In a DC (Direct Current) power system, the DC link refers to the intermediate circuit between the AC (Alternating Current) input and the DC output of a power converter, like a variable frequency drive (VFD) or inverter. A DC link undervoltage occurs when the voltage in this DC circuit falls below the acceptable operating range.

Here's a breakdown of DC link undervoltage, its causes, effects, and troubleshooting steps:

Causes:

Several factors can contribute to a DC link undervoltage:

• AC Input Power Issues: The most common cause is a problem with the AC mains power supply feeding the converter. This could be:

• Voltage Sags or Dips: Temporary drops in the AC line voltage can cause the DC link voltage to fall below the minimum threshold.
• Single-phase Operation: If one phase of a three-phase AC supply is lost, it can significantly impact the DC link voltage in a three-phase converter.
• Undersized AC Supply: An AC power source with insufficient capacity for the converter's requirements can lead to undervoltage during periods of high load.

• Converter Malfunction (Less Likely): In rare cases, a malfunctioning component within the converter itself, like a faulty rectifier bridge, could be causing the undervoltage issue.

Effects:

A DC link undervoltage can have several consequences:

• Reduced Output Power: The converter might not be able to provide the necessary DC voltage to the motor or load at the desired power level. This can lead to a decrease in motor speed or torque.
• Drive Shutdown: If the undervoltage condition persists or becomes severe, the converter might shut down to protect itself and the motor from damage.
• Potential Equipment Damage: In extreme cases, prolonged undervoltage can damage the converter or the connected motor.

Troubleshooting:

Here's how to troubleshoot a DC link undervoltage:

1. Verify AC Input Power: Check the AC mains voltage using a voltmeter. Ensure it falls within the acceptable range specified for the converter.
2. Inspect AC Connections: Look for any loose or damaged connections in the AC wiring feeding the converter.
3. Consult User Manual: The converter's user manual will provide specific troubleshooting steps for undervoltage faults. It might include procedures for checking internal fuses or diagnostics for potential converter malfunctions. You can typically find user manuals on the manufacturer's website or through authorized distributors.
4. Contact a Qualified Technician: If you're unsure about the cause of the undervoltage or hesitant to perform any internal checks on the converter, consider contacting a qualified technician familiar with the specific model for further assistance. They can diagnose the issue and ensure safe operation of the converter and motor.

Additional Tips:

• Some converters have built-in features like undervoltage ride-through (UVRT) capability, which allows them to tolerate temporary voltage dips without shutting down.
• If the issue is caused by frequent voltage sags or dips in the AC supply, consider using a voltage regulator or uninterruptible power supply (UPS) to provide a more stable AC input for the converter.

By following these steps and addressing the underlying cause of the DC link undervoltage, you can ensure the proper operation of your converter and prevent potential damage to your equipment.