imx test types - automated e-motor evaluation
Test types for testing electric motors generate the information that developers and quality assurance experts need for evaluation. They provide the basis for a deeper, more thorough understanding of the motor being tested and allow conclusions to be drawn about quality and motor characteristics.
Our wide range of standardized test types and a number of customized tests are used to determine a wide variety of parameters on complex modern motor designs and make them available to the user for further work.
Modular and flexible motor testing
Test types for electric motors consist of an automated test sequence and associated measurement data evaluation. Depending on the test type, different load profiles or load points of an electric motor are automatically approached during the test sequence. The DUT is operated actively as a motor or passively with the help of the load machine.
All settings required for a test can be parameterised using our imx OMEGA test bench software. No programming skills are required.
The evaluation of a test is usually performed automatically after a test sequence. All raw data and results of a test are stored on the test bench PC and documented in a report. If required, the data can also be transferred to a database.
Which test is the right one? With imx OMEGA, around 15 active and passive standardised test types as well as the load-free imx Parameter Identification (imx PI) can be used to gain specific insights. These automated tests are modular and can be integrated into a test bench at a later stage.
Typical imx test types are divided into active and passive tests. The main electric motor tests from both categories can be found below and also on our YouTube channel.
Active testing of electric motors
Overview of imx test types
Flux table EC (static/dynamic test)
The active standard test type Flux Table EC is used to record and “map” the magnetic flux in electric motors in detail under various operating conditions. The word “mapping” comes from the 3-dimensional result displays, which show the dependencies of the flux at different load points.
The determination of magnetic fluxes and the presentation of the results are more complex than many active test types. Unlike currents and voltages, fluxes are not measured directly. In modern permanent-magnet synchronous motors, they are determined by calculation in the d and q directions as a function of the total current.
The calculation is based on detailed models that take into account factors such as non-linearities, temperature dependencies, and, if necessary, geometric arrangements of the electric motor. Measurement results from other types of tests are used to feed these models. Ohmic losses or lead resistances to determine the temperature of the test object are also included in the calculations, as are voltages and currents.
These additional but necessary measurements allow the flow table to provide other interesting insights. These include
- the efficiency of the motor,
- electrical and mechanical power information,
- the breakdown of individual losses, and
- the representation of total losses over speed and torque.
Similar to the characteristic curve measurement, the flux characteristics can be recorded statically or dynamically. In the static measurement of an electric motor, the test object is energized at the same speed and voltage for several predefined operating points. The total current is held constant for one operating point. However, to calculate the fluxes d and q, however, the current angle of this constant total current is changed in defined steps. The variation of the current angle, i.e. the ratio of the components d and q of a total current to each other, leads to the desired result due to the direct dependence of the d and q flows on the d and q currents.
Small steps between the previously defined total currents and small changes in the current angle steps help to obtain more detailed information about the d and q flows of a motor. The current distribution is displayed in a polar plot, while colored 3D maps show the d and q flows as a function of Id and Iq.
The Flux Table is primarily used to analyze the efficiency of modern electric motors. The results of the flux table are particularly useful for optimizing application-oriented motor control by means of controllers. The results of the flux table are particularly useful for optimizing application-oriented motor control.
Examples of derived results of the test type
- Magnetic flux in d-direction as a function of d- and q-current
- Magnetic flux in q-direction as a function of d- and q-current
- Internal electrical torque
- Maximum torque for the given current and voltage limits as a function of speed
- d-current as a function of speed to generate the optimum torque characteristic curve
- q-current as a function of speed to generate the optimum torque characteristic curve
- d-current as a function of speed and torque for the minimum total current at the operating point
- q-current as a function of speed and torque for the minimum total current at the operating point
- Total current as a function of speed and torque at minimum total current at operating point
- Electrically absorbed power as a function of speed and torque at minimum total current at the operating point
- Mechanical power output as a function of speed and torque at minimum total current at operating point
- Efficiency as a function of speed and torque with minimum total current at the operating point
- Ohmic losses as a function of speed and torque with minimum total current at the operating point
- Friction losses as a function of speed and torque
- Total losses as a function of speed and torque with minimum total current at the operating point
Torque ripple test
Electric motors are not perfectly symmetrical per se. This leads to irregularities in the torque during operation of the motor. Such uneven torque curves during one revolution of the rotor can cause severe vibration and noise, shorten the life of the motor and the driven machines, or simply reduce the performance of the system.
