About the size of a watermelon, with a lot more juice.
FIRST, CONSIDER THE ALTERNATIVE
The Internal Combustion Engine is a complex, amazing machine. In perfect concert, valves open, spark plugs ignite, pistons move, and the crankshaft turns. Every fourth cycle an air-fuel mix explodes and a piston is forced down. The crankshaft converts the linear motion of the piston and connecting rod to rotational motion that eventually propels the vehicle.
Unfortunately, Internal Combustion Engine complexity results in wasted energy. At best, only 30% of the energy stored in gasoline is converted to forward motion. The rest is wasted as heat and noise. When the engine is not spinning, there is no torque available. In fact, the engine must turn at several hundred revolutions per minute (RPM) before it can generate enough power to overcome its own internal losses – that’s why cars idle around 1,000 RPM when at rest.
An Internal Combustion Engine does not develop peak torque until many thousand RPM. Once peak torque is reached, it starts to drop-off quickly. To overcome this narrow torque range, multi-speed transmissions are employed to create gear ratios that keep the engine spinning where it's most effective.
Internal Combustion Engine power output could be improved with faster rotation. However, combustion engines have a limit to how fast they can spin - as RPM exceeds 5,000 or 6,000 it becomes challenging and costly to keep the timing of the engine on track and keep all of the parts together. For example think of the springs that push the valves closed: a spring can only bounce back into place so quickly. As the engine RPM increases the springs can fall behind and the valve could end up striking the piston, leading to catastrophic engine failure.
Replace the Engine with a Motor
Electric motors are profoundly simple. The motor converts electricity into mechanical power and also acts as a generator, turning mechanical power into electricity. Compared to the myriad parts in an engine, the Roadster motor has only one moving piece — the rotor. The spinning rotor eliminates conversion of linear motion to rotational motion and has no mechanical timing issues to overcome.
With an electric motor, instant torque is available at any RPM. The entire rotational force of the motor is available the instant the accelerator is pressed. Peak torque stays constant to nearly 6000 RPM, only then does it start to slowly roll-off.
The wide torque band, particularly the torque available at low RPM, eliminates the need for gears – the Roadster has only a single speed gear reduction; one gear ratio from zero to top speed. Switch two of the phases (this can be done electronically), and the motor runs in reverse. No need for a reverse gear. Not only is this design incredibly simple, reliable, compact, and lightweight, but it allows a unique and exhilarating driving experience. The Roadster accelerates faster than most sports cars and whether driving on windy mountain roads or cruising down the highway, there’s always instant torque.
Tesla's electric motor is not just a great generator of torque - it is able to create torque efficiently. The Roadster achieves an overall driving efficiency of 88%, about three times the efficiency of a conventional car.
As driving conditions permit, the Roadster motor acts as a generator to recharge the battery. When the accelerator pedal is released, the motor switches to ‘generator’ mode and captures energy while slowing the car. The experience is similar to ‘engine braking’ in a conventional car, but far more intuitive – the driver controls the car’s speed with nuanced adjustments of the right foot.
Electric motors come in many varieties, each with a different approach to creating mechanical force (torque) from the simple interaction of two magnetic fields. The Tesla Roadster uses a three-phase Alternating Current (AC) Induction motor. The AC Induction motor was first patented by Nikola Tesla in 1888. AC Induction motors are widely used in industry for their reliability, simplicity, and efficiency.
The Roadster motor has two primary components: a rotor and a stator. The rotor is a shaft of steel with copper bars running through it. It rotates and, in doing so, turns the wheels. The stationary stator surrounds, but does not touch, the rotor. The stator has two functions: it creates a rotating magnetic field and it induces a current in the rotor. The current creates a second magnetic field in the rotor that chases the rotating stator field. The end result is torque. Some motors use permanent magnets, but not the Roadster motor -- the magnetic field is created completely from electricity.
The stator is assembled by winding coils of copper wire through a stack of thin steel plates called laminations. The copper wire conducts the electricity fed into the motor from the Power Electronics Module. There are three sets of wires – each wire conducts one of the three phases of electricity. Think of a phase as a wave with peaks and valleys (a sine wave). The wave is “alternating” between peaks and valleys. The three phases are offset from each other such that combing the rises and falls of each phase creates a smooth supply of current—and therefore power. The flow of alternating current into the copper windings creates a magnetic field. This is electromagnetism. And just as the current in each phase constantly rises and falls, the magnetic field also varies between “North” and “South”.
Because of the way the copper coils are placed within the stator, 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 nearby fans.
The copper bars mentioned earlier are “shorted” to each other (referred to as a “squirrel cage”) which allows current to flow with little resistance from one side of the rotor to the other. The rotor does not have a direct supply of electricity. When a conductor (the copper bars) is moved through a magnetic field (created by the Alternating Current in the stator), a current is induced. This is induction.
Because the stator magnetic field is moving (remember the wave), the rotor is always trying to catch up. The interaction of the magnetic fields creates torque. The amount of torque produced is related to the relative position of the rotor field to the rolling ‘wave’ of magnetism in the stator (the stator field). The further the rotor field is from the ‘wave’, the more torque is produced. Since the stator field is always ahead of the rotor when the accelerator is depressed, the rotor is always spinning to catch up, and it is continuously producing torque.
When the driver releases the accelerator pedal, the Power Electronics Module immediately changes the position of the stator field to behind the rotor field. Now, the rotor must slow down to align its field with the stator field. The direction of current in the stator switches direction, and energy starts to flow, through the Power Electronics Module, back to the battery. This is energy regeneration.
How is the motor controlled? How does the motor know when to be a motor and when to be a generator? How does it know how much torque to supply?
It depends on the driver and their interaction with the accelerator pedal. When the accelerator pedal is pressed, the Power Electronics Module interprets a request for torque. Flooring it means a request for 100% of the available torque. Half-way? A request for partial torque. Letting off the accelerator pedal means a request for re-gen. The Power Electronics Module interprets the accelerator pedal input and sends the appropriate amount of alternating current to the stator. Torque is created in the motor and the car accelerates.
The Tesla Advantage
Though it shares a history with industrial machines, the Roadster motor is extremely unique. ‘Traction’ motors must remain small and light. About the size of a watermelon (but a little heavier), the Roadster motor is a fraction of the size of an industrial machine capable of similar power levels. Aluminum aircraft alloys are used to maintain an advantageous strength to weight ratio and ceramic bearings are used for long-life and reduced friction, even at high speed. High-strength alloy steels are required to manage the massive torque.
The Power Electronics Module supplies as much as 900 amps of current to the stator. To handle such high current levels, the stator coils in a Tesla motor employ significantly more copper than a traditional motor of its size. The copper is tightly packed in a proprietary winding pattern to optimize efficiency and power.
The copper loops are encapsulated by special polymers that facilitate heat transfer and ensure reliability under the demands of high-performance driving in extreme conditions. The motor is, as are all parts of the Roadster, tested to withstand both Arctic winters and Phoenix summers.
High stator currents mean high rotor currents. Unlike typical induction motors which employ aluminum for its conductors, the Roadster rotor conductors are made of copper. Copper, while harder to work with, has a much lower resistance and can therefore handle higher currents. Special care is taken in the motor design to handle the high speed (14,000 RPM).
Though highly efficient, the motor still generates some heat. To keep within acceptable operating temperatures, specially engineered cooling fins have been integrated into the housing and a fan is employed to blow air across the fins to most effectively extract the heat. This helps keep the overall package light and tight.