain wrap?

 

[Apartunis]

And I have a 10-tooth sprocket installed. I once had an 11-tooth one installed to get a little more speed, and it was a bit quieter right away. An odd number of teeth is also an advantage because then the top and bottom teeth aren’t pulling on the chain at the same time. But I’m back to 10 teeth because of the better acceleration and because that’s what’s specified in the registration.

You would be much better off with a set of larger gears with the same ratio. 16t is a good minimum for less noise and wear.

 

[offGridDownUnder]

'Noise' means vibrations. That's what sound is.

A violin bow clutches the string, then releases it, then repeats. This is regular enough to make a recognizable tone, sustained over enough time for a human brain to perceive a note.

A rattle, a knock, a grind, any sound are all symptoms of something vibrating, perhaps regularly, like a violin string, a gear whine, a repetitive impact or scraping.

If you hear noise, you're also hearing wasted energy, and probably wear leading to shorter operating life. I like DD motors - often the only wearing part is the two bearings.