Characteristic detection and test methods in industrial inverter development

1 Introduction

As a means of energy saving, variable frequency drive has been widely used in the industrial field, and the market for frequency converters is huge. Before a qualified frequency converter product enters the market, it must undergo various characteristic detection tests, which can be called evaluation tests. The implementers of these tests include the R&D department, the quality assurance department, and the factory inspection department. The test conditions, test methods, and result judgment of the evaluation test are very important. Generally speaking, the types of tests include:

(1) Type test: This test is to check whether the rated conditions, specifications and other requirements given by the inverter manufacturer in the product meet the requirements. It is carried out during the prototype trial stage of the inverter.

(2) Routine test: Check whether the product characteristics are consistent with the characteristics obtained by formal test.

(3) Option test: A test that has no direct relationship with the product style or rated parameters and is conducted only to understand certain capabilities of the product during use.

(4) Combined test: This is a test to evaluate the entire system including the motor and the load.

Due to limited space, this article roughly gives the various characteristic evaluation tests required in the research and development stage of industrial inverters.

2Electromagnetic compatibility tests required for product certification

Electromagnetic compatibility test is also the immunity test of inverter equipment. At present, it mainly refers to electrostatic immunity test, radiation immunity test caused by radio frequency field, electrical fast transient pulse immunity test, lightning surge immunity test, conducted immunity test caused by radio frequency induction, and voltage instantaneous drop and short interruption immunity test. Electromagnetic compatibility test is a relatively important test in the product development stage. Since it is a series of product certification tests, the test conditions and specifications should be strictly carried out in accordance with certain standards (such as national standards).

The electrostatic discharge test simulates the actual discharge process through a discharge gun, which is divided into direct discharge and indirect discharge. Direct discharge simulates the damage that may be caused to the device by static electricity when a person or object touches the device. Indirect discharge simulates the impact of a person or object discharging electricity to an object near the device on the device's operation.

Numerous mechanical switches in actual power grids will generate large interference when switching inductive loads. Fast transient pulse immunity test is conducted to simulate this interference. During the test, the pulse can be superimposed on the power line through the coupling/decoupling network, or on the communication line through the capacitive coupling clamp, so as to simulate the interference to the main power circuit and communication circuit of the inverter.

The lightning surge immunity test simulates the impact of natural lightning strikes on the power supply and communication lines of the inverter. The characteristics of the surge test are low pulse repetition rate, high amplitude, and huge energy, so the impact on the equipment may be destructive.

The voltage drop and instantaneous interruption test is a test to simulate the interference caused by sudden changes from the power supply side and the load side. In practice, the probability of these phenomena is random, and the characteristics are deviation from the rated voltage and lasting for a certain period of time. The main purpose of this test is to detect whether the inverter can work normally or respond in time under the above conditions to ensure that data is not lost.

The RF radiation electromagnetic field test simulates the working condition of the equipment being interfered by RF radiation, and the RF conduction test simulates the impact of RF conduction interference on the equipment. Both tests cannot be completed by a single machine, but require a set of instruments. They cannot be carried out in ordinary laboratories either. The RF radiation electromagnetic field test should be carried out in an anechoic chamber. The RF conduction test should generally be carried out in a shielded room.

3 Evaluation Tests in Hardware Circuit Design

There are many test items in hardware design. Strictly speaking, functional evaluation tests are required for various circuits designed in the product, such as confirmation of the initial current limiting circuit action, fan action and other auxiliary circuits. Quality evaluation tests are required for various components used in the product, such as ripple current and life estimation of electrolytic capacitors, shaft voltage of fans, etc.

Since the most important parts in hardware circuit design are IGBT (IPM) peripheral circuit design and switching power supply design, this article will focus on the relevant evaluation tests in these two aspects.

The key points of IGBT design include selection, drive circuit design, protection circuit design, and heat dissipation design. For large-capacity inverter design, the parallel application design of the device is also a very important content. The main evaluation tests include:

[page] (1) Selection test

Generally speaking, the selection of IGBT includes considerations of voltage, current and cost. But strictly speaking, loss is also a consideration in the selection stage. There are three basic methods for analyzing loss: calculation method, software simulation method and experimental method. Since the collector-emitter saturation voltage Vce (sat) and the diode forward voltage VF are important parameters for calculating the loss of IGBT and anti-parallel diode respectively. Therefore, in module selection and replacement, the evaluation test of these two voltages is also very important. When testing, it is necessary to pay attention to the saturation voltage, which is only a low value of several volts. In order to eliminate the influence of the saturation of the internal amplifier of the oscilloscope during measurement, the external voltage clamping circuit method shown in the figure below can be used.

