Current measurement of motor drives and inverters - IEEE Xplore

PCIM Europe 2015, 19 – 21 May 2015, Nuremberg, Germany

Current measurement of motor drives and inverters – Influence of shielded and not-shielded motor cable Andreas E. Neuhold, Dewetron GmbH, Austria, andreas.neuhold@dewetron.com Bernhard Grasel, Dewetron GmbH, Austria, bernhard.grasel@dewetron.com Michael Oberhofer, Dewetron GmbH, Austria, michael.oberhofer@dewetron.com

Abstract For increasing the efficiency of electric drive trains, measurement and testing plays a key role. The requirements to the measurement system concerning hardware (high accuracy, high bandwidth, high sampling rate, isolated high-voltage inputs, etc.) as well as to the software (power calculation for variable frequencies, for DC and AC 1-, 2-, 3- up to 7-phase systems, multiple power calculations, FFT, etc.) are really high. A very important part of the measurement chain for such applications is the current measurement. There are different technologies for measuring the current. The first aim of this article is to determine the difference between an iron-core current clamp, a rogowsky coil, a hall-compensated AC/DC current clamp and a zero-flux current transducer at a measurement of an inverter. The second focus of this article is the analysis of measuring the current between a frequency inverter and a motor of an electric car with and without shielded motor cable.

1. 1.1.

Measurement with different types of current transducers Measurement system

Figure 1: Dewetron DEWE 2600 measurement device Hardware: DEWE 2600, 5 x HSI-LV modules Clamp 150 DC Zerofluxtransducer IT-S 400 with Shunt BR5 PNA Ampflex A100-200-45 PNA-Clamp 20 Electric Vehicle: Mitsubishi iMiev

ISBN 978-3-8007-3924-0

1928

© VDE VERLAG GMBH · Berlin · Offenbach


1.2.

Measurement with different types of current transducers

Central Issue: What’s the difference between measuring motor and inverter of an electric car with different current sensors (iron-core current clamp, rogowsky coil, hall-compensated AC/DC current clamp and zero-flux current transducer)?

Figure 2: Using different sensors for current measurement The following table shows the results of the analysis for different parts of the measured signal:

PNA-Clamp 20

Technology DC measurement

150DC

waveform moderate accuracy

Zero-flux

compensation

transducer

Not possible

high accuracy

high accuracy

Oscillations

high accuracy

high accuracy

high accuracy

high accuracy

high accuracy

(bandwidth

high accuracy

high accuracy up

depends on

up to 100 kHz

to 300 kHz

high accuracy Higher

Frequency no signals higher

components

than 10 kHz

PM-ITS400

Hall-

Not possible

Frequency up to 15 Hz

and 400 Hz

A100-200-45

Rogowsky coil

distorted

Frequency between 15

PNA-Clamp-

Iron-core

Fundamental

Fundamental

PNA-Ampflex

-

integrator)

Table 1: Comparison of different current sensors

ISBN 978-3-8007-3924-0

1929



¾ PNA-Clamp 20 Due to the technology of that current transducer, it is not possible to measure DCsignals. The specification of the current clamp starts at 40 Hz. Below 40 Hz the measured signal is partly very distorted, see:

Figure 3: 8Hz – Measurement blue – 150DC clamp without shield, red – Clamp20 with shield, turquoise – zero-flux transducer without shield As you can see in this picture, the waveform of the current measured with the ironcore clamp (red) at a frequency of 8 Hz is much distorted. The turquoise line is the reference signal of a zero-flux transducer. The accuracy for the measurement of the fundamental signal between 30 Hz and 400 Hz is moderate. If you consider the low-price of this product, it’s quite good in that range.