Torque ripple, determined by an active imx standard test, is an important parameter when optimising an electric motor. The test is performed by operating and measuring the test motor at constant speeds and torques close to those of the application. The relevant torque fluctuations, superimposed on the load of the load machine, are then recorded using high-precision sensors. A sampling or digitisation rate adapted to the speed supports the subsequent magnitude and frequency analysis.
The measurement of specimens at different temperatures can also lead to different results and should therefore be included in the investigations. Early detection of excessive torque ripple is worthwhile.
This type of test should not be missing from any development or quality assurance test bench.
Results of the test
- Curve of the resulting shaft torque
- Curve of the angle of rotation
- Curve of the torque versus angle of rotation
- Order analysis of the shaft torque
Performance curve EC (static/dynamic test)
The characteristic curve measurement, an active imx standard test, is one of the best-selling test types. For good reason, as this type of test provides valuable and comprehensive insight into the practical behavior of an electric motor.
The results of a characteristic curve measurement describe the motor as a converter of electrical into mechanical energy. Currents and voltages on the input side and torques and speeds on the mechanical output side are compared, losses and efficiency are calculated, and all results and dependencies are displayed. The results of such a measurement should always be part of the data sheet of an electric motor.
In practice, the performance curve measurement is subdivided. Should the test be speed controlled or torque controlled? The correct control strategy depends on the subsequent use of the motor. For example, if the DUT is to be operated under torque control in its future environment, the test should be designed accordingly.
Measuring a “static” characteristic curve determines the behavior of a motor at specific operating points, i.e. specified constant speed/torque combinations. Measurements are taken over a defined time interval, starting as soon as the desired speed is reached and the motor current has stabilized. The dwell time of typically a few seconds at the operating point allows for precise and undisturbed recording of the measured values. This means that the results of each operating point are based on maximum accuracy and a high degree of noise immunity.
The disadvantage of this method results from the above mentioned measurement strategy. Many operating points require a lot of time. On the one hand, because the measurement has to be performed slowly from one operating point to the next, and on the other hand, because of the changing thermal conditions of the motor, adapted cooling phases must always be provided.
The dynamic characteristic curve measurement enables the entire working range of an electric motor to be measured quickly, usually in just a few seconds. The process minimizes the heating of the motor and thus ensures that the results are obtained under almost identical thermal conditions. The prerequisite for this is a fast control speed on the test bench. As a result, the dynamic measurement procedure is fast and reliable. All typical motor characteristics are generated within less than a minute in most cases. The disadvantages of this measurement also result from the method. Possible control inaccuracies of the controller or interference on the measurement signals can only be filtered out with difficulty or not at all. The measurement and the resulting results are therefore slightly more prone to error.
The results of the characteristic curve measurement can be used to assess the suitability of a motor for specific tasks. They give every user a good overview of defined operating points (speed/torque combinations). However, they also help with development, for example when defining different control strategies.
In summary, both types of tests provide the user with valuable results for evaluating a motor. If precision is an absolute priority and time is not a critical factor, the static method is recommended. If a large number of motors are to be measured or an overview is to be obtained in a short time, the dynamic method should be selected. In practice, in our experience, a mix is almost always recommended.
Results of the test
- Torque curve versus speed
- Current curve versus speed
- Voltage curves versus speed
- Efficiency versus speed
- Input power versus speed
- Output power versus speed
- Current curve versus speed in the d-axis
- Current curve versus speed in the q-axis
- Voltage curve versus speed in the d-axis
- Voltage curve versus speed in the q-axis
Inductance test
Unlike the measurement of a simple coil, the measurement of motor inductance is dynamic. It must be performed at a constant speed with an adapted sampling rate. The frequency of the AC current applied during the measurement is also essential. It should vary for different measurements in order to evaluate the test motor.
Through the test and subsequent analysis, designers can assess the overall quality of the above-mentioned motor components and, more importantly, their interaction. It also identifies potential problems or irregularities that affect the overall performance of an electric motor. These findings help to optimize the efficiency, reliability and lifetime of electric motors.
The standard passive test inductance measurement is an important tool for evaluating a complex electric motor. Design features such as conductor geometries and cross sections, coil winding adjustments, and material selection have a significant impact on the inductance and therefore the performance of electric motors. They contribute to the goal of creating an optimal magnetic field that generates torque.