Figure 1 IGBT saturation voltage measurement method

In order to safely test the characteristics, it is necessary to pay attention to the power-on sequence. In addition, in order to ensure that the voltage is measured at the specified junction temperature, a pulse test method with a low duty cycle must be used.

Switching loss represents the loss from the start of switching to the return to a stable state after the switching action is completed. Switching loss is related to the DC side voltage, collector current, gate voltage and drive resistance. Therefore, the above test conditions must be paid attention to when testing and measuring switching loss. The waveform of switching loss includes the waveform of voltage and current. Please use the graphic area calculation function of the oscilloscope to calculate it. Pay attention to using effective filtering measures to eliminate the distortion of the current waveform collected by the oscilloscope. The figure below shows the measured waveform of IGBT switching loss.

(a) Turn-on loss (b) Turn-off loss

Figure 2 Measured waveform of IGBT switching loss (Vce: 200V/div, Vge: 10V/div, Ic: 200A/div)

(Test conditions: DC voltage 600V, collector current 600A, gate drive resistance 3.3Ω, drive voltage +15V and -10V)

(2) Driving circuit evaluation test

In the design of gate drive circuit, the measurement of drive voltage, current and power is an important project. The figure below shows the instantaneous waveform of gate drive voltage in a large-capacity inverter. Due to the use of a positive and negative dual power supply mode, it can be seen that the voltage value is about +15V when turned on and about -10V when turned off.

(a) ON (b) OFF

Figure 3 Measured gate voltage waveform

(3) Protection circuit evaluation test

One of the most important aspects of IGBT use is that the voltage and current of the device cannot exceed the safe working area: ① When turned off, including any required overload conditions, the collector peak current must be within the provisions of the switch safe working area ② When short-circuited, the action time of the protection circuit cannot exceed the short-circuit tolerance. The relevant evaluation tests include the shutdown safe working area confirmation test and the short-circuit test. The following figure shows the measured waveforms in the two tests.

(a) Surge voltage test waveform (b) Short-circuit current test waveform

Figure 4 Measured waveform of safe working area confirmation test

When measuring, please pay attention to the following: ① Use a detector and oscilloscope with sufficient frequency band ② Adjust the sensitivity of the oscilloscope in advance and adjust the frequency correction of the detector ③ Connect the measuring detector directly to the terminal of the component ④ Pay attention to the detection sensitivity and current conversion ratio of the CT. When measuring surge voltage, pay special attention to the direction of the current, because different current directions represent whether the surge voltage comes from the IGBT or the anti-parallel diode.

(4) Heat dissipation performance evaluation test

Another important aspect of IGBT use is that the operating junction temperature of IGBT must always be kept below Tjmax under any circumstances. To achieve this requirement, accurate loss calculation is required to design an excellent heat dissipation system. The evaluation of heat dissipation performance is mainly based on temperature testing. Of course, temperature testing is not entirely for IGBTs. Other components in the inverter also need temperature testing, including electrolytic capacitors, transformers, contactors, and major components in the drive circuit, etc.

The temperature test must first determine the temperature measurement point of the device, and then simulate various overload conditions to ensure that the junction temperature of the device does not exceed the maximum value under the worst conditions.

[page] (5) Device parallel evaluation test

When IGBTs are used in parallel, a measuring current transformer is required to detect the current passing through each device to confirm whether the current is balanced in the on state.

The design of the switching power supply is another important aspect in the design of the inverter hardware circuit. There are many evaluation tests after the design of the switching power supply is completed. Here we only introduce the more important test items. Including the action confirmation test of the switching power supply, the device characteristic confirmation of the switching power supply, the high and low temperature start-up measurement test, the electrolytic capacitor ripple current confirmation test, and the switching power supply capacity confirmation test. As the switching power supply used in the inverter, the main power supplies provided are: 5V power supply for the control circuit, power supply for the detection sensor, power supply for the relay, power supply for the fan, and power supply for the drive.

(1) Switching power supply operation confirmation test

It mainly refers to whether the variation range of each output voltage of the switching power supply is within the designed allowable error, which includes static and dynamic aspects. Static refers to the stable state of the inverter main power supply, and includes two load states: light load and heavy load. Dynamic mainly measures the output peak pulsation of each power supply in the initial state of the inverter main power supply, as shown in the figure below.

