For enterprises to ensure the normal operation of production lines, predictive maintenance is particularly important. Regular manual maintenance is to prevent unexpected motor shutdowns that could cause huge losses to the production line.
Many companies perform predictive insulation maintenance regularly. However, despite regular insulation testing, motors can still be damaged due to insulation problems. Why is this? Let's find out.
Background Introduction
The production department of a water utility group in a city in Fujian Province is responsible for the safety of water supply production in various cities and counties. The department conducts routine temperature and insulation performance checks on motors before they are put into operation. However, these routine checks did not prevent the motors from accidentally burning out.
The motor installed in 2005 burned out in 2012 after running for 6 years, and the motor installed in 2012 burned out in June 2018 after running for 6 years. Both motors had their windings burned out.
Both motors had burnt-out windings. Why couldn't a traditional insulation tester detect the insulation problem in the motor windings?
Let's first take a look at the voltage conditions at the output terminals of the frequency converter and the motor.
Real-world test cases
Tested using Fluke MDA-550
Tests ①-④ were conducted on a power frequency motor. The cable length between the inverter and the motor was 20m. As shown in the diagram above, the burrs at the motor end were obvious, the peak value increased from 1240V to 1790V, and the overshoot increased from 14.7% to 80.1%.
⑤ Waveform analysis of inverter output and motor terminals
Figure 5 also shows the test of a power frequency motor, which was repaired after burning out in June 2018. The heat resistance rating of the insulation material of this motor is F, and the maximum allowable operating temperature is 155℃. The cable length between the inverter and the motor is 2m. According to the MDA-500 analysis, the peak voltage at the motor end reaches 900V, and the overshoot coefficient is 67%. This pulse impulse voltage has reached the "severe" level of the IEC standard.
Case Analysis
As shown in Figure 5, the peak-to-peak voltage of the motor was 1640V, while the maximum allowable voltage for a frequency converter motor with "ordinary IVICA" insulation class was 1254V. It was this long-term pulse overvoltage that caused the inter-turn insulation performance of the motor to deteriorate, eventually leading to breakdown and short circuit, and the motor to burn out.
For this operating condition, it is recommended to use a variable frequency motor with a pulse insulation class of at least "medium IVICB".
in conclusion
Traditional insulation resistance testers can only measure the insulation performance between phase and ground, and between phases, but cannot assess the inter-turn insulation of each phase. This is why, even with traditional periodic insulation testing, motor burnout due to insulation problems still occurs. Therefore, pulse voltage and overshoot coefficient testing at the motor terminals are particularly important. Adding this test is essential for predictive maintenance, preventing significant economic losses from production line downtime.
Using Fluke MDA-500 series motor drive analyzer
Easily determine the health status of the motor
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