An Introduction into Electric Power Testing

It can be quite complex and requires a real-time power analyser for high channel counts, dynamic load changes, raw data acquisition and advanced analysis. Electrical motor and inverter testing can be found in automotive, transportation, railways, aerospace, industrial, wind energy and also in power generation. Typical application areas are electric ship motor, electric fork lifter, wind energy generator, electric or hybrid car, high speed train, high-performance pumps, electric drive in Airbus A320, industrial VF inverter, multiphase AC industrial motor and much more.

The Importance of Testing

The main purpose of electric power testing is to improve the efficiency of the electric drive train. There are certain possibilities as to improve the inverter, the motor, the inverter / motor matching and improve the drive “strategy”. The better the inverter and the motor are „matched“, the higher the efficiency of the drive train. To do so, the motor needs to be carefully characterized with the inverter. This can be achieved with raw data being stored and analysed continuously by using appropriate first-class equipment.

Testing Electric Drive Trains Requires High-End Data Acquisition System

No matter which drive train we are testing the voltages, currents, torque, speed and temperatures to be measured are quite similar. The task is always the same: At the inverter input battery voltage and current needs to be measured. Typically, we have at the inverter output 3 or more phases (for example 6 phases in aerospace, 6 phases can also be found outside aerospace, for example in elevators) and pulse width modulated voltages up to +/-1000 Volts and currents up to a few hundred amps to be measured. Measuring each of these voltages and currents individually allows us to calculate the electrical Power from the battery and from the inverter. Using a torque transducer the motor output signals torque, speed and position can be measured which allows to calculate mechanical power of the motor. Then if we calculate the ratios of each of these sections we get the efficiency of the frequency inverter, the electrical motor and the Efficiency of the entire electric drive train.

Currently wired setup looks like this: DMMs (Digital Multimeter) are used to measure the battery voltage, a power analyzer is measuring the three-phase voltage and currents, maybe a scope is used if the signals want to be seen as they occur, and a small DAQ (data acquisition system) is used to measure the torque, speed and angle of the electric motor.

But this setup with different measuring devices makes it almost impossible to obtain reliable results. There are several causes as:

  1. Conventional power analyzers deliver few calculations only and are not reliable in dynamic load change situations.
  2. Limited channel count for power and torque/speed.
  3. Difficult time synchronization between different systems.
  4. Data storage is limited in different systems & different formats.
  5. No raw data are available for verification or analysis.

Therefore, the demand for a high-end single system solution is bigger than ever as electric drive trains will become more important.

Market Requirement: A Single System Does All the Job

To simplify electric motor testing the market is requiring a single system that allows a flexible system configuration to store raw data and continuously analyse them. If required the data can be used later for in-depth analysis.   

A single system acquires and displays the battery voltage and current (input to the inverter), the three-phase (or six-phase) pulse width modulated voltages and currents (output from the inverter), plus the torque, speed and angle of the electric motor.

The advantage of such a system is:

  • Scalable from small to high channel counts.
  • One system for power, mechanical, control and NVH
  • High-speed acquisition and sampling of all signals without phase shift
  • Real-time results and raw data storage in one file & format.
  • Simultaneous acquisition and recording of all signals.
  • Easy system integration trough software interfaces and open data format.
  • Real-time result transfer to automation system.
  • Perform real-time power calculations per half cycle (even based on your own formulas for “special” setups).
  • Display all the recorded and computed signals live, safely and conveniently in the control room
  • Continuous recording or snapshots per setpoint for mapping, verification and analysis
  • Advanced analysis capabilities like space vector, dq0 transformation or air gap torque

The result of this setup is: No matter what happens during testing raw data is stored and the operator can carefully analyse the data and make amendments and modifications.

Maximum safety can be achieved by placing the Data Acquisition system inside the test cell and connect it with fiber optical ethernet to a control PC outside the test cell.

Further Advantage of Storing Raw Data

Storing raw data has further advantages. The data can be used for additional, complex analysis of the electric drive train. All results can be stored (CSV File) or streamed to an automation system.

Some examples of inverter analysis:

  • THD
  • Switching frequency
  • Modulation method
  • Inverter control behaviour

Some examples of motor analysis:

  • Equivalent circuit diagram
  • Iron & copper losses
  • Starting currents
  • Air gap torque
  • Torque ripple / Cogging torque
  • Saturation effects

Some examples of drive analysis

  • WLPT Testing

Summary

As electric power testing is quite complex a single system simplifies testing and allows an in-depth analysis of data. A high-end power analyser (as the eDrive Power Analyzer) is able to acquire all signals, including high voltage, current, torque, speed, temperatures, CAN, vibration, sound, etc. And can sample all signals simultaneously, with high sample rate and high resolution.

Raw data will be stored continuously or as snapshot per set points for verification and analysis. A Test setup in one simple menu with preconfigured applications is possible. Power results, traces, FFTs and mappings are calculated in real time. The eDrive creator and user formulas allow adaption to complex applications. Due to a formula database a detailed motor and inverter analysis is possible. The results can be transferred and controlled via soft-/hardware interfaces to automation system.