Particularly important in electric motors is the angle-dependent consideration of the change during a mechanical revolution, since the inductance is closely correlated with the position of the rotor.
Results of the test
- Inductance over one mechanical revolution of the motor
- Line-to-line inductance
- Phase inductance
Encoder test
Encoder testing is one of the standard passive test methods.
In many modern BLDC motors (EC motors), an encoder is used to determine the exact position of the rotor in the stator and send it to the motor controller. These signals are then used by the controller to electronically commutate the motor. Encoders can be of various types. Their purpose, to determine the position of the rotor in the motor, is always the same.
If the encoder is not correctly aligned or its position signals are unreadable or defective, this can lead to malfunction or damage to the motor. Typical symptoms are uneven running, increased noise and reduced efficiency. Encoder errors can also lead to premature wear. These are good reasons to test the encoder.
The encoder test evaluates the quality of the encoder. This is done by comparing the angle signal output by the tested encoder with that of a reference encoder. In addition, the angle signal is examined in relation to the generator voltage and any angular misalignment between the rotor and stator is detected. Calculation of the corresponding offset value can then be used to offset the angle during operation, provided modern controllers are used.
The encoder test is often carried out together with the generator voltage and reactive torque measurements. It is also typical to use the test in the end of line area.
Results of the test
- Error of encoder angle to reference angle over the reference angle
- Encoder angle over the reference angle
- Generator terminal voltages of the DUT over the reference angle
Cogging torque measurement
The cogging torque (previously often referred to as “pole sensitivity”) is a typical characteristic of permanent magnet motors. It is caused by the interaction between the permanent magnets and the grooved electrical laminations of the rotor and stator. It is particularly noticeable at low speeds. The cogging torque is measured and evaluated using a standard passive imx test method.
Cogging is often undesirable and can be almost completely eliminated by design measures. On the other hand, strong cogging can be used for starting, braking and positioning the rotor. Reason enough to take a closer look at cogging in various applications.
The cogging torque is particularly evident when a test specimen is dragged slowly through the load machine in the range of 1-10 rpm. The angle-dependent measurement of the torque generated by cogging provides insight into the structure of the motor and the quality of various components. In addition to measuring the cogging torque, the uniformity of the air gap, the teeth and grooves of the electrical laminations, and the magnetization, quality, and orientation of the magnets can be analyzed.
One challenge in performing the test is the absolutely necessary uniformity of the test speed. Despite high cogging in some cases, the load machine and its controller must not be influenced by the DUT (and must not influence the tested motor).
The measurement of the cogging torque is often performed together with the measurement of the generator voltage and the drag torque.
Results of the test
- Curve of the measured torque
- Plot of the torque versus the angle of rotation
- Order spectrum of the measured torque
- Average frictional torque over one revolution
- Peak-to-peak value of cogging torque
Drag torque determination
The thermal rise of an electric motor is one of the undesirable characteristics of all motor types. It depends on the size of the motor, the operating load, the speed and the efficiency. Heat influences the service life of a motor so significantly that its measurement is often part of development and quality control in the end-of-line area.
The passive standard test type drag torque can provide insights into the partial determination of the cause of unwanted heat. This passive test makes it possible to determine the loss torque of a dragged test motor as a function of the speed, whereby the average torque over one mechanical revolution is always considered.
The drag torque is mainly determined by the bearing friction, but magnetization losses or the air friction of a permanently connected fan wheel are also included in the measurement result. It is divided into a speed-dependent and speed-independent torque.
The drag torque measurement allows conclusions to be drawn about defective bearings, bearing tension, mechanical fan defects and grinding parts as well as faults in the magnetization components.
The drag torque test is also frequently carried out to determine the mass moment of inertia of the test specimen (together with the coupling and the torque sensor).
Results of the test
- Curve of the drag torque
- Curve of the speed
- Drag torque above speed
- Table of individual operating points
- Static friction
- Dynamic friction
- Mass moment of inertia
Back EMF test
One of the most commonly used passive standard tests is the generator voltage test or back EMF. It is relatively easy to perform and provides important information about the design and quality of an electric motor. Generator voltage tests can be carried out on the finished motor or during motor production by temporarily “marrying” the rotor and stator at a test station.
Permanently excited synchronous motors that are externally driven produce a measurable voltage at the terminals, called the generator voltage. This is directly related to the speed (at which the test object is driven) and the excitation or structure of the motor components responsible for it (the magnets, the stator and the windings).
