Delta MS300 "bb" External Base Block Fault
Is your Delta MS300 VFD showing a "bb" code? Learn what the External Base Block fault means, why it triggers, and how to reset it to get your motor running.
If you are operating a Delta MS300 series drive and the display suddenly shows "bb" while the motor coasts to a stop, you might think the drive has failed. However, the "bb" code is unique—it is often a status indicator rather than a permanent hardware fault.
In the world of Delta inverters, "bb" stands for Base Block. When this appears, the drive has "blocked" the firing of its internal power transistors (IGBTs). Power to the motor is cut instantly, and the motor is left in a "free running" or coasting state. Here is a human-made guide to help you understand why this happened and how to clear it.
What is an External Base Block?
The Base Block function is an interlock feature. It is typically used for safety or sequencing. When a specific Multi-Function Input (MIx) terminal is programmed for the Base Block function and that terminal is activated (closed), the drive is forced to stop outputting power immediately, regardless of the current speed or command.
Common Causes of the "bb" Code
- External Interlock Triggered: A physical safety switch, guard door, or emergency stop relay wired to a digital input has been activated.
- PLC Logic: Your master controller (PLC) has sent a signal to the drive's digital input to "disarm" the output during a specific part of the machine cycle.
- Wiring Issues: A loose wire or a short circuit in the control wiring is tricking the drive into thinking the Base Block command is active.
- Parameter Misconfiguration: A digital input terminal (MI1 through MI7) was accidentally programmed for the Base Block function (Value 08) without actually having hardware wired to it.
Step-by-Step Troubleshooting Guide
1. Identify the Triggering Input
The first step is to find out which terminal is telling the drive to block the output. Look at your parameter settings for the Multi-Function Inputs:
- Check Pr.02-01 through Pr.02-06 (Settings for MI1 through MI6).
- Look for any parameter set to 08 (External Base Block, normally open) or 09 (External Base Block, normally closed).
- Once you find the terminal (e.g., if Pr.02-03 is set to 08, it's terminal MI3), you know exactly where to look for the physical signal.
2. Check the Physical Switch/Sensor
Go to the device wired to that terminal. Is a safety gate open? Is a relay tripped?
- Action: Ensure the external system is in a "Safe" or "Normal" condition.
- Action: Use a multimeter to verify if 24V DC is arriving at the terminal. If the drive is set for NPN logic, check if the terminal is being pulled to common.
3. Verify Wiring Integrity
Vibration in industrial cabinets can loosen small control wires.
- Action: Tug-test the wires at the digital input terminal block.
- Action: Ensure there are no stray wire strands "whiskers" bridging the 24V (or COM) terminal to your MIx input.
How to Reset the "bb" Fault
Unlike a standard overcurrent or overvoltage fault, the "bb" code often won't disappear just by fixing the switch. You must perform a specific sequence:
- Deactivate the Signal: Restore the external contact (Open the switch if it's NO, or close it if it's NC).
- Press RESET: Press the "RESET" key on the MS300 keypad.
- Check Status: The "bb" should disappear, and the drive should return to its default display (usually showing the target frequency like 60.00 or 0.00).
Summary Checklist
| Checkpoint | Action Required |
|---|---|
| Parameters | Check Group 02 to see which MI terminal is set to "08" or "09". |
| Physical Logic | Ensure the external safety device or PLC relay is reset. |
| Wiring | Ensure the signal wire hasn't fallen out of the MI terminal. |
| Keypad | Press the RED reset button after the signal is restored. |
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