Modeling optimizes drive systems

Roy Davis
Simulation Engineering Manager
Ecostar Electric Drive Systems L.L.C.
Dearborn, Mich.

Edited by Miles Budimir

Software tools for modeling complex electromechanical systems have grown sophisticated. The state of the art has progressed to a point where simulation programs can model combinations of power transmissions, electric motors, motion controls, and power drives (inverters, converters, and so forth).

An example of current simulation capabilities comes from Ecostar Electric Drive Systems L.L.C. in Dearborn, Mich. Engineers there modeled an integrated electric powertrain system that combined a motor, drive electronics, and a mechanical transmission. Integrating electronics such as sensors, actuators, and power converters into a hostile automotive environment makes modeling a necessity.

In the electric powertrain, the ac induction motor is driven by an inverter and controller employing a vector control algorithm. Insulatedgate-bipolar transistors (IGBTs) power the motor with a peak power rating of 65 kW and a continuous power rating of 45 kW. A pulse-width-modulation (PWM) current drive is used with the input dc bus voltage varying from 250 to 400 V.

System modeling and optimization starts by identifying input and output signals. Both digital logic and actual analog measurements are identified in the overall component model. The inverter model uses the input voltage command signals, Vd and Vq, to establish the desired motor-phase voltages. A 12-Vdc supply (LVDC) is used to power the inverter logic circuitry. The inverter is liquid cooled. The coolant flow rate, min, and temperature, Tcool, are measured as the water enters the inverter.

The inverter sends signals back to the motor controls for monitoring inverter heat-sink temperature, Theat sink, three-phase current levels (Ia, Ib, and Ic), and the dc bus voltage, Vdc. It also puts out four signals to permit evaluation of vehicle parameters such as power lost through switching and conduction, coolant pressure drop, Pout, inverter drive current, Idc, and inverter logic current.

The three-phase voltages (Va, Vb, and Vc) from the inverter drive the motor. The output shaft speed, mot, and position, mot, are measured and used to control the motor. Simulation software models the motor equivalent circuit model, and finite-element-analysis programs assist in identifying its thermal and magnetic qualities. The motor's electrical and mechanical losses are identified by testing and other simulation programs.

Key ac induction motor outputs include airgap torque, Mairgap, rotor position, mot, and speed, mot. Motor temperature, Tmot, and rotor position signals are sent to the motor control simulation. The motor's coolant pressure drop and measurements of power loss (copper, core, and friction losses) are available to evaluate system level performance. Motor-cooling jacket qualities are modeled using a fluid-dynamics simulation program.

The mechanical transmission integrates the transaxle and motor. It is a single-speed transaxle with four gear ratios available and inter-faces the motor to the front or rear wheels. Simulation software helps identify mechanical elements such as gear ratio, inertias, backlash, stiffness, friction and viscous damping. Transmission losses are simulated and available to evaluate system performance.

The motor controller is the central processing unit for the electric drivetrain. Modeling evaluates and optimizes the current control algorithms and different PWM schemes used by the inverter to drive the motor. The electrical, mechanical, and environmental signals provide the continuous information needed to optimally control the electric powertrain, and communicate with the vehicle system controller via the CAN bus. Performance can be measured in dollars per kilowatt, kilowatts per liter, kilowatts per kilogram, system efficiency, or durability.