Accelerating EV Traction Inverter Analysis
Traction inverters and motors are at the heart of the EV powertrain. Efficiency improvements within these subsystems translate directly into better range, performance, and cost of the vehicle.
Adoption of SiC power semiconductors is contributing to better efficiency and more compact traction inverters. Control algorithms and motor construction are being optimized to achieve demanding efficiency and cost targets
EV architects are combining new traction inverter designs with different motor designs, creating new hybrid structures uniquely suited to the demands of electric mobility.
Techniques for Repeatable Inverter Measurements
Traction Inverter and Motor Technology
EVs employ different types of electric motors but they all require the application of PWM voltage signals to the motor stator to develop three sinusoidal currents spaced 120° apart. The modulation of the high-voltage input is usually performed by high-voltage IGBTs or MOSFETs switching at frequencies ranging from 20 to 100 kHz. Designers work hard to minimize energy loss during switching while maintaining safe timing.
Gate drivers are controlled by a microcontroller (MCU) subsystem and determine the timing of the switching devices. The control circuits must be galvanically isolated from the high-voltage sections.
Inverter controllers often use DSP algorithms, such as field-oriented control (FOC), to precisely vary the PWM output. Based on the driver's input and current speed of the motor, the inverter’s MCU controls the angle between the poles of the direct axis of the rotor (D) and the magnetic field, or quadrature axis (Q) to deliver smooth, optimal torque. Sensors such as encoders or resolvers on the motor’s rotor provide feedback on rotor angle.
Analyzing Critical Inverter Signals
Pulse-width modulation and multi-phase current and voltage waveforms have historically presented challenges for oscilloscopes and the engineers who rely on them. Yet being able to see and measure these waveforms is critical to optimizing an inverter’s reliability, robustness, power density and efficiency.
The introduction of 6 and 8-channel oscilloscopes has made it much easier to study 3-phase systems, but for inverters special measurement techniques are also needed:
- PWM signals are difficult to trigger on – making it hard to get stable, repeatable measurements. Special attention must be paid to ensure a stable time reference.
- Analyzing 3-phase systems requires voltage, current, angle and power measurements for individual phases as well as the total system. Phasor diagrams are ideal for observing magnitudes, angles and balance.
Inverter, Motor and Drive Analysis software on 4/5/6 Series oscilloscopes simplifies triggering on PWM outputs and setting up 3-phase measurements. Phasor diagram displays help you visually understand and debug 3-phase electrical problems.
Understanding System Behavior Under Changing Motor Loads
In the quest for power density and efficiency, it is important to understand and analyze the dynamic performance of the drive and motor under many different test conditions including:
- Motor startup
- Different motor loads
- Motor stop
Test times can vary from a few seconds to several minutes depending on the test plan. An oscilloscope with long record length stores all of the relevant information during the run and presents the results as waveforms and plots. Capturing high-speed data gives the engineer an ability to zoom into a particular region of the waveform to pinpoint a problem. In contrast, power analyzers typically support calibrated 3-phase measurements, but without access to high sample rate data.
Visibility into Vector Control Parameters such as DQ0
Closed loop inverter and motor systems use feedback to provide superior control of speed and torque compared to open loop systems. Closed loop “vector” controllers perform real-time computations to transform angular and current feedback into simpler variables (D and Q) which can be linearly scaled in real time. The scaled D and Q parameters are then inverse-transformed to provide input to the modulators used to drive the switches.
Since these important calculations occur deep within the controller, it is difficult to study D and Q in relation to other system parameters. The IMDA application on the 5/6 Series B MSOs supports a unique measurement – DQ0 (Direct Quadrature Zero) that helps engineers gain insight into controllers. It mathematically computes D and Q from the inverter’s output waveforms by applying a combination of Park’s and Clarke’s transform. The results are displayed as numeric measurements and as a phasor diagram with a resultant vector. By incorporating encoder angle, engineers can observe DQ0 vectors aligned with rotor magnet zero position when used with the QEI index pulse. These visual tools provide unique visibility into controller performance during actual operation of the motor.
Correlating Mechanical and Electrical Measurements
In order to understand the effects of decisions in electronics and algorithms, engineers must be able to correlate the motor’s mechanical performance with electrical measurements. The motor’s angle, direction, speed, acceleration and torque are key to understanding system performance. Being able to measure both electrical parameters at the input of the traction inverter and the mechanical output of the motor enables engineers to determine overall system efficiency.
Mechanical measurements like speed, direction and angle depend on sensor signals which must be decoded and displayed by the test equipment. Many BLDC motors come equipped with built-in Hall sensors which can be accessed using digital or analog probes. Other systems may rely on QEI (Quadrature Encoder Interface) sensors.
Torque measurements may be performed using a special-purpose torque sensor on the output of the motor. Torque may also be approximated by applying a scale factor to rms current.
With Tektronix IMDA software sensor signals can be decoded, enabling 5 and 6 Series B MSO oscilloscopes to display speed, acceleration, direction, angle and torque.
Understand the Impact of Wide Bandgap Power Device Integration
The transition to 800 V architectures is unlocking benefits such as lower cable and battery costs, reduced thermal loss and higher system efficiency. SiC MOSFETs are enabling higher switching voltages and lower switching loss, but traditional test plans based on silicon devices no longer apply.
Key challenges in testing wide bandgap semiconductors include:
- Current and voltage probing at high power levels
- Accurately measuring signals on high-side MOSFETs in the presence of very high common mode voltages
- Measuring switching loss with standardized tests such as double pulse tests
Tektronix provides solutions for testing traction inverters based on SiC MOSFETs including oscilloscopes, high voltage differential probes, current probes, optically isolated probes, signal sources and precision power supplies.
EV Traction Inverter and Motor Reference System
Testing an EV Powertrain design requires an oscilloscope, appropriate probes, signal source, and application software. This system may be customized to suit your application.
This traction inverter test system is available as SOLN-IMDA-EV.
|MSO58B-BW1000*||1||1 GHz, 8 Channel Scope|
|5-PRO-AUTOMOTIVE-3Y||1||Automotive Solution Bundle including Inverter, Motor and Drive Software Analysis Options 5-IMDA, 5-IMDA-DQ0 and 5-IMDA-MECH and decoding for automotive serial buses|
|THDP0200||3||200 MHz, +/-750 V, high voltage differential probe|
|TCP0030A||3||120 MHz, 30 Arms, spilt-core AC/DC current probe|
|TLP058||1||8-channel logic probe|
|TEKSCOPE-ULTIMATE||1||TekScope PC Software for off-line analysis, including IMDA Analysis and comprehensive serial bus support|
|TEKDRIVE-STARTER||1||TekDrive Data Storage Subscription, Individual Tier, Annual user license|