General Question

LuckyGuy's avatar

For those of you with electric vehicles, do you notice a vibration when you are moving at low speeds?

Asked by LuckyGuy (40261points) October 19th, 2020

Electric motors using permanent magnets are efficient but do not have perfectly smooth torque at low speeds. This variation is called “cogging torque” and it results in a mild vibration that is felt through the car at very slow speeds.
If you have an EV, do you notice this cogging when you’re driving slowly like creeping in traffic or in a parking lot, or moving in and out of your garage? Do you find it objectionable? Did you even notice it?

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18 Answers

gorillapaws's avatar

I haven’t felt this. My Model 3 is buttery-smooth unless it’s going over rough pavement. You feel the road (by design), but never the motor. I have driven a Model X and haven’t experienced this either. Those are the only EV’s I’ve ever driven.

LuckyGuy's avatar

@gorillapaws Thanks. The Model 3 electric motor does not use permanent magnets so it is slightly less efficient and slightly lower cost. But theoretically it can be made to run as smooth as silk.
If you are creeping (under 5 mph) in traffic you don’t feel anything? The cogging torque pulsations in a permanent magnet design are in the 4 to 5 Hertz range.

gorillapaws's avatar

@LuckyGuy “If you are creeping (under 5 mph) in traffic you don’t feel anything?”

I honestly don’t. I’m usually in Autopilot though when I’m creeping like that, so I might be more disconnected from those kinds of vibrations. My experience though really has been silent and smooth except for the Millennium Falconesque noise it makes when you stomp it. Maybe they’ve isolated the vibrations? or maybe it’s tuned to prevent them somehow? I saw a video of what’s inside a Model S motor and and I was shocked at how many PCB’s were in the casing. It’s like a Bitcoin mining operation. Maybe some of those 1’s and 0’s are being used to address the vibrations?

I’ll creep around sometime this week, paying close attention to vibrations and report back if I feel anything that you’re describing.

LuckyGuy's avatar

@gorillapaws i think you just explained why Tesla stock keeps going up even though they are not making money and have such small production. When the cars are not driving Elon is using the idle processing capability in the transmission to mine bitcoins. Each night the car phones home to the mother ship and downloads the results. How else can you explain the mandatory connectivity requirement?
(I expect to see this on QAnon shortly.)

crazyguy's avatar

@LuckyGuy LOL

I am always in autopilot mode when stick in slow and stop driving. In four years of driving a Model S, and a year or so with the Y and once with a Model 3, I never felt vibrations as you describe, although my old Model S did have some noise issues. Funnily enough, my wife never heard anything in the original Model S even though her hearing is far better than mine.

LuckyGuy's avatar

To be clear, the older electric motors are not permanent magnet type and should not have this kind of issue. Permanent magnet motors are on the horizon due to their higher efficiency which translates to higher power, longer battery life, and extended range. But, they necessarily have the cogging torque issue. There is some serious R&D taking place now to reduce this effect.
i just wondered if anyone with any EV has noticed it.

gorillapaws's avatar

@LuckyGuy I’m pretty sure the Model 3 uses permanent magnet motors unless I’ve got bad info.

This article talks about the motor also.

Brian1946's avatar

@LuckyGuy

Do you drive a Tesla?

LuckyGuy's avatar

@Brian1946 No. i do not. I live in western NY – a cold climate with snow. I have Subaru Forester.

SquirrelEStuff's avatar

I do not notice this in any of my EVS. I have a Volt, Clarity, and I-Pace.

Are you sure you are not feeling the regenerative braking?

gorillapaws's avatar

I just came across this video that goes in-depth on Tesla’s model 3 motors. IPM-SynRM motors.

crazyguy's avatar

@LuckyGuy Permanent magnet motors became the standard for Tesla in the first quarter of 2020 – see
https://arstechnica.com/cars/2019/04/motor-technology-from-model-3-helps-tesla-boost-model-s-range-10/

The permanent motors are more efficient than the older induction motors. My model S has PM motors, I have not noticed any extra noise.

Also, living in a cold climate does not disqualify you from owning an EV.

gorillapaws's avatar

Just noticed my youtube link went bad. Here’s the fixed link: Tesla Model 3’s motor – The Brilliant Engineering behind it. It’s a pretty neat video that gets into the engeneering behind the design. Skip ahead if you’re already well-versed in the basics of electric motors.

crazyguy's avatar

@gorillapaws That is a good video that has a good explanation. However, even though I am an engineer, the explanation went right over my head!

LuckyGuy's avatar

@crazyguy Temperatures here regularly drop below -20C -4F. A quick search for temperature effects on LIIon batteries will produce many articles with details. Source: Science Direct
I keep my cars outdoors. I suppose If I stored it inside and left a heater on in the garage it would be ok. But why bother? My gas Subaru starts and runs great. Turn the key and Vroom!.. I don’t have to think about it.

“Graphical abstract

Lithium-ion batteries (LIBs), with high energy density and power density, exhibit good performance in many different areas. The performance of LIBs, however, is still limited by the impact of temperature. The acceptable temperature region for LIBs normally is −20 °C ~ 60 °C. Both low temperature and high temperature that are outside of this region will lead to degradation of performance and irreversible damages, such as lithium plating and thermal runaway. Therefore, understanding the temperature effects and accurate measurement of temperature inside lithium-ion batteries are important for the proper battery management. The state-of-art achievements in monitoring the temperature inside the LIBs can be divided into contact measurement and contactless measurement. This review overviews recent development in both the understanding of the temperature effects and the temperature monitoring, and discusses the challenges and possible future directions in achieving optimum battery performance.”

crazyguy's avatar

@LuckyGuy Thanks for the education. Living in California we consider freezing temperatures (32F or lower) a serious problem!

However, given the fact that Tesla is very popular in China and parts of Europe, I feel certain that Tesla has figured out a solution for the problem. I did some digging and found out that Tesla does warm up the battery before charging it. I do not know if actual driving at low temperatures hurts the battery or not. I believe I saw someplace that the range goes down at low temperatures, but I don’t think that was your concern.

gorillapaws's avatar

@LuckyGuy Was that video helpful in understanding your original question about cogging torque at least as it might apply to Model 3 engines? It was mostly over my head but it sounds like they’re doing some weird things with the permanent magnets and the geometry of the fields. Might that explain why I’m not experiencing the vibrations you describe?

LuckyGuy's avatar

@gorillapaws That is a great video! Thanks! I can now see how they have reduced the cogging torque by playing with the reluctance portion of the system,. They can’t eliminate it completely but by using clever magnet placement and going to 6 poles from 4 they reduced the magnitude of the effect. I’m guessing most people don’t see it.
The condition where it will show up is a slow crawl at constant speed like in traffic. Maybe the 6 pole design has reduced it to unnoticeable levels. Be on the lookout for it.

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