Efficiency Map of 100‐kW PMSM Drive
Overview
The efficiency map of a motor drive system is important design information as it gives system engineers a picture of the overall efficiency of the drive and what operating point to operate at for maximum efficiency.
The generation of the efficiency map of a PMSM drive system, however, is a significant task. It involves comprehensive load testing of the complete PMSM drive system with a motor dynamometer. The workload, time, and cost prohibit such activity most times. It is particularly true for a large power size PMSM drive system. The alternative way to obtain an efficiency map of a large power PMSM drive system is often through simulation. The FEA-based motor model with accurate motor control as well as with automated simulations is required to obtain the efficiency map of the complete PMSM drive system. The simulation setup of a motor control system for an FEA-based motor is a non-trivial task by itself, and script programming is also required for the automation of simulations.
The Motor Control Design Suite and automation script features make this task considerably easier. Pre-built design templates are provided for FEA-based motors from JMAG (in JMAG-RT format) and motor control functions. Based on minimum system specifications, the Motor Control Design Suite automatically designs all the controllers for speed and current loops with properly specified crossover frequencies and stability margins and implements advanced control algorithms such as Maximum-Torque-Per-Ampere (MTPA) control, field weakening control, and Maximum-Torque-Per-Volt (MTPV) control. It will generate a complete system that is operational and ready to simulate in minutes. The script programming code can help to automate numerous simulations without users’ intervention for generating efficiency data of the PMSM drive system.
This application note describes how to generate the efficiency map of a 100-kW PMSM motor drive system using the PMSM (IPM) Drive (JMAG-RT)(torque control) template and the script code efficiency map – speed-torque limit script code. Files for this application note can be found in the folder “Application Notes\Efficiency map of 100-kW PMSM Drive” in PSIM.
Please refer to a companion application note Control Design of 100-kW PMSM Drive (AN001-01).pdf for speed control design of the same motor.
In the PMSM (IPM) Drive (JMAG-RT) template, the IPM motor is modeled in a JMAG-RT model. A JMAG-RT model is derived from JMAG finite element analysis. It delivers a very high level of fidelity and accuracy as compared to the actual motor, with all nonlinear effects (such as saturation, spatial harmonics) included.
Unlike the motor in the linear PMSM drive system, the JMAG-RT motor in this system is nonlinear, and d‐axis and q‐axis inductances are functions of the motor currents Id and Iq. The nonlinear inductances Ld and Lq are conveniently obtained from the JMAG-RT model in real-time and are fed back to various control blocks to achieve the optimal control performance.
The power losses of the PMSM motor including copper loss and core loss can be obtained from the JMAG-RT model, and the power loss of the drive including switching loss and conduction loss can also be obtained from IGBT thermal module during the simulation. Hence the PMSM drive system power loss and efficiency can be easily derived from the power losses of the PMSM and its drive.
System Specifications
The 100-kW PMSM is based on the JMAG-RT motor model PMSM RTML-034 available from https://www.jmag-international.com/modellibrary/034/). It has the following parameters:
A 3-phase inverter with a 500-V dc bus and 283A current will be used to drive the motor.
Step-by-Step Design Procedure
Several major steps are needed in order to derive the efficiency map of a PMSM drive system using the Motor Control Design Suite:
- Create an operational torque control system for the PMSM motor drive
- Use the thermal module of the inverter to obtain the IGBT power loss, and add efficiency calculation
- Run the script code to sweep the speed and torque and automatically simulate the motor drive at different operating points and obtain the efficiency map of the drive system
- Plot the efficiency map contour
Follow the steps below to perform the efficiency map generation.
Step 1: Launch the Design Suite template
In PSIM, go to Design Suites >> Motor Control Design Suite, and select PMSM (IPM) Drive (JMAG-RT)(torque control). Choose the folder to unpack the files. The template will be loaded to PSIM as shown below.
The Input Parameter Panel of the template and the PMSM block in the JMAG-RT model are highlighted in red. The advanced Torque Control block (also highlighted in red) includes MTPA, field weakening, and MTPV control all in one.
Step 2: Load the JMAG-RT model file
Double click on the PMSM block. The following dialog menu will appear:
Click on the Browse button (highlighted in red above). Select and load the correct JMAG-RT file for the 100-kW PMSM, as shown below:
For this JMAG-RT model, the spatial harmonics model is available. Set the Accuracy Type to Spatial Harmonics to take into account the spatial harmonics effect.
Step 3: Enter system parameters
Enter parameters of DC Bus, Inverter, Load, Motor, and Motor Controller in the Parameter Panel based on design requirement, operation conditions, and motor parameters, as shown below:
One needs to decide the proper values of the switching frequency fsw, the sampling frequency fsam of the current loop, and the sampling frequency fsam_w of the speed loop. One also needs to decide the crossover frequencies fcr_i and fcr_Te of the current loop and the torque loop. Typically, fcr_i should be roughly fsam/10 or less, and fcr_Te should be roughly fcr_i/10 or less.
