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Vibration In Medium Voltage Motor Caused By Misfiring MV Drive

Medium Voltage Variable Frequency Drives are “intelligent” to a point. Advanced fault sensing firmware makes modern drives fairly user-friendly and easy to troubleshoot; however, there are times when issues leave operators and maintainers scratching their heads! By the way, we’re thankful for this because it is job security for us. 

A customer of ours recently had an issue that was unique and only able to be resolved by an advanced troubleshooting tool.  

The Problem:

A 4500 horsepower, 4160V motor running an ID fan was vibrating excessively, causing noise and overall plant nervousness as operators and maintainers were unsure what was causing it. 

The Troubleshooting:

Multiple tests were done on the motor and load to determine the cause of the vibration, including balancing the blades of the fan and balancing of the motor platform; nothing helped. The motor was run by a 4500 HP Toshiba T300MVi medium voltage variable frequency drive and since it was not faulting, it was assumed that the drive was operating as it should. The plant ran several months with this issue without resolution. 

The Verification:

The plant decided to install a DriveScan remote monitoring tool on the VFD in hopes of shedding some additional light on the situation.  

DriveScan remote monitoring solution helps identify issues with medium voltage drives

Once DriveScan was up and running, the plant was immediately notified by it that there was a very high imbalance on the output of the drive. Typically on VFDs, current imbalance over 5% is an indication of a problem. In this particular case, the imbalance was over 15%! 

Another thing immediately noticeable was the ripple ratio on the DC bus. DriveScan’s unique “medium voltage modules” measure the ripple ratio of the DC bus of individual cells; this helps with determining failing DC Bus capacitors and/or sensing circuits within the cell. In this particular case, the U phase power cell had much higher ripple than the other two, so it was the cell that needed to be investigated first. 

The Solution:

The drive was powered down, locked out, and the U-phase power cell inspected. Upon further inspection, it was determined that an out-of-tolerance resistor on one of the bleeding boards and a failed voltage detection (VDET) board was causing the cell to fire its IGBTs incorrectly, causing the extreme imbalance in output current.  

The 5-level topology of a Toshiba T300MVi utilizes multiple IGBTs at different levels to produce an output as opposed to your typical, single-step topology utilized by most low voltage drives. Because of this, these drives can “miss” a step and still produce an output to the motor; however, it causes issues like the imbalance in this case. 

The faulty cell was replaced with a spare and the imbalance immediately dropped to below 5%, further confirming that the cell was in-fact faulty. The vibration on the motor was also significantly reduced, lowering the potential mechanical wear and tear on the motor. 

By utilizing DriveScan, the problem cell was identified without taking dangerous, onsite “hot” readings. DriveScan’s unique, independent sensors monitor all aspects of VFD health and give users insight into internal and external factors affecting drive-related issues, all from their desk. DriveScan is not limited to just Toshiba drives either; we have installed it all over the world on multiple medium voltage VFD manufacturers and brands.  

DriveScan not only saved the customer thousands in downtime and troubleshooting, it also gave us valuable insight into VFD operation.  

Are you having issues troubleshooting your medium voltage VFDs? Call us, Email Us, or Chat with us to schedule a personal demonstration of DriveScan in order to see if it is the right solution for you! 

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