Practical, High-Precision Current Sensing for EV Testing Benches

Introduction

The electrification of vehicles (EVs) is increasingly recognized as a key initiative to advance Sustainable Development Goals (SDGs) and environmental preservation efforts, with expectations high for their role in achieving carbon neutrality. Evaluation tests aimed at promoting the EV shift necessitate a comprehensive array of parameter measurements, including assessments of power efficiency, waveform characteristics, and monitoring of physical signals like vibrations. In particular, an evaluation test using a power measuring instrument and a waveform measuring instrument which is capable of various analyses is a crucial factor in the quest to achieve high efficiency.

Challenges

The landscape of electrified systems is witnessing a surge in the deployment of high-frequency power electronics aimed at bolstering the power efficiency of various applications, including EV inverters. Consequently, the evaluation process in EV development necessitates measuring instruments with the capacity to conduct measurements across wide bandwidths, spanning from low to high frequencies, all while maintaining high accuracy. Furthermore, as motor drive currents increase, it becomes necessary to utilize current sensors external to the instrument. Closed loop current sensors are ideal for high-precision measurements; however, they require disconnection of large-current cabling which may be impractical. In these situations, open-core-type current sensors may be the only viable option.
Split-core-type (clamp-type) current sensors have drawbacks such as notable variations and errors in measurement data, posing a threat to evaluation reliability even when paired with high-precision measuring instruments. Thus, the main priority in a measurement system with a split core current sensor is to minimize sensor related variations between tests. Moreover, as measurements are conducted under noisy environments, excellent noise resistance is required for both measuring instruments and sensors.

Solving Challenges using AC/ DC Split-Core Current Sensor

• Convenient and precise current sensor eliminating the need for cable removal
• Current accuracy (50/60 Hz): ±(0.2% of reading + 0.01% of full scale) Measurement of large-current up to 1000 Amps AC/1500 Amps DC*
• Easy connection to Power Analyzers and Waveform Measuring Instruments
• Carrier frequency measurement at high bandwidth up to 300 kHz (−3 dB)
• Screw mounting for measurement reproducibility
• Conductor position adjuster to reduce cable shake effect
• Excellent CMRR characteristics enabling accurate measurement under harsh noisy environments
• World-class accuracy when paired with the WT5000
• Long duration multi-channel data capture with the DL950 ScopeCorder

* 1500 Amps DC (continuous) at maximum operating environment temperature, +40˚C

Power and Waveform Measurement with a Large- Current Sensor

Easy Connection to Power Analyzers and Waveform Measuring Instruments

The CT1000S excels in measuring large currents, accommodating up to 1000 Amps AC/1500 Amps DC* with remarkable accuracy (50/60 Hz: ±(0.2% of reading + 0.01% of full scale)).
Its open-core configuration enables seamless measurement without the need to disconnect cables. Moreover, unlike the common practice of requiring separate current sensor models for interfacing with power analyzer or waveform measuring instruments, the CT1000S is compatible with both instrument types.
* 1500 Amps DC (continuous) at maximum operating environment temperature, +40˚C

Figure 1. CT1000S connected with various measuring instruments

Carrier Frequency Measurement at High Bandwidth up to 300 kHz (−3 dB)

In power electronics, the carrier frequency refers to the highfrequency signal used to modulate the amplitude of a lowerfrequency signal. This technique is employed in switching power converters, such as PWM (Pulse Width Modulation) inverters.
To measure carrier frequency components (several kHz to 100 kHz in general), a current sensor with wide bandwidth is required. With measurement bandwidth of 300 kHz (−3 dB), the CT1000S has excellent frequency response that is flat up to 100 kHz.

Figure 3. Secure screw mount

Mitigating the Effect of Off-Axis Cable Position

According to the principle of current sensor measurement, optimal performance requires the measured cable to pass through the center of the sensor’s primary hole. Achieving this consistently during actual testing can be challenging, leading to positional deviations that affect measurement accuracy. The CT1000S resolves this issue by incorporating a conductor position adjuster, which restricts the cable’s axis position to minimize these deviations.

Figure 4. Conductor position adjuster (left) and in use (right)

Excellent CMRR Characteristics

The CT1000S has an exceptional Common-Mode Rejection Ratio (CMRR), exceeding 150 dB (equal to or less than 0.0016% of full scale). This feature ensures precise measurements even in noisy environments.

Power Measurement and Motor Analysis

World-Class Power Measurement Accuracy with the WT5000

The WT5000 delivers world-class measurement accuracy of ±0.03% (50/60 Hz). It is capable of the highest accuracy for inverter efficiency measurements.

Figure 5 WT5000 Precision Power Analyzer

Power for Current Sensors

t features a modular chassis with up to seven power input elements. Yokogawa’s design technology accumulated over many years enables extremely precise measurement circuits inside a compact form factor. Each element also incorporates a power supply for driving current sensors, allowing large-current measurement without the need for a separate power supply.

Figure 6. WT5000 Rear side view

Figure 7. 760903 Current Sensor Element

Simultaneous Analysis of up to Four Motors

Adding Motor Analysis Option (/MTR1, /MTR2) enables the WT5000 to perform simultaneous evaluations of four motors in one unit. These options also enable the measurement of motor rotation speed, rotational direction, and electric angle of two motors by connecting their A, B, and Z phase signals.

Figure 8. Motor input terminals

When using an AC/DC current sensor or a current clamp probe, correcting the phase shift of the current signals provides additional power measurement accuracy. The WT5000 can correct the phase difference between the voltage and current input with 1ns resolution, significantly reducing phase error. It also supports amplitude correction (gain correction) for high frequency signals.

Figure 9. Phase correction image

Long Duration Multi-channel Data Capture with the DL950 ScopeCorder

The DL950 is a modular high-speed data logger used for logging a variety of signals. It delivers high-speed sample rate of up to 200 MS/s, high-resolution A/D converter of up to 16 bits, and long-duration and multi-channel measurements. Channel to channel isolation enables input of multiple signals at different ground levels. Moreover, its high noise resistance simplifies on-site wiring.

Figure 10. DL950 ScopeCorder

Measurement of Various Physical Signals

Input modules for thermocouples, accelerometers, strain sensors, etc. are available, and the ScopeCorder chassis supports up to eight modules. A wide variety of physical parameters can be captured simultaneously with electrical signals.

Figure 11. DL950 side view (left), List of modules (right)

To capture even more parameters simultaneously, an optional feature allows up to five DL950s to be synchronized, expanding the number of input channels to 160.

Figure 12. Connecting multiple units

Real-Time Math Computation

The real-time math computation function enables various calculations on captured signals, displaying results instantly with no perceptible delay. This functionality supports triggering based on computation results, automatic measurement of waveform parameters, and cursor measurement. With independent input terminals for input modules, it allows simultaneous display and analysis of up to 32 input channels and real-time computation results for up to 16 channels.

Real-Time Comparison of Motor Parameters and CAN Measurement Values

By leveraging the real-time math computation function, motor parameters such as dq-axis voltage, current, and rotation position can be calculated and displayed as waveforms. This function supports cycle-by-cycle average and RMS value analysis of fluctuations in active power and dq-axis current voltage during rise time. Additionally, it can simultaneously display motor parameters and other signals, such as CAN, on a single screen, facilitating real-time comparisons.

Figure 13. Motor analysis with real-time math