The ubiquitous VSD, AC type, consists of three main parts: a rectifier, a DC capacitor, and an on-off switch (IGBT).
The working principle is that the VSD adjusts the frequency (f) so that the motor rotates at the desired speed. At the same time, it will increase - decrease the pressure to suit the automatic use. In order to achieve constant magnetic flux values according to the motor's iron core design (V/f), for example, to slow the motor down by 50%, the voltage must be reduced by 50% from 400V to 200V with a DC capacitor used. To smooth the pressure Before joining the inverter, the on-off switch commands the rectifiers such as IGBT, SCR to make the output voltage look square-wave and the current entering the motor is close to the sine wave with proper switching on and off. Both the converter and the inverter will allow us to generate electrical power (Variable Frequency Power) that can control the motor speed as needed. with energy saving
Different types of VSD malfunctions and methods of determining their primary cause
Fault from the rectifier
We can stop the VSD system to take electrical measurements to see if the rectifier is working properly. In addition, we now have a technique to look at the pattern's response changes through machine learning.
Fault from the DC capacitor
The deterioration of the DC capacitor causes the voltage at the DC bus to be uneven, causing the speed control to be unstable. We can measure capacitance when power is off, or measure online via DC bus voltage, or monitor the capacitor's thermal conductivity online. this abnormal
Fault from the power switch (IGBT and controller)
3.1 Malfunction from controller malfunction
This malfunction may be an error from a miscalculated software or a microcontroller/microprocessor set. May be damaged in itself, including capacitors and various components in electronic circuits that have deteriorated. This causes the wrong output of the switching command to turn on and off. We can use thermal imaging to identify abnormalities on the controller.
3.2 Anomalies from IGBT
The IGBT malfunction directly affects the VSD's output voltage, making it difficult to control the motor. and often encounter problems with the ripple of the output current Fault checks are performed using the method of measuring the resistance between the gate pins. which can only be measured during downtime and is often detected after significant damage has occurred.
Example of “VSD failure problem from defective IGBT”
VSD can view anomalies online
From looking at the pattern of voltage and current of VSD
Motor current measurements with VSD drives can detect faults, e.g. in this case we can see that the current is overshooted in a straight line across multiple waves rather than a sine wave.
Motor voltage measurements with VSD drives can also detect faults. But they are more difficult to observe than current measurements. For example, in this case we can see that the voltage has a linear undershoot down one phase.
After measuring both items 1 and 2, it was found that approximately less than 1 month, this VSD caused a short circuit caused by a defective IGBT.
