Low Voltage VFD preventive maintenance is best performed on a periodic schedule, typically once per year. Consists mainly of visual inspection, cleaning with dry compressed air, checking connections for correct torque and setting up a replacement schedule for those components with the shortest operational life cycles.
Don’t retorque the connections. Use a thermal camera and check the variable frequency drive under load. If any connections show hot, THEN shut it down and check the torque with a torque wrench/screwdriver. I’ve seen more problems caused by people cranking down too hard on motor and power leads (and even control connections) than I have from connections that have worked loose.
Also, DON’T disassemble the VFD to perform the visual inspection (I’ve seen service contract specifications that required that). Get a bright light source and look as deep as you can for components showing excessive heating, for debris blown into the body of the drive by the cooling fans, for dust accumulation, for discoloration or corrosion due to corrosive gasses, for oil from leaking capacitors, etc. Only disassemble the VFD if REQUIRED to access something that doesn’t appear right.
- Contaminants: Many VFD failures are heat related. A natural byproduct of switching components (like those found in VFDs), is heat and the drive’s ability to dissipate that heat can be restricted by contaminants such as dust, oil, or debris. For every 10 degrees C rise in temperature, the life of electronic components is reduced by half. A PM program should involve cleaning the drive, especially the heat sink and checking for proper operation of all cooling fans.
- Loose Connections: Loose connections are another very common reason for drive failures. As current “jumps” between 2 loose connections, a small arc can occur. Over time, the oxide build up in that gap will essentially terminate the connection. Not to mention, that a loose connection is in essence a high resistance connection. Run current through a resistance, and you get heat. A PM program should involve checking all connections and tightening loose ones.
- The “eye” check: One of the most commonly used troubleshooting techniques (don’t tell my boss I’m letting you in on these industry secrets) is a simple visual inspection. A large number of problems are found by simply looking for discolored or bulged components on printed circuit boards. Electrolytic capacitors, another common failure item, tend to bulge and leak before they completely go bad; this is usually fairly obvious to see, as are burnt resistors and ICs. A PM program should include visually inspecting all printed circuit boards.
- Get ahead of future problems by taking measurements: By taking simple readings such as input/output voltage, DC Buss voltage, DC Buss ripple, control power supply voltage, and input/output current, one can see indications of future problems. The general rule on the input and output current is balance. For example: If you read 40 amps on the T1 motor lead, you want to have around 40 amps on the T2 and T3 motor leads as well. If you’ve got a phase much higher or lower than the others, you have a potential problem on your hands. A PM program should include taking live measurements.
WARNING: If you are not familiar or comfortable with high voltage electrical equipment, you should have a trained technician, such as one from EMA, take these readings.
- Check for proper operation: An often overlooked benefit to PM is having someone make sure the drive is in fact working as it should. A common finding is changed parameters which affect drive operation. In cases like this, there isn’t anything actually wrong with the VFD, it just isn’t running as it should. A PM program should include an operational check of the drive by someone who understands the application.