Troubleshooting Delta MS300 Fault Lvn (Low Voltage)

Getting Lvn or related low-voltage faults on your Delta MS300 VFD? Learn what "Low Voltage at Constant Speed" means, how to check your input power, and steps to stabilize your drive.

 

If your Delta MS300 drive has stopped and displays an Lvn fault code, it is telling you that the drive's internal DC bus voltage has dropped below the safe operating threshold while the motor was running at a constant speed.

Think of the DC bus as the "fuel tank" of your drive. When that fuel level drops too low, the drive cannot maintain the power needed to keep the motor turning, so it trips to protect the internal electronics. Here is how to diagnose why your drive is "running out of gas."

What do Lvn and related codes mean?

The MS300 has several low-voltage fault codes depending on when the trip occurs:

• Lvn (Low voltage at constant speed): The voltage dropped while running at a steady setpoint.
• LvA (Low voltage during acceleration): The voltage dropped while the drive was trying to speed up.
• Lvd (Low voltage during deceleration): The voltage dropped while the drive was slowing down.
• LvS (Low voltage at stop): The voltage is too low even when the motor is stopped.

Common Causes of Low Voltage Faults

• Unstable Input Power: Your facility’s incoming AC voltage is sagging or fluctuating significantly.
• Phase Loss: One of the three input phases is missing or has a poor connection (loose terminal).
• Excessive Load: A sudden increase in the mechanical load on the motor can cause the DC bus voltage to "dip."
• Overloaded Power Supply: The power transformer supplying the drive may be undersized, causing a voltage drop when the motor starts or when other machines in the facility turn on.

Step-by-Step Troubleshooting Guide

Safety Warning: Always ensure power is disconnected and wait the required time for the DC capacitors to discharge before touching any internal wiring or terminals.

1. Check Incoming Power Supply

Measure the voltage at the input terminals (R/L1, S/L2, T/L3) of the MS300 while the drive is running.

• Are all three phases balanced?
• Is the voltage consistently within the range specified on the drive's nameplate? If your supply is significantly lower than the rated voltage, you may need a buck-boost transformer or a line reactor to stabilize the input.

2. Inspect Terminations

Vibration is the enemy of electrical connections. Check the tightness of all input power terminals and any fuses or contactors upstream. A loose connection often acts like a resistor, causing a voltage drop only when current starts flowing.

3. Check for Sudden Load Changes

If the Lvn fault happens when another large machine starts in your facility, you are likely experiencing a facility-wide voltage sag. If the fault happens when your own motor load spikes, verify if your motor is overloaded or if the drive capacity is simply too small for the application.

4. Parameter Adjustments

If the input power is verified to be stable and within specification, you can check the following:

• Pr.06-00: This parameter determines the low-voltage detection level. Ensure it is set to the default value unless your specific application requires a change. Warning: Do not lower this value to "hide" the fault, as this can damage the drive.

Summary: When to seek further help

If you have verified that your input power is solid, all connections are tight, and your load is within the drive's rated capacity, but the drive still trips on Lvn, it is possible that the internal rectifier bridge or DC bus capacitors in your MS300 have reached the end of their service life.