Unlike low voltage variable frequency drives where the topology is consistent from manufacturer to manufacturer; medium voltage variable frequency drives come in a wide variety of topologies. The majority of medium voltage VFD manufactures utilize some sort of power cell technology in their drives. By making the output section modular, the user is able to get up and running quickly by simply swapping the power module instead of troubleshooting down to individual components within that module. The design varies a bit from drive to drive, but essentially power cells (or power modules, power arms, etc.) act as miniature drives with the DC bus bank, driver circuitry, and transistors all part of the cell itself. Those cells then work in tandem with the other cells within the drive to create the output to the motor.
When a new medium voltage drive is purchased, the user will oftentimes buy a spare cell to avoid costly downtime when a cell failure occurs. This is certainly advisable, but the question is, “what happens when my cell fails and I am now using what was my spare?” EMA can repair it for you! To make this even easier, we’ve set up a cell repair hotline. By using EMA to repair your cell, you avoid the cost of buying a brand new one, the repaired cell becomes your spare.
Outside of the unforeseen such as flooding, major power spikes, and mechanical damage, there are factors that will affect the life of your medium voltage power cell.
What Shortens Medium Voltage Drive Power Cell Life?
1. Environment: Your power cell is no different than any other electronic component, its environment has a direct and significant impact on its lifespan. Some of the environmental factors are within your control, some are not. Either way, the environment affects your power cells, causing more frequent need for maintenance, repair or replacement.
Within your control:
- Temperature – Heat Kills!
- Humidity – Moisture rusts!
- Wildlife – Rodents chew up wires!
- Filth – dust, dirt and grime!
Not within your control:
- Storms – lightning strikes, floods, tornadoes, hurricanes, and earthquakes!
- Accidents – trucks, forklifts, and cranes!
2. Loose Connections: A study by IEEE showed that loose connections caused almost one-third of all failures in electrical systems. The power and control connections to the power cell can come loose due to normal vibrations around the drive. When a connection isn’t tight, arcing occurs which builds up oxide which eventually will destroy the connection, leading to failure.
3. Capacitors: Capacitors are a common failure item on medium voltage cells. Electrolytic capacitors, which are used by many manufacturers, have a useable life of around 7-10 years, so if your cell is at or past that time frame, your chances of failure increase dramatically. Higher operating temperature can accelerate your capacitor degradation as well. Other manufactures use oil-filled or film type capacitors, which have longer useful lives, but still will degrade over time. Even if your power cell uses oil filled or film type capacitors on the DC Bus, there are still small electrolytic capacitors on the printed circuit boards that have a 7-10 year shelf life.
4. Age vs. Run Hours: The amount of time you’ve had your cell isn’t as important as run time on the cell. Run hours on the cell is the determining factor of its true “age”.
Preventative Maintenance is Designed to Prolong MVD Cell Life
Electrical systems, including medium voltage VFDs without a Preventative Maintenance Program are three times more likely to fail than systems that are on a regularly scheduled Preventative Maintenance program. If your Medium Voltage drive is part of a good MVD Cell Preventative Maintenance Program, the cells inside should have a longer useable life. Preventative Maintenance can be a headache, but it will pay off in the end.
At EMA, we’ve seen many examples of power cell failures and learned how to fix them. If you have a medium voltage power cell that needs repair, call us! Let us prove to you that No One, ANYWHERE, is better at drives than we are.