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29 May 2001

Basic VFD Maintenance

by Dave Polka

It's not hard to keep a drive running, if you follow a few simple steps.

Do you know how to maintain a variable frequency drive (VFD)? It's easier than you might think. By integrating some simple, logical steps into your preventive maintenance (PM) program, you can ensure your drives provide many years of trouble-free service. Before looking at those steps, however, let's quickly review what a VFD is and how it works.

A VFD controls the speed, torque, and direction of an AC induction motor. It takes fixed-voltage, fixed-frequency AC input and converts it to a variable-voltage, variable-frequency AC output. In very small VFDs, a single power-pack unit might contain the converter and inverter.

Intricate control circuitry coordinates the switching of power devices, typically through a control board that dictates firing power components in the proper sequence. A microprocessor or digital signal processor meets all the internal logic and decision requirements.

From this description, you can see a VFD is essentially little more than a computer and a power supply. For that reason, the same safety and equipment precautions you'd apply to a computer/power supply combination apply here. VFD maintenance requirements fall into three basic categories:

• Keep it clean.
• Keep it dry.
• Keep the connections tight.

Let's look at each of these.

Keep It Clean

Most VFDs fall into either the NEMA 1 (side vents for cooling airflow) or the NEMA 12 (sealed, dust-tight enclosure) category. Drives categorized as NEMA 1 are susceptible to dust contamination (see Figure 1). Dust on an electronic device (such as VFD hardware) can cause a malfunction or even failure. It contributes to a lack of airflow, resulting in diminished performance from heat sinks and circulating fans.

Dust absorbs moisture, which also plays a role in failure. Periodically spraying air through the heat sink fan is a good PM measure. However, although discharging compressed air into a VFD is a viable option in some environments, typical plant air contains oil and water. To use compressed air for cooling, you must use dry, oil-free air or you're likely to do more harm than good. This, in turn, requires a specialized, dedicated, and expensive air supply. Moreover, you still run the risk of generating electrostatic discharges (ESDs). A non-static-generating spray or a reverse-operated ESD vacuum reduces static buildup. Common plastics are prime generators of static electricity. The material in ESD vacuum cases and fans is a special, non-static-generating plastic. These vacuums, and cans of non-static-generating compressed air, are available through companies that specialize in static control equipment.

Keep It Dry

Figure 2 shows a control board that was periodically subjected to a moist environment. Initially, this VFD was wall mounted in a clean, dry area of a mechanical room, and moisture wasn't a problem. However, as is often the case, a well-meaning modification led to problems. An area of the building required a dehumidifier close to the mechanical room. Because wall space was available above the VFD, this is where the dehumidifier went. Unfortunately, this was a NEMA 1-style VFD. The obvious result: Water dripped from the dehumidifier into the drive. In six months, the VFD accumulated enough water to produce circuit board corrosion.

What about condensation? Some VFD manufacturers included a type of "condensation protection" on earlier product versions. When the mercury dipped below 32°F, the software logic wouldn't allow the drive to start. VFDs seldom offer this protection today. If you operate the VFD all day, every day, the normal radiant heat from the heat sink should prevent condensation. However, unless the unit is in continuous operation, use a NEMA 12 enclosure and thermostatically controlled space heater if you locate it where condensation is likely.

Keep Connections Tight

While this sounds simple, checking connections is a step many people miss entirely or do incorrectly-and the requirement applies even in clean rooms. Heat cycles and mechanical vibration can lead to substandard connections, as can standard PM practices. Retorquing screws isn't a good idea, and further tightening an already tight connection can ruin it (see sidebar).

Bad connections eventually lead to arcing. Arcing at the VFD input could result in nuisance overvoltage faults, clearing of input fuses, or damage to protective components. Arcing at the VFD output could result in overcurrent faults or even damage to the power components. Figures 3 and 4 demonstrate potential results.

Loose control wiring connections can cause erratic operation. For example, a loose START/STOP signal wire can cause uncontrollable VFD stops. A loose speed reference wire can cause the drive speed to fluctuate, resulting in scrap, machine damage, or personnel injury.

Additional Steps

1. As part of a mechanical inspection procedure, don't overlook internal VFD components. Check circulating fans for signs of bearing failure or foreign objects-usually indicated by unusual noise or shafts that appear wobbly (Figure 5).
2. Inspect DC bus capacitors for bulging and leaking (Figure 6). Either could be a sign of component stress or electrical misuse.
3. Take voltage measurements while the VFD is in operation. Fluctuations in DC bus voltage measurements can indicate degradation of DC bus capacitors. One function of the capacitor bank is to act as a filter section (smoothing out any AC ripple voltage on the bus). Abnormal AC voltage on the DC bus indicates the capacitors are headed for trouble. Most VFD manufacturers have a special terminal block for this type of measurement and for connecting the dynamic braking resistors. Measurements more than 4 VAC may indicate a capacitor filtering problem or a possible problem with the diode bridge converter section (ahead of the bus). If you have such voltage levels, consult the VFD manufacturer before taking further action.

   With the VFD in START and at zero speed, you should read output voltage of 40 VAC phase to phase or less. If you read more than this, you may have transistor leakage. At zero speed, the power components shouldn't be operating. If your readings are 60 VAC or more, you can expect power component failure.

4. What about spare VFDs? Store them in a clean, dry environment, with no condensation allowed. Place this unit in your PM system so you know to power it up every six months to keep the DC bus capacitors at their peak performance capability. Otherwise, their charging ability will significantly diminish. A capacitor is much like a battery: It needs to go into service soon after purchase or suffer a loss of usable life.
5. Regularly monitor heat sink temperatures. Most VFD manufacturers make this task easy by including a direct temperature readout on the keypad or display. Verify where this readout is, and make checking it part of a weekly or monthly review of VFD operation.

You wouldn't place your laptop computer outside, on the roof of a building, or in direct sunlight, where temperatures could reach as high as +115°F or as low as -10°F. A VFD, which is essentially a computer with a power supply, needs the same consideration. Some VFD manufacturers advertise 200,000 hours—almost 23 years—of mean time between failures. Such impressive performance is easy to obtain if you follow these simple procedures. MC

Figures and Graphics

• Basic VFD Maintenance [PDF file]
• Figure 1: This VFD shows evidence of fan-injected dust particles.
• Figure 2: Corrosion caused by excessive moisture.
• Figure 3: Loose input power connections.
• Figure 4: Loose output power connections.
• Figure 5: A fan with a foreign object in it.
• Figure 6: An obvious capacitor stress problem.

Sidebars

• Retorquing: A Screwy Practice

Author Information

Dave Polka is the drives training manager for ABB's Drives & Power Products Group. Contact him at 16250 West Glendale Drive, New Berlin, WI 53151; tel: (414) 785-3517; fax: (414) 789-8608; www.abb.com/usa.