Variable Frequency Drives (VFDs) are the workhorses of modern motor control, and at the heart of most VFDs lies a component that often goes unnoticed—electrolytic capacitors. These components play a critical role in the performance and reliability of the drive. If you’re a technician working with VFDs, understanding how these capacitors function—and fail—can save you time, money, and headaches.
What Do Electrolytic Capacitors Do In A VFD?
Electrolytic capacitors are primarily used in the DC bus section of a VFD. AC is rectified to DC using diodes, SCRs, or other semiconductors and the DC is stored by the capacitors. The diagram below is a layout of a typical Siemens Medium Voltage Power Cell. Notice the capacitors are the main component of the “DC Link” also known as the “DC Bus.”
Capacitors smooth out the DC voltage by filtering ripple from the rectifier section and store energy to be used by the inverter section. They are like the VFD’s energy reservoir—stabilizing voltage and ensuring consistent power delivery to the motor.
Why Are Electrolytic Capacitors A Common Failure Point In A VFD?
Electrolytic capacitors are electrochemical devices, which means they degrade over time. Their lifespan is influenced by:
- Heat: High ambient temperatures or poor ventilation accelerate aging.
- Ripple current: Excessive ripple can cause internal heating and breakdown.
- Voltage stress: Operating near or above rated voltage shortens life.
- Time: Even under ideal conditions, they have a limited service life (typically 7–10 years).
Signs Your Capacitors Are Failing:
- Bulging or leaking capacitors: Internal pressure from electrolyte breakdown can cause the capacitor to bulge or leak out; this indicates a capacitor is past its useable life and needs to be replaced.
- Random overvoltage and/or undervoltage faults: As the capacitor loses its ability to store charge, it can rapidly charge and then discharge again, causing the drive to trip.
- A distinct smell: Failing capacitors have a very distinct smell that some describe as “burning fish”. The smell is unique and tells you the capacitors are failing.
Capacitor Best Practices:
- Visually inspect capacitors during scheduled shutdowns and/or preventative maintenance.
- Keep the ambient temperature around the drive as low as possible. As with any electronic device, heat rapidly reduces useable life and capacitors are especially vulnerable.
- If you replace 1 capacitor, replace them all. Never just replace one capacitor within a VFD, even if only one appears bad. The DC bus will not remain balanced if the capacitors are at different life cycles.
- Reform VFDs and medium voltage power cells that use electrolytic capacitors if they’ve been on the shelf for over a year. When new capacitors have been on the shelf for an extended period of time, the oxide layer deteriorates and if it isn’t reformed, it can be catastrophic when put into service. You can read more about that here.
Final Thoughts:
Electrolytic capacitors are small components within a much larger drive, but they’re vital to the health and overall performance of your VFD. Understanding that they have a useable life and the signs of their deterioration can greatly reduce catastrophic failures in the future.
Do you need the capacitors replaced in your Medium Voltage Variable Frequency Drive? We can help! Call us, email us, or use our online chat to get in touch with us. Let us prove to you that No One, ANYWHERE, Is Better At Drives Than We Are!