TESLA ROADSTER: THE NEW STANDARD OF ELECTRIC AUTOMOBILES

Cara Hutter
Tyler Starmack
functions as a current monitor. If the current going to the
motor is inconsistent with the acceleration pedal, it will stop
the system [6]. This prevents the Roadster from accelerating
more than the driver intends.
The last system in the PEM is the line filter. The line
filter is a series of inductors (devices that store energy in
magnetic fields) called chokes that are placed between the
charge port and the IGBTs. Their purpose is to filter out
electrical noise which is a result of the IGBTs are turning off
and on at a rapid rate while the Roadster is charging. This
noise, if allowed to conduct back through the power lines,
would cause interference in other electronic devices such as
radios and cell phones [6].
The PEM is indeed a complex system that is incredibly
important to the Roadster. It, in combination with the ESS,
provides the necessary power to run the electric motor,
which will be discussed, in detail, in the next section.
The Electric Motor
The ESS and PEM are vital to the functioning of the
Roadster-they would mean nothing if the electric motor did
not exist. In order for the car to drive, the motor is essential
because it is connected to the back axel and therefore the
wheels. The type of motor that the Roadster uses is called a
three-phase AC induction motor, and is one of the most
common types of electric motors (Figure 6).
FIGURE 6
ELECTRIC MOTOR [7]
The motor consists of two main parts: the rotor and the
stator (Figure 7). The rotor consists of a steel shaft with
copper bars running through it. As the rotor turns, the wheels
do as well, moving the car. The stator is stationary and
encases the rotor, but does not touch it. 900 amperes (amps)
of current are delivered to the stator through copper wires
(used for their low resistance, and therefore can endure more
current) that are wound through a stack of steel plates. There
are three sets of these wires, each corresponding to the three
phases of the motor [7].
FIGURE 7
CROSS SECTION OF ELECTRIC MOTOR. ROTOR:
INNER BLUE CIRCLE, STATOR: OUTER RING [8]
As the alternating current from the PEM flows through
the copper wires in the stator, a magnetic field is produced
that, like the current, alternates between a North and South
Pole. The three phases of the motor occur because of the
three sets of alternating currents in the three sets of wires.
The magnetic fields resulting from each set of wires are
slightly out of time with each other. This creates a ripple of
magnetic field travelling around the stator. Tesla Motors
describes this by way of analogy, “The magnetic field
appears to move in a circular path around the stator- similar
to the way spectators in a sports stadium create the illusion
of a ‘wave’ by alternating between standing or sitting in
concert with other fans” [7].
The magnetic field from the stator then induces a current
in the copper bars within the rotor, which then creates an
opposite magnetic field around the rotor, due to Lenz’s Law.
This law states, “An induced current has a direction such
that the magnetic field due to the current opposes the change
in magnetic flux that induces the current” [9]. This means
that the rotor will have a magnetic field that is opposite of
the stator, and because the magnetic field of the stator is
constantly moving around in a circle, the rotor will spin to
follow it. This, in turn, will provide the torque that is
necessary to spin the wheels. The rotor is also positioned in
such a way that its magnetic field is always “behind” the
stator’s. This ensures that the rotor keeps spinning. This also
means that the farther behind the rotor is from the stator, the
more torque is being produced (when accelerating) [7].
Torque, then, is always being produced as long as the
rotor is spinning. This means that there is no need for this
type of automobile to have a transmission with gears since it
produces effective torque at a wide range of rpms (rotations
per minute). This simplifies the running process of the
Roadster to an extreme degree since there are little to no
timing issues possible (unlike a car with an ICE). There is
also no reverse “gear” in the Roadster. All that needs to be
done in order to put the car in reverse is to switch two of the
phases of the motor so the magnetic field runs in the
opposite direction. This completely eliminates the need for a
transmission, and thus increases the Roadster’s efficiency
and contributes to its sustainability, which will be discussed
in the next section [7].
University of Pittsburgh April 2, 2013
Swanson School of Engineering
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