(Second teardown article in a series)

By John Scott-Thomas, TechInsights

The Chevrolet Volt, GM’s entry in the electric car market,  is one of the most complex vehicles on the road, using almost 100 microprocessors that are controlled by about 10 million lines of code. By comparison, a Boeing 787 Dreamliner  gets by with only 6.5 million lines. One of the real wizards in the Volt is the Power Inverter Module. Sophisticated hardware and control systems sit in this breadbox-sized component. In this article we’ll take a look inside.

The Power Inverter is responsible for transferring power between the main battery pack, the gas engine, and the wheels of the vehicle. To understand why this is important we first need to appreciate the severe limits of the battery pack.

In a previous article, we looked at the Lithium Ion battery pack inside the Volt. The 435-pound battery stores 10 kiloWatt-hours  of energy (the maximum capacity is actually 16 kW-h, but it uses only a fraction of this  in order to extend the battery life), and can move the vehicle about 35 miles on a full charge.  In contrast, a single gallon of gas has about 36.6 kW-h of energy and can move a car about the same distance.    

From this comparison two things follow: gas  engines are terribly inefficient, and battery packs are, even today, poor reservoirs of energy. We can make a rough equivalence;  the 435 pound Volt battery pack is about the same as a gallon of gas weighing only 6 pounds.  

Driving a  Volt with a  charged battery is a bit like driving a gas car  with the “Fuel Empty” warning light always on! It’s no wonder that electric car drivers often suffer from “Range Anxiety”- the nagging fear that you are going to run out of juice. To get around this concern, GM has put an auxiliary 1.4 L gas engine in the Volt that kicks in when the battery charge is low.

Job 1: using energy efficiently

The basic job of the Power Inverter Module is to make the best use of every bit of energy in the battery pack, and finally when the battery is discharged to use the gas engine to move the car and recharge the battery. Here’s how it does this.  Looking at Figure 1 the Inverter sits at the center of the battery pack, gas engine, traction motor, generator motor, and drive unit (transmission). There are four different modes of operation for getting energy to the wheels, and a fifth mode for transferring energy from the wheels to the battery in a procedure called regenerative braking:

1. Drive Mode 1: The Volt usually operates in this mode.  The Power Inverter Module takes 360 Volt Direct Current power from the battery pack and converts it to three phase Alternating Current energy that powers the wheels via the traction motor.
2. Drive Mode 2: At high speeds, the traction motor is spinning too fast and becomes inefficient. The Power Inverter then engages the generator motor in parallel with the traction motor. In this way, both electric motors operate at a slower, more efficient speed. This can add a mile or two to the range of the battery pack.
3. Drive Mode 3: As the battery discharges, the gas engine turns on. The engine powers the generator motor, which the Power Inverter uses to simultaneously recharge the battery and power the traction motor to move the car.
4. Drive Mode 4: In this mode, the gas engine  powers the generator motor which also mechanically couples to the wheels.  This is the drive mode that makes the Volt a hybrid  more than a pure electric vehicle. It can improve fuel efficiency by 10%.
5. Regenerative Braking: When the brakes are applied, energy can be harvested from the kinetic energy of the car. The Power Inverter Module runs in reverse; the wheels turn the traction motor, the Inverter converts AC energy to DC energy that charges the battery. This technique can improve range by up to 10 %.  There are interesting control issues here. The Volt has standard calliper brakes as well as regenerative braking. To stop the car rapidly, the calliper brakes are used. A brake control module decides when to switch between regenerative braking and calliper braking; an intuitive display on the dashboard tells the driver when the vehicle is using different braking modes. We’ll look at that in a future article.

The Power Inverter  Module is GM’s name for the control electronics that co-ordinate all these modes. Its primary function is to operate as a basic electrical inverter, which is a standard electrical circuit that  takes DC power and converts it to AC power.  Electrical inverters are common in solar power panels and switching power converters.  In addition to this inverter operation, the Power Inverter Module converts AC power to DC power much in the way an alternator is used in a gas car to charge the lead acid battery. This function is required during regenerative braking.  The Module also configures the transmission,  traction motor, and generator motor to enable the different Drive Modes as required.

Stripping off the beauty cover

A deeper look required removal of the Power Inverter Module from the car. The Module is located under the front hood of the Volt on the driver’s side.  GM  protects the device with a “beauty” cover having a safety interlock that disables the high voltage cables if the cover is removed. Two high voltage cables (one to the battery, and one to the A/C compressor) are connected along with two sets of three phase power cables that go to the traction and generator motors.  Additional inputs include two resolvers used to measure the rotation of the two electrical motors , temperature sensors for the motors, an engine crank sensor, and standard 12 V power and ground inputs.

Opening the Inverter Module  shows a number of notable features.  The module and the electrical motor manufacturing  is subcontracted to Hitachi Automotive Systems. The module is clearly divided into a side having low voltage control electronics and a side with power electronics.  A coolant loop runs through the middle of the Module, and is cooled by one of the four radiator segments.

Switching converters step the 360 Volt DC battery power down to DC levels that can drive the motors. The power electronics boards use large IGBTs ( Insulated Gate Bipolar Transistors) to generate three phase AC power for the motors.  Current sensing resistors are placed   on the inverter  output to adjust the AC phase, ensuring optimal power transfer.

On the low voltage/control side, two  low current connectors (40 pins each)  use at least seven communication lines to connect to the rest of the vehicle. The main control Printed Circuit Board (PCB) is shown in Figure 2. The PCB is clearly marked with Hitachi branding. There are four Freescale microprocessors that constitute the brain of the Invert Module.  The three smaller processors control the traction motor, generator motor, and transmission hydraulics.

A surprise

We were initially surprised to see a dedicated transmission controller, but further analysis of the transmission operation clarified this need.  Three clutches control the engagement of the electrical motors, wheels, and gas engine as the Volt switches between different Drive Modes and Regenerative Braking; this controller co-ordinates the clutch switching.

The largest microprocessor is the main Supervisory controller. It takes the charge state of the battery, the position of the accelerator and brake pedals, the gas engine state  and the electric motor speeds and determines the optimal configuration for the power train. The packaging of the controller was removed in a process called “decapping” (basically treating the IC with aggressive acid) to show the bare die seen in Figure 2.  The supervisory processer is a Freescale Qorivva 32-bit MPC6230 embedded controller, using a 3.3V power supply and 80-144 MHz clock. A single channel Analog to Digital Converter is on board. Three MB of flash memory are used to hold the firmware; almost half the die area is used for this. The Supervisory controller has the largest memory of any IC we have seen on the Volt powertrain.

Besides   safety, the success of the Volt depends on the ability to drive efficiently and smoothly using the battery energy as much as possible. The Power Inverter Module is responsible for much of this task. The module is subcontracted to Hitachi, and the control electronics use Freescale microprocessors. A delicate interplay of two electric motors, a gas engine and a battery pack combine to create a driving experience that accurately resembles a conventional car .  In the next article, we’ll look at the electronics behind the dashboard that make up the Infotainment system.

Figure 1: The Powertrain of the Chevrolet Volt. The arrows show power connections; control signals are not shown.

Figure 2: Main Control Printed Circuit Board on the Power Inverter Module. The left inset show the Module mounted in the Volt.

Click here to see our Design West slide show presentation of the Volt's charge/powertrain system.

Teardown Video

Here are Steier and Scott-Thomas presenting the charge system and powertrain electronics at Design West here (32:14).