For this motor, considering the power level of 100 kW, we choose fsw, fsam, and fsam_w to be 20 kHz, fcr_i to be 2kHz, and fcr_Te to be 200 Hz.
Click on the button Update Parameter File to update the system parameters for the simulation.
Step 4: Run simulation to validate a functional and stable motor drive system
Once the parameter file is updated, it’s ready to run the simulation. It is important that the system is functionally stable and reached a steady state.
Again, one may need to adjust fsw, fsam, fsam_w, fcr_i, fcr_Te, and the simulation time step to achieve the desired performance. Also, adjust the simulation time so that motor torque and system efficiency reach a steady state.
One key simulation waveform to watch is the modulation signal Vma at the input of the PWM block. The waveform Vma should not be severely out of the range of the carrier wave.
Simulation results from this step are shown below.
The waveforms show the start-up transient of the 100-kW IPM drive from 0 to 160 Nm at the speed of 6000 rpm.
The traces in the first window are the motor currents.
The traces in the second window are the torque reference and the actual torque. It reaches 160 Nm around 0.2 s.
The trace in the third window is the modulation signal Vma.
Step 5: Run simulation again to validate system efficiency calculation
Make changes to the simulation schematics highlighted in red shown below and run the simulation:
- Replace the inverter with an inverter from the Thermal Module for loss calculation
- Add circuit to obtain motor power loss from the JMAG-RT motor model
In this example, we select the Infineon HybridPACK 2 IGBT FS800R07A2E3 for the inverter, and two HybridPACK 2 connected in parallel are used.
The motor copper loss and iron loss are obtained from the JMAG-RT model. A low-pass filter is used at each power loss output to smooth out the waveform.
The motor-developed torque is also obtained from the JMAG-RT model. This torque contains a significant amount of spatial harmonics, and a low-pass filter is used to smooth out the waveform.
The efficiency calculation subcircuit is given in the PSIM folder “Application notes\Efficiency Map of 100-kW”.
Simulation results from this step are shown below.
The waveforms show a steady-state of the 100-kW IPM drive.
The trace in the first window is the motor speed (6000 RPM).
The trace in the second window is the steady-state motor torque of around 160 Nm.
The trace in the third window is the steady-state system efficiency of around 96.2%.
Step 6: Run automated simulations with the script code and plot the efficiency map
After the simulation is fully set up and ready to generate the efficiency map, one can run simulations by sweep the torque command and the load speed, and record the system efficiency. In this example, Te_cmd is set as 20, 40, …, 200 Nm (10 points), and nm_load is set as 1000, 2000, …, 9000 RPM (9 points). For this example, a total of 90 simulations (i.e. 9×10) are required to capture the complete system efficiency data over the entire speed-torque range, and this is a big and time-consuming task.
PSIM’s script function can be used to automate the process. A script code “efficiency map – speed-torque limit.script” is prepared to run all the simulations, record the efficiency map data (system efficiency eff_Drive, motor efficiency eff_motor, and inverter efficiency eff_inv vs. the speed nm and torque Tem), and create three Matlab m files for plotting the contour maps of the motor, inverter and drive system efficiencies in the Matlab software.
Before running the script code, one needs to disable the speed and torque commands from the parameter file in the simulation schematic as shown below:
To run the script, load the script “efficiency maps – speed-torque limit.script” into PSIM, and select Run >> Run Script as shown below.
After automated simulations are completed, three Matlab m-files (“efficiency_map_drive.m”, “efficiency_map_motor.m”, and “efficiency_map_inverter.m”) that contain the efficiency map data for the drive system, motor, and inverter respectively will be created. Launch Matlab, and load and run the m-files. The efficiency maps for this example are shown below (top left: motor; top right: inverter; bottom: drive system).
From these efficiency maps, one can conclude that if the PMSM motor is operated in the region of 1500-2500 rpm and 120-185 N*m, or 5000-9000 rpm and torque along the limit of the speed-torque curve, it achieves the highest efficiency. But the highest efficiency region for the inverter is when the speed is 5000 to 9000 rpm, and torque along the limit of the speed‐torque curve. Overall, for the drive system, the highest efficiency region is when the speed is 5000 to 9000 rpm, and torque is along the limit of the speed-torque curve.
To generate an efficiency map with finer resolution, more speed and torque points can be selected.
Conclusions
With the Motor Control Design Suite, JMAG-RT motor model, and the script function in PSIM, the process of generating the efficiency map of a motor drive system, which is difficult, tedious, and time-consuming, is made considerably easier. This greatly helps motor design engineers in evaluating the motor performance in a motor drive environment and helps system engineers in determining the optimum operation of the motor drive system.