Efficiency Measurement of Traction Motors (E)

Efficiency measurementsof DC-AC convertersis a key part of theEV traction systemdevelopment process,accurate and reliable measurement is a key priority forany test and development engineer.This article discusses important aspects of efficiency measurement that apower analyzerwillneed to address

Traction system analysis involves DC Bus to AC Inverter e lectrical efficiency measurement al ong with e lectrical to m echanical efficiency between DC input/AC output and output shaft. Synchronisation of these measurements is vitally important, so that the instantaneous efficiency is correctly calculated. Some power analyzers will not synchronise th e measurement acquisition window of the input and the output power. In this application , synchronisation can be defined as the common start and end time of the data acquisition window across all phases of the measurement instrument. This ensures that the i nstantaneous efficiency calculated using P out (AC output power) and P in (DC Input Power) utilise data points from a time synchronised acquisition window, where the start time and end time match respectively

It is common for a power analyzer to use the nominal update rate set by the user ( eg. 500ms) as its internal data acquisition window size , regardless of the motor frequency of the motor drive unit being evaluated . However, o ptimum measurement requires synchronisation of the measurement windo w to the fundamental time period or "motor frequency" (with respect to time) . If the measurement window is correctly sized to a single or even multiple fundamental time periods, RMS calculations of Voltage, Current and Power will be correctly computed . If the acquisition window d oes not contain an exact integer number of cycles, this will cause significant measurement error when update rate s approach the time period of the motor frequency, figure 1 illustrates the issue.

N4L power analyzers utilise a proprietary windowing algorithm based upon the discrete Fourier transform (DFT) as well as dual FPGA/DSP signal processing techniques to adjust the acquisition window size proportional to the fundamental time period of the power waveform. For example, if a 1 second update rate is selected by the user, N4L power analyzers are capable of adjusting the acquisition window size in 500n s step s (1 sample point) around the 1 second selected update rate in order to ensure that complete cycles are captured . This is preferred to simply "chopping" off part of the final cycle in a particular window to achieve a given update rate , this improves accura cy for both power measurement and harmonic measurement.

There are pro's and con's when selecting a single 4 phase instrument chassis for measurement of DC Bus to 3 Phase Output power of an inverter module. The advantages are that only a single instrument needs to be querie d by softw are , simplifying programming and system integration time. Another advantage is size, as an example the N4L PPA3500 is a 2U high, high accuracy multi - channel power analyzer supporting up to 6 phase (12 channel) analysis within a single chassis.

This occupies minimum space within a test system , leaving space for other equipment while still providing market leading 0.03% nominal accuracy. A second option is to utilise two separate instruments, a single phase unit to measure the DC bus and a 3 Phase unit to measure the output power of the inverter. An advantage of this setup is flexibility, since the instruments can be separated at any time and used in separate applications . Furthermore, the high specification hardware required to analyse PWM waveforms is not necessary to measure DC bus power , therefore c ost savings can be made on selection of DC bus instrumentation . The disadvantages of a 4 phase analysis system comprising 2 instruments is greater complexity with regards to software programming, as well as great er rack space requirements.

Whilst making the choice between a four phase instrument and two separate instruments is important, eng ineers should ensure best practice synchronisation techniques and inte ger cycle windowing are carried out to avoid waveform "chopping". N4L Power Analyzers perform all of the above to ensure optimum measurement in demanding applications.

Das ist eine Applikations-Schrift von Newtons4th.