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Pumps & Systems, October 2007

The service department of a variable frequency drives manufacturer frequently sees the following scenario: A frustrated user calls with what he perceives to be a defective piece of equipment. As the technician begins probing for information, the user's frustration boils over, often with an exclamation along the lines of "what a piece of junk!" As the service technician asks the pertinent questions, the exasperated user relays the details of a drive that is continually tripping on a fault until the user is at the end of his rope, not knowing what to do.

If we rule out internal problems with the drive, what else can cause a drive to run poorly?

Remember that a drive is a sensitive electronic device. Unlike equipment that is run across the line, it is not designed to provide maximum power to the load until the motor or the system collapses. The drive will respond to fluctuations in system conditions and eventually drop off on a fault indication, depending on what part of the system is malfunctioning.

So how does one go about diagnosing this type of drive problem?

The best way to start looking for problems in the system is to take a system-wide approach. This sounds a bit simplistic, but let's look at the approach. When diagnosing fault tripping in a drive system, begin with the basic preventative maintenance overview. Laying out the steps of a good PM schedule is essential. These steps are as follows:

1. Check the system with a good visual inspection. Look for running or dripping water, high humidity, excessive temperature extremes, excessive dirt or contaminant debris, and corrosive agents located near or under the equipment.

Here's a good rule of thumb: If you would not place a television near the drive because of the physical environment, then the drive may have problems. If the drive does not have a sealed enclosure to handle rough environmental conditions, care must be taken to protect the drive assembly.

2. Clean the drive of dirt, dust and corrosion. Depending on the environment, there may be significant problems with contaminants. The drive should be relatively clean. Do not allow significant amounts of dirt to build up on the heatsink of the drive. This may prevent sufficient cooling of the drive semiconductors and may damage cooling fans and cause overtemperature problems.

3. Check all wiring connections for tightness. Loose connections in the wiring of the drive to the incoming power and to the motor are a major cause of drive failure. As the drive performs day in and day out, the constant cycle of increasing in temperature and subsequent cooling can cause the connections to loosen over time.

Depending on the manufacturer of the unit, the wire used may be highly stranded for flexibility. This type of wire may be difficult to keep tight. Loose connections can contribute to overcurrent trips, destroy IGBTs, cause input rectifiers to fail, and burn terminals on contactors and switches.

4. Check the line voltages and currents coming into the drive. These voltages should be balanced within five percent. Unbalanced line voltage can cause significant problems. Check the current coming into the input of the drive next.

Current levels can differ a bit more from phase to phase without causing too much concern, but there is the possibility of finding one line completely dead. Remember, most drives today can still run the motor with one phase of the incoming power missing.

5. Check the drive output for voltage and current. The drive produces the waveform going to the motor. On most drives, the voltage from the inverter section should be balanced within a couple of volts and the current should be balanced, as well. Large variations cause the motor to shake violently and can cause motor problems.

These are the basic first steps to determining the problem with any given drive. This process should be done on a periodic basis. If these procedures are followed, most problems can be eliminated and the drive should deliver trouble-free service for many years.

Modern drives are amazingly reliable. With advances in semiconductor technology and increased buss capacitor performance, many of the problems that previously plagued drive manufacturers are all but gone. All of the major drive manufacturers build a relatively sturdy and reliable drive. Minimum internal failures ever occur. Problems outside of the drive now contribute to a large number of drive failures and are a major cause of nuisance tripping.