Power Inverters 101

The power inverter used in a hybrid or electric vehicle controls the power that the battery pack transfers to the three phase (3Ø) electric machine (motor or generator).  It also rectifies or, inverts, alternating current (AC) to direct current (DC) energy the electric machines generate and transfer back to the battery pack.  The power inverter system (Figure 1) is one of the most important systems on the hybrid vehicle because can serve as the central control point for distributing high voltage to other components in the high voltage system.  In many respects the power inverter system provides the same functionality to the electric machine as the fuel and ignitions provide to the internal combustion engine.  The power inverter is responsible for varying the frequency, voltage level, and type of waveform to the electric machine for controlling its torque and speed. 

Figure 1.  Toyota Prius power inverter location (engine compartment)

The primary reason of why AC energy is used instead of DC in contemporary production hybrid or electric vehicles is that AC is approximately 10% - 20% more efficient than a DC electric machine when the same load is applied.  Also, DC electric machines are limited to a much lower speed range and operating voltage levels.  This is due to the DC machine commutator brushes that cannot change their contact angle on the commutator bars to advance or retard electric current timing at higher speeds, and higher operating voltages would result in significant brush arcing.  At most load ranges the DC machines can use significantly higher levels of electrical current when compared to its AC counterpart (e.g., 625 amps DC machine vs. ≈ 300 amps AC machine @ 150 lb/ft torque).   Additionally, a DC motor is rpm limited to lower rpm ranges; it typically would need a multispeed gear box to limit the rpm band to supply the required torque levels.  The AC motor does not require a multi-speed gear box to operate in any speed or torque range.  Finally, depending on the motor design, the cost of an AC motor can be significantly lower than its DC counterpart, due mostly to the reduced copper requirement and steel (content and mass) necessary to provide a comparable AC machine rpm and torque band.  In summary, AC electric machines are comparatively much smaller, less expensive (in most cases) have less mass (weight), can operate at significantly higher voltage and speed ranges, and are infinitely variable throughout these ranges.

Power Inverter Building Blocks

The power inverter is the basic component of an electric propulsion system and is controlled by a microprocessor that receives inputs from various input sensors, as illustrated in Figure 2.  It should be noted that most of the sensors illustrated in Figure 2 are identical to the sensors that can be found in the typical ICE emission and fuel control systems.  For example, the throttle position sensor system, speed (rpm) sensor, and brake sensor (switch) are typical sensing systems found on traditional ICE systems.  The sensor that would not be found as part of an ICE control system would be the electrical current sensor (three sensors shown in Figure 2). 

Figure 2.  IM and PM motor, power inverter, and controller

The basic design of the 3Ø power inverter consists of the six power switches (transistors) configured as an H bridge with gate drive circuits.  Note in Figure 2 that there were three cables connected from the power inverter to the three electric machine fields that will transfer 3Ø energy during propulsion or regenerative braking modes. Power devices, usually an insulated gate bipolar transistor (IGBT), control the energy at proper amplitudes to the traction motor that will result in the desired torque level (Figure 3).   The IGBT illustrated in Figure 3 will control and provide electrical power to one electric machine.

Figure 3.  Insulated Gate BiPolar Transistor (IGBT) – Ford Escape

Using an “H” Bridge circuit will be an important first step in understanding the operation of how a power inverter systems uses an IGBT transistor circuit to control energy through the stator windings of an electric machine.  Therefore, to assist in the understanding of either induction/permanent magnet (IM/PM) motor control strategy, the H bridge circuit will be examined to form a point of reference.  Automotive technicians are currently engaged in working with H bridge circuits on a daily basis.  This type of design can be found in motor drive circuits that require bi-directional (clockwise and counter-clockwise) rotation or stop and hold the motor at a specific position.  As an example, the H bridge circuit can be found in the idle air control (IAC) circuit and automatic air conditioning circuits to control the idle speed of an ICE or the blend/defrost/air delivery doors (see Figure 4).  Both the IAC and air conditioning motors are bi-directional PM machines.  This is accomplished by turning ON the H bridge transistors in tandem, and electrical current flow direction through the coil can be changed allowing the motor to run in two directions.  By turning ON Q2 and Q3 the motor will turn in one direction.  Turning OFF Q2 and Q3, and turning ON Q1 and Q4 will permit the motor to rotate in the opposite direction.

Figure 4.  H bridge circuit

The IM/PM for a hybrid and electric is also bi-directional (i.e., the rotor shaft can be driven by electrical current to rotate in a clockwise or counter-clockwise direction depending on which direction electrical current is applied to the field windings). The only difference between a power inverter and an IAC circuit is the amount of electrical power being controlled in the circuit and how the power is applied.

Next Month: Power Inverters 101 Part Two:  Creating Sine Waves for the AC Machine

Until next time remember: Knowledge is Power

- AR&D Tech Team -