Power Control

Controlling power during driving and charging.Tesla's Power Electronics control power during driving and charging.

The Power Electronics Module functions as a bridge for energy between the charge port, battery, and motor.  Every electron ever used in a Roadster, from the motor drive to the dome light, flows through the Power Electronics Module.

All of the power electronics systems in the Roadster have the same basic function, routing power from a source to either be converted to mechanical power or stored as energy in the battery. The Roadster battery, as with all batteries, stores electricity at a DC voltage. The motor uses energy in the form of an AC voltage. The voltage is varied by turning switches called IGBTs on and off very quickly. As the IGBTs allow more current from the battery to the motor, the AC waveforms grow in amplitude until peak torque is produced in the motor. In the video below, watch how the AC intensifies as the Roadster accelerates from 0 to 60mph.

In addition, the voltage that is supplied to the Roadster when charging is AC. Therefore, the charger in the Power Electronics Module functions to convert AC to DC when charging the battery and then the Power Electronics Module switches it back from DC to AC when the car is driving.

In drive mode, the Power Electronics Module responds to information from the accelerator pedal, motor speed sensor, ABS speed sensors, and other powertrain sensors.  The Power Electronics Module determines requested torque from the pedal position and monitors the ABS speed sensors to detect if tires are slipping. Based on sensor feedback, it produces torque by converting the DC voltage stored in the battery to the appropriate AC voltage at the motor terminals. As the driver steps on the accelerator pedal, the Power Electronics Module begins to control increasing motor current and voltage to produce the torque required to accelerate from 0 to 60 mph in 3.7 seconds.  

In charge mode, the Power Electronics Module converts AC voltage from the grid at between 90V and 265V, into DC voltages between 250V and 425V.   The wide allowable input AC voltage range enables the car to charge from nearly any outlet in the world. Input frequency can be either 50Hz or 60Hz, again for global compatibility. The Vehicle Management System (VMS) calculates the battery’s state of charge and sends charge requests to the PEM which then outputs power to the battery based on the requested charge current.


Under the PEM Cover

Inside the Power Electronics Module, there are three major systems – power stages, a controller, and a line filter. The most complex is the power stages, called Megapoles. The Megapoles are large semiconductor switch arrays that connect the charge port or motor to the battery depending on if the car is charging or driving.

Within the Megapoles, there are six different switches, grouped into three pairs known as half bridges. In drive mode, each bridge forms a phase. Each phase connects to a phase of the 3-phase AC induction motor. In charge mode, only two bridges are required, one for each wire in the AC line. The charge and drive modes are configured using a set of four large relays known as contactors. The contactors allow the semiconductor switches to be used to connect the battery to either the charge port or the motor. When the Roadster is turned on, a series of clicking sounds can be heard as the contactors close the connection to the motor.

Each switch is composed of fourteen insulated gate bipolar transistors, or IGBTs. Without IGBTs, today's advanced electric vehicles would not be possible. In total, 84 IGBTs are used in the Power Electronics Module. Each IGBT is less than one square inch and about a quarter of an inch thick. Inside the IGBT package is a small piece of silicon, about the thickness of a few sheets of paper and a quarter of an inch per side. The total area of IGBT silicon in the entire Power Electronics Module is less than five square inches (about the size of a business card).

The second major component of the Power Electronics Module is the controller board that turns the switches on and off. The switches can turn on and off up to thirty-two thousand times per second. The controller contains two processors: the main DSP and a secondary safety processor. The DSP controls torque, charge behaviors and interprets requests from the Vehicle Management System. The safety processor monitors the accelerator pedal and the motor current to detect unexpected behaviors. If the safety processor measures motor current inconsistent with accelerator pedal position, it can stop the system. While this behavior is extremely unlikely, this redundancy means a glitch in the main DSP can’t produce unexpected torque.

The third major component of the Power Electronics Module is the charge input filter. When the Roadster is charging and the IGBTs are switching at 32kHz, a large amount of electrical noise is created on the AC side of the power stages. If the noise was allowed to conduct back into the power lines, it could interfere with other appliances, radios, cell phones, etc. A group of large inductors called chokes are placed between the IGBTs and the charge port to filter out the noise and avoid unwanted interference.

Putting this all together, it is amazing to consider that a bunch of tiny pieces of silicon, totaling less area than a business card, can turn on and off tens of thousands of times per second, and control the flow of over 900 amps of current to the Roadster motor. The amazing driving experience is simply controlled by silicon chips and a spinning rotor instead of pistons, fuel injectors, camshafts, a crankshaft, valves, and a whole lot of gasoline.

Traction Control

Tesla power control enables a traction control system with amazing improvements over systems in internal combustion cars.  Traction control systems for ICE vehicles have a few options to maintain traction at prescribed levels: kill engine spark, reduce fuel supply, or use electronic throttle control to actively modulate throttle requests. Fundamentally, it is practically impossible to maintain near-zero output torque from an ICE, whereas zero torque is simple to maintain in an electric drivetrain. In the Roadster, the motor torque can be accurately reduced either gradually or quickly - resulting in better control with less noticeable loss of power.  With onboard sensors, the car predicts achievable traction when cornering before the driver can even command a change in acceleration. It’s much safer to avoid loss of traction than react to it. Expert test drivers have found they are able to achieve higher performance with the Roadster traction control system than in comparable gas-powered vehicles.


Traction Control: On

Traction Control: Off