Accelerating EV Traction Inverter Analysis

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.

Pulse-width modulation and multi-phase current and voltage waveforms have historically presented challenges for automotive 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.

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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.

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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.

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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.

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.

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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.