These components can then be evaluated using the results of the generator voltage measurement as a function of the rotor angle, which is also recorded. Asymmetrical windings, missing, defective or incorrectly installed magnets, different magnetizations or air gap dimensions lead to measurable deviations and are displayed by calculating the measurement signals.
Generator voltage measurements are often performed together with short-circuit current measurements.
Results of the test
- Induced motor voltages over the angle of rotation
- Order spectrum of motor voltages
- Distortion factor of the motor voltages
- Course of the induced voltage over the speed
Phase resistance measurement
The measurement of lead resistance is one of the typical tests that electric motors are subjected to. It is used both in development and in quality assurance, for example at the end of motor production, to check and ensure certain motor parameters.
The purpose of string resistance measurement is to determine the resistances of the different wiring harnesses in a multiphase motor. However, its importance goes far beyond the mere determination of the measured values and the often associated assessment of the winding dimensioning.
By comparing the measured resistances, winding symmetries can be assessed and potential winding faults identified. In addition, a number of contact resistances between terminals and windings as well as between windings themselves are usually evaluated and errors such as incorrect number of windings and possible insulation problems are detected.
A key factor in string resistance measurement is the inclusion of temperature. Temperatures and lead resistance are known to be directly related. Therefore, resistance must always be evaluated in relation to temperature, and comparative measurements should always be made under the same thermal conditions.
On the other hand, a measured resistance, for example at the end of a load cycle, can provide information about the winding temperature. Resistance measurement can therefore be used to determine the thermal load on a motor, which in turn is critical to the life and efficiency of the motor.
String resistance measurement thus provides a small but important insight into the structure of an electric motor and its behavior under real conditions.
Results of the test
- Phase resistances of the motor windings
- Phase resistances of the motor windings scaled to 20 °C
- Average phase resistance
- Phase resistance variation
- Terminal resistances of the motor
- Terminal resistances of the motor scaled to 20 °C
- Average terminal resistance
- Scattering of the terminal resistances
Operating temperature test
The custom operating temperature test literally heats things up. The active test type and its results determine and show the relationship between the operating temperature and the way an electric motor is used. The operating temperature in conjunction with a usage profile is particularly important in estimating the continuous load capacity and life of an electric motor.
In most cases, the permissible temperatures for motors are designed on the assumption that they will be used in continuous operation (S1 according to IEC 60034-1) at rated power. In short-time operation or intermittent load patterns, the same motors (at the same power) will not reach their temperature limit. This means that motors for such short-time operation could well be used in higher power ranges.
A special test is therefore recommended to avoid excessive temperatures in possibly unknown load scenarios, as the negative effects of excessive temperatures in the everyday life of a motor are generally known:
- accelerated thermal ageing
- increased losses
- premature degradation of lubricants
- deformation and expansion
The test bench automation plays an important role in determining the operating temperature of an engine in the various operating states. The accuracy of the load specifications in terms of time and amount is important for the usually complex measurements. The load patterns should be configurable prior to measurement so that customer-specific requirements can be accurately reproduced. Automated test execution ensures the necessary repeatability and reduces the number of personnel.
Using imx GraphiPEd (Graphical Program Editor), load profiles can be easily created graphically or using spreadsheets. Cyclic repetitions, load sequences and loops can be created in minutes. Limits to monitor during the test and abort criteria can be defined and customized. Test plans can be used to load the test object with a sequence of different tests that the user does not have to explicitly start, allowing tests to run overnight, over the weekend, or over a longer period of time (endurance testing).
e-Motor custom tests
With the rise of electric drives in vehicles, electric motors are also subject to increased requirements in terms of electrical robustness. Test specifications such as LV124/148 require new or additional types of tests. This is less about the motor and its performance per se, but more about the possible interactions between the components in the vehicle, which of course includes the electric motor.
With the help of partners and suppliers of high voltage, current and voltage test technology, electric motors and, if required, motor/control units (inverters) can be tested on the imx test benches. This makes it possible to test motors holistically on imx test benches during the development process.
Typical test types for electric motors (also with customer control) are listed below as examples of a large number of different tests.
- Insulation test
- Resistance measurements
- Overvoltage and undervoltage behavior & power supply variations etc. (LV124 E01…/LV148 E4801…)