Figure 5 Measured waveform of 24V instantaneous start of switching power supply

(2) High and low temperature start-up test The design of the switching power supply must ensure normal start-up and operation at the specified operating temperature of the inverter. This test is to check the working performance of the switching power supply at high and low temperatures. The test waveform is shown in the figure below (MOSFET tube DS voltage).

(a) Normal oscillation waveform (b) Oscillation failure waveform

Figure 6: Oscillation waveform of the switching power supply at -15℃

(3) Component quality confirmation test refers to confirming whether the operating voltage and current of each major component, such as MOSFET and diode, exceeds the maximum operating range of the device under the most severe overload conditions.

(4) Ripple current confirmation test is to confirm whether the ripple current of the filter electrolytic capacitor used in the multi-channel switching power supply design meets the design requirements.

(5) Other tests

As a high-quality switching power supply design, it is also necessary to conduct input ON/OFF tests, abnormal state startup tests, load short-circuit tests, sudden changes in input voltage and load current tests, feedback circuit safety operation confirmation, temperature rise tests of each circuit, and destruction tests.

[page]4 Functional evaluation project of software design

Each inverter has some general software functions and its own special software functions. General software functions include some common functions, such as acceleration and deceleration function, DC braking function, torque boost function, speed tracking function, instantaneous stop restart function, etc. Special software functions are more suitable for different applications. After the design of the inverter is completed, the evaluation of these software functions is called software function evaluation. The purpose of these tests is to verify the correctness of the software design so as to better correct some parameters in the software. The items, conditions and methods of these tests are different due to the different inverters produced by various manufacturers. Among them, vector control is a function that most high-performance inverters now have, and it is an important indicator to distinguish the quality of inverters. Now I will mainly talk about the evaluation plan of this function.

From the control strategy itself, the control characteristics of vector control are mainly current control, torque control, speed control and flux control. Therefore, the technical indicators of vector control inverter are speed control range, speed control accuracy, torque control response speed and torque control accuracy. The figure below shows several typical test waveforms for vector control performance evaluation.

(a) Static speed characteristics (b) Rapid acceleration and deceleration test

(c) Load sudden change characteristics (d) Torque control characteristics

Figure 7 Typical test waveform for vector control performance evaluation

Figure (b) shows the rapid acceleration/deceleration test waveform. The main purpose of this test is to confirm the rapidity of speed response, the output torque control performance during acceleration/deceleration time, and the overshoot of speed response under certain parameters.

Figure (c) is the waveform of the load sudden change characteristic test. This test is actually the dynamic response process of the system when the speed returns to the original equilibrium state when the system is disturbed (external load). The main evaluation indicators are: speed change, speed recovery time and current control performance.

Figure (d) is the torque control performance test waveform. The inverter has torque limiting function and torque control function. The torque limiting function is to limit the output torque of the inverter within a certain range during speed control in vector control. Torque control means that the given signal of the inverter does not control the output frequency of the inverter, but controls the size of the torque generated by the motor. The performance of torque control is divided into static characteristics and dynamic characteristics of torque control. This figure is the waveform of the dynamic characteristic test, which mainly examines the response speed and accuracy of the torque, especially the operation under low speed and low torque setting.

5 Reference test

Reference tests mainly include: speed-torque and speed-current characteristic tests, leakage current measurement tests, vibration characteristic tests, shaft voltage characteristic tests, noise characteristic tests and temperature rise characteristic tests. These tests are mainly conducted to understand the possible problems of the inverter in actual applications. Since the conditions of the reference test are high and do not directly involve the product specifications, general inverter manufacturers do not conduct them.

The leakage current measurement circuit is shown in the figure below.

Figure 8 Schematic diagram of leakage current measurement circuit

The leakage current measurement requires a leakage current meter. Attention should be paid to the test conditions such as the length of the input wire, the length of the output wire, and the wire diameter. Generally speaking, the leakage current of the inverter is the largest when it is running at 50Hz to 60Hz, and it remains at a certain level below 50Hz, and is at a lower level above 60Hz.

6 Conclusion

There are many types of inverter tests. Due to limited space, this article introduces various evaluation tests done on inverters during the R&D stage. To complete the evaluation test, not only good laboratory and test instruments are required, but also the purpose of the test must be clear and the test method must be standardized. The test personnel are required to be careful and patient enough. Only in this way can the accuracy and reliability of the evaluation test be guaranteed, and qualified inverter products can be guaranteed.

References

[1] Fuji IGBT Module User Manual

[2] Inverter Drive Manual Inverter Drive Manual Editorial Committee Nikkan Kogyo Shimbun