- A problem is occurring on some Model S cars regarding a "hum at 65-75 mph."
- Tesla's responses have varied from calling it normal to changing the motor and inverter, sometimes to a "refurbished unit."
- Tesla so far is on its third "fix" for this problem, and it is still occurring on some new cars.
- The problems are listed with descriptions on a Tesla website. Google "65-75 mph Hum."
- After reviewing the descriptions of the problem, it seems that Tesla has the age-old problem of "Closing the Loop."
The Advantage of the EV
Although Tesla (NASDAQ:TSLA) owners like to brag about saving money on gas, the reality of the Model S that makes it exceptional is its acceleration and regenerative braking. The ability of the car to instantly jump when you hit the accelerator pedal and the ease of driving with regenerative braking is what the drivers love. In order for this to happen, full available power must be applied to the drive inverter instantly. The acceleration occurs and as you reach your new desired speed, the inverter output is adjusted to maintain the new speed and you go on your way.
The Problem: On some cars, however, as your speed levels out, you hear a hum as if there is a truck passing you. This hum remains as you drive in the 65-85 mph range. Owners have reported it starting as low as 44 mph. It seems to come with age.
The Answer: The answer is probably the one engineers call "Closing the Loop," and on the Model S, this might be the problem.
What is the Loop?
The Loop, or feedback loop, is the means of maintaining a device's output by comparing it to a known reference and keeping it at a desired output based on this reference. If the loop speed is too fast, it can overshoot, and then undershoot when trying to recover, causing an oscillation.
In an Internal Combustion Engine (ICE) vehicle, when you step on the gas there is a delay before the ICE can accelerate. The car accelerates from 55-65 mph; at 65, you back off the gas and maintain a steady 65. In this case, you closed the loop when you backed slowly off the gas; your feedback reference was provided by the speedometer and your foot action.
Suppose Tesla did the same thing. You hit the accelerator, and suddenly, you are at 70 mph before you can take your foot off. You take your foot off and you drop to 60; foot back on - bingo, back to 68 - you keep bouncing or oscillating. Tesla realizes this problem and adjusts the response time of the inverter to correct for it, yet it wants to maintain the "instant acceleration" thrill, so it still tries to keep it as fast as possible. It probably does this by closing the loop itself based on the position of the accelerator pedal. As an example, if the accelerator pedal produced a voltage that would vary between 0 and 10 volts and the speedometer provided a voltage of 0-10 volts corresponding to 0-100 mph, a control circuit to the inverter could maintain any speed depending upon the accelerator pedal's position.
The accelerator pedal is pressed down to a 60 mph level, the inverter turns on full power, the three phase induction motor roars to life, and as the car approaches 60 mph, the inverter output backs off and the car settles at a steady 60 mph.
What happens after tens of thousand of miles and the bearings are a bit tight slowing the ability of the motor's response, just enough that the error signal overreacts and the speed overshoots slightly. It then begins a slow speed oscillation at a low frequency, which creates a hum.
This description is my own guess based upon reading the tales of woe on the Tesla 65-75 mph hum problems and spending a lifetime stabilizing power circuits with bode plots.
Conclusion: Closing the loop in a complete electronic circuit is relatively easy to do because the component values never change. When you add a three-phase inverter and induction motor and a 5000 lb. car to the loop, response times will vary with age and torque settings, etc. If what is described is the case, Tesla will probably have to slow the response time in future designs. If the problem requires changing bearings or motor mounts, Tesla might face another recall on all its cars, as the hum problem seems to come with age and miles driven. This problem could also haunt the Model X with four-wheel drive.
I have no positions in Tesla stock.
Disclosure: I have no positions in any stocks mentioned, and no plans to initiate any positions within the next 72 hours. I wrote this article myself, and it expresses my own opinions. I am not receiving compensation for it (other than from Seeking Alpha). I have no business relationship with any company whose stock is mentioned in this article.