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ABB Maintaining Variable Frequency Drives Can Be Simple, If You Understand and Follow Some Basic Practices (By Dave Polka, Drives Training Manager, ABB Inc., Drives & Power Electronics, Automation Technology Products Division, New Berlin, WI) Do you know how to maintain Variable Frequency Drives (VFDs)? Doing so is easier than you might think. By integrating some simple, logical steps into your preventative maintenance program, you can ensure your drives provide many years of trouble-free service. Before looking at those steps, let’s quickly review what a VFD is and how it works. A quick overview A VFD controls the speed, torque and direction of an AC Induction motor. It takes fixed voltage and frequency AC input and converts it to a variable voltage and frequency AC output. The Table shows what the three main sections of a VFD are, and what they do. In very small VFD’s, a single power pack unit may contain the converter and inverter. Fairly involved control circuitry coordinates the switching of power devices, typically through a control board that dictates the firing of power components in the proper sequence. A microprocessor or Digital Signal Processor (DSP) meets all the internal logic and decision requirements. From this description, you can see a VFD is basically a computer and power supply. And the same safety and equipment precautions you’d apply to a computer and to a power supply apply here. VFD maintenance requirements fall into three basic categories:
Let’s look at each of these. Keep it Dry What about condensation? Some VFD manufacturers included a type of "condensation protection" on earlier product versions. When the mercury dipped below 32 degrees Fahrenheit, the software logic would not 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 heatsink should prevent condensation. 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 basic, checking connections is a step many people miss or do incorrectly—and the requirement applies even in clean rooms. Heat cycles and mechanical vibration can lead to sub-standard connections, as can standard PM practices. Re-torquing screws is not a good idea, and further tightening an already tight connection can ruin the connection (see Sidebar, below). Bad connections eventually lead to arcing. Arcing at the VFD input could result in nuisance over voltage faults, clearing of input fuses, or damage to protective components. Arcing at the VFD output could result in over-current faults, or even damage to the power components. 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. 2. Inspect DC bus capacitors for bulging and leakage. 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 also for connection of the dynamic braking resistors. Measurements more than 4VAC 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 40VAC phase-to-phase or less. If you read more than this, you may have transistor leakage. At zero speed, the power components should not be operating. If your readings are 60VAC 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 6 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 115 degrees Fahrenheit or as low as –10 degrees Fahrenheit. A VFD, which is basically 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 (MTBF). Such impressive performance is easy to obtain, if you follow these simple procedures. Sidebar: Re-torquing -- A screwy practice Although "re-torquing" as a way of checking tightness is common in many PM procedures, it violates basic mechanical principles and does more harm than good. A screw has maximum clamping power at a torque value specific to its size, shape, and composition. Exceeding that torque value permanently reduces the clamping power of that screw by reducing its elasticity and deforming it. Loosening and then re-torquing still reduces elasticity, which still means a loss of clamping power. Doing this to a lock washer results in a permanent 50% loss. What should you do? Use an infrared thermometer to note hot connections. Check their torque. If they have merely worked loose, you can try retightening them. Note which screws were loose, and be sure to give them an IR check at the next PM cycle. If they are loose again, replace them. Finally, don’t forget the "tug test." This checks crimps, as well as screw connections. Don’t do this with the drive online with the process, though, or you may cause some very expensive process disturbances. For more information on ABB Drives & Power Electronics, contact: Becky Nethery, Manager, Marketing Communications, ABB Inc., Automation Technology Products Division, Drives & Power Electronics, 16250 West Glendale Drive New Berlin, WI 53151-2840, Tel: (262) 785-8363, Fax: (262) 780-5120 e-mail: becky.nethery@us.abb.com |
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