Figure 4: 30Hz to 400Hz – Measurement blue – 150DC clamp without shield, red – Clamp20 with shield, turquoise – zero-flux transducer without shield For measurement of higher-frequency components it’s again not possible to use ironcore clamps. The bandwidth of iron-core current clamps ends at least at 20 kHz (10 kHz for this clamp). The following FFT chart shows the missing signal parts at higher frequencies (red = iron-core clamp; turquoise = zero-flux-transducer):

ISBN 978-3-8007-3924-0

1930



Figure 5: FFT Spectrum of Clamp20 (red) and zero-flux transducer (turquoise) Conclusion: This current clamp can’t be used for the measurement at inverters. ¾ PNA-Ampflex-A100-200-45 Due to the technology of that current transducer it’s not possible to measure DCsignals. The specification of the current clamp starts at 10 Hz. Below 10 Hz the measured signal is strongly oscillating, see:

Figure 6: Oszillating signal below 10Hz blue – 150DC clamp without shield, red – Clamp20 with shield, turquoise – zero-flux transducer without shield The violet line in the picture shows the current measured with a rogowsky coil at a frequency of 8 Hz. There are strong deviations of the waveform and the phase angle (oscillating). At frequencies above 10 Hz the PNA-Ampflex has a very high accuracy. Also the higher-frequency parts can be measured very well. The bandwidth of the rogowsky coils only depends on the external integrator. So this should be considered for measurements with high–frequency components.

ISBN 978-3-8007-3924-0

1931



Conclusion: No DC-measurement, oscillations at frequencies below 10 Hz, high accuracy at frequencies above 10 Hz up to the bandwidth of the external integrator ¾ Clamp 150 DC Using the Clamp 150-DC for measurement of inverters achieve high accuracy for all parts of the signal.

Figure 7: 150DC clamp signal comparison blue – 150DC clamp without shield, turquoise – zero-flux transducer without shield If you compare the FFT chart to a zero-flux transducer, you can see that all signals are captured very well up to a frequency of 100 kHz.

Figure 8: FFT Spectrum of 150DC Clamp and zero-flux transducer Conclusion: High accuracy for all parts of the signal up to 100 kHz. ¾ Zero-Flux transducer PM IT-S 400 Zero-Flux transducers are used to achieve highest accuracy over a wide frequency bandwidth. The sophisticated technology allows measuring all parts of a signal, starting at the DC part up to higher frequency parts of up to 300 kHz, very accurate.

ISBN 978-3-8007-3924-0

1932



Figure 9: FFT Spectrum of zero-flux transducer Conclusion: Highest accuracy for all parts of the signal up to 300 kHz.

2.

Measurement with and without shielded motor cable

Central Issue: Is there a difference of measuring the electric car with and without shielded motor cable?

Figure 10: Idea of detecting the influence of shielded cables

ISBN 978-3-8007-3924-0

1933



This test-measurement was done with four current transducers. Two different current transducers with shielded motor cable (Zero-flux transducer and Clamp 150DC) and two without shielded motor cable (Zero-flux transducer and Clamp 150DC). This should ensure reliable results. There was no difference in amplitude and phase of the measured signal of the current sensors which was placed on the same position.

Figure 11: Comparison 150DC Clamp (red) with shield and zero-flux transducer without a shield (turquoise) In this chart you see the current signal measured with shielded motor cable (red) and without shielded motor cable (turquoise). As you can see the signals are different. The difference is that that the waveform of the red line is smoothed, and there is a little phase shift on that signal. The next picture shows the FFT of both signals. The red line is again the current transducer with shielded motor cable, the turquoise one the transducer without shielded motor cable.

Figure 12: FFT Spectrum of 150DC Clamp with shield (red) and zero-flux transducer without a shield (turquoise) The spectrum of both signals is again very different. There are a lot more higherfrequency components at the measurement at the motor cable without shield. Respective there are signals at the measurement with shield which are damped. So

ISBN 978-3-8007-3924-0

1934



the signal of the current transducer with shielded motor cable has a typical Low-pass characteristic (phase shift, no higher frequency parts). Due to the frequency depended phase shift of both signals the reason for this behaviour are very likely capacitive leakage currents over the shield of the motor cable, which acts as a low-pass filter for the measured signal with shielded motor cable. The frequency depended phase shift has its peak of 10° at the maximal fundamental frequency of 190 Hz. The following chart shows the correlation of the phase shift between both current signals and of the frequency:

Figure 13: Correlation of the phase shift, green signal without a shield, red signal with shield Another effect of the measurement of the current with shielded motor cable is the higher DC current. The amount of the DC current can be seen in the FFT chart above at a frequency of 0 Hz. Note: All this effects appears at a certain electric car. For other electric cars the influence of these things could be a lot lower. It always depends on the realisation of the electric drive train.

ISBN 978-3-8007-3924-0

1935
