If you're on your fourth motor repair, it's time for a new test technique.
When a motor drive goes down, important production equipment is sure to follow. Troubleshooting the drive can be stressful work, but the pressure can be especially bad if you're on the second or third repair in a short period of time. A gold mining company's technique for dealing with the task can offer some insight into how to handle the pressure and implement a quick and accurate repair.
Round Mountain Gold, Round Mountain, Nev., uses a 500-hp motor and 12-pulse SCR-based variable frequency drive to operate the number one pump at one of its mine operations. While testing the pump's drive, the company's maintenance team noticed the drive did not sound right. It wasn't “singing,” the way its sister drive was. Instead, it was emitting a buzzing noise. The technicians brought their motor tester in to investigate the problem.
With the tester connected to its line side, the drive exhibited high third harmonics and other multiples of the third harmonics. The presence of harmonics alone, however, doesn't indicate a problem. In fact, it's normal for an SCR to produce the 5th, 7th, and 11th harmonics. You can speed up troubleshooting by understanding the harmonics signature that is typical for your drive and application. You should take a “snapshot” of baseline harmonics readings so you can compare them to harmonics readings when troubleshooting later.
Where do you look for these signatures? Many of today's drives are “harmonics-compensated,” so they don't introduce harmonics to the load or supply. Does this mean you can't attain harmonics signatures? No, because you are looking for the harmonics signature of the individual SCRs themselves or the circuitry between the SCRs and the onboard noise filtering.
It's rare to find the 3rd, 9th, or 15th harmonics when viewing the harmonics signature of SCRs, but that's what the technicians found in this case. The presence of these harmonics indicated a problem, but anything from the circuitry in the drive to the SCRs themselves could have been affected. The crew decided more measurements would be necessary to find the culprit.
Next, the maintenance team looked at the voltage and current vs. time signature. The measurements showed the SCRs were not turning off at the zero crossover point. They were 5 years old, which is the typical life span of SCRs in heavy service. Heavy service refers to continuous operation in an environment with high heat and high vibration. The junctions in SCRs of this age do not have sufficient cooling to maintain a temperature conducive to long service life in these environments. As the junctions age, they fail to turn off at the zero crossover.
Common sense might lead you to replace the failed SCR — after all, you can pinpoint the dead SCR with proper measurement. However, experience shows it's best to replace them all at once. Because SCRs age so rapidly under heavy service, replacing only one at a time results in an unmatched set. In this case, the remaining two SCRs were due to fail anyway. It's much cheaper to install a new set than go back four days later when a second failed SCR has halted production. By replacing the set, you avoid serial episodes of downtime, and you increase the average time between failure of the SCRs in one set.
After replacing the full set of SCRs, the maintenance team used the motor tester to read the electrical signature. The harmonics readings revealed the third harmonics and its multiples had subsided, and the fifth harmonic was more pronounced than before. However, its magnitude was normal for this type of drive. The loss of the third and multiples shows up also in lower negative sequence currents.
The voltage and current vs. time signature now shows the SCRs shut off at the zero crossing. With the new set of SCRs, the drive was “singing” instead of “buzzing.”
In most cases, you should take readings from both the line and load side of the drives. This helps determine the drive condition. It also proves useful in other kinds of drives, such as transistor drives. Transistor drives have low fifth harmonics and high third harmonics when they are operational, and they require a different type of troubleshooting. It is a good idea to compare the signatures of similar drives to each other to determine what is normal for that drive or motor.
This comparison will enable you to detect motor and drive problems. It will also allow you to find such things as power factor correction capacitor failure, drive belt misalignment, wet cables, and high-resistance wires and connections. These are all root causes of downtime, and they can contribute to premature drive failure. By knowing the signatures, you can determine and eliminate root causes of failure.
Maintenance efforts aren't always scientific. But by starting with a scientific methodology and a team approach to your entire maintenance philosophy, you can reduce unplanned downtime and cut back on maintenance costs.
Testing and troubleshooting go hand-in-hand. In industries, like gold mining, that are experiencing depressed prices for the goods produced, paying careful attention to these things isn't just a way to ensure a golden future — it's a matter of survival.
Travers is a manager at Round Mountain Gold, Round Mountain, Nev.