Precision power analysis in electric vehicle testing

From R&D to manufacturing and compliance testing, measurement of powertrain not only require progressively greater accuracies but also consistency in measurement over the specified ranges and conditions. Modern drive systems with integrated inverters do not allow access to the AC signals. Here, one of the main measurement tasks is the overall drive train efficiency from DC to mechanical power.

Yokogawa's new WT5000 precision power analyser is designed to combine exceptional accuracy (claimed to be the highest available - ±0.03% of total at 50/60 Hz) with stability, noise immunity and flexibility.

An essential element for determining the performance of a power measuring instrument is the A/D converter. To obtain the stated accuracy, the WT5000 uses an 18-bit converter with a sampling frequency of maximum 10MS/s. As a result, it becomes possible to accurately capture waveforms from the latest high-speed inverter devices.

While the WT5000 has the same dimensions as existing models in Yokogawa’s WT series, it incorporates up to seven input channels, allowing it to support applications that previously could only have been measured by synchronising several separate instruments.

For larger current devices, an external current sensor input function is standard in both the 30A and 5A input elements. For much higher currents (up to 2000Arms), dedicated high-current sensors are available.

For electric vehicles or fuel-cell vehicles, the 30A and 5A elements can be switched. Using the WT5000 equipped with the /MTR1 and /MTR2 options and torque and rotation sensors, it is thus possible to evaluate up to four motors simultaneously with one unit; flexible measurement of the A, B, C and Z phases of each motor can be carried out.

Users can obtain differential voltages, line and phase voltages from the sums and differences of the instantaneous values of voltage and current in each element. The WT5000 enables conversion of measurement values for a three-phase star wiring system to those for a delta wiring system and vice versa.

From the analogue or pulse outputs of torque sensors or pulse outputs of rotation sensors, the motor evaluation function enables measurements of rotation speed and direction, synchronous speed, slip, torque, mechanical power, electrical angle and motor efficiency.

Harmonic measurement

The WT5000 can carry out two harmonic measurement functions simultaneously, each at up the 500th order and up to 300kHz fundamental waveform. This makes it possible to measure the carrier frequency component from the rotational speed of the motor in the inverter drive and also to check the influence of the carrier frequency on the motor drive. With a lower frequency limit of 0.5Hz, the WT5000 enables measurement of harmonics and total harmonic distortion (THD) even at very low motor rotation speeds.

Users can view harmonics alongside conventional RMS values of voltage and current or compare different inputs side by side (particularly useful when comparing input and output of inverters, ballasts, emergency power supplies etc.).

Whether for custom synchronization of measurements, smoothing of signal fluctuations or simultaneous wideband and harmonic power analysis, advanced filtering options put the user in control of measurements without compromising on accuracy:

- Synchronisation source filter: instead of synchronising to zero crossings, select any specific point of the synchronisation source signal.

- Enhanced frequency filter: simultaneously measure fundamental and switching frequencies without influencing any other parameter.

- Digital parallel path filters: together with a high frequency anti-aliasing filter, two separate line filters for normal and harmonic measurements ensure accuracy without aliasing in wide band and harmonic measurements. Users can limit the number of harmonic orders to eliminate attenuation in low bandwidth measurements.

Finally, Modbus allows the WT5000 to be connected to Yokogawa's data acquisition system, which reads the data and saves it in sync with the acquired temperature data.