Is your VFD tripping on DC Link Overvoltage? Learn about regenerative energy, braking resistors, and how to stabilize your DC bus for peak performance.

 

High Voltage Alert: Troubleshooting DC Link Overvoltage

It’s one of the most common—and frustrating—faults in the world of Variable Frequency Drives (VFDs). You’re running a process perfectly, and then as soon as the machine tries to slow down or stop, the drive trips.

The screen reads: DC Link Overvoltage (or codes like F0002, OV, or OU).

This isn't usually a sign that your drive is "broken." Instead, it’s a sign that the drive has become an accidental battery charger, and the battery (the DC Bus) is full.

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The "Why": Understanding the DC Bus

To understand overvoltage, you have to look at the internal anatomy of the drive. A VFD converts AC to DC, stores it in a "pool" of capacitors (the DC Link), and then converts it back to AC for the motor.

The voltage in that DC "pool" is typically $1.35\times \text{Input AC Voltage}$. For a 480V system, that’s about 650V DC. If that number climbs too high (usually toward 800V-900V), the drive trips to prevent the capacitors from physically exploding.

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The 3 Main Culprits

1. Regenerative Energy (The #1 Cause)

When a motor slows down a heavy, spinning load faster than it would naturally coast, the motor turns into a Generator. It starts pushing electricity back into the drive. Since the input side of the drive is a "one-way street" (the rectifier won't let power go back to the grid), that energy piles up in the DC Link capacitors.

2. Overhauling Loads

This happens when gravity or another force is "pulling" the motor.

• Example: A downhill conveyor or a crane lowering a heavy crate. The load is spinning the motor faster than the drive wants it to go, constantly pumping energy into the DC bus.

3. High Supply Voltage

If your facility's incoming power is running "hot" (e.g., 500V instead of 480V), your DC link starts at a higher baseline. This leaves very little "headroom" for even minor spikes before the drive trips.

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How to Fix It

Strategy A: Adjust the "Ramp"

The easiest fix is software. Increase your Deceleration Time. By slowing the motor down more gradually, you reduce the amount of regenerative energy pushed back at once.

Strategy B: Use a Braking Resistor

If you must stop fast, you need somewhere for that energy to go. A Braking Resistor acts like a heating element. When the DC bus voltage gets too high, a "Chopper" circuit kicks in and dumps the excess energy into the resistor to be burned off as heat.

Strategy C: Enable "Voltage Regulation" Features

Most modern drives have a "Vdc_max" or "Overvoltage Avoidance" parameter. When enabled, the drive will automatically ignore your stop command and extend the ramp-down time just enough to keep the voltage below the trip point.