[Guide]Test cables are generally regarded as ideal conductors in textbooks, but in reality, if you ignore the influence of cables, you will often get wrong test results. For example, the glitch signal on the output voltage of the inverter may be caused by the cable.
Engineers accidentally found glitches on the PWM waveform of the voltage during the inverter motor test. Is this glitch signal the true output of the inverter?
Theoretically, a normal inverter output has no glitch signal, so what is the problem with the power analyzer or the inverter itself?
After comparative verification test, it is found that there is no problem with the power analyzer and the inverter. The problem lies in the cable connecting the inverter to the motor. Test different pick positions, the results obtained are actually different.
1.5m wire is used to connect the inverter and the motor, and the modulation frequency of the inverter is 16kHz.
1. Select the voltage measurement point close to the output terminal of the inverter. At this time, the voltage waveform is normal, relatively clean and free of burrs, which is a normal state.
Figure 1 Test diagram of inverter output terminal
Figure 2 Magnified view of the output terminal test of the inverter
2. Select the voltage measurement point close to the motor side. At this time, the voltage waveform has glitches, which does not meet the normal expectations.
Figure 3 Test diagram of motor input
Figure 4 Magnified view of motor input test
Through the above comparison, it is found that the waveforms measured at different points on the cable are quite different, and it can be basically confirmed that it is the cause of the cable.
The measured 1.5m wire inductance is 1.4μH. The frequency converter is a low-impedance source, and the overvoltage at the output end will be clamped by the internal DC bus, so the waveform stability burr is few. The PWM waveform loaded on the motor has steep edges and rich high-frequency components. The motor is an inductive load and has a back-EMF. When the connecting wire is long, the wire inductance prevents the inverter from absorbing the overvoltage, and the voltage at the motor input produces an overshoot. You will see that the waveform has many glitches.
There are two situations:
● When paying attention to the output power of the inverter, select the measurement point on the side of the inverter to eliminate the influence and loss caused by the cable. At this time, the focus is on the inverter;
● When paying attention to the conversion efficiency of the motor, efficiency = Pm/P motor input, and the measurement point is selected on the motor input side. At this time, the main focus is on the motor.
The overall test of the inverter motor mainly focuses on the input and output power and efficiency of the inverter, the input power, mechanical power and efficiency of the motor; the PA5000H power analyzer has a high precision of 0.05%, up to 7 boards can be freely matched with motor cards, and Comprehensive analysis function. One power analyzer can complete the parameter measurement and analysis of the entire system of the inverter motor.
Figure 5 PA5000H power analyzer
Source: ZLG Zhiyuan Electronics