Drive Unit / Motor / Rotor

Sarah Schumm, Tesla Rotor Machinist

Sarah Schumm, Tesla Rotor Machinist

On an excursion to Yosemite, I happened to  meet the machinist who built my Model S rotor. Her name is Sarah Schumm and she built and machined all the rotors till around serial number 5,000.

This wonderful chance meeting has lead me to discuss the electric motor of the Model S.  As I mentioned previously, I have a degree in Electrical Engineering.  I have to admit my least favorite class was Electromagnetics but that was largely due to both a very poor professor and a bad book.

Electric cars are driven by very simple magnetic principles.  Anytime electricity runs through a wire, a magnetic field surrounds that wire.  By turning on and off electricity on different places around a cylinder, the magnetic field can essentially move in a circle.

An electric motor contains two parts:  the fixed outside stator (represented in grey below) and the rotating rotor (in yellow and purple) with the magnetic polarities of north (N) and south (S). The rotor is very simplistically a large magnet. The accelerator causes a series of electrical pulses along the stator to generate a moving magnetic field.  The rotor will respond by trying to line up the magnets as the magnetic field moves along the stator over time.

Very Simplistic Motor

Very Simplistic Motor

The stator has various wires in different locations.  By controlling the electricity in these fixed wires, the magnetic field can essentially move in a circle causing the stator to physically rotate clockwise.  In the first circle, a electricity through a wire creates a south pole magnetic field.  The bottom of the stator with the matching pole is repelled by this field, and the upper north pole is attracted to the field causing the magnet to turn clockwise as in the second picture.  The stator then turns on a different wire further along the circle to continue the movement of the rotor causing an additional turn in the same direction.

This example is very simplistic.  The actual rotor that Sarah machines for both the Model S and the upcoming Model X is also much more complex that one large magnet. The Model S uses a 3 phase four pole alternate current (A/C) system.

To learn more about the actual motor, you can read an official Tesla whitepaper about the Roadster’s motor.  Or watch the following discovery channel video.

An electric motor is more efficient than a gas engine.  An electric motor simply has less moving parts.  By eliminating the engine, an electric car needs no fuel, engine-oil, spark plugs and emits no exhaust.

Another driver has been blogging about his Model S during the same period I have.   With 30,000 miles he has had four drive units replaced.  Since this blogger is Dan Edmunds of the car website he has received a fair amount of attention.

The drive unit contains the motor, some simple gears and the inverter.  When the car is built the drive unit is completely sealed.  So when anything goes wrong with any of those components, the whole unit needs to be replaced.

I think his experience is somewhat unique.  A poll occurred on the Tesla Motors Club Forum that found 61 other owners have had their drive units replaced with eleven having repeat replacements.  Most of the owners report just simple noise issues not catastrophic failures.

There is no way to statistically compare this data in any accurate way.  But from my observations, the problems with inside edge tire wear seem to be a higher level of concern amongst the owners.


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