YORK YMC2 — MBC - V24 LOW FREQUENCY DISPLACEMENT

The display shows MBC - V24 LOW FREQUENCY DISPLACEMENT, in exactly this capitalisation. This is the only wording the manuals print for the message, with no variants. It appears in the cycling shutdown messages group of the fault tables and is classified as a fault.

Excessive low-frequency shaft displacement at the opposite-impeller-end radial bearing, on the axis designated V. The MBC measured shaft displacement greater than the trip level from center in the orthogonal radial axis designated V at the opposite-impeller-end radial magnetic bearing while the machine was in Levitation or Rotation mode, and the displacement persisted over a 7 second window. The manuals print different trip levels: one gives greater than 75 µm from center, approximately 1/3 of the air gap space to the touchdown bearing; the other gives greater than 90 micron from center, approximately 2/3 of the air gap space to the touchdown bearing. When the level is exceeded, the MBC contacts open — one manual names them the MBC ESD Fault contacts, the other simply the MBC Fault contacts. Although this is a cycling shutdown, the shaft does not drop out of levitation: the MBC remains in Levitation mode with the motor shaft levitated until it is commanded to de-levitate.

• Shaft displacement greater than 75 µm (per one manual) or greater than 90 micron (per the other) from center in the orthogonal radial axis designated V at the opposite-impeller-end radial magnetic bearing.
• The displacement is present during Levitation or Rotation mode and holds above the trip level over a 7 second window — approximately 1/3 of the air gap space to the touchdown bearing per one manual, approximately 2/3 per the other.

• Trip level, one manual: shaft displacement greater than 75 µm from center — approximately 1/3 of the air gap space to the touchdown bearing.
• Trip level, other manual: shaft displacement greater than 90 micron from center — approximately 2/3 of the air gap space to the touchdown bearing.
• Evaluation window: 7 seconds, during Levitation or Rotation mode.
• Release level: displacement less than 75 µm (one manual) or less than 90 micron (the other).

The manuals describe no separate reset procedure: the message is released when the measured displacement is less than the trip level — less than 75 µm per one manual, less than 90 micron per the other. While the message is active, the MBC stays in Levitation mode with the motor shaft levitated. One manual prints that it remains so until commanded to de-levitate over serial comms or VSD DC buss power depletes; the other prints until commanded to de-levitate over serial comms.