Few manufacturers of measurement technology are able to keep pace with the rapid development of electromobility.
With the LTTsmart, you are on the safe side.
The efficiency of electric motors
The ever-changing history of automotive technology began almost exactly 135 years ago. Over the years, developments gradually led to increasingly more efficient and powerful combustion engines. But now, due to the sudden demand for a transition to electromobility, progress has to come in major leaps rather than small steps – and this tremendous pace often brings developers to the limits of their own experience. Very few manufacturers of measurement technology are able to keep pace with the demanding requirements of electromobility. Labortechnik Tasler GmbH is at the forefront of this development.
To determine the efficiency of an electrical engine, the mechanical power output has to be compared against the electric power input. Both measurement variables involve potential pitfalls.
Fortunately, this problem can be confined to the electrical measurement variables, as a great deal of experience and suitable measurement technology are already available to record the mechanical power thanks to the long history of combustion engines.
The challenges of measurement accuracy
What is the situation when it comes to electrical power? To start with, electrical power consumption is difficult to measure directly. In addition, high measurement accuracy is required in accordance with the regulations of the DIN IEC 60034-2-3 Standard:
“The nominal accuracy of power meters must be 0.3% or better with respect to the apparent power at the measuring point of the inspected engine. This applies for the total uncertainty of the power meter including any converters.”
Point 7.2 of this standard, Additional losses due to the voltage drop in the frequency converter, defines tighter regulations, meaning that the accuracy must actually be better than 0.3%.
So we have two measurement variables (current and voltage) with errors that accumulate according to the error propagation law when multiplied to calculate the power (U*I).
Each variable individually (current and voltage considered separately) is a chain of at least four errors:
Amplitude errors (over all frequencies)
Phase errors during sampling (over all frequencies)
Bandwidth restriction in the number of harmonics (at least up to the 29th harmonic of the voltage switching frequency for 0.1% accuracy of the voltage signal (and up to the 7th harmonic for the current signal))
Errors in connection lines and transformers if used
The sum of these 4 errors for each variable determines the resolution of the two measurement variables (voltage and current).
Since all 4 errors are cumulative for both measurements according to the error propagation law, this means we have a total of 8 error sources which may not exceed 0.3% when added together. As mentioned above (in the reference to point 7.2 of the standard), the performance must actually be a bit better than that.
On average, each error source by itself can only amount to an error of roughly 0.03%.
That’s quite ambitious!
This means it is necessary to cleanly record a square wave signal up to the 53rd harmonic wave, and a triangular signal must be cleanly recorded up to the 9th harmonic wave.
At the same time, the amplitude error must not exceed 0.03% over this frequency range and the synchronicity must be better than 3 ns (between the sampling of the voltage U(t) versus sampling of the current I(t)).
Due to this strict synchronization requirement and due to the necessity of being able to multiply U(t) and I(t) for each sample without additional phase errors (of filters, etc.), the two measurement variables (U and I) must be recorded at the same sampling rate.
For the operation of an electric engine, the frequency and amplitude of the sinusoidal supply signal have to match: a low rotational speed corresponds to a small amplitude. A high rotational speed corresponds to a large amplitude.
This means that if you want to cover a speed range from of between 1% and 100% of the engine’s maximum rotational speed, the amplitude in the slow speed range will also correspond to just 1% of the amplitude at full speed.
Accordingly, the final value of the measurement range must have a resolution of not just 1/0.03% = 3333 steps, but in fact 333333 steps. This corresponds to an effective resolution of 18.3 bits for an AD converter. And this value is obtained after deducting all noise and distortion from this AD converter itself.
Demanding requirements for measuring instruments
With that, we come to the heart of this article: There’s no point even trying to measure the performance of a PWM-operated electric motor without at least a 20-bit AD converter, or preferably a 24-bit AD converter, with a sampling rate of at least 1 MHz (preferably 2 MHz or 4 MHz).
At the same time, the synchronicity across all measurement channels must be better than 3 ns (preferably 1 ns)!
In addition, high signal voltages are involved, which pose a considerable safety risk as well as extreme requirements for the input capacity of the measurement technology.
Consider a 400 V battery-operated electric engine that is brought from a voltage of 0 V to 400 V at a 10 kHz pulse rate (and then immediately falls back to 0 V). If you only have a stray capacitance of 10 pF in the signal path, this 10 pF
must be charged or discharged 20,000 times per second at 400V.
In the slowest case, the capacitor only reaches a full charge when it starts to discharge again. Then you have precisely 50 µs to charge the capacitor with a 50% pulse width.
But in reality, the slopes are much steeper. The capacitor is completely charged after roughly 500 ns. At this time, the charging current of this hypothetical stray capacitance is already 8 8 mA.
Harmless?
Unfortunately not! For slow speeds, the rule of thumb when determining the maximum permissible measurement error was given above: “Measurement range / 333333”.
The 8 mA charging current would be just barely acceptable as an error if the measurement current is in the range of 2.7 kA (or larger).
In short: the permitted stray capacitance of the measurement channels must be extremely low!
And coupled with the required high-voltage protection and the necessity of completely excluding ground current loops (because these would suffer sensitive disruptions due to the magnetic field of the electrical engine), the channels must be galvanically isolatedfrom one another and at the same time have an extremely low stray capacitance with respect to the housing.
If just one of these requirements (sampling rate, resolution, synchronicity, galvanic isolation with low stray capacitance) is not implemented perfectly, the efficiency measurement will fail in accordance with the DIN IEC 60034-2-3 Standard.
All of these points have been carefully coordinated in the LTTsmart measurement system developed by Labortechnik Tasler GmbH. This specialist in measurement technology has been established on the market for over 25 years and was already operating with high-resolution megahertz bandwidths when the market for measurement technology gradually transitioned from DOS to Windows.
The requirements for electromobility are now driving the market into this niche from two directions: first, slow PC measurement technology is far from sufficient when it comes to bandwidth. And the resolution delivered by rapid oscilloscopes is far from adequate for detecting all the necessary details.
But determining efficiency means bringing together two signal views:
(rapid) electrical and (slow) mechanical performance measurements.
Of course, the LTTsmart is also able to handle slow mechanical measurements with excellent synchronicity.
Nevertheless, in many cases it is useful to record the numerous types of (slow) additional signals, such as temperatures, using the additional measurement tools provided by the company Gantner Instruments, which work together seamlessly with the LTTsmart.
Summary
Users benefit from the LTTsmart’s perfect coordination for demanding measurement tasks and its ability to work together smoothly with other measurement tools.
The comprehensive functions of the LTT Power Analyzer are available both in the cross-industry software solution LTTpro as well as in Gantner’s software GI.bench, combining the two fields (electrical and mechanical power analysis) with optimal synchronicity. Slow, medium and high-speed measurement signals are compiled online for the required efficiency calculation.
The large quantity of data required for the calculation of the electrical performance figures indicated above remain inconspicuously in the background without disrupting the user. At the same time, entire sections of this fast raw data can be extracted by the user or a decision criterion configured in the software, and then the raw data can be analysed at any point in time, for example to discuss potential discrepancies in the recorded and computed variables.
Whether for research purposes, process monitoring or quality assurance: as a user, you have full access to all signal views at all times. From the overview down to the finest details.
This makes LTT’s Power Analyzer solution (alongside many other LTT solutions) the perfect, one-of-a-kind holistic package to help you keep up with the rapid development of electromobility.
