VFD Annual Maintenance Contract

VFD demands the same safety and equipment prudence  applied to computers and power supplies because of its similitude to both: Set it clean & dry; and clinch the connections tight.

Clinch it clean. Uttermost VFDs fall into a NEMA 1 (side vents for cooling airflow) or NEMA 12 (sealed, dust-tight enclosure) category. Side vents in NEMA 1 drives make VFDs nonresistant to dust taint, which can cut down airflow and extenuate attainment from heat sinks and circulating fans.

Dust on an electronic device can cause glitch or even wreck by absorbing moisture. Discharging compressed air into the VFD is a viable choice in some environments, but conventional plant air hold oil and water. Oil-free, dry air demand a specialized, dedicated, and upscale air supply — and you still run the peril of generating static charges. A nonstatic generating spray or a reverse-operated ESD vacuum will slash static build-up. Common plastics are zenith generators of static electricity. The material in ESD vacuum cases and fans is a special, nonstatic generating plastic.

Set it dry. The photo above (not available online) shows what can occur to a control board consistently subjected to a moist environment. Initially, this VFD was wall- braced in a clean, dry area of a mechanical room. However, problems emerges when maintenance personnel installed a dehumidifier in an open space above the VFD. Unfortunately, the VFD was a NEMA 1 enclosure style (side vents and no seal around the cover). Water dripped from the dehumidifier into the drive. In six months, the VFD took on adequate water to corrode the circuit board.

VFDs seldom aid condensation protection today. If you operate the VFD regularly, the normal radiant heat from the heat sink should avert condensation. Except the unit is in continuous operation, use a NEMA 12 enclosure and a thermostatically controlled space heater if you place it where condensation is likely.

Maintain connections tight. This may seem apparent, but checking connections is a step many people miss or do wrongly — and the fundamental applies even in clean rooms. Heat cycles, mechanical vibration, and standard PM process can lead to substandard connections. Also, retorquing screws and over tightening can wreck connections.

Bad connections finally lead to arcing. Arcing at the VFD input could end in overvoltage faults, clearing of input fuses, or ravage to protective components. Arcing at the VFD output could result in overcurrent faults, or even deterioration to the power components.

Loose control wiring connections can cause abnormal operation. For example, a loose speed reference wire can cause the drive speed to fluctuate, resulting in chunk, machine damage, or injury.

Further steps. The specified perpetuation requirements enhance a good PM program.

1. When organizing a mechanical inspection, don’t let slide internal VFD components. Check circulating fans for warning of bearing collapse of foreign objects.
2. Observe DC bus capacitors for bulging and leakage, which could be trace of component stress or electrical misuse.
3. Take voltage measurements while the VFD is in operation. Fluctuations in DC bus voltage measurements can evince decline of DC bus capacitors. One purpose of the capacitor bank is to deed as a filter section (smoothing out any AC ripple voltage on the bus). Unusual AC voltage on the DC bus specify potential capacitor failure. Analysis of more than 4VAC may point out a capacitor filtering problem or a realizable complication with the diode bridge converter section (ahead of the bus). If you have specific voltage levels, consider the manufacturer before taking further process.

With the VFD in ONSET and at zero speed, you should view output voltage of 40VAC phase-to-phase or less. Higher voltages could signify transistor discharge. At zero speed, the power components should not be operating. Readings of 60VAC or more can imply power component failure.

1. Stockpile spare VFDs in a clean, dry circumstances. Fix this unit in your PM system so you know to power it up every six months to keep the DC bus capacitors at their crest performance efficacy. Elseways, their charging ability will abate certainly.
2. Systematically supervise heat-sink temperatures. Most VFD manufacturers make this chore obvious by including a direct temperature readout on the keypad or display. Find this readout, and analysis it periodically.

You wouldn’t keep your laptop outside, on the ceiling of a building, or in direct sunlight. Besides a VFD needs the same attention. Some manufacturers allege their VFDs offer 200,000 hr — almost 23 yr — of mean time between wreck (MTBF). Such influential performance is easy to attain, if you pursue these simple strategy.