Electrical Troubleshooting Quiz — Dec. 7, 2021

At the heart of the plant’s most profitable production line is a 50-hp motor. A variable-frequency drive (VFD) allows for precise control of the line speed based on sensor data fed to a PLC. A few months ago, the VFD began having problems. It was an old drive that preceded today’s ubiquitous use of IGBTs. These old transistors seemed to be running awfully hot, thus making the plant engineer conclude they had been damaged by transients or some similar anomaly. Spare parts were not available, so the decision was made to replace the drive.

The new drive has IGBTs. It also has a much better interface for set-up tuning. Unlike its predecessor, it is corrected for harmonics and power factor so that it does not introduce excess harmonics into the power distribution system and does not reduce the power factor.

In the two months since this drive was installed, the line has had one problem after another. The drive sometimes becomes erratic, the IGBTs have already had to be replaced, and the motor burned up so that it also had to be replaced.

The plant engineer told you they’ve checked for voltage imbalance and it’s negligible. They also checked the drive’s set-up, going step by step through the instructions several times. It sure is a mystery.

You work for an electrical services firm that could finally land a nice contract with this company. It all rides on whether you can solve this problem. What could be going on here?

What they didn't check

So, let’s recap, a bit. They replaced an old, outdated drive with a modern drive. But they didn’t check:

• To see if there’s a motor/drive mismatch, else they would have told you they checked that.
• Whether the motor had power factor correction on it. The old drive likely was not power-factor-corrected, and the new drive is. That means now the power factor may be grossly incorrect. If the motor is power-factor-corrected, then removing the power factor correction capacitor(s) may solve the performance issues.
• Whether there was harmonics mitigation installed. In drives that correct for harmonics, the strategy is to increase the effective source impedance with either an isolation transformer or line reactor (inductor coil) at the supply. If this is done twice, what would have been a correction introduces new error.

Check the bonding & grounding system

Once you check for these three issues, your next step is to identify the equipment grounding (bonding) used for the VFD and then for the motor. A short visit with Art. 250, Part V of the 2020 National Electrical Code (NEC) before doing this check will help ensure you know what to look for. The goal of this bonding is to provide a low-impedance path to the power source (e.g., the service ground) [Sec. 250.4(A)(2)]. Presumably, the equipment grounding (bonding) conductor (EGC) already provides that path from some point near the motor. It is at this “some point” where you need to look.

If, for example, the EGC is the raceway system (e.g., EMT), is there a continuous electrical path to the bonding connection point on the motor case and the bonding connection point on the drive cabinet? One insulating bushing is all it takes to disrupt that. Look at Sec. 250.5 to see the permitted methods of making connections.

You should also read the list of acceptable EGCs [Sec. 250.118] and see if, somewhere in that entire EGC, there is something that’s not on that list. It is not necessarily so that something wrong with the drive or motor is causing these problems. An error could have been introduced through unrelated work or an unrelated incident, and the timing is coincidental.

Check the motor conditions while it's operating

Power quality can’t be taken for granted. While the motor is running, use a power analyzer to check for waveform issues (e.g., flat-topping), frequency cycles, active power, apparent power, power factor, and power consumption. Identify anything that is outside acceptable limits.

Temperature issues also cannot be taken for granted. Is the drive properly installed so that it has adequate cooling? At this size, it should have vents. Are those able to use the minimum space requirements as stated in the installation instructions? Or are some vents blocked from getting adequate circulating air? If the drive is mounted in a cabinet with other equipment, what’s the temperature in the cabinet? Has anyone thought to change the cabinet air filters?

Once all of these issues are investigated and corrected as necessary, study that set-up and tuning process, then carefully go over the settings. If the problems still plague this line, contact the drive manufacturer. They should have a way for you to troubleshoot the drive and the application in general. Or they may conclude the drive needs to be replaced. Be sure to share with them your findings and corrections to this point; a short write-up e-mailed to them could be very helpful.

Be sure to photodocument your work, so this client understands what you found, what you changed, and what effects